We characterized mutants in two novel genes of Bacillus subtilis, cheC and cheD. Mutants in CheC had a high smooth swimming bias and exhibited poor adaptation to positive stimuli. Analysis of tethered cells revealed two distinct subpopulations which differ in their prestimulus bias and extent of adaptation. The receptors, the methyl-accepting chemotaxis proteins (MCPs), of this mutant strain were overmethylated, as a result of an increase in CheR activity. We speculate that CheC helps to control tumbling frequency by regulating CheR, perhaps by a feedback mechanism through the MCPs. In contrast, a cheD mutant exhibited very tumbly behavior, and many of the MCPs were unmethylated. It seems that some B. subtilis MCPs require the presence of CheD for CheR to methylate them, a unique feature of B. subtilis chemotaxis. It is hypothesized that CheD is part of a complex that facilitates methylation of some of the MCPs, and dissociation of CheD from this complex affects CheA activity and may help bring about adaptation.

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In this study, we have demonstrated that two unique proteins in Bacillus subtilis chemotaxis, CheC and CheD, interact. We have shown this interaction both by using the yeast two-hybrid system and by precipitation of in vitro translated products using glutathione-S-transferase fusions and glutathione agarose beads. We have also shown that CheC inhibits B. subtilis CheR-mediated methylation of B. subtilis methyl-accepting chemotaxis proteins (MCPs) but not of Escherichia coli MCPs. It was previously reported that cheC mutants tend to swim smoothly and do not adapt to addition of attractant; cheD mutants have very poorly methylated MCPs and are very tumbly, similar to cheA mutants. We hypothesize that CheC exerts its effect on MCP methylation in B. subtilis by controlling the binding of CheD to the MCPs. In absence of CheD, the MCPs are poor substrates for CheR and appear to tie up, rather than activate, CheA. The regulation of CheD by CheC may be part of a unique adaptation system for chemotaxis in B. subtilis, whereby high levels of CheY-P brought about by attractant addition would allow CheC to interact with CheD and consequently leave the MCPs, reducing CheA activity and hence the levels of CheY-P.

We report the sequence and characterization of the Bacillus subtilis tlpC gene. tlpC encodes a 61.8 kDa polypeptide (TIpC) which exhibits 30% amino acid identity with the Escherichia coli methyl-accepting chemotaxis proteins (MCPs) and 38% identity with B. subtilis MCPs within the C-terminal domain. The putative methylation sites parallel those of the B. subtilis MCPs, rather than those of the E. coli receptors. TlpC is methylated both in vivo and in vitro although the level of methylation is poor. In addition, the Em coli anti-Trg antibody is shown to cross-react with this membrane protein. Inactivation of the tlpC gene confirms that TlpC is not one of the previously characterized MCPs from B. subtilis. Capillary assays were performed using a variety of chemoeffectors, which included all 20 amino acids, several sugars, and several compounds previously classified as repellents. However, no chemotactic defect was observed for any of the chemoeffectors tested. We suggest that TlpC is similar to an evolutionary intermediate from which the major chemotactic transducers from B. subtilis arose.