O'gara F, Shanmugam K T
Biochim Biophys Acta. 1976 Dec 21;451(2):342-52. doi: 10.1016/0304-4165(76)90129-x.
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).
本通讯涉及对感染豆科植物的根瘤菌（根瘤细菌）铵（NH₄⁺）同化调控的生理、生化和遗传学研究。主要结论如下：（i）生理研究表明，根瘤菌只有在补充某些有机氮源（如L-天冬氨酸、L-亮氨酸、L-丝氨酸）时才能同化NH₄⁺以实现生长。添加低至2微克/毫升的L-天冬氨酸就能支持以NH₄⁺作为氮源的生长。相反，添加谷氨酸与NH₄⁺结合会阻碍NH₄⁺的利用；（ii）生化分析表明，谷氨酸合酶活性（与NADP和NAD相关）总是存在于能够同化NH₄⁺的细胞中；此外，发现没有谷氨酸合酶活性的细胞无法利用NH₄⁺。观察到谷氨酸合酶水平根据可用氮源和培养物的生长阶段而显著波动；（iii）筛选出了NH₄⁺同化不再受谷氨酸抑制（阻遏？）的突变体。（在存在谷氨酸的情况下）谷氨酸合酶活性（与NADP相关）的水平比亲本菌株中的水平大约增加了两倍。这些突变体在含有无机氮（如NH₄⁺或NO₃⁻）的培养基中生长时不再需要补充少量有机氮；（iv）结合最近提出的共生固氮工作模型（奥加拉和山穆加姆（1976年），《生物化学与生物物理学报》437，313 - 321）对这些发现进行了讨论。
