Control of synbiotic nitrogen fixation in Rhizobia. Regulation of NH4+ assimilation.

This communication is concerned with physiological, biochemical, and genetic studies of the regulation of ammonium (NH4+) assimilation by Rhizobia (root nodule bacteria) that infect leguminous plants. The major conclutions are (i) physiological studies show that Rhizobia are able to assimilate NH4+ for growth only when supplemented with certain organic nitrogen sources (e.g., L-aspartate, L-leucine, L-serine). Addition of as little as 2 mug/ml of L-aspartate supported growth on NH4+ as nitrogen source. In contrast, addition of glutamate in combination with NH4+-blocked NH4+ utilization; (ii) biochemical analysis show that glutamate synthase activity (NADP- and NAD-linked) is always present in cells capable of assimilating NH4+; also cells without glutamate synthase activity were found to be incapable of NH4+ utilization. Glutamate synthase levels were observed to fluctuate markedly depending on the available nitrogen source and on the growth stage of the culture; (iii) mutants were selected in which assimilation of NH4+ is no longer subject to inhibition (repression?) by glutamate. The levels of glutamate synthase activity (NADP-linked) (in the presence of glutamate) show approximately a two-fold increase over the level in the parent strain. The mutants no longer require supplementation with small amounts of organic nitrogen for growth in medium containing inorganic nitrogen (e.g., NH4+ or NO3-); (iv) these findings are discussed in relation to the working model of symbiotic nitrogen fixation recently proposed (O'Gara and Shanmugam (1976), Biochim. Biophys. Acta 437, 313--321).