CHEMOTACTIC ADAPTATION IS ALTERED BY CHANGES IN THE CARBOXY-TERMINAL SEQUENCE CONSERVED AMONG THE MAJOR METHYL-ACCEPTING CHEMORECEPTORS

In Escherichia coli and Salmonella typhimurium, methylation and demeth ylation of receptors are responsible for chemotactic adaptation and ar e catalyzed by the methyltransferase CheR and the methylesterase CheB, respectively. Among the chemoreceptors of these species, Tsr, Tar, an d Tcp have a well-conserved carboxyterminal motif (NWET/SF) that is ab sent in Trg and Tap. When they are expressed as sole chemoreceptors, T sr, Tar, and Tcp support good adaptation, but Trg and Tap are poorly m ethylated and supported only weak adaptation, It was recently discover ed that CheR binds to the NWETF sequence of Tsr in vitro, To examine t he physiological significance of this binding, we characterized mutant receptors in which this pentapeptide sequence was altered, C-terminal ly-mutated Tar and Tcp expressed in a receptorless E. coli strain medi ated responses to aspartate and citrate, respectively, but their adapt ation abilities were severely impaired, Their expression levels and at tractant-sensing abilities were similar to those of the wild-type rece ptors, but the methylation levels of the mutant receptors increased on ly slightly upon addition of attractants. When CheR was overproduced, both the adaptation and methylation profiles of the mutant Tar recepto r became comparable to those of wild-type Tar. Furthermore, overproduc tion of CheR also enhanced adaptive methylation of wild-type Trg, whic h lacks the NWETF sequence, in the absence of any other chemoreceptor, These results suggest that the pentapeptide sequence facilitates effe ctive adaptation and methylation by recruiting CheR.