Cloning, Sequence Analysis and Overexpression of the Rhodanese Gene of Azotobacter vinelandii

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Abstract
A gene encoding rhodanese (rhdA) was cloned from Azotobacter vinelandii on a 2.3‐kb SphI fragment. This fragment was identified by its hybridization to a PCR product obtained by amplification of genomic DNA using degenerate primers encoding the N‐terminal sequence of rhodanese purified from A. vinelandii. The sequence of a 1.2‐kb region revealed an 813‐bp open reading frame that encoded a polypeptide of 271 amino acids, the N‐terminal sequence of which was identical to that of A. vinelandii rhodanese. In a search of database entries, eukaryotic rhodaneses and rhodanese‐like proteins from bacteria gave the highest scores of identity (27–30%) with the predicted product of the 813‐bp open reading frame. A. vinelandii RhdA shows less sequence similarity to vertebrate rhodaneses than it does to prokaryotic rhodanese‐like proteins which did not show typical rhodanese activity. Basic residues thought to be catalytically important in bovine rhodanese are not conserved in A. vinelandii rhodanese. The sequence similarity between the two structurally similar domains of rhodanese is more pronounced for the A. vinelandii enzyme than the bovine enzyme, and supports the hypothesis that the complete structure was originally generated by gene duplication. When rhdA was overexpressed in Escherichia coli, rhodanese represented 30% of total cell protein and thiosulfate:cyanide sulfurtransferase activity increased >600 fold in cell‐free extracts. A. vinelandii rhdA insertion/deletion mutants had no discernible phenotype distinct from the wild‐type strain with respect to growth on various sulfur sources or nitrogenase activity. Mutants retained 20% of wild‐type rhodanese thiosulfate: cyanide sulfurtransferase activity suggesting the presence of redundant sulfurtransferase enzymes in A. vinelandii.