charge of regulating the expression of the other nif genes. Rhodospirillum rubrum—a free-living anaerobic photosynthetic bacterium which, in addition to the transcriptional controls described above, regulates expression of the nif genes also in a metabolic way through a reversible ADP-ribosylation of a specific arginine residue in the nitrogenase complex. The ribosylation takes place when reduced nitrogen is present and it causes a barrier in the electron transfer flow and thereby inactivates nitrogenase activity. The enzymes catalyzing the ribosylation are called DraG and DraT. Rhodobacter capsulatus—a free-living anaerobic phototroph containing a transcriptional nif gene regulatory system. R. capsulatus regulates nif gene expression through nifA in the same manner described before, but it uses a different nifA activator which initiates the NtrC. NtrC activates a different expression of nifA and the other nif genes. Rhizobium spp.—Gram-negative, symbiotic nitrogen fixing bacteria that usually form a symbiotic relationship with legume species. In some rhizobia, the nif genes are located on plasmids called 'sym plasmids' (sym = symbiosis) which contain genes related to nitrogen fixation and metabolism, while the chromosomes contain most of the housekeeping genes of the bacteria. Regulation of the nif genes is at the transcriptional level and is dependent on colonization of the plant host. See also Nif regulon (Klebsiella pneumoniae) References External links nif gene—definition from Biology-Online.org Prokaryote genes Molecular biology