[29] The Protein Carboxylmethyltransferase Involved in Escherichia coli and Salmonella typhimurium Chemotaxis

This chapter deals with carboxylmethyltransferase, which is involved in chemotaxis of Escherichia coli and Salmonella typhimurium. The transferase enzyme responsible catalyzes the transfer of methyl groups from S-adenosylmethionine (SAM) to specific glutamyl residues in the methylated 60,000-dalton membrane proteins. The reaction is catalyzed by a single transferase which is encoded by the cheR gene. The transferase shows a broad pH optimum between 6.5 and 8.5 and its activity depends on the ionic composition of the assay medium. Assays of the enzyme can be performed by a variety of methods in vivo, in situ, or in vitro. Transferase activity may be assessed in vivo by incubating cells in the presence of chloramphenicol and [methyl ³H]methionine which is rapidly transported into cells and converted into S-adenosyl[³H-methyl]methionine. Toluene treatment allows free access of S-adenosylmethionine to the cell interior so that transferase may be assayed in situ. The extent of methyl group incorporation depends on the exact conditions of toluene treatment. Transferase activity may be assayed in vitro by combining a cytoplasmic preparation containing the enzyme with membranes containing the methyl accepting proteins, and S-adenosyl[methyl-³H]methionine.