- Research article
- Open Access
Characterization of the dsDNA prophage sequences in the genome of Neisseria gonorrhoeae and visualization of productive bacteriophage
- Andrzej Piekarowicz†1,
- Aneta Kłyż1,
- Michał Majchrzak1,
- Monika Adamczyk-Popławska1,
- Timothy K Maugel3 and
- Daniel C Stein2Email author
© Piekarowicz et al; licensee BioMed Central Ltd. 2007
- Received: 10 October 2006
- Accepted: 05 July 2007
- Published: 05 July 2007
Bioinformatic analysis of the genome sequence of Neisseria gonorrhoeae revealed the presence of nine probable prophage islands. The distribution, conservation and function of many of these sequences, and their ability to produce bacteriophage particles are unknown.
Our analysis of the genomic sequence of FA1090 identified five genomic regions (NgoΦ1 – 5) that are related to dsDNA lysogenic phage. The genetic content of the dsDNA prophage sequences were examined in detail and found to contain blocks of genes encoding for proteins homologous to proteins responsible for phage DNA replication, structural proteins and proteins responsible for phage assembly. The DNA sequences from NgoΦ1, NgoΦ2 and NgoΦ3 contain some significant regions of identity. A unique region of NgoΦ2 showed very high similarity with the Pseudomonas aeruginosa generalized transducing phage F116. Comparative analysis at the nucleotide and protein levels suggests that the sequences of NgoΦ1 and NgoΦ2 encode functionally active phages, while NgoΦ3, NgoΦ4 and NgoΦ5 encode incomplete genomes. Expression of the NgoΦ1 and NgoΦ2 repressors in Escherichia coli inhibit the growth of E. coli and the propagation of phage λ. The NgoΦ2 repressor was able to inhibit transcription of N. gonorrhoeae genes and Haemophilus influenzae HP1 phage promoters. The holin gene of NgoΦ1 (identical to that encoded by NgoΦ2), when expressed in E. coli, could serve as substitute for the phage λ s gene. We were able to detect the presence of the DNA derived from NgoΦ1 in the cultures of N. gonorrhoeae. Electron microscopy analysis of culture supernatants revealed the presence of multiple forms of bacteriophage particles.
These data suggest that the genes similar to dsDNA lysogenic phage present in the gonococcus are generally conserved in this pathogen and that they are able to regulate the expression of other neisserial genes. Since phage particles were only present in culture supernatants after induction with mitomycin C, it indicates that the gonococcus also regulates the expression of bacteriophage genes.
- Phage Particle
- Strain FA1090
- Gonorrhoeae Strain
- Phage Tail
The sequencing of bacterial genomes has revealed the presence of integrated viral genomes (prophages) in most of the sequenced bacterial genomes [1–3] Prophage DNA sequences can constitute up to 10–20% of the bacterial genome and are major contributors for differences between individual species . Prophage gene expression may influence the pathogenicity or the general fitness of the bacterium [4, 5]. The list of genes regulated by bacteriophage is very long and represents a broad group of genes (For review, see Brüsow et al. [1, 6]).
Analysis of prophage DNA suggests that integration into bacterial genomes can lead to changes including inactivating point mutations, genome rearrangements, modular exchanges, invasion by further mobile DNA elements, and massive DNA deletion [2, 6, 7]. Bacteriophage have been described that can produce plaques on certain commensal Neisseria [8–10]. Bacteriophage able to propagate in N. meningitidis have been identified, but they were not able to propagate on other Neisseria strains . Similarly, the presence of autoplaquing in N. gonorrhoeae was observed but no phage propagation was seen .
DNA sequence analysis has identified prophage DNA sequences in the genomes of most bacteria. In N. meningitidis, they belong to two groups of phages. The presence of the Mu-like prophage sequences was detected in the genomes of serogroup A strains of the epidemic subgroups I, III, IV-1 and VI of N. meningitidis [13–15]. Two additional Mu-like sequences were found in N. meningitidis serotype A . A ~39.3-kb region named as Pnm1  looks as if it could also encode for a functional bacteriophage. A sequence homologous to Pnm1 was found in the genome of serogroup B N. meningitidis but not in genome of N. gonorrhoeae FA1090 . The second group consists of filamentous prophage sequences homologous to f1 and CTXΦ . These prophages can excise, albeit with very low frequency, from the bacterial genome resulting in the production of biologically active phages .
In this paper we describe properties of prophage sequences present in N. gonorrhoeae genomes (NgoΦ1 – 5) that all belong to dsDNA tailed group of bacteriophage. We show the biological activity of some of the prophage genes and the presence of the prophage DNA sequences in bacterial cultures. We further demonstrate the production of phage particles by gonococci. However, we were unable to demonstrate the production of plaques on any of the N. gonorrhoeae or on non pathogenic Neisseria strains tested.
