SUMMARY: A dissimilatory sulphite reductase was isolated from the extremely thermophilic dissimilatory sulphate-reducing archaeon In common with other dissimilatory sulphite reductases thus far characterized, the enzyme has an αβ-structure and contains sirohaem, non-haem iron atoms and acid labile sulphide. The oxidized enzyme exhibited absorption maxima at 281, 394, 545 and 593 nm with a weak band around 715 nm. We have cloned and sequenced the genes for the α and β subunits of this enzyme, which we designate and respectively. They are contiguous in the order and probably comprise an operon, since is preceded by sequences characteristic of promoters in methanogenic archaea, and is followed by a sequence resembling termination signals in extremely thermophilic sulphur-dependent archaea. and encode 47.4 kDa and 41.7 kDa peptides, which have 25.6% amino acid sequence identity, indicating that they may have arisen by duplication of an ancestral gene. Each deduced peptide contains cysteine clusters resembling those postulated to bind sirohaem-[FeS] complexes in sulphite reductases and nitrite reductases from other species. The encoded peptide lacks a single cysteine residue in one of the two clusters, suggesting that only the α subunit binds a sirohaem-[FeS] complex, and chemical analyses showed the presence of only two sirohaems per αβ enzyme molecule. Both deduced peptides also contain an arrangement of cysteine residues characteristic of [FeS] ferredoxins, and chemical analyses were consistent with the presence of six [FeS] clusters per αβ enzyme molecule, two of which would be expected to be associated with sirohaem while the other four could bind to the ferredoxin-like sites.


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