Genetics of Nitrogen-fixation

The genetics of nitrogen-fixation is known in detail in Klebsiella pneumoniae. There are twenty genes required in organising the complete N₂-fixing apparatus. Interestingly, it has been shown that if all the twenty genes of Klebsiella are transferred to E coli, a non-N₂-fixer, the latter becomes a N₂-fixer. Some nif genes code for the structural components of nitrogenase while two genes (nif L and nif A) are regulatory. The fast growing Rhizobia like R. leguminosarum are knouin to contain three kinds of genes for N₂-fixation. They are

1. nif-genes required for the formation of functional nitrogenase as in Klebsiella,
2. Nod-genes required for the formation of nodules in root and
3. Fix-genes required for the maintenance of N₂-fixation by the nodule.

They control oxygen protection of nitrogenase as well as ATP and reductant supply. There are about 18 nod-genes known which are activated by the inducer - flavonoid (leutolin) present in the exudate of roots. They are supposed to be involved in controlling various stages of Rhizobium infection leading to the formation of nodule. The nif, the nod and the fix genes are located on a plasmid in the fast growing rhizobia and on the chromosome in slow growing rhizobia. Nod-genes are known to control the host-range of the bacterium and it should be genetically possible to increase its host range, thus enabling it to infect cereals and produce root nodules on them. Recently cultured tissues of rice and wheat have been successfully infected with Rhizobium although the association does not have the capability of fixing nitrogen. This achievement, however, is a step closer to Rhimbium-wheat or Rhizobium-rice N₂-fixing system.