Table_1_The Signaling Molecule Indole Inhibits Induction of the AR2 Acid Resistance System in Escherichia coli.pdf

Induction of the AR2 acid response system of Escherichia coli occurs at a moderately low pH (pH 5.5) and leads to high levels of resistance to pH levels below 2.5 in the presence of glutamate. Induction is mediated in part by the EvgAS two component system. Here, we show that the bacterial signaling molecule indole inhibits the induction of key promoters in the AR2 system and blocks the development of glutamate-dependent acid resistance. The addition of tryptophan, the precursor for indole biosynthesis, had the same effects, and this block was relieved in a tnaA mutant, which is unable to synthesize indole. Expression of a constitutively active EvgS protein was able to relieve the inhibition caused by indole, consistent with EvgS being inhibited directly or indirectly by indole. Indole had no effect on autophosphorylation of the isolated cytoplasmic domain of EvgS. This is consistent with a model where indole directly or indirectly affects the ability of EvgS to detect its inducing signal or to transduce this information across the cytoplasmic membrane. The inhibitory activity of indole on the AR2 system is not related to its ability to act as an ionophore, and, conversely, the ionophore CCCP had no effect on acid-induced AR2 promoter activity, showing that the proton motive force is unlikely to be a signal for induction of the AR2 system.

大肠杆菌（Escherichia coli）的AR2酸应答系统（AR2 acid response system）可在中度偏低的酸碱度（pH 5.5）下被诱导，进而使菌体在谷氨酸（glutamate）存在的条件下获得对pH 2.5以下酸碱度的极高抗性。该系统的诱导过程部分由EvgAS双组分系统（two component system）介导。本研究表明，细菌信号分子吲哚（indole）能够抑制AR2酸应答系统中关键启动子的诱导，并阻断谷氨酸依赖性酸抗性的形成。吲哚的生物合成（biosynthesis）前体色氨酸（tryptophan）的添加可产生相同效应，且这一抑制作用可在无法合成吲哚的tnaA突变体（tnaA mutant）中被解除。组成型激活的EvgS蛋白的表达可抵消吲哚介导的抑制作用，这与吲哚直接或间接抑制EvgS的结论一致。吲哚对分离得到的EvgS胞质结构域（cytoplasmic domain）的自磷酸化（autophosphorylation）并无影响，这支持如下模型：吲哚通过直接或间接方式，影响EvgS感知诱导信号或跨细胞质膜传递该信号的能力。吲哚对AR2酸应答系统的抑制活性与其作为离子载体（ionophore）的功能无关；与之相反，离子载体CCCP对酸诱导的AR2启动子活性无影响，这表明质子动力势（proton motive force）不太可能是AR2酸应答系统诱导的信号。