PURIFICATION AND PROPERTIES OF A BACTERIAL-TYPE FERREDOXIN FROM THE NITROGEN-FIXING CYANOBACTERIUM ANABAENA-VARIABILIS ATCC29413

Three soluble ferredoxins were purified to homogeneity from nitrogen-f ixing cultures of Anabaena variabilis (ATCC 29413) and characterized. The purified proteins have different absorption spectra, molecular mas s, iron content, amino-acid composition and resistance to O2 inactivat ion. Two were plant-type ferredoxins FdI and FdxH, corresponding to th e previously reported ferredoxins II and I (Bohme, H. and Schrautemeie r, B. (1987) Biochim. Biophys. Acta 891, 1-7). The third ferredoxin (f erredoxin III) (previously not described in cyanobacteria) was a bacte rial-type ferredoxin. Ferredoxin III has a molecular mass of about 6 k Da and contains 3-4 atoms Fe/mol. Native (oxidized) ferredoxin Ill sho ws an EPR-signal at g = 2.014 that disappears after reduction by dithi onite, characteristic of ferredoxins containing three-iron clusters. F erredoxin III, like ferredoxin FdxH, is inactivated by oxygen. Ferredo xin III supports higher rates of C2H2 reduction by Rhodobacter capsula tus nitrogenase than FdI and higher rates of H-2 evolution by clostrid ial hydrogenase than FdI and FdxH. Combined nitrogen supresses the syn thesis of both nitrogenase and ferredoxin III. These data suggest a po ssible role of ferredoxin III (bacterial-type) in nitrogen fixation by A. variabilis.