The hydBGDA genes, which encode the four subunits /?, y, S and a of the [Ni-Fe] hydrogenase from the archaeon Pymcoccus furiosus, have been isolated and sequenced using a PCWIPCR-based strategy. From the sequence analysis it appears that the four structural genes are tightly linked and organized in a single transcription unit. The hydD and hydA gene products are related to the small and the large subunits of several archaeal and eubacterial [Ni-Fe] hydrogenases with an overall degree of sequence relatedness ranging from 35 YO to 50% (identity +similarity). In particular, the amino acid sequence motifs involved in the accommodation of nickel and iron-sulfur clusters are conserved. In addition, the database search revealed that the hydB and hydG gene products are homologous to the asrA-and asrB-encoded subunits of the sulf ite reductase enzyme from Salmonella typhimurium . This is particularly interesting in view of the recent finding that the P. furiosus hydrogenase appears t o be a bifunctional enzyme endowed with both proton-and sulfurreducing activities.