Regulation of nitrogen fixation by Rhizobia. Export of fixed N2 as NH+4.

The metabolic fate of gaseous nitrogen (15N2) fixed by free-living cultures of Rhizobia (root nodule bacteria) induced for their N2-fixation system was followed. A majority of the fixed 15N2 was found to be exported into the cell supernatant. For example, as much as 94% of the 15N2 fixed by Rhizobium japonicum (soybean symbiont) was recovered as 15NH+4 from the cell supernatant following alkaline diffusion. Several species of root nodule bacteria also exported large quantities of NH+4 from L-histidine. Evidence is presented that overproduction and export of NH+4 by free-living Rhizobia may be closely linked to the control of several key enzymes of NH+4 assimilation. For instance, NH+4 was found to repress glutamine synthetase whereas L-glutamate repressed glutamate synthase. Assimilation of NH+4 as nitrogen source for growth of Rhizobia was inhibited by glutamate. The mechanism of regulation of NH+4 production by root nodule bacteria is discussed.