Overall genetic structure of NgoΦ1 – NgoΦ5 prophages
Localization of the prophage islands on the genome sequence of N. gonorrhoeae FA1090
Length of the DNA sequence (bp)
CDS annotations (Acc. No AE004969)
Number of CDS#
ds DNA prophage sequences
455173 – 498100
1044447 – 1078281
1583028 – 1599049
1606828 – 1609808
972837 – 984008
721256 – 729870
Filamentous ss DNA prophage sequences
1080185 – 1088420
NGO1137 – NGO1146
1215967 – 1223383
NGO1262 – NGO1270
1103017 – 1109444
NGO1164 – NGO1170
1599378 – 1607537*
NGO1641 – NGO1648
Properties of the NgoΦ1 CDS' sequences
Significant matches to proteins in GenBank, scores represent filtered BLASTP, % identities, over stated number of amino acid)
Identity with CDSs
VC0847 [Vibrio cholerae] 31, 51% on 406, Int A E. coli K12 32, 51% on 412
B3 CDS5 [Bacteriophage B3] 29, 46% on 227
CDS8 [Streptococcus phage Φ-O1205] (pfam SiphoGp 35, 56% on 144, CDS157 [Streptococcus 157 family) Thermophilus phage Sfi21] 31, 51% on 160
BBTVs1gp1 [Banana bunchy top virus] replication protein 26, 43% on 109 primase A2p38 [Lactobacillus casei phage A2] 32, 44% on 87
NMB0904 [N. meningitidis MC58] 45, 63% on 61, gp30 [Burkholderia cepacia phage Bcep22] 37, 54% on 48
IrgB [V. cholerae strain N16961 serogroup O1] 26, 62% on 50
EF1886 [Enterococcus faecalis (pfam HTH 3 family) regulator V583] 45, 70% on 44, NMB1204 [N. meningitidis MC58] 32, 59% on 62
repressor protein cI
cI [Salmonella typhimurium (pfam peptidase 1630 phage T104] 29, 52% on 222 S24family), HK022 p43 [Enterobacteria phage HK022] 30, 49% on 223 [Pseudomonas phage D3] 26, 42% on 221
Bcep22p22 [B. cepacia phage Bcep22] 32, 52% on 71
DNA replication protein
[Phage Aaphi23]33, 45% on 342 Gp54 [Phage HK97] 26, 42% on 123 [Enterobacteria phage P22] 24, 17% on 125
replicative DNA helicase
(pfam DnaB family, DnaB-C family) [Enterobacteri a phage P1] 34, 53% on 446, CDS74 [Pseudomona s phage D3 ] 32, 55% on 430
SA1787 [Staphylococcus phage phiN315] 41, 51% on 58
rus [Phage 82] 34, 47% on102 (pfamRusA family) RUS Sb45 [S. typhimurium phage ST64B ] 31, 44% on 113, rus [Phage HK620] 30, 47% on 96
gp18 [B. cepacia phage Bcep22] 46, 58% on 41
SfVp44 [Shigella flexner i phage V] 50, 67% on 34
terminase small subunit
f6p01 [Enterobacteria phage Sf6] 45, 69% on 122, 3 [Phage HK620] 45, 69% on 122
terminase large subunit
T1p5324 [Enterobacteria phage T1] (COG 24, 42% on 505, TerL [Phage terminase Aaphi23] 28, 45% on 438,
phage portal protein
gp34 [Phage phi-C31] (pfam Phage Mu F family) 25, 43% on 254, gp34 [Phage phiB 25], 43% on 258
protease structural protein
T5 150 [Phage T5] 31, 48% on 137 psiM100p19 [Methanothermobacter wolfeii prophage psiM100], 46% on 140
2 [Equine herpesvirus 1] 23, 30% on 18
gp8 [Phage phiE125] 30, 44% on 125
SO690 protein, prophage MuSo2, morphogenesis protein [Shewanella oneidensis MR-1] 47, 60% on 53
PHG31p11 protein [Aeromonas phage 31] 28, 46% on 88
gp83 [Phage phi JL001] 25, 46% on 80
gp84 [Phage phi JL001] 42, 58% on 103, D3112p50 [Phage D3112] 32, 47% on 50
BMEI1342 protein) (pfam06940, DUF1287 [Brucella melitenis] 33, 47% on 119
CDS114 [Pseudomonas phage phiKZ 43, 60%on 81 transcriptional regulator PP0276 [P. putida KT2440] Cro/cI family 39, 65% on 63
gp86 [Phage phi JL001] 22,39% on 459 CDS97 [Lactobacillus plantarum phage LP65] 26, 42% on 173 putative phage tail protein HCM2.005c protein [S. enterica] 27, 44% on 180
(pfam SpoVT AbrB family)
NMB0912 protein [N. meningitidis MC58] 32, 45% on 53
NMB0989 protein [N. meningitidis MC58] 55, 61% on 47
tail length tape
JL00p82 [Phage phi JL001 measure protein 29, 50% on 407 putative tail component [Z6034] protein cryptic prophage CP-933P [E. coli] 27, 47% on 259
NMB0899 [N. meningitidis MC58] 100, 100% on 118
[Haemophilus influenzae R2866] (pfam phage integrase family 49, 62% on 200
Modular genome organization of phage NgoΦ1
The DNA sequence of phage NgoΦ1 contains 62 CDSs that we annotated as phage related genes (Fig. 2). A large number of the CDSs show significant homology to other known phage genes (Table 3). Some of the CDSs encode putative proteins with no current functional annotation. Based on available data, sequence similarity, and domain and motif searches, information indicating for modular organization of the genome was obtained. Both ends of the phage DNA encoded two bacteriophage P4-like integrases belonging to pfam 00589 family (NGO0462 and NGO0524) with no homology between either of them. The P4 integrase mediates integrative and site-specific recombination between two attachment sites, located on the phage genome and the bacterial chromosome. NGO0462, encoding the first of these integrases, is preceded by the sequence encoding serine tRNA (NGO0461) and a 151 bp noncoding region. Similarly, a noncoding region is also present downstream of the last phage gene (NGO0524). We were unable to identify any repetitive sequences at either end of the integrated phage DNA.
Analysis of the CDSs encoded by this phage allowed us to clearly identify two blocks of genes encoding structural and assembly proteins. The longer block spanning from NGO0494 to NGO0505 includes such genes as NGO0494 and NGO0495 which encode small and large terminase subunits respectively, NGO0496 which encodes phage portal protein and NGO0510, which encodes a phage tail protein. The smaller block of structural genes is located at the end of phage genome (NGO0522 and NGO0523) and encodes the tail protein. The remaining CDSs encode either proteins of unknown function or other structural proteins. It is interesting that gene NGO0509, which interrupts the larger block, belongs to the transcriptional regulator family of genes. The regulatory modules include one well defined region (NGO0474 to NGO0483) with NGO0479 encoding a protein similar to cI-like repressor protein. Two less defined regulatory modules are located in the region between NGO0515-0517 and in the region of NGO0464. The main regulatory module are predicted to encode not only cI repressor but also other proteins that show some homology to phage and cell transcriptional regulatory proteins, (NGO0474, NGO0477, and NGO0478), or the presence of a domain specific for regulatory protein (NGO0483). NGO0479 contains well defined helix-turn-helix motif (E value 6-4) between I30 and F75 and high scoring (E value 5-6) from V147 to D219 peptidase family S24. These motifs are present in DNA binding proteins, including the cro and cI proteins of phage lambda. NGO0477 also contained a helix-turn-helix motif (E value: 4-10).
The special control region includes two CDSs, NGO0516 and NGO0517, which encode PemK and PemI-like proteins. The pemI-pemK system is an addiction module present on plasmid R100 that helps to maintain the plasmid by post-segregational killing of E. coli cells that have lost the plasmid [19, 20]. PemK, the toxin encoded by the pemI-pemK addiction module, inhibits protein synthesis in an E. coli cell-free system, whereas the addition of PemI, the antitoxin against PemK, allows for the resumption of protein synthesis. These systems are also known to operate by stabilizing the presence of autonomously replicating prophages, like P1 .
The third module includes genes that play a role in the replication of the phage genome. This module includes two well defined CDSs, NGO0484 and NGO0485, and a third CDS, NGO0469, located to left end of the genome. NGO0485 encodes a helicase with high homology to the phage P1 gene ban or gp12 of Salmonella typhimurium bacteriophage ST64T (Table 3). All these proteins belong to pfam DnaB, DnaB-C family. NGO0484 shows high homology to a DNA replication protein of bacteriophage AAphi23 as well as other phages. Finally, the NGO0469 probably encodes the primase.
