Illustration showing the proposed horizontal gene transfer (HGT) of a type 3 hydrogenase/protease from an archaeal organism to a bacterial organism. The result form the phylogenetic tree indicates that it has been at least one major HGT event within the evolution of [NiFe]-hydrogenases and the hydrogenase specific proteases. Our results suggest that the root may be placed between group 3a and 4 of the hydrogenase specific proteases which would mean that the proteolytic cleavage of the hydrogenase large subunit by a protease originated within the archaean superkingdom. This illustration indicates the proposed HGT that transferred the protease to bacteria, which could then have been incorporated to the maturations process of type 1 and 2 hydrogenases. This theory does not rule out that additional HGT might have occurred and in this illustration type 4 hydrogenases within proteobacteria, together with their specific protease, are shown as the result of a similar HGT. This is still unclear though and the type 4 hydrogenases might have existed in both bacteria and archaea from the start. Large circle; hydrogenase, small circle; protease, red/orange colour; suggested archaean origin, blue colour; suggested bacterial origin.