  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.