Modular genome organization of phage NgoΦ2
Properties of the NgoΦ2 CDS' sequences
Significant matches to proteins in GenBank (Scores represent filtered BLASTP % identities over stated number of amino acid) CDSs
prophage antirepressor protein 933N], 37, 56% on 20
Ant [Phage Aaphi23], 41, 56% on 258 (pfam02498, Bro N family) [E. coli prop CP933N], 37, 56% on 20
F116p45 [P. aeruginosa phage F116] 31, 42% on 148
phage structural protein
F116p44 [P. aeruginosa phage F116] 42% on 140
phage structural protein
F116p43 [P. aeruginosa phage F116] 56, 72% on 415
F116p42 [P. aeruginosa phage F116] 34, 49% on 141
possible DNA methylase
[Sinorhizobium meliloti phage PBC5] 33, 52% on 584
[S. meliloti phage PBC5] 33, 52% on 584 hypothetical protein F116 p60
phage portal protein
F116p40 [P. aeruginosa phage F116] 58, 71% on 756
phage terminase large subunit Terminase 3 family
ZMO0379 [Zymomonas mobilis subsp.mobilis ZM4]phage terminase large subunit 35, 50% on 397
[H. influenzae RdKW20] 44, 67% on 168
prophage terminase small subunit
STY1046 [S. enterica subsp. enterica serovar Typhi] small subunit 33, 57% on 45
F116p37 [P. aeruginosa phage F116] 38, 59% on 81
endonuclease of the HNH family
17R [Xanthomonas oryzae phage endonuclease Xp10] 41, 53% on 167 [S. thermophilus phage ST3] 47, 55% on 85
protein Nin B
HK022 p48 [Enterobacteria phage HK022]30, 50% on 121, 933Wp33 [Phage 933W] 27, 49% on 99
F116 p35 [P. aeruginosa F116] 42, 57% on 98 pfam DUF 1364 CDS-136 [S. typhimurium phage ST64T] 53, 66% on 30
NMB1116 [N. meningitidis] 88, 96% on 27
NMB0910 [N. meningitidis regulator MC58] 83, 87% on 215, F116p29 [P. aeruginosa phage F116] 39, 54% on 157 putative CI protein cI [Phage Aaphi23]37, 54% on 191, cI [Pseudomonas phage D3]30, 44% on 218
CDS21 [Phage bIL311] GepA [phage-like] 37, 53% on 135, SpyM3_1207 [S. pyogenes phage 315.4] 40, 66% on 71
33 [Enterobacteria phage epsilon15] 68% on 44
The structural and assembly module, extending from NGO1105 to NGO1086, contains the genes encoding the large and small terminase proteins (NGO01098 and NGO01101 respectively) and the portal protein (NGO01097). The structural module contains one very large protein (1977 aa) encoded by NGO1092, whose C-terminal part shows some homology to DNA methyltransferases. NGO1085 would seem to encode a protein with homology to phage antirepressors. While the structural module of NgoΦ2 (NGO1102 to NGO1186) shows high identity with P. aeruginosa phage F116, the structural block of NgoΦ1 does not show high identity with any particular phage sequence.
Modular genome organization of phage NgoΦ3
The DNA sequence of NgoΦ3 is much smaller than NgoΦ1 and NgoΦ2 and is disrupted by the insertion of the DNA sequence of ssDNA phage NgoΦ9. The first part of the sequence (16021 bp) encodes 28 CDSs while the second part of (2980 bp) encodes only four CDSs. The genome sequence starting with NGO1613, encodes a P-22-like integrase but without homology to the integrase encoded by NgoΦ1. The majority of the DNA sequence shows homology to DNA sequence of NgoΦ1 and NgoΦ2. Two regions without homology include: (i) NGO0475 – NGO0409 responsible for the maintenance of lysogenic state (encoding a repressor) and (ii) NGO0482-NGO0487, genes responsible for DNA replication. There is also lack of the NgoΦ1 DNA region encoding PemK-PemI proteins. However, the most important difference between the genomic sequences of NgoΦ3 and NgoΦ1 and NgoΦ2 is the lack of CDSs encoding the structural protein (structural-assembly module). The second part of the genome of NgoΦ3 encodes the antirepressor protein (NGO1652) which is identical with NGO1085 encoded by the phage NgoΦ2.
Modular genome organization of phages NgoΦ4 and NgoΦ5
Properties of the NgoΦ3 CDS' sequences
Significant matches to proteins in GeneBank (Scores represent filtered BLASTP % identities over stated number of amino acid)
[Xylella fastidiosa Dixon] (phage integrase family) 0524 35,52% on,367, [Pseudomonas syringe tomato str. DC3000] 24,38% on 340, int [Azoarcus sp. EbN1] 25,42% on 365
conserved hypothetical protein
CDS21 [Bacteriophage bIL311] (pfam GepA,) 29,48% on 116
HP2p14 protein [Haemophilus phage (pfam UPFO150 family) HP2] 34,54% on 129
conserved hypothetical protein
[HP1p18] protein [Haemophilus (COG1724) phage HP1] 60,74% on 51
hypothetical protein [P27p17] [Bacteriophage P27] 30,51% on 147 replication protein [SfVp87] [Shigella flexneri bacteriophage V] 32,54% on 87
[STM2625] protein [Phage Gifsy-1] (pfam IstB family) 37,51% on 201, replication protein DnaC [Bacteriophage P27] 37,56% on 192
NMB1116 [Neisseria meningitidis MC58] 88,96% on 27
Neisseria-specific protein, uncharacterized
protein Ant [Bacteriophage (pfam Bro-N family) 1085 Aaphi23]
Properties of the NgoΦ4 CDS' sequences
Significant matches to proteins in GeneBank (Scores represent filtered BLASTP % identities over stated number of amino acid)
RecB family exonuclease
RecB family exonuclease
[Exiguobacterium sp. 255-15] antiterminator 27,53% on 77 (traA) [Lactococcus lactis] 20,46% on 100
BH0339 [Bacillus halodurans C-125] (pfam DUF694 family) 36,55% on 180
[NMA1601b][Neisseria meningitidis Z2491 79,83% on 43
VT2-Sap81 [Bacteriophage VT2-Sa] (pfam zf-dskA-traR family) 42,55%on69, phi4795p07 [Phage phi4795]40,53% on 69
NMB1087 [Neisseria meningitidis MC58] 44,56% on 67
NMB1014 [Neisseria meningitidis MC58] 93,94% on 75
conserved hypothetical protein
Pseudomonas aeruginosa 32,51% on 137
[NMB1007] [Neisseria meningitidis pfam HTH 3 family phage repressor MC58] 60,72% on 126
NMB1005 [Neisseria meningitidis MC58] 79,84% on 73 B3CDS17 [Bacteriophage B3] 46,59% on 47
NMB1003 [Neisseria meningitidis MC58] 77,90% on 95 hypothetical protein B3CDS14 [Bacteriophage B3]32,61% on 90
[Bacteriophage B3] 43,61% on 585 (pfam rve family) integrase protein [Salmonella typhimurium LT2] 25,41% on 383
Properties of the NgoΦ5 CDS' sequences
Significant matches to proteins in GeneBank (Scores represent filtered BLASTP % identities over stated number of amino acid)
NMB1387 [Neisseria meningitidis MC58] 99,99% on 44
NMA1603 [Neisseria meningitidis Z2491] 72,76% on 173
NMA1602 [Neisseria meningitidis Z2491] 94,97% on 36
NMA1216 [Neisseria meningitidis Z2491] 94, 100% on 37 HP2p14 [Haemophilus phage HP2] 31, 61% on 2
