BphA3 from Pseudomonas sp. KKS102 is a Rieske-type [2Fe-2S] ferredoxin that transfers electrons from an NADH-dependent oxidoreductase, BphA4, to a biphenyl dioxygenase complex. A high-level expression and purification system for the recombinant BphA3 in Escherichia coli was constructed. Two histidine ligands of the Rieske-type cluster in BphA3, were each replaced with serine, cysteine, asparagine and tyrosine. The single mutants, in which either His44 or His65 was replaced with a cysteine residue (CH and HC mutants respectively), and the double mutant, in which both histidine residues were replaced with cysteine residue (CC mutant), accumulated to high levels in the E. coli cells, while the other single mutants did not. The purified WT (wild-type) protein showed characteristic near-UV and visible absorption and CD spectra of Rieske-type clusters. The X-ray absorption spectra were suggestive of the existence of [2Fe-2S] clusters, with one histidine and three cysteine ligands in the CH and HC mutants, and an [2Fe-2S] cluster with four cysteine ligands in the CC mutant. The BphA4-dependent cytochrome c reductase activities of the mutants were less than 0.3% of that of the WT protein. The redox potential of the WT protein determined by cyclic voltammetry was −180±5 mV compared with the standard hydrogen electrode, and that of the CH mutant was approx. 175 mV lower. The changes in the near-UV and visible absorption spectra of the mutants showed that the reduced iron–sulphur clusters in the mutants were unstable. His44 and His65 in BphA3 can be replaced with cysteine residues, but are required for the stabilization of the reduced form of the cluster.

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