3.D.12. The Nitrogen Fixation Complex, FixABCX (FixABCX) Family
The components participating in electron transport to nitrogenase in Rhodospirillum rubrum have been studied (Edgren and Nordlund 2004). A Tn5 mutant, SNT-1, exhibited a decreased growth rate and about 25% of the in vivo nitrogenase activity compared to the wild-type value. The in vitro nitrogenase activity was essentially wild type, indicating that the mutation affects electron transport to nitrogenase. Sequencing showed that the Tn5 insertion is located in fixC, and extended sequencing revealed additional putative fix genes, in the order fixABCX. Complementation of SNT-1 with the whole fix gene cluster in trans restored wild-type nitrogenase activity and growth. Using Western blotting, it was demonstrated that expression of fixA and fixB occurs only under conditions under which nitrogenase is expressed. The SNT-1 mutant was shown to produce larger amounts of both ribulose 1,5-bisphosphate carboxylase/oxygenase and polyhydroxy alkanoates than the wild type, indicating that the redox status was affected by this mutation. Using Western blotting, FixA and FixB proved to be soluble proteins, whereas FixC most likely is a transmembrane protein. Possibly the fixABCX genes encode a membrane protein complex that plays a central role in electron transfer to nitrogenase in R. rubrum. FixC may br the link between nitrogen fixation and the proton motive force generated in the photosynthetic reactions. If so, this would imply that the FixABCX complex is responsive to the pmf by virtue of its proton transport capability. However, this has not been demonstrated. FixX is a ferridoxin-like protein while FixC has an N-terminal TMS plus up to 7 possiible TMSs of low hydrophobicity. FixA and B have characteristics of soluble (cytoplasmic) proteins.