NMA1216 [Neisseria meningitidis Z2491] 97, 97% on 34
NMB1116 [Neisseria meningitidis MC58] 54,62% on 90
phage baseplate assembly
[o0965] [Escherichia coli CFT073] (pfam Baseplate J family) 54, 67% on 146, gpJ [Enterobacteria phage P2] 48, 61% on 146
phage baseplate assembly protein
CDS17 [bacteriophage phi CTX] (pfam Phage base V family) gpW [Enterobacteria phage P2] (pfam GPW gp25 family 37,57% on 90
putative cI-like repressor
[Streptococcus pyogenes phage 315.3] 45,67% on 31
[Neisseria meningitidis MC58] 78, 82% on 217 protein V6-Haemophilus influenzae 55,69% on 175
NMA1219 [Neisseria meningitidis Z2491] 97, 97% on 82
Distribution of the dsDNA prophage sequences among the N. gonorrhoeae strains
Biological activity of the prophage repressor genes
The maintenance of the lysogenic state is most often maintained by the action of a repressor gene. Functionally, the repressor is silencing the activity of most of prophage genes. However, the expression of several genes, whose transcription is constitutive, is independent of its control. Recently it was shown that host genes can be regulated by phage repressors active in the lysogenic cells . If this were true for the dsDNA prophages integrated into N. gonorrhoeae FA1090, we would expect that the phage repressors could influence the expression of cellular genes. We tested for their biological activity in E. coli.
The influence of expression of CDS1116 on the different promoter activity.
XylE activity (U)a
E. coli Top10 (pMPMT4::CDS1116, pMPMK6::PRHP1) - ara
22.3 +/- 10.2
E. coli Top10 (pMPMT4::CDS1116, pMPMK6::PRHP1) + ara
20 +/- 9.5
E. coli Top10 (pMPMT4::CDS1116, pMPMK6::PLHP1) - ara
268.2 +/- 33.1
E. coli Top10 (pMPMT4::CDS1116, pMPMK6::PLHP1) + ara
20.0 +/- 9.2
The putative holin proteins
Most bacteriophage must lyse their host cells to liberate the progeny virions. The decision of when to terminate the infection and to lyse the host is the only major decision made in the vegetative cycle . Thus, if the prophage sequences present in the genome of N. gonorrhoeae are able to excise from the bacterial genome and produce progeny, they should encode a lytic system. Phages with double-stranded nucleic acid genomes use the "holin-endolysin" strategy . In this scheme, the phage encodes murein-degrading enzyme, an endolysin, and a second membrane-embedded protein, holin, which serves to activate the endolysin at the defined time. Holins are small membrane proteins that accumulate in the cytoplasmic membrane of the host. The holins are represented by more than 250 members belonging to 50 gene families with no recognizable detectable sequence similarity . The common property of all holins is the presence of one or more trans-membrane domains , which allows for their identification in the phage sequences. Our analysis of the proteins encoded by the prophage NgoΦ1-NgoΦ3 sequences suggested that the NGO0488, NGO1106 and NGO1622 could potentially encode a holin. NGO0488 and 1106 are identical and encode for a protein of 49 amino acids, while NGO1622 encodes for a protein of 91 amino acids. These proteins do not showhomology to any other proteins and each possesses one TM domain. To test whether the NgoΦ1 putative holin can act as a true holin, we tested to see if this CDS could complement mutations in the phage λ S gene. For controlled expression of NGO0488, this gene was inserted downstream of the pBAD promoter of pMPMK6  resulting in the construct pMPMK6hol. The E. coli MM 294 cells (λc I857 S am7), carrying the plasmid pMPMK6::hol were grown at 30°C to the optical density of OD600 of 0.6. At that time, the heat shock was carried out (45°C for 20 min) and arabinose was added to a final concentration of 1.0%. In the presence of hol gene of NgoΦ1 phage, even without its induction, the cells grew very slowly. We were unable to detect the lysis of these cells after induction by arabinose (data not shown). Thus, in these experiments the uninduced level of holin was sufficient to inhibit the growth of E. coli cells, carrying an uninduced λ prophage.
Presence of the dsDNA phages in N. gonorrhoeae supernatants
To detect the presence of phage DNA in phage particles released from the cells during growth, the same method was used as described previously for the detection of the filamentous ssDNA phages in N. meningitidis cultures . In this method, the presumptive phage particles are precipitated with PEG and the precipitate intensively treated with DNase and RNAse to remove contaminating bacterial genomic DNA and RNA. The DNA is then isolated from the phage and used as a template in PCR reactions. The presence of the particular phage DNA sequence in such phage preparations means that the prophage genome must be excised from the bacterial genome, and after a replication cycle, packaged into the phage particles and released from the cells.
The existence of prophage DNA sequences in the bacterial chromosomes is very common . It is also very common that several different prophage sequences can be present in the genome of a particular bacterial strain. For example, in eleven Salmonella enterica serovars Typhi and Typhimurium, two Yersinia pestis strains, Shigella flexnerii, two Xylella fastidiosa, and four E. coli strains that have been sequenced, each carries between 7 to 20 prophages (see review by Casjens ). Cryptic prophage DNA sequences have been found in several N. meningitidis strains as well [15, 16]. Our analysis of nine DNA sequences of N. gonorrhoeae allowed us to identify the presence of two different types of bacteriophage DNA sequences in this species. The first group is represented by sequences that are homologous to the filamentous ssDNA prophages recently discovered in the chromosome of N. meningitidis [16, 18]. The second group is represented by the sequences that show the high homology to the tailed dsDNA phages present in diverse groups of bacteria. The overall genetic organization of these phages resembles mostly that of P2 genome ld MACROBUTTON endnote+.cit . This is in sharp contrast to N. meningitidis where dsDNA prophage sequences show homology to Mu-like phages representing a very specific group of phages .
