Salmonella chemoreceptors McpB and McpC mediate a repellent response to L-cystine: a potential mechanism to avoid oxidative conditions

TL;DR

Salmonella's chemoreceptors McpB and McpC detect L-cystine as a repellent, potentially helping the bacteria avoid oxidative conditions, with distinct responses to reduced and oxidized forms of cysteine.

Contribution

This study reveals that McpB and McpC mediate a repellent response to L-cystine, providing insight into redox sensing mechanisms in Salmonella chemotaxis.

Findings

Cystine acts as a repellent via McpB/C receptors.

Cysteine is an attractant primarily via Tsr.

Opposing responses to cysteine and cystine may help Salmonella avoid oxidative environments.

Abstract

Chemoreceptors McpB and McpC in Salmonella enterica have been reported to promote chemotaxis in LB motility-plate assays. Of the chemicals tested as potential effectors of these receptors, the only response was towards L-cysteine and its oxidized form, L-cystine. Although enhanced radial migration in plates suggested positive chemotaxis to both amino acids, capillary assays failed to show an attractant response to either, in cells expressing only these two chemoreceptors. In vivo fluorescence resonance energy transfer (FRET) measurements of kinase activity revealed that in wild-type bacteria, cysteine and cystine are chemoeffectors of opposing sign, the reduced form being a chemoattractant and the oxidized form a repellent. The attractant response to cysteine was mediated primarily by Tsr, as reported earlier for E. coli. The repellent response to cystine was mediated by McpB / C. Adaptive recovery upon cystine exposure required the methyl-transferase/-esterase pair, CheR / CheB, but restoration of kinase activity was never complete (i.e. imperfect adaptation). We provide a plausible explanation for the attractant-like responses to both cystine and cysteine motility plates, and speculate that the opposing signs of response to this redox pair might afford Salmonella a mechanism to gauge and avoid oxidative environments.