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.