NgoΦ1 and NgoΦ2 could encode functional bacteriophages since they seem to encode all of genes necessary for lytic growth. The ability to produce functional bacteriophage is further supported by the fact that we were able to detect the presence of some of these genes outside of the cells in a DNase resistant form (phage particles). We were able to visualize phage particles. We showed that some of the genes are biologically active. Since we were unable to further propagate phages on any of the strains tested, it suggests that each strain is expressing the appropriate lysogenic control genes.
It is much less probable that the phage belong to gene transfer agents that are tailed phage-like particles that encapsidate random fragments of the bacterial genome  since we were unable to detect the chromosomal sequences in the DNase resistant forms. Three of the dsDNA prophages present in N. gonorrhoeae FA1090 are probably defective forms of NgoΦ1 and NgoΦ2, being in the stage of complex decay of prophages. We believe that NgoΦ5 can be classified as a bacteriocin since it seems to encode mainly the genes of phage tail.
The presence of two types of biologically active prophage in the N. gonorrhoeae may form the molecular explanation for the observation of the formation of autoplaquing . While Campbell and coworkers were clearly able to induce gonococci to produce products capable of inhibiting their growth, they were unable to demonstrate the propagation of phage. On the other hand, it is possible that autoplaquing does not result from phage propagation, but rather an alteration in an autoylytic mechanism within the cell, as suggested for autoplaquing in Myxococcus . It could also be the result of down-regulation of the phage repressors and up-regulation of the phage encoded holin-endolysin lytic system that seem to be encoded by NgoΦ1 and NgoΦ2 prophages, in response to an environmental change.
To test for the presence of prophage sequences in different gonococcal strains we determined the presence of three different genes present in NgoΦ1 and NgoΦ2 using a PCR method, with the specificity of the PCR product verified by DNA sequence analysis. The results indicate that the prophage sequences detected in strain FA1090 are not present in all N. gonorrhoeae strains. Although, the specific PCR products encoding the large terminase subunits of NgoΦ1 and NgoΦ2 were formed using the chromosomal DNA from FA1090, MS11,1291 and WR302, the DNA encoding the holin gene (NGO0488) was not detected in 1291 and WR302. Similarly, we were unable to detect the presence of the DNA sequences representing phages NgoΦ3-NgoΦΦ5 in strains as WR220 or 1291. However, the lack of corresponding PCR products or very low amount of product formation, as in case of strain WR220 and 1291 the DNA encoding holin gene could be due to the differences in the DNA sequence encoding the genes in particular strains, thus changing the formation of the hybrids between primers and template DNA used for PCR reaction and the chromosomal DNA.
Presence of the prophage sequences in the bacterial genomes may have a profound effect on the pathogenicity of the host cell (see review by Wagner & Waldor, ) as well as on the population fitness . Over time, a number of toxin genes have been found to be phage-encoded and it has become clear that toxin genes are only a part of diverse group of virulence factors encoded by bacteriophages . We do not know whether any of the identified prophage genes of N. gonorrhoeae can be recognized as encoding bacterial toxins. On the other hand the properties of the NgoΦ1 and NgoΦ2 genes encoding putative repressors could have a profound effect on the fitness of N. gonorrhoeae strains. In E. coli the expression of the λ repressor inhibits the growth of cells in energy-poor environments, probably as an adaptive response to a host predation system . We have shown that when E. coli is grown on a rich LB medium, the bacteria that express the repressor grows poorly and that induction of these genes inhibits of growth of cells, with high level of induction leading to cell death. This effect can be manifested by acting through the influence on the expression of different genes. The repressor of NgoΦ1 seems to inhibit the expression of the λ phage since even very low level of expression blocks the development of λ particles. These results suggest the tight control of genes encoding these repressors that would allow only very low levels of these proteins. The need for tight control of prophage gene expression is also evident from the activity of the NGO0488 encoding the holin. The activity of the gene has to be tightly controlled otherwise it is lethal for the cells and will results in the cell death.
Among different types of bacterial toxins that are phage encoded are the R- and F-type pyocins produced by P. aeruginosa. Pyocins are derived from a common ancestral origin with P2 phage and the λ phage respectively . The gene organization of the R2 and F2 pyocins suggest that they are phage tails that have evolutionarily specialized to become bacteriocins. While the data presented in this paper do not show directly that these prophage sequences act as this type of toxin, the genes of NgoΦ4 encoding mainly the phage tail structural proteins could play such role.
To our knowledge, this is the first demonstration of bacteriophage production by the gonococcus. Their role in pathogenicity is not known and has to be understood. The activity of the repressors on the chromosomal gene activity implies strict control of the level of their production. The ability to manipulate this level could be a potential method by which bacteria regulate their growth in the human body.
Bacterial strains, plasmids, phages and growth conditions
Escherichia coli Top 10: F- mcrA Δ(mrr-hsdRMS-mcrBC) φ80 Δlac ΔM15 Δlac X74 deoR recA1 araD139 Δ(araA-leu) 7697 galU galK λs-rpsL endA1 nupG, E. coli strain MM294 λ111 (cI 857 Sam7), and E. coli 3102 λcIts 857 were grown in Luria-Bertani broth (LB) at 37°C or 30°C . Antibiotics included in media were used at the following final concentrations (μgml-1): ampicillin 100, kanamycin 10. Neisseria strains were grown in standardgonococcal medium (designated GCP if broth and GCK if agar) (Difco laboratories) plus Kellogg's growth supplements  and 0.042% sodium bicarbonate if in broth or in a 37°C CO2 incubator. Haemophilus influenzae strain Rd was grown in BHI (Difco) supplemented with 2 μg of NAD ml-1 and 10 μg of hemin ml-1 at 37°C . Bacteriophage HP1 was originally obtained from R.D. Herriot. All HP1 phage manipulations were carried out as previously described  while those with λ phage as described by Sambrook et al., . Plasmid pUC19 was purchased from MBI Fermentas. Plasmid pXYL20 was described previously . Plasmid pMPMK6Ω was obtained from P. Mayer .
