A protein methylesterase involved in bacterial sensing

A protein methylesterase has been identified in soluble extracts of Salmonella typhimurium and Escherichia coli. This enzyme catalyzes the hydrolysis of γ-glutamyl methyl ester residues from membrane-bound 60,000-molecular weight proteins that are essential for chemotaxis. Analyses of methylesterase activity in a variety of chemotactically defective strains suggest that the methylesterase is a product of the cheX gene in Salmonella and the cheB gene in E. coli. In addition, the cheT gene product in S. typhimurium seems to play a role in expression of methylesterase activity. Mutant strains lacking the protein methylesterase tumble incessantly in the absence of attractant gradients. This behavior is the converse of that shown by mutant strains defective in methyltransferase activity, which swim smoothly in the absence of repellent gradients. This finding indicates that reversible methylation acts as a control mechanism and that both a methyltransferase and a protein methylesterase are instrumental in bacterial sensing.