Nitrogen Control of Nitrogen Assimilation

N₂-fixer like Klebsiella pneumoniae and Nostoc can grow with N₂, NO^-₃ or NH⁺₄ as nitrogen source. You would like to know how these organisms manage to assimilate one of the three forms of N₂ when given all the three simultaneously. It is known that NH⁺₄  is preferred over NO^-₃ or N₂ as nitrogen source. Now the question is how this preference is realised by such N₂-fixers? Ammonia can readily enter the cells by diffusion and the cells assimilate ammonia thus available into glutamine and glutamate. Under such conditions the ratio of glutamine to α-ketoglutarate rises which is a signal for sufficient nitrogen and causes repression of both NO^-₃ assimilation and N₂-fixation system. This is analogous to ATP/ADP ratio which signals the energy-state of a cell.

A high ATP/ADP ratio indicates that the cell has sufficient energy to perform its metabolic functions. That is why legumes grown in the medium containing NH⁺₄ do not form nodules with Rhizobium. In this connection it is important to point out that recognition mechanics as explained earlier of Rhizobium-legume symbiosis is not seen in root hairs of legume supplied with NH⁺₄. Similarly, when a N₂-fixer is exposed to NO^-₃  and N₂ it preferentially assimilates NO^-₃  and such NO^-₃ assimilating organisms do not produce N₂-fixing apparatus. The mechanism of NO, inhibition of N₂-fixation is the same as that described for NO^-₃ inhibition of N₂-fixation. The mechanism of ammonium repression of NO^-₃ assimilation occurs through the same mechanism as ammonium repression of N₂-fixation. This explains why N₂-fixation, nodule formation and heterocyst formation occur under conditions of limited nitrogen and not under conditions when nitrogen is available to the plant.