Cloning of N. gonorrhoeae DNA fragments carrying the NgoΦ1 and NgoΦ2 prophage genes
A DNA fragment carrying ORF NGO0479 of N. gonorrhoeae FA1090 was amplified using primers RNGF1c and FNGF1CI. The resulting amplicon (717 bp) was cloned into pMPMK6Ω at the EcoRI and the PstI sites, resulting in the formation of plasmid pMPMK6cds479. A DNA fragment carrying ORF NGO1116 of N. gonorrhoeae FA1090 was amplified using primers F2Reg8L2 and F2Reg8Rt. The resulting amplicon (711 bp) was cloned into pMPMK6Ω and pMPMT4Ω at the EcoRI and PstI sites, resulting in the formation of plasmids pMPMT4cds1116 and pMPMK6cds1116 respectively. A DNA fragment carrying ORF NGO0488 was amplified using primers H0488T4F and HO488T4B. The resulting amplicon (150 bp) was cloned into pMPMK6Ω DNA into EcoRI and PstI sites, resulting in the formation of plasmid pMPMK6CDS488. All amplicons lacked their native promoters. Protein expression was from an inducible pBAD promoter.
Cloning of the H. influenzae HP1 phage p L and p R promoters
The XylE cassette without any promoter sequence was derived from the plasmid pXYL20 and introduced into EcoRI and HindIII sites of pMPMK6Ω, resulting in the plasmid pMPMK6::XylE. The DNA fragments of phage HP1 carrying the pL and pR promoters  were amplified using HPPL-F and HPPL-R and HPPR-F and HPPR-R primers respectively. The resulting amplicons of 510 bp and 588 bp respectively were cleaved with EcoRI and BamHI and cloned into pMPMK6::XylE plasmid, resulting in the formation of plasmids pMPMK6::XylE::pL and pMPMK6::XylE::pR respectively. In both of these plasmids the pBAD promoter was removed and the XylE expression placed under control of the p L or p R promoters of HP1 phage.
Detection of the prophage and phage sequences
List of primers used in this study
Amplifies the 5' end of NGO0495 (putative terminase) without any restriction sites
94°C, 5 min.; 2× (94°C, 48° 45 sec, 72°C 1 min); 30× (94°C, 45 sec.; 63°C,45 sec.; 72°C, 3 min.); 72°C, 10 min.
Amplifies the 3' end of NGO0495 (putative terminase) without any restriction sites
GCGAATTC GATGTCTGAA TTTAAAGACCGCCTGAAAG AG
Amplifies the 5' end of NGO1116 (putative repressor cI) with EcoRI site
94°C, 5 min.; 2×(94°C, 30 sec.; 48°C, 45 sec.; 72°C, 2 min.); 28× (94°C, 15 sec.; 50°C, 45 sec.; 72°C, 2 min.); 72°C, 10 min.
ATTCTGCAG TCACATCAAT CCAACACGCTCCACCAAA AG
Amplifies the 3' end of NGO1116 (putative repressor cI) with PstI site
Amplifies the 5' end of NGO1098 (putative terminase) without any restriction sites
94°C, 5 min; 30× (94°C, 45 sec.; 63°C, 45 sec.); 72°C, 3 min.; 72°C, 10 min.
Amplifies the 3' end of NGO1098 (putative terminase) without any restriction sites
GCCGGAATTC TCCAAGTT TTAAACTTTCAAC
Amplifies the 5' end of NGO1085 (putative antirepressor) with EcoRI site
94°C, 5 min.; 3×(94°C, 1 min.; 44°C, 45 sec.; 72°C, 3 min.); 27× (94°C, 1 min.; 59°C, 45 sec.; 72°C,3 min.); 72°C, 10 min
TTATCTGCAG TTACCTTAC CGTAGCCTTGCC
Amplifies the 3' end of NGO1085 (putative antirepressor) with PstI site
Amplifies the 5' end of NGO1636 (putative replication protein DnaC) without any restriction sites
94°C, 2 min; 3× (94°C, 1 min.; 48°C, 45 sec.; 72°C, 3 min.28× (94°C, 45 sec.; 52°C, 45 sec.; 72°C, 2 min.); 72°C, 10 min.
Amplifies the 3' end of NGO1636 (putative replication protein DnaC) without any restriction sites
Amplifies the 5' end of NGO1013 (putative repressor cI) without any restriction sites
94°C, 2 min; 3× (94°C, 1 min.; 48°C, 45 sec.; 72°C, 3 min), 28×(94°C, 45 sec.; 50°C, 45 sec.; 72°C, 2 min.); 72°C, 10 min.
Amplifies the 3' end of NGO1013 (putative repressor cI) without any restriction sites
Amplifies the 5' end of NGO0729 (putative repressor cI) without any restriction sites
94°C, 2 min; 28× (94°C, 45 sec.; 52°C, 45 sec.; 72°C, 2 min.); 72°C, 10 min.
Amplifies the 3' end of NGO0729 (putative repressor cI) without any restriction sites
Amplifies the 5' end of NGO0477 (putative repressor cI)
94°C, 2 min; 28× (94°C, 45 sec.; 52°C, 45 sec.; 72°C, 2 min.); 72°C, 10 min.
Amplifies the 3' end of NGO0477
Amplifies the 5' region of PL promoter of phage HP1
94°C, 5 min; 27×(94°C, 45 sec.; 56°C, 45 sec.; 72°C, 1 min.); 72°C, 10 min.
Amplifies the 3' region of PL promoter of phage HP1
Amplifies the 5' region of PR promoter of Φ HP1
94°C, 5 min; 27× (94°C, 45 sec.; 56°C, 45 sec.; 72°C, 1 min.); 72°C, 10 min.
Amplifies the 3' region of PR promoter of Φ HP1
Amplifies the 5' region of putative holin of NgoΦ1
94°C, 2 min; 2× (94°C, 48° 45 sek, 72°C 1 min); 28 × (94°C, 45 sec.; 56°C, 45 sec.; 72°C, 1 min.); 72°C, 10 min.
Amplifies the 3' region of putative holin of NgoΦ1
Enzymes and chemicals
Restriction enzymes were purchased from MBI Fermentas and New England Biolabs. T4 DNA ligase, Pfu DNA polymerase and DNA and protein size markers were purchased from MBI Fermentas. Kits for the DNA purification and plasmid DNA isolation were purchased from A&A Biotechnology (Gdansk, Poland). All the chemicals used were reagent grade or better and were obtained from Sigma (St. Louis, MO), unless otherwise noted.
