The [2Fe-2S] protein I (Shetna protein I) from Azotobacter vinelandii is homologous to the [2Fe-2S] ferredoxin from Clostridium pasteurianum

The [2Fe-2S] protein from Azotobacter vinelandii that was previously known as iron-sulfur protein I, or Shethna protein I, has been shown to be encode d by a gene belonging to the major nif gene cluster. Overexpression of this gene in Escherichia coli yielded a dimeric protein of which each subunit c omprises 106 residues and contains one [2Fe-2S] cluster. The sequence of th is protein is very similar to that of the [2Fe-2S] ferredoxin from Clostrid ium pasteurianum (2FeCpFd), and the four cysteine ligands of the [2Fe-2S] c luster occur in the same positions. The A. vinelandii protein differs from the C. pasteurianum one by the absence of the N-terminaI methionine, the pr esence of a five-residue C-terminal extension, and a lesser number of acidi c and polar residues. The UV-visible absorption and EPR spectra, as well as the redox potentials of the two proteins, are nearly identical. These data show that the A. vinelandii FeS protein I, which is therefore proposed to be designated 2FeAvFdI, is the counterpart of the [2Fe-2S] ferredoxin from C. pasteurianum. The occurrence of the 2FeAvFdI-encoding gene in the nif ge ne cluster, together with the previous demonstration of a specific interact ion between the 2FeCpFd and the nitrogenase MoFe protein, suggest that both proteins might be involved in nitrogen fixation, with possibly similar rol es.