Genetics of methyl-accepting chemotaxis proteins in Escherichia coli: cheD mutations affect the structure and function of the Tsr transducer.

The tsr gene specifies a methyl-accepting membrane protein involved in chemotaxis to serine and several repellent compounds. We have characterized a special class of tsr mutations designated cheD which alter the signaling properties of the Tsr transducer. Unlike tsr null mutants, cheD strains are generally nonchemotactic, dominant in complementation tests, and exhibit a pronounced counterclockwise bias in flagellar rotation. Several lines of evidence showed that cheD mutations were alleles of the tsr gene. First, cheD mutations were mapped into the same deletion segments as conventional tsr mutations. Second, restriction site analysis of the transducing phage deletions used to construct the genetic map demonstrated that the endpoints of the deletion segments fell within the tsr coding sequence. Third, a number of the cheD mutants synthesized Tsr proteins with slight changes in electrophoretic mobility, consistent with alterations in Tsr primary structure. These mutant proteins were able to undergo posttranslational deamidation and methylation reactions in the same manner as wild-type Tsr protein; however, the steady-state level of Tsr methylation in cheD strains was very high. The methylation state of the Tar protein, another species of methyl-accepting protein in Escherichia coli, was also higher than normal in cheD strains, suggesting that the aberrant Tsr transducer in cheD mutants has a generalized effect on the sensory adaptation system of the cell. These properties are consistent with the notion that the Tsr protein of cheD mutants is locked in an excitatory signaling mode that both activates the sensory adaptation system and drowns out chemotactic signals generated by other transducer species. Further study of cheD mutations thus promises to reveal valuable information about the functional architecture of the Tsr protein and how this transducer controls flagellar behavior.