Quantitative assays were performed as described by Braun & Stein, . Briefly, 20 ml of LB was inoculated with an E. coli Top10 strain carrying the appropriate plasmid and grown at 37°C until a culture density of about 1 × 108 was achieved. The induction of the NGO1116 gene was achieved by the addition of arabinose to a final concentration of 0.1%, with additional incubation for 60 min at 37°C. Cells were harvested and resuspended in 2.5 ml of 50 mM potassium phosphate (pH 7.5), 20 mM EDTA-10% acetone (vol/vol) (pH 7.2) and 0.01% of Triton X-100. After 5 min of incubation on ice, the resulting crude lysate was clarified of cell debris by centrifugation at 4,000 × g for 5 min and then in a microfuge at 10,000 × g for 10 min. Assays were performed by diluting cell extracts in assay buffer (100 mM potassium phosphate, 0.2 mM catechol). Dilutions were chosen such that a linear change in absorbance at 375 nm was seen over time. XylE activity was calculated by linear regression of the slope over six time points. One microunit of XylE activity corresponds to the formation of 1 mM of 2-hydroxymuconic semialdehyde per min at 22°C. XylE activity was normalized against total protein concentration, as determined by the method of Bradford et al. , with bovine serum albumin (MBI Fermentas) as the standard.
The secreted form of phage and its DNA was prepared by standard phage preparation techniques . Bacteria were collected by centrifugation from 200 ml of exponentially growing culture in GC medium, and DNA that was extracted from the cells was dissolved in 200 μl of TE buffer (10 mM Tris-HCl, 1 mM EDTA-Na pH, 8.0). After filtration through a 0.45 μM filter, the supernatant was treated for 3 h at 20°C with DNase I and RNaseA, (25 μg ml-1/ml each). Particles were precipitated by the addition of NaCl to a final concentration of 1 M and polyethylene glycol 8000 to 10%, incubation at 4°C overnight, and centrifugation at 12,000 g for 30 min. DNA from the presumptive phage particles was extracted with phenol, and the precipitated material was redissolved in 200 μl of TE buffer.
In silico analysis
DNA and protein sequences were compared with the GenBank and SWISS-PROT databases on the BLAST server hosted by the National Center for Biotechnology Information . The N. gonorrhoeae strain FA1090 genomic sequence was obtained from the University of Oklahoma's Advanced Center for Genome Technology  and the N. meningitidis strain Z2491 (serogroup A) genomic sequence (and the genomic sequence of the N. meningitidis strain FAM 18 from the Sanger Institute . Other comparisons were performed using the BLAST tools at the NCBI web site . Codon usage and codon frequency comparisons were performed using the CUSP and CODCMP programs at . The G+C content of various chromosomal fragments was determined using the program COMPOSITION at . ORFs were identified using GeneMark.hmm for PROKARYITIC (Version2.4) , EMBOSS , Glimmer , EasyGene 1.0 , ORF Finder, NCBI Conserved Domain Search , GeneImage Map for Neisseria gonorrhoeae , and EMBL-EBI (CpG content) ).
An overnight culture of N. gonorrhoeae was diluted 30 times in fresh media and incubated with shaking for 2 hr. Mitomycin C was added (20 ng ml-1 final concentration) and culture incubated in the dark for 3 hr. Chloroform was added and the culture shaken for 20 min. The cells and debris were removed by centrifugation for 20 min at 5000 rpm and the supernatant was filtrated through a 0.45 μm. Phage particles were precipitated by the addition of NaCl to a final concentration of 1 μM and polyethylene glycol 8000 to 10%, in the presence of CaCl2 to a final concentration of 1 mM and DNase I and RNaseA, (25 μg ml-1 each) for 16 h at 4°C. The precipitate was centrifuged for 30 min at 12 500 rpm at 4°C. The pellet was resuspended in TE buffer. Phage were stained with uranyl acetate (2%) for 30 sec prior to visualization on a Zeiss EM10CA microscope (80 kv).
This work was supported by the Faculty of Biology Grant no. 501/64-1055/4 to AP, and a grant from the National Institutes of Health (AI24452) to DCS. We acknowledge the Gonococcal Genome Sequencing Project supported by USPHS/NIH grant #AI38399, and B.A. Roe, L. Song, S. P. Lin, X. Yuan, S. Clifton, Tom Ducey, Lisa Lewis and D.W. Dyer at the University of Oklahoma.
- Brussow H, Canchaya C, Hardt WD: Phages and the evolution of bacterial pathogens: from genomic rearrangements to lysogenic conversion. Microbiol Mol Biol Rev. 2004, 68: 560-602. 10.1128/MMBR.68.3.560-602.2004.PubMed CentralView ArticlePubMedGoogle Scholar
- Canchaya C, Fournous G, Brussow H: The impact of prophages on bacterial chromosomes. Mol Microbiol. 2004, 53: 9-18. 10.1111/j.1365-2958.2004.04113.x.View ArticlePubMedGoogle Scholar
- Casjens S: Prophages and bacterial genomics: what have we learned so far?. Mol Microbiol. 2003, 49: 277-300. 10.1046/j.1365-2958.2003.03580.x.View ArticlePubMedGoogle Scholar
- Waldor MK, Friedman DI: Phage regulatory circuits and virulence gene expression. Curr Opin Microbiol. 2005, 8: 459-465. 10.1016/j.mib.2005.06.001.View ArticlePubMedGoogle Scholar
- Groisman EA, Casadesus J: The origin and evolution of human pathogens. Mol Microbiol. 2005, 56: 1-7. 10.1111/j.1365-2958.2005.04564.x.View ArticlePubMedGoogle Scholar
- Casjens SR: Comparative genomics and evolution of the tailed-bacteriophages. Curr Opin Microbiol. 2005, 8: 451-458. 10.1016/j.mib.2005.06.014.View ArticlePubMedGoogle Scholar
- Tinsley CR, Bille E, Nassif X: Bacteriophages and pathogenicity: more than just providing a toxin?. Microbes Infect. 2006, 8: 1365-1371. 10.1016/j.micinf.2005.12.013.View ArticlePubMedGoogle Scholar
- Steinberg VI, Hart EJ, Handley J, Goldberg ID: Isolation and characterization of a bacteriophage specific for Neisseria perflava. J Clin Microbiol. 1976, 4: 87-91.PubMed CentralPubMedGoogle Scholar
