Expression in Escherichia coli and characterization of a reconstituted recombinant 7Fe ferredoxin from Desulfovibrio africanus

Desulfovibrio africanus ferredoxin III is a monomeric protein (molecular mass of 6585 Da) that contains one [3Fe-4S](1+/0) and one [4Fe-4S](2+/1+) cluster when isolated aerobically. The amino acid sequence consists of 61 amino acids, including seven cysteine residues that are all involved in co-ordination to the clusters. In order to isolate larger quantities of D. africanus ferredoxin III, we have overexpressed it in Escherichia coli by constructing a synthetic gene based on the amino acid sequence of the native protein. The recombinant ferredoxin was expressed in E. coli as an apoprotein. We have reconstituted the holoprotein by incubating the apoprotein with excess iron and sulphide in the presence of a, reducing agent. The reconstituted recombinant ferredoxin appeared to have a lower stability than that of wildtype D. africanus ferredoxin III. We have shown by low-temperature magnetic circular dichroism and EPR spectroscopy that the recombinant ferredoxin contains a [3Fe-4S](1+/0) and a [4Fe-4S](2+/1+) cluster similar to those found in native D. africanus ferredoxin III. These results indicate that the two clusters have been correctly inserted into the recombinant ferredoxin.