Jayakumar A, Schulman I, MacNeil D, Barnes E M
J Bacteriol. 1986 Apr;166(1):281-4. doi: 10.1128/jb.166.1.281-284.1986.
Escherichia coli expresses a specific ammonium (methylammonium) transport system (Amt) when cultured with glutamate or glutamine as the nitrogen source. Over 95% of this Amt activity is repressed by growth of wild-type cells on media containing ammonia. The control of Amt expression was studied with strains containing specific mutations in the glnALG operon. GlnA- (glutamine synthetase deficient) mutants, which contain polar mutations on glnL and glnG genes and therefore have the Reg- phenotype (fail to turn on nitrogen-regulated operons such as histidase), expressed less than 10% of the Amt activity observed for the parental strain. Similarly, low levels of Amt were found in GlnG mutants having the GlnA+ Reg- phenotype. However, GlnA- RegC mutants (a phenotype constitutive for histidase) contained over 70% of the parental Amt activity. At steady-state levels, GlnA- RegC mutants accumulated chemically unaltered [14C]methylammonium against a 60- to 80-fold concentration gradient, whereas the labeled substrate was trapped within parental cells as gamma-glutamylmethylamide. GlnL Reg- mutants (normal glutamine synthetase regulation) had less than 4% of the Amt activity observed for the parental strain. However, the Amt activity of GlnL RegC mutants was slightly higher than that of the parental strain and was not repressed during growth of cells in media containing ammonia. These findings demonstrate that glutamine synthetase is not required for Amt in E. coli. The loss of Amt in certain GlnA- strains is due to polar effects on glnL and glnG genes, whose products are involved in expression of nitrogen-regulated genes, including that for Amt.
当以谷氨酸或谷氨酰胺作为氮源进行培养时，大肠杆菌会表达一种特定的铵（甲基铵）转运系统（Amt）。超过95%的这种Amt活性会被野生型细胞在含氨培养基上的生长所抑制。利用在glnALG操纵子中含有特定突变的菌株研究了Amt表达的调控。GlnA-（谷氨酰胺合成酶缺陷型）突变体，其在glnL和glnG基因上含有极性突变，因此具有Reg-表型（无法开启氮调节操纵子，如组氨酸酶），其表达的Amt活性不到亲本菌株的10%。同样，在具有GlnA+ Reg-表型的GlnG突变体中也发现了低水平的Amt。然而，GlnA- RegC突变体（一种组氨酸酶组成型表型）含有超过亲本Amt活性70%的活性。在稳态水平下，GlnA- RegC突变体逆着60至80倍的浓度梯度积累化学性质未改变的[14C]甲基铵，而标记的底物在亲本细胞内以γ-谷氨酰甲基酰胺的形式被捕获。GlnL Reg-突变体（正常的谷氨酰胺合成酶调节）的Amt活性不到亲本菌株的4%。然而，GlnL RegC突变体的Amt活性略高于亲本菌株，并且在细胞在含氨培养基中生长期间不被抑制。这些发现表明，谷氨酰胺合成酶对于大肠杆菌中的Amt不是必需的。某些GlnA-菌株中Amt的丧失是由于对glnL和glnG基因的极性效应，其产物参与氮调节基因的表达，包括Amt的基因表达。
