NITROGEN FIXING GENES BY MUHAMMAD NADEEM ASHRAF SOIL & ENVIRONMENTAL SCIENCES email@example.com
NITROGEN FIXING GENES The nif gene is the gene responsible for the coding of proteins related and associated with the fixation of atmospheric nitrogen into a form of nitrogen available to plants. These genes are found in nitrogen fixing bacteria and cyanobacteria.
• Specific expression of plant and bacterial genesaccompanies the development of rhizobial plantsymbiosis.• The genes involved in the root nodule formationcollectively called Nodulin genes, encodes a series ofNod proteins that serves specific functions in theestablishment of nodules that permit symbioticnitrogen fixation.
Classification of Genes These genes are classified into 2 classes The first class include the genes that specify the biochemical composition of bacterial cell surface such as genes determining the synthesis of exoploysaccharides (exo genes), lipopolysaccharides (lps genes), capsular polysaccharides of K-antigen and β-1,2 glucans (ndv genes)
The exo and lps genes may play a role in determining the host specificity but this has yet to be firmly established.The second class of the geses consists of the nodulation genes (nod or nol genes).Inactivation of the nodulation genes can result in various plant phenotypes, such as the absence of nodulation, or delayed but effective nodulation, or changes in the host range.
Fast growing rhizobia : sym plasmid Slow growing Brady rhizobia : bacterial chromosomes. With in the gene cluster essential for process of nitrogen fixation are nif and fix genes, which includes the structural genes for the nitrogenase enzyme. The host specific nod genes determines the specificity of nodulation on a particular host.
Location of genes Nod Genes : sym plasmid Specificity genes : sym plasmid Nif genes : may or may not be plasmid associated in various rhizobial strains.
nif and fix genes Beside nod genes the symplasids carry the nif and fix clusters including the structural genes for nitrogenase. Most rhizobium nod genes are not expressed in cultured cells but are induced in the presence of plant. This induction required falvonoids secreted by the plant and also the transcriptional activator protein NodD. The NodD gene is the only nod gene that is constitutively expressed in both free living and symbiotic state of Rhizobium