Conditional inversion of the thermoresponse in Escherichia coli

Mutants in Escherichia coli having defects in one of the methyl-accepting chemotaxis proteins, Tsr protein, which is the chemoreceptor and transducer for L-serine, showed a reduced but similar type of thermoresponse compared with wild-type strains; the cells showed smooth swimming upon temperature increase and tumbling upon temperature decrease. However, when the mutant cells were adapted to attractants such as L-aspartate and maltose, which are specific to another methyl-accepting chemotaxis protein, Tar protein, the direction of the thermoresponse was found to be inverted; a temperature increase induced tumbling and a temperature decrease induced smooth swimming. Consistent with this, the mutant cells showed inverted changes in the methylation level of Tar protein upon temperature changes. Wild-type strains but not Tar protein-deficient mutants exhibited the inverted thermoresponse when the cells were simultaneously adapted to L-aspartate and L-serine, indicating that Tar protein has a key role in the inversion of the thermoresponse. Thus, besides Tsr protein, Tar protein has a certain role in thermoreception. A simple model for thermoreception and inversion of the thermoresponse is also discussed.