Adaptation and methylation kinetics in Escherichia coli chemotaxis

Transmembrane chemoreceptors of Escherichia coli bind periplasmic ligands and transduce the signal to the flagella motors, thereby adjusting the swimming behaviour of the cell according to the chemical nature of the ligand. Cell movement, directed either towards nutrients or away from toxic compounds, is known as chemotaxis. An important property of the chemotaxis signalling pathway essential for navigation in complex gradients of nutrients is adaptation, mediated by methylation of specific glutamate residues in the chemoreceptors cytoplasmic domain. The aspartate chemoreceptor Tar possesses four such sites, but it is still unclear why several sites of methylation are needed and if a certain hierarchy among these sites exists. In this study, we systematically and quantitatively characterized the efficiency of chemotaxis and the precision of adaptation for cells expressing Tar mutated at one or more modification sites as the only chemoreceptor. Therefore, we constructed Tar chemoreceptors with all possible combinations of alanine substitutions at