Multiple lateral transfers of dissimilatory sulfite reductase genes between major lineages of sulfate-reducing prokaryotes

A large fragment of the dissimilatory sulfite reductase genes (dsrAB) was P CR amplified and fully sequenced from 30 reference strains representing all recognized lineages of sulfate-reducing bacteria. In addition, the sequenc e of the dsrAB gene homologs of the sulfite reducer Desulfitobacterium deha logenans was determined. In contrast to previous reports, comparative analy sis of all available DsrAB sequences produced a tree topology partially inc onsistent with the corresponding 16S rRNA phylogeny. For example, the DsrAB sequences of several Desulfotomaculum species (low G+C gram-positive divis ion) and two members of the genus Thermodesulfobacterium (a separate bacter ial division) were monophyletic with delta -proteobacterial DsrAB sequences . The most parsimonious interpretation of these data is that dsrAB genes fr om ancestors of as-yet-unrecognized sulfate reducers within the B-Proteobac teria were laterally transferred across divisions. A number of insertions a nd deletions in the DsrAB alignment independently support these inferred la teral acquisitions of dsrAB genes. Evidence for a dsrAB lateral gene transf er event also was found within the delta -Proteobacteria, affecting Desulfo bacula toluolica. The root of the dsr tree was inferred to be within the Th ermodesulfovibrio lineage by paralogous rooting of the alpha and beta subun its. This rooting suggests that the dsrAB genes in Archaeoglobus species al so are the result of an ancient lateral transfer from a bacterial donor. Al though these findings complicate the use of dsrAB genes to infer phylogenet ic relationships among sulfate reducers in molecular diversity studies, the y establish a framework to resolve the origins and diversification of this ancient respiratory lifestyle among organisms mediating a key step in the b iogeochemical cycling of sulfur.