- Stone RL, Culbertson CG, Powell HM: Studies of a bacteriophage active against a chromogenic Neisseria. J Bacteriol. 1956, 71: 516-520.PubMed CentralPubMedGoogle Scholar
- Phelps LN: Isolation and characterization of bacteriophages for Neisseria. J Gen Virol. 1967, 1: 529-536.View ArticlePubMedGoogle Scholar
- Cary SG, Hunter DH: Isolation of bacteriophages active against Neisseria meningitidis. J Virol. 1967, 1: 538-542.PubMed CentralPubMedGoogle Scholar
- Campbell LA, Short HB, Young FE, Clark VL: Autoplaquing in Neisseria gonorrhoeae. J Bacteriol. 1985, 164: 461-465.PubMed CentralPubMedGoogle Scholar
- Klee SR, Nassif X, Kusecek B, Merker P, Beretti JL, Achtman M, Tinsley CR: Molecular and biological analysis of eight genetic islands that distinguish Neisseria meningitidis from the closely related pathogen Neisseria gonorrhoeae. Infect Immun. 2000, 68: 2082-2095. 10.1128/IAI.68.4.2082-2095.2000.PubMed CentralView ArticlePubMedGoogle Scholar
- Morgan GJ, Hatfull GF, Casjens S, Hendrix RW: Bacteriophage Mu genome sequence: analysis and comparison with Mu-like prophages in Haemophilus, Neisseria and Deinococcus. J Mol Biol. 2002, 317: 337-359. 10.1006/jmbi.2002.5437.View ArticlePubMedGoogle Scholar
- Masignani V, Giuliani MM, Tettelin H, Comanducci M, Rappuoli R, Scarlato V: Mu-like Prophage in serogroup B Neisseria meningitidis coding for surface-exposed antigens. Infect Immun. 2001, 69: 2580-2588. 10.1128/IAI.69.4.2580-2588.2001.PubMed CentralView ArticlePubMedGoogle Scholar
- Bille E, Zahar JR, Perrin A, Morelle S, Kriz P, Jolley KA, Maiden MC, Dervin C, Nassif X, Tinsley CR: A chromosomally integrated bacteriophage in invasive meningococci. J Exp Med. 2005, 201: 1905-1913. 10.1084/jem.20050112.PubMed CentralView ArticlePubMedGoogle Scholar
- Los Alamos National Laboratory Bioscience Division. [http://stdgen.northwestern.edu/]
- Kawai M, Uchiyama I, Kobayashi I: Genome Comparison In Silico in Neisseria Suggests Integration of Filamentous Bacteriophages by their Own Transposase. DNA Res. 2005, 12: 389-401. 10.1093/dnares/dsi021.View ArticlePubMedGoogle Scholar
- Tsuchimoto S, Ohtsubo H, Ohtsubo E: Two genes, pemK and pemI, responsible for stable maintenance of resistance plasmid R100. J Bacteriol. 1988, 170: 1461-1466.PubMed CentralPubMedGoogle Scholar
- Zhang J, Zhang Y, Zhu L, Suzuki M, Inouye M: Interference of mRNA function by sequence-specific endoribonuclease PemK. J Biol Chem. 2004, 279: 20678-20684. 10.1074/jbc.M314284200.View ArticlePubMedGoogle Scholar
- Abeles AL, Friedman SA, Austin SJ: Partition of unit-copy miniplasmids to daughter cells. III. The DNA sequence and functional organization of the P1 partition region. J Mol Biol. 1985, 185: 261-272. 10.1016/0022-2836(85)90402-4.View ArticlePubMedGoogle Scholar
- Nakayama K, Takashima K, Ishihara H, Shinomiya T, Kageyama M, Kanaya S, Ohnishi M, Murata T, Mori H, Hayashi T: The R-type pyocin of Pseudomonas aeruginosa is related to P2 phage, and the F-type is related to lambda phage. Mol Microbiol. 2000, 38: 213-231. 10.1046/j.1365-2958.2000.02135.x.View ArticlePubMedGoogle Scholar
- Chen Y, Golding I, Sawai S, Guo L, Cox EC: Population fitness and the regulation of Escherichia coli genes by bacterial viruses. PLoS Biol. 2005, 3: e229-10.1371/journal.pbio.0030229.PubMed CentralView ArticlePubMedGoogle Scholar
- Mayer MP: A new set of useful cloning and expression vectors derived from pBlueScript. Gene. 1995, 163: 41-46. 10.1016/0378-1119(95)00389-N.View ArticlePubMedGoogle Scholar
- Young I, Wang I, Roof WD: Phages will out: strategies of host cell lysis. Trends Microbiol. 2000, 8: 120-128. 10.1016/S0966-842X(00)01705-4.View ArticlePubMedGoogle Scholar
- Breyen SA, Dworkin M: Autoplaquing in Myxococcus strains. J Bacteriol. 1984, 158: 1163-1164.PubMed CentralPubMedGoogle Scholar
- Wagner PL, Waldor MK: Bacteriophage control of bacterial virulence. Infect Immun. 2002, 70: 3985-3993. 10.1128/IAI.70.8.3985-3993.2002.PubMed CentralView ArticlePubMedGoogle Scholar
- Sambrook J, Fritsch EF, Maniatis T: Molecular Cloning: a laboratory manual. 1989, Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2Google Scholar
- White LA, Kellogg DS: Neisseria gonorrhoeae identification in direct smears by a fluorescent antibody counterstain method. Appl Microbiol. 1965, 13: 171-174.PubMed CentralPubMedGoogle Scholar
- Barcak GJ, Chandler MS, Redfield RJ, Tomb JF: Genetic systems in Haemophilus influenzae. Methods Enzymol. 1991, 204: 321-342.View ArticlePubMedGoogle Scholar
- Braun DC, Stein DC: The lgtABCDE gene cluster, involved in lipooligosaccharide biosynthesis in Neisseria gonorrhoeae, contains multiple promoter sequences. J Bacteriol. 2004, 186: 1038-1049. 10.1128/JB.186.4.1038-1049.2004.PubMed CentralView ArticlePubMedGoogle Scholar
- Esposito D, Fitzmaurice WP, Benjamin RC, Goodman SD, Waldman AS, Scocca JJ: The complete nucleotide sequence of bacteriophage HP1 DNA. Nucleic Acids Res. 1996, 24: 2360-2368. 10.1093/nar/24.12.2360.PubMed CentralView ArticlePubMedGoogle Scholar
- Bradford MM: A rapid and sensitive method for the quantitation of microgram quantities of protein utilizing the principle of protein-dye binding. Anal Biochem. 1976, 72: 248-254. 10.1016/0003-2697(76)90527-3.View ArticlePubMedGoogle Scholar
- National Center for Biotechnology Information. [http://www.ncbi.nlm.nih.gov]
- University of Oklahoma's Advanced Center for Genome Technology. [http://www.genome.ou.edu]
- Sanger Institute. [http://www.sanger.ac.uk]
- EMBOSS tools. [http://www.ebi.ac.uk]
- On line analysis tools. [http://molbiol-tools.ca]
- GeneMark. [http://exon.gatech.edu]
- Glimmer microbial gene finder system. [http://cbcb.umd.edu/software/glimmer/]
- Easy Gene. [http://www.cbs.dtu.dk]
- ORF finder. [http://myhits.isb-sib.ch]
- GC content. [http://www.ebi.ac.uk]
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.