Abstract
The structural gene of the ferredoxin : sulphite reductase (EC 1.8.7.1) from the cyanobacterium Synechococcus PCC7942 (formerly ‘ Anacystis nidulans’) was cloned and sequenced. The gene termed ‘ sir’ was detected by heterologous Southern hybridisation with the structural gene cysI from Escherichia coli encoding the iron-sulphur haemoprotein of the NADPH : sulphite reductase. The open reading frame is comprised of 1875 bp encoding for a polypeptide of M r 70.028. The deduced amino acid sequence is 35.6% identical with the enterobacterial iron-sulphur haemoprotein. This putative fd-dependent sulphite reductase is only distantly related to the fd-dependent nitrite reductase (binary matching coefficient S AB: 0.23) or with the NADPH-sulphite reductase ( S AB: 0.32). Highly conserved residues are found within the two Cys clusters forming the reactive Fe 4S 4-sirohaem centre of the enzyme. Expression of the sir gene using a fusion vector gave a single gene product which is immunologically related with the fd-sulphite reductase from the wild-type bacterium.
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