Purification and properties of a bacterial-type ferredoxin from the nitrogen-fixing cyanobacterium Anabaena variabilis ATCC29413

Abstract

Three soluble ferredoxins were purified to homogeneity from nitrogen-fixing cultures of Anabaena variabilis (ATCC 29413) and characterized. The purified proteins have different absorption spectra, molecular mass, iron content, amino-acid composition and resistance to O 2 inactivation. Two were plant-type ferredoxins FdI and FdxH, corresponding to the previously reported ferredoxins II and I (Böhme, H. and Schrautemeier, B. (1987) Biochim. Biophys. Acta 891, 1–7). The third ferredoxin (ferredoxin III) (previously not described in cyanobacteria) was a bacterial-type ferredoxin. Ferredoxin III has a molecular mass of about 6 kDa and contains 3–4 atoms Fe/mol. Native (oxidized) ferredoxin III shows an EPR-signal at g = 2.014 that disappears after reduction by dithionite, characteristic of ferredoxins containing three-iron clusters. Ferredoxin III, like ferredoxin FdxH, is inactivated by oxygen. Ferredoxin III supports higher rates of C 2H 2 reduction by Rhodobacter capsulatus nitrogenase than FdI and higher rates of H 2 evolution by clostridial hydrogenase than FdI and FdxH. Combined nitrogen supresses the synthesis of both nitrogenase and ferredoxin III. These data suggest a possible role of ferredoxin III (bacterial-type) in nitrogen fixation by A. variabilis.