Desensitization by covalent modification of the chemoreceptor of Escherichia coli

Chemosensing in Escherichia coli: two regimes of two-state receptors

The chemotaxis network in Escherichia coli is remarkable for its sensitivity to small relative changes in the concentrations of multiple chemical signals. We present a model for signal integration by mixed clusters of interacting two-state chemoreceptors. Our model results compare favorably to the results obtained by Sourjik and Berg with in vivo fluorescence resonance energy transfer. Importantly, we identify two distinct regimes of behavior, depending on the relative energies of the two states of the receptors. In regime I, coupling of receptors leads to high sensitivity, while in regime II, coupling of receptors leads to high cooperativity, i.e., high Hill coefficient. For homogeneous receptors, we predict an observable transition between regime I and regime II with increasing receptor methylation or amidation.

Covalent modification regulates ligand binding to receptor complexes in the chemosensory system of Escherichia coli

In the Escherichia coli chemosensory pathway, receptor modification mediates adaptation to ligand. Evidence is presented that covalent modification influences ligand binding to receptors in complexes with CheW and the kinase CheA. Kinase inhibition was measured with serine receptor complexes in different modification levels; Ki for serine-mediated inhibition increased 10,000-fold from the lowest to the highest level. Without CheA and CheW, ligand binding is unaffected by covalent modification; thus, the influence of covalent modification is mediated only in the receptor complex, a conclusion supported by an analogy to allosteric enzymes and the observation of cooperative kinase inhibition. Also, the finding that a subsaturating serine concentration accelerates active receptor-kinase complex assembly implies that the assembly/disassembly process may also contribute to kinase regulation.

Receptor clustering as a cellular mechanism to control sensitivity

Chemotactic bacteria such as Escherichia coli can detect and respond to extremely low concentrations of attractants, concentrations of less than 5 nM in the case of aspartate. They also sense gradients of attractants extending over five orders of magnitude in concentration (up to 1 mM aspartate). Here we consider the possibility that this combination of sensitivity and range of response depends on the clustering of chemotactic receptors on the surface of the bacterium. We examine what will happen if ligand binding changes the activity of a receptor, propagating this change in activity to neighbouring receptors in a cluster. Calculations based on these assumptions show that sensitivity to extracellular ligands increases with the extent of spread of activity through an array of receptors, but that the range of concentrations over which the array works is severely diminished. However, a combination of low threshold of response and wide dynamic range can be attained if the cell has both clusters and single receptors on its surface, particularly if the extent of activity spread can adapt to external conditions. A mechanism of this kind can account quantitatively for the sensitivity and response range of E. coli to aspartate.

Uncoupling of ligand-binding affinity of the bacterial serine chemoreceptor from methylation- and temperature-modulated signaling states

The Escherichia coli chemoreceptor Tsr mediates tactic responses to serine, repellents, and changes in temperature. We have previously shown that the serine-sensing ability of Tsr-T156C, which has a unique cysteine in place of threonine at residue 156, is specifically inactivated by thiol-modifying reagents and that L-serine protects the receptor from modification. In this study, we demonstrated the correlation between protective effects of various attractants and their potencies to elicit attractant responses. This indirect binding assay was used to monitor the affinity of the receptor for L-serine under various conditions. It has been demonstrated by in vitro assays that the ligand-binding affinities of Tsr and the related chemoreceptor Tar are unaffected by changes in the methylation state of the receptor. Using the serine protection assay, we re-examined this issue both in vitro and in vivo. The methylation levels of Tsr-T156C did not affect its ligand-binding affinity. We also showed both in vitro and in vivo that the ligand-binding affinity was unaffected by temperature. These results suggest that the structure of the periplasmic domain of the receptor is uncoupled from the signaling states of the cytoplasmic domain. This ligand-binding assay system should be applicable to other receptors.

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.

Hybrid Escherichia coli sensory transducers with altered stimulus detection and signaling properties

The tar and tap loci of Escherichia coli encode methyl-accepting inner membrane proteins that mediate chemotactic responses to aspartate and maltose or to dipeptides. These genes lie adjacent to each other in the same orientation on the chromosome and have extensive sequence homology throughout the C-terminal portions of their coding regions. Many spontaneous deletions in the tar-tap region appear to be generated by recombination between these regions of homology, leading to gene fusions that produce hybrid transducer molecules in which the N terminus of Tar is joined to the C terminus of Tap. The properties of two such hybrids are described in this report. Although Tar and Tap molecules have homologous domain structures, these Tar-Tap hybrids exhibited defects in stimulus detection and flagellar signaling. Both hybrid transducers retained Tar receptor specificity, but had reduced detection sensitivity. This defect was correlated with the presence of the C-terminal methyl-accepting segment of Tap, which may have more methylation sites than its Tar counterpart, leading to elevated steady-state methylation levels in the hybrid molecules. One of the hybrids, which carried a more extensive segment from Tap, appeared to generate constitutive signals that locked the flagellar motors in a counterclockwise rotational mode. Changes in the methylation state of this transducer were ineffective in cancelling this aberrant signal. These findings implicate the conserved C-terminal domain of bacterial transducers in the generation or regulation of flagellar signals.

Chemotaxis in Escherichia coli: construction and properties of lambda tsr transducing phage

The tsr gene of Escherichia coli, located at approximately 99 min on the chromosomal map, encodes a methyl-accepting protein that serves as the chemoreceptor and signal transducer for chemotactic responses to serine and several repellents. To determine whether any other chemotaxis or motility genes were located in the tsr region, we constructed and characterized two lambda tsr transducing phages that each contain about 12 kilobases of chromosomal material adjacent to tsr. lambda tsr70 carries sequences from the promoter-proximal side of tsr; lambda tsr72 carries sequences from the promoter-distal side of tsr. Restriction maps of the bacterial inserts in these phages and Southern hybridization analyses of the bacterial chromosome indicated that the tsr gene is transcribed in the counterclockwise direction on the genetic map. Insert deletions were isolated in lambda tsr70 and transferred into the host chromosome to examine the null phenotype of tsr. All such strains exhibited wild-type swimming patterns and chemotactic responses to a variety of stimuli, but were specifically defective in serine taxis and other Tsr-mediated responses. In addition, UV programming experiments demonstrated that Tsr and several of its presumptive degradation products were the only bacterial proteins encoded by lambda tsr70 and lambda tsr72 that required host FlbB/FlaI function for expression. These findings indicate that there are probably no other chemotaxis-related genes in the tsr region. A series of tsr point mutations were isolated by propagating lambda tsr70 on a mutD host and used to construct a fine-structure map of the tsr locus. These mutations should prove valuable in exploring structure-function relationships in the Tsr transducer.