CheB is required for behavioural responses to negative stimuli during chemotaxis in Bacillus subtilis

The methyl-accepting chemotaxis protein, McpB, is the sole receptor mediati ng asparagine chemotaxis in Bacillus subtilis. In this study, we show that wildtype B. subtilis cells contain approximate to 2000 copies of McpB per c ell, that these receptors are localized polarly, and that titration of only a few receptors is sufficient to generate a detectable behavioural respons e. In contrast to the wild type, a cheB mutant was incapable of tumbling in response to decreasing concentrations of asparagine, but the cheB mutant w as able to accumulate to low concentrations of asparagine in the capillary assay, as observed previously in response to azetidine-2-carboxylate. Furth ermore, net demethylation of McpB is logarithmically dependent on asparagin e concentration, with half-maximal demethylation of McpB occurring when onl y 3% of the receptors are titrated. Because the corresponding methanol prod uction is exponentially dependent on attractant concentration, net methylat ion changes and increased turnover of methyl groups must occur on McpB at h igh concentrations of asparagine. Together, the data support the hypothesis that methylation changes occur on asparagine-bound McpB to enhance the dyn amic range of the receptor complex and to enable the cell to respond to a n egative stimulus, such as removal of asparagine.