The expression and regulation of
nif genes, while sharing common features in all or most of the nitrogen-fixing organisms in nature, have distinct characters and qualities that differ from one diazotroph to another. Examples of
nif gene structure and regulation in different diazotrophs include:
Klebsiella pneumoniae—a free-living, facultatively anaerobic nitrogen-fixing bacterium. It contains a total of 20
nif genes located on the chromosome in a 24-Kb region.
nifH,
nifD, and
nifK encode the nitrogenase subunits, while
nifE,
nifN,
nifU,
nifS,
nifV,
nifW,
nifX,
nifB, and
nifQ encode proteins involved the assembly and incorporation of
iron and
molybdenum atoms into the nitrogenase subunits.
nifF and
nifJ encode proteins related to electron transfer taking place in the reduction process and
nifA and
nifL are regulatory proteins in 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 ==