Thermosensing properties of Escherichia coli tsr mutants defective in serine chemoreception

Abstract

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.

Aspartate and maltose-binding protein interact with adjacent sites in the Tar chemotactic signal transducer of Escherichia coli

“…Lee and Imae (21) proposed that the alpha-amino group of an amino acid ligand interacts with Thr-154 of Tar (or Thr-156 of Tsr) (22) and that the alpha-carboxyl group of the ligand interacts with Arg-64. Binding specificity for a particular amino acid ligand would be determined by the interaction of its R group with other residues in the two segments.…”

“…The primary effect of mutations altering these residues is to reduce affinity for aspartate (37). Replacing Thr-154 of Tar with isoleucine interferes with aspartate sensing (21), and altering the corresponding Thr-156 residue in the Tsr transducer disrupts serine sensing (22).…”

Transmembrane signalling by a hybrid protein: communication from the domain of chemoreceptor Trg that recognizes sugar-binding proteins to the kinase/phosphatase domain of osmosensor EnvZ

“…EnvZ is the transmembrane sensor protein, and OmpR is the phosphorylation-activated DNA-binding protein (1,9,15). EnvZ has a transmembrane domain made up of two segments, one near the amino terminus and the other near the middle of the polypeptide chain, a periplasmic domain thought to be involved in sensing osmotic pressure (32), and a cytoplasmic domain that has both kinase and phosphatase activity specific for OmpR (10,18 There is substantial identity among the amino acid sequences of the four receptors in their cytoplasmic domains but little in their periplasmic or transmembraqe domains (13), yet mutational analyses of ligand binding imply that there is a common three-dimensional organization even in those domains (12,19,21,22,28,(34)(35)(36).…”

Mutational Analysis of Ligand Recognition by Tcp, the Citrate Chemoreceptor of Salmonella enterica Serovar Typhimurium

“…1A and B). The corresponding Arg residues of Tsr are predicted to interact with the ␣-carboxyl or the hydroxyl group of serine (19,22) (Fig. 1A and B).…”