Tsr, a chemoreceptor for serine and repellents in Escherichia coli, also functions as a thermoreceptor. The relationship between the chemoreceptor and thermoreceptor functions of Tsr was examined in five tsr mutants with altered serine detection thresholds. The thermosensing abilities of the mutant Tsr proteins were not affected by the alterations in their affinities to serine. In contrast, the ability of serine to inactivate thermoreceptor function was altered in these mutants. The minimal serine concentration required for thermoreceptor inactivation was directly related to the decreased affinity of the mutant Tsr for serine. The amino acid replacements in the mutant receptors were deduced from DNA sequence analyses and occurred at two different locations in the presumed periplasmic domain of Tsr. Two mutations caused histidine or cysteine replacements at arginine 64, whereas three others caused isoleucine or proline replacements at threonine 156.Escherichia coli cells can sense changes in their environment and respond by modulating their swimming pattern; attractant stimuli cause smooth swimming, and repellent stimuli cause tumbling. Genetic and biochemical studies of chemotaxis have shown that E. coli has four transmembrane chemoreceptors, called Tsr, Tar, Trg, and Tap (for reviews, see references 8 and 19). Many chemical stimuli are detected by these receptors, which then transmit sensory information into the cell, generating signals that modulate the swimming pattern of the organism. Adaptation to stimuli is brought about through the reversible methylation of specific glutamic acid residues in the cytoplasmic domain of these receptors.We found that E. coli can also sense thermal stimuli (10). Cells show smooth swimming upon a temperature increase and tumbling upon a temperature decrease. Further studies showed that two major chemoreceptors, Tsr and Tar, which detect serine and aspartate, respectively, also function as thermoreceptors (4,9,14). Thus, these receptors are able to detect two quite different stimuli, one chemical and the other physical.Competition experiments between chemical and thermal stimuli demonstrated that the thermosensing ability of these receptors was altered by first adapting the cells to serine or aspartate (4,9,14). Tsr in serine-adapted cells loses its ability to detect thermal stimuli, and therefore, cells with only Tsr show no thermoresponse after adapting to serine. In contrast, Tar in cells which have been adapted to aspartate retains its ability to detect thermal stimuli but generates an inverted sensory signal. Thus, after adapting to aspartate, cells with only Tar tumble in response to a temperature increase and swim smoothly in response to a temperature decrease. In wild-type cells in which Tsr is present at approximately twice the level of Tar, aspartate reduces, but does not invert, thermoresponses.
