A sulfite-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 α α β structure and contains high-spin sirohaem, non-haem iron and acid-labile sulfide. The oxidized protein exhibits absorption maxima at 280, 392, 578 and 710 nm with shoulders at 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 α- and β-subunits ( and ) are contiguous in the order and most probably comprise an operon with the directly following and genes. and encode products which are homologous to eukaryotic glutathione -transferases and the proposed α-subunit of sulfite reductase, respectively. and 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 protein and the dissimilatory sulfite reductases from sulfate-reducing and sulfide-oxidizing prokaryotes. However, the protein from and the sulfite reductases from sulfate-reducers and from sulfur-oxidizers most probably evolved into three independent lineages prior to divergence of archaea and bacteria.


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