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

Nathaniel Boon,Kenta Shibayama,Yoko Eguchi,Peter A Lund,Morissa Bradnick,Amina Aziz,Manpreet Kaur

doi:10.3389/fmicb.2020.00474

Nathaniel Boon, Kenta Shibayama + Show 5 more

Open Access

https://doi.org/10.3389/fmicb.2020.00474

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Abstract

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.

Highlights

The enteric bacterium Escherichia coli has multiple systems that can protect it against low pH, the best characterized of which is the AR2 or GAD system
Phosphorylated EvgA activates expression of YdeO which, in turn, activates expression of GadE. Both GadE and YdeO activate other components of the AR2 pathway, with GadE responsible for activating expression of the decarboxylase proteins GadA and GadB, the glutamine/GABA antiporter GadC, and the periplasmic chaperones HdeA and HdeB. The dynamics of this process are complex but the net effect is the induction of the expression of a large number of genes that act together to enable E. coli survival at low pH, a phenomenon often referred to as extreme acid resistance
We screened a range of small molecules which we considered to be potential candidates for modulators of EvgS activity, based both on consideration of the predicted EvgS periplasmic domain structure (Sen et al, 2017) and knowledge of metabolites produced by E. coli

Summary

Introduction

The enteric bacterium Escherichia coli has multiple systems that can protect it against low pH, the best characterized of which is the AR2 or GAD system (for recent reviews, see Foster, 2004; Slonczewski et al, 2009; Kanjee and Houry, 2013; Lund et al, 2014; de Biase and Lund, 2015). Both GadE and YdeO activate other components of the AR2 pathway, with GadE responsible for activating expression of the decarboxylase proteins GadA and GadB, the glutamine/GABA antiporter GadC, and the periplasmic chaperones HdeA and HdeB The dynamics of this process are complex but the net effect is the induction of the expression of a large number of genes that act together to enable E. coli survival at low pH (typically pH 2.5), a phenomenon often referred to as extreme acid resistance. The relevant substitutions are all in the cytoplasmic PAS domain, consistent with a model where this domain mediates interactions in an EvgS dimer that switch it between active and inactive states (Itou et al, 2009; Johnson et al, 2014)

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