Yan D, Ikeda T P, Shauger A E, Kustu S
Department of Plant Biology, University of California, Berkeley 94720, USA.
Proc Natl Acad Sci U S A. 1996 Jun 25;93(13):6527-31. doi: 10.1073/pnas.93.13.6527.
In many bacteria, accumulation of K+ at high external osmolalities is accompanied by accumulation of glutamate. To determine whether there is an obligatory relationship between glutamate and K+ pools, we studied mutant strains of Salmonella typhimurium with defects in glutamate synthesis. Enteric bacteria synthesize glutamate by the combined action of glutamine synthetase and glutamate synthase (GS/GOGAT cycle) or the action of biosynthetic glutamate dehydrogenase (GDH). Activity of the GS/GOGAT cycle is required under nitrogen-limiting conditions and is decreased at high external ammonium/ammonia ((NH4)+) concentrations by lowered synthesis of GS and a decrease in its catalytic activity due to covalent modification (adenylylation by GS adenylyltransferase). By contrast, GDH functions efficiently only at high external (NH4)+ concentrations, because it has a low affinity for (NH4)+. When grown at low concentrations of (NH4)+ (< or = 2 mM), mutant strains of S. typhimurium that lack GOGAT and therefore are dependent on GDH have a low glutamate pool and grow slowly; we now demonstrate that they have a low K+ pool. When subjected to a sudden (NH4)+ upshift, strains lacking GS adenylyltransferase drain their glutamate pool into glutamine and grow very slowly; we now find that they also drain their K+ pool. Restoration of the glutamate pool in these strains at late times after shift was accompanied by restoration of the K+ pool and a normal growth rate. Taken together, the results indicate that glutamate is required to maintain the steady-state K+ pool -- apparently no other anion can substitute as a counter-ion for free K+ -- and that K+ glutamate is required for optimal growth.
在许多细菌中，细胞外高渗透压下钾离子（K⁺）的积累伴随着谷氨酸的积累。为了确定谷氨酸池与K⁺池之间是否存在必然联系，我们研究了鼠伤寒沙门氏菌中谷氨酸合成存在缺陷的突变菌株。肠道细菌通过谷氨酰胺合成酶和谷氨酸合酶（GS/GOGAT循环）的联合作用或生物合成型谷氨酸脱氢酶（GDH）的作用来合成谷氨酸。在氮限制条件下，GS/GOGAT循环的活性是必需的，并且在细胞外高铵/氨（(NH₄)⁺）浓度下，由于GS合成减少以及其催化活性因共价修饰（被GS腺苷酰转移酶腺苷酰化）而降低，该循环活性会下降。相比之下，GDH仅在细胞外高(NH₄)⁺浓度下才能有效发挥作用，因为它对(NH₄)⁺的亲和力较低。当在低(NH₄)⁺浓度（≤2 mM）下生长时，缺乏GOGAT因而依赖GDH的鼠伤寒沙门氏菌突变菌株谷氨酸池较低且生长缓慢；我们现在证明它们的K⁺池也较低。当突然提高(NH₄)⁺浓度时，缺乏GS腺苷酰转移酶的菌株会将其谷氨酸池排空到谷氨酰胺中，生长非常缓慢；我们现在发现它们也会排空其K⁺池。在浓度变化后期这些菌株中谷氨酸池的恢复伴随着K⁺池的恢复和正常生长速率。综上所述，结果表明谷氨酸是维持K⁺池稳态所必需的——显然没有其他阴离子可以替代作为游离K⁺的抗衡离子——并且谷氨酸钾是最佳生长所必需的。
