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Nitrogen Fixation Nitrogen Fixation Part 1 Structure of Nitrogenase Components Fe Protein Mo-Fe Protein Mechanism of Nitrogenase Action Hydrogen Evolution Factors Affecting Nitrogenase Activity Nitrogen Fixation Genes Nitrification Nitrate Reduction Glutamate Synthase Pathway

Nitrogen Fixation Genes

	Nitrogen Fixation Genes - Klebsiella pneumoniae (Enterobacteriaceae) has been used for detailed genetic analyses of the genes involved in nitrogen fixation (nif genes). This organism is widely distributed in water, soil, grain and intestine of mammals. In the presence of available fixed nitrogen it can grow in aerobic as well as anaerobic conditions. Nitrogen fixation, however, takes place only in an anaerobic environment. All nif mutations are located near the his (histidine) operon. There are at least 17 nif loci in the duster. No non nif genes appear to be present within the nif region. The genes are organized in seven distinct operons, all of which transcribe towards the his genes. The nif region is about 24 kilobases (kb) long. As mentioned previously the two components of nitrogenase are the Fe protein and MoFe protein. The latter contains the FeMo cofactor (FeMo-co). The nifH gene encodes the proto Fe protein. This is then processed by the nifM protein, and possibly also the nifV and nifS products to form the active Fe protein.
Two genes, nifD and nifK code for the structural proteins of the MoFe protein. The gene nifD codes for one subunit and nifK for the other. The products form proto FeMo protein. The two subunits are distinct proteins. The similarities between the two are perhaps the result of gene duplication in the course of evolution. Lesions in genes nifB, Nor E cause the cells to produce an inactive MoFe protein. This can be activated in vitro by FeMo. During aerobic growth, no nitrogenase is synthesized by K pneumoniae. It seems that oxygen rapidly turns off all nif encoded proteins except nifRLA proteins. These are the regulatory proteins. Gene nifA is responsible for the purpling of 6 cyanopurine, an analogue of adenine. Its product converts the colourless 6 cyanopurine into a purple pigment. (Test for nifA deletion).
The genes nifL, nifW and nifU do not appear to be essential for nitrogen fixation in growing cells under laboratory conditions. Mutations in these genes are detected only when they have a polar effect on an essential gene. The gene nifQ is not absolutely essential for nitrogen fixation. Mu insertions in this gene only reduce nitrogen fixing ability. Scientists of the University of Sussex, England, have assembled a bacterial plasmid which contains all the 17 known nitrogen fixation genes (nif) from Klebsiella pneumoniae. Transfer of this plasmid into E. coli converted the bacterium into a nitrogen fixing organism. Szalay and coworkers at Cornell University, and scientists at the Pasteur. Institute and the University of Paris have transferred the 17 nif genes of Klebsiella pneumoniae into yeast, a eukaryote organism.
The yeast cells carrying the nitrogen fixation genes were, however, not able to fix atmospheric nitrogen. This may be due to the many difficulties involved in gene functioning after transfer to foreign cells. The yeast cells must interpret the start and stop signals on bacterial DNA for RNA transcription. The transcribed RNA must then be able to translate proteins on the yeast ribosomes. The translated proteins must be able to function in the foreign cell where they may come across host cell proteins. Enough iron must be available in yeast cell for incorporation into the nitrogenase molecule, without interfering with the iron requirements of the yeast cell. Considering these difficulties, it is not surprising that the bacterial nitrogen fixation genes were not able to function in the yeast cells

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Nitrogen Fixation Part 1 Ammonification Denitrification Denitrifying Bacteria Ecological Importance of Denitrification Nitrogen Assimilation Transamination Glutamate Dehydrogenase (GDH) Glutamate Synthase Pathway Glutamine Synthetase (GS) Reaction Glutamate Synthase (GOGAT) Reaction Transaminase Reactions Glutamine Synthtase Adenylylation and Deadenylylation

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