Thermosensing properties of Escherichia coli tsr mutants defective in serine chemoreception

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.