The ribose and galactose chemoreceptors of Escherichia coli have previously been identified as the riboseand galactose-binding proteins. We now report the discovery of a methyl-accepting chemotaxis protein that functions in the transfer of receptor signals from these two binding proteins to the flagella. This protein is distinct from previously described methyl-accepting chemotaxis proteins. Its level of methylation is influenced by. D-ribose, D-galactose, and certain structural analogues of them. This methyl-accepting protein is required for chemotaxis toward those attractants; mutants in the trg gene, which do not methylate this protein, are devoid of taxis toward D-ribose, D-galactose, and their analogues. In addition, methylation of the methyl-accepting protein in response to each of these attractants requires the appropriate binding protein. The binding protein's chemoreceptor function is required for such methylation; but its transport activity is not. Because the function of this methyl-accepting chemotaxis protein involves two of the best-characterized chemoreceptors, the discovery of this protein represents a promising base for further study of the linkage between chemoreceptors and flagella in bacteria. The chain of events that links bacterial chemoreceptors to flagella brings about the cell's chemotactic responses. Involved in this chain is a set of cytoplasmic membrane proteins that can be methylated and demethylated: the methyl-accepting chemotaxis proteins (MCPs) (1-4). This report identifies an MCP of Escherichia coil whose function is linked to the galactose and ribose chemoreceptors. The galactose receptor (5) has been identified (6, 7) as the galactose-binding protein (8)(9)(10)(11), and the ribose receptor (5) has been identified as the ribose-binding protein (6,(12)(13)(14). Because these are two of the best-characterized bacterial chemoreceptors, this particular receptor-MCP system promises to be useful in our study of ); however, the MCP species specific to the ribose receptor was not defined. In addition, a tsr-tardouble mutant, previously described as totally defective in MCP methylation (2, 3) and in chemotaxis (2), has since been found to exhibit some chemotactic response to D-ribose (M. S. Springer and M. F. Goy, cited in ref. 24).We reinvestigated the effect and specificity of D-ribose, and of D-galactose, on MCP methylation. Our study focused on "trg-" mutants of E. coli, which are specifically defective in taxis to ribose and galactose (25). The location of trg-mutants on the E. coli genetic map (25) is distinct from the known locations of the structural genes for the ribose-(14) and galactose-(25) binding proteins. Therefore, trg-mutants are distinct from known ribose and galactose chemoreceptor mutants.We now report the discovery of a MCP species, called "MCP III," whose methylation is absent in trg-mutants. Chemicals detected by the ribose and galactose chemoreceptors affect MCP III methylation in wild-type cells. This effect on MCP III methylation requires the chemor...

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