Adaptational "crosstalk" and the crucial role of methylation in chemotactic migration by Escherichia coli.

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Abstract
We investigated roles of methylation in bacterial chemotaxis by characterizing a methyl-accepting transducer protein incapable of methylation because of amino acid substitutions at the modification sites. Mutant Trg protein recognized ligand and generated excitatory signals that affected flagella but was unable to mediate efficient adaptation or net cellular migration in a relevant chemical gradient. Defects caused by lack of methyl-accepting sites on Trg were suppressed by a sufficient cellular content of other transducer molecules with functional methyl-accepting sites. These observations establish directly that methylation is crucial for transducer-mediated chemotaxis and that neither phosphotransfer reactions among the soluble Che proteins nor other interaction among those chemotactic components can effectively fulfill the functions of methylation. Suppression was correlated with adaptational "crosstalk" in which unoccupied methyl-accepting transducers acquired methyl groups, thus apparently substituting effectively for blocked methyl-accepting sites on the transducer. A plausible model for this phenomenon is that increased methylation of unstimulated transducers results from global inhibition of the demethylating enzyme in a cell with a normally active methyltransferase and no available methyl-accepting sites on the stimulated, mutant transducer. Thus methylation can perform its roles in adaptation and gradient sensing even if modification occurs on molecules different from those that recognize the stimulating compound. This observation emphasizes the central role of methylation and the modular nature of the chemosensory system.