A sulf ite-reductase-type protein was purified from the hyperthermophilic crenarchaeote Pyrobaculum islandicum grown chemoorganoheterotrophically with thiosulfate as terminal electron acceptor. In common with dissimilatory sulfite reductases the protein has an a#* structure and contains high-spin sirohaem, non-haem iron and acid-labile sulfide. The oxidized protein exhibits absorption maxima a t 280,392, 578 and 710 nm with shoulders a t 430 and 610 nm. The isoelectric point of pH 8 4 sets the protein apart from all dissimilatory sulfite reductases characterized thus far. The genes for the a-and P-subunits (dsrA and dsrB) are contiguous in the order dsrAdsrB and most probably comprise an operon with the directly following dsrG and dsrC genes. dsrG and dsrC encode products which are homologous to eukaryotic glutathione S-transferases and the proposed y-subunit of Desulfovibrio vulgaris sulf ite reductase, respectively. dsrA and dsrB encode 44.2 kDa and 41.2 kDa peptides which show significant similarity to the two homologous subunits DsrA and DsrB of dissimilatory sulfite reductases. Phylogenetic analyses indicate a common protogenotic origin of the P. islandicum protein and the dissimilatory sulfite reductases from sulfate-reducing and sulfideoxidizing prokaryotes. However, the protein from P. islandicum and the sulfite reductases f rom sulfate-reducers and from sulf ur-oxidizers most probably evolved into three independent lineages prior to divergence of archaea and bacteria.

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