Adenylylsulphate (adenosine-5′-phosphosulphate, APS) reductase from the extremely thermophilic sulphate-reducing archaeon is an iron-sulphur flavoprotein containing one non-covalently bound flavin group, eight non-haem iron and six labile sulphide atoms per molecule. Re-evaluation of the enzyme structure revealed the presence of two different subunits with molecular masses of 80 and 18.5 kDa. The subunits are arranged in an αβ subunit structure. We have cloned and sequenced a 2.7 kb segment of DNA containing the genes for the α and β subunits, which we designate and , respectively. The two genes are separated by 17 bp and localized in the order . While a putative promoter could not be identified in the vicinity of a probable termination signal was found just downstream of the translation stop codon of . The codon usage for shows strong preferences for G and C in the third codon position. encodes a 73.3 kDa polypeptide, which shows significant overall similarities with the flavoprotein subunits of the succinate dehydrogenases from and and the corresponding flavoprotein of fumarate reductase. Part of the homologous peptide stretches could be assigned to domains that are involved in the binding of the substrate or of the FAD prosthetic group. encodes a 17.1 kDa polypeptide representing an iron-sulphur protein, seven cysteine residues of which are arranged in two clusters typical of ligands of the iron-sulphur centres in {[FeS][FeS]} 7-Fe ferredoxins.


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