Examples in Nature
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 differed from one diazotroph to another. Examples of nif genes structure and regulation in different diazotrophs:
Klebsiella pneumoniae – a free living anaerobic nitrogen fixing bacteria. It contains a total of 20 nif genes located on the bacteria's chromosome in a 24kb region. nifH, nifK and nifD encode the nitrogenase's subunits, while nifE, nifN, nifU, nifS, nifV, nifW, nifX, nifB and nifQ encode proteins involved the assembly and incorporation of the Fe and Mo into the nitrogenase's subunits. nifF and nifJ encode proteins related to electron transfer taking place in the reduction process and nifA is a regulatory protein in-charge of regulating the nif genes expression. Regulation of the nif genes expression is done through nifA and nifL.
Rhodospirillum rubrum – a free living anaerobic photosynthetic bacteria which in addition to the transcriptional control described before, 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 by so, inactivates nitrogenase's activity. The enzymes catalyzing the ribosylation are called DRAG and DRAT.
Rhodobacter capsulatus - free living anaerobic phototroph containing a transcriptional nif genes regulatory system. R. capsulatus regulates nif genes expression through nifA in the same manner described before, but it uses a different nifA activator called factor which initiates the RcNtrC. RcNtrC activates a different Expression of nifA and the- nif genes.
Rhizobium spp. - Gram negative, symbiotic nitrogen fixing bacteria. It usually forms symbiotic relationship with Legum species. In some Rhizobia, the nif genes are located on plasmids called the sym plasmids (sym = symbiosis) which contain genes related to nitrogen fixing and some housekeeping genes, while the chromosomes contain most of the housekeeping genes of the bacteria. Regulation of the nif genes expression is done at the transcription level inside the symbiotic plant.
Read more about this topic: Nif Gene
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