PURIFICATION AND PROPERTIES OF A NIF-SPECIFIC FLAVODOXIN FROM THE PHOTOSYNTHETIC BACTERIUM RHODOBACTER-CAPSULATUS

A flavodoxin was isolated from iron-sufficient, nitrogen-limited cultu res of the photosynthetic bacterium Rhodobacter capsulatus. Its molecu lar properties, molecular weight, UV-visible absorption spectrum, and amino acid composition suggest that it is similar to the nif-specific flavodoxin, NifF, of Klebsiella pneumoniae. The results of immunoblott ing showed that R. capsulatus flavodoxin is nif specific, since it is absent from ammonia-replete cultures and is not synthesized by the mut ant strain J61, which lacks a nif-specific regulator (NifR1). Growth o f cultures under iron-deficient conditions causes a small amount of fl avodoxin to be synthesized under ammonia-replete conditions and increa ses its synthesis under N2-fixing conditions, suggesting that its synt hesis is under a dual system of control with respect to iron and fixed nitrogen availability. Here we show that flavodoxin, when supplemente d with catalytic amounts of methyl viologen, is capable of efficiently reducing nitrogenase in an illuminated chloroplast system. Thus, this nif-specific flavodoxin is a potential in vivo electron carrier to ni trogenase; however, its role in the nitrogen fixation process remains to be established.