Tucker Don L, Tucker Nancy, Ma Zhuo, Foster John W, Miranda Regina L, Cohen Paul S, Conway Tyrrell
Advanced Center for Genome Technology, The University of Oklahoma, Norman, Oklahoma 73019, USA.
J Bacteriol. 2003 May;185(10):3190-201. doi: 10.1128/JB.185.10.3190-3201.2003.
Acid in the stomach is thought to be a barrier to bacterial colonization of the intestine. Escherichia coli, however, has three systems for acid resistance, which overcome this barrier. The most effective of these systems is dependent on transport and decarboxylation of glutamate. GadX regulates two genes that encode isoforms of glutamate decarboxylase critical to this system, but additional genes associated with the glutamate-dependent acid resistance system remained to be identified. The gadX gene and a second downstream araC-like transcription factor gene, gadW, were mutated separately and in combination, and the gene expression profiles of the mutants were compared to those of the wild-type strain grown in neutral and acidified media under conditions favoring induction of glutamate-dependent acid resistance. Cluster and principal-component analyses identified 15 GadX-regulated, acid-inducible genes. Reverse transcriptase mapping demonstrated that these genes are organized in 10 operons. Analysis of the strain lacking GadX but possessing GadW confirmed that GadX is a transcriptional activator under acidic growth conditions. Analysis of the strain lacking GadW but possessing GadX indicated that GadW exerts negative control over three GadX target genes. The strain lacking both GadX and GadW was defective in acid induction of most but not all GadX target genes, consistent with the roles of GadW as an inhibitor of GadX-dependent activation of some genes and an activator of other genes. Resistance to acid was decreased under certain conditions in a gadX mutant and even more so by combined mutation of gadX and gadW. However, there was no defect in colonization of the streptomycin-treated mouse model by the gadX mutant in competition with the wild type, and the gadX gadW mutant was a better colonizer than the wild type. Thus, E. coli colonization of the mouse does not appear to require glutamate-dependent acid resistance.
胃中的胃酸被认为是肠道细菌定植的一个屏障。然而，大肠杆菌有三种耐酸系统，可克服这一屏障。其中最有效的系统依赖于谷氨酸的转运和脱羧作用。GadX调控两个基因，这两个基因编码对该系统至关重要的谷氨酸脱羧酶同工型，但与谷氨酸依赖性耐酸系统相关的其他基因仍有待确定。分别单独和联合突变gadX基因以及第二个下游类araC转录因子基因gadW，并将突变体的基因表达谱与在有利于诱导谷氨酸依赖性耐酸的条件下于中性和酸化培养基中生长的野生型菌株的基因表达谱进行比较。聚类分析和主成分分析确定了15个受GadX调控的酸诱导基因。逆转录酶定位表明这些基因以10个操纵子的形式组织。对缺乏GadX但拥有GadW的菌株的分析证实，GadX在酸性生长条件下是一种转录激活因子。对缺乏GadW但拥有GadX的菌株的分析表明，GadW对三个GadX靶基因发挥负调控作用。同时缺乏GadX和GadW的菌株在大多数但并非所有GadX靶基因的酸诱导方面存在缺陷，这与GadW作为某些基因的GadX依赖性激活的抑制剂和其他基因的激活剂的作用一致。在某些条件下，gadX突变体的耐酸性降低了，而gadX和gadW的联合突变导致的耐酸性降低更明显。然而，在与野生型竞争时，gadX突变体在链霉素处理的小鼠模型中的定植没有缺陷，并且gadX gadW突变体比野生型更善于定植。因此，大肠杆菌在小鼠中的定植似乎不需要谷氨酸依赖性耐酸能力。
