This document discusses nitrogen fixing genes in bacteria and cyanobacteria. It describes the nif gene which codes for proteins involved in fixing atmospheric nitrogen into a form available to plants. These genes are found in nitrogen fixing bacteria and cyanobacteria. It also discusses nodulin genes which encode proteins that establish root nodules and allow for symbiotic nitrogen fixation. These nitrogen fixing genes are classified into those involved in bacterial cell surfaces and those involved in nodulation. Location of these genes varies between fast and slow growing rhizobia.

1. NITROGEN FIXING GENES
BY
MUHAMMAD NADEEM ASHRAF
SOIL & ENVIRONMENTAL SCIENCES
nadeemawan17@gmail.com

2. 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.

3. • Specific expression of plant and bacterial genes
accompanies the development of rhizobial plant
symbiosis.
• The genes involved in the root nodule formation
collectively called Nodulin genes, encodes a series of
Nod proteins that serves specific functions in the
establishment of nodules that permit symbiotic
nitrogen fixation.

4. 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)

5.  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.

6.  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.

7. Location of genes
 Nod Genes : sym plasmid
 Specificity genes : sym plasmid
 Nif genes : may or may not be plasmid associated in
various rhizobial strains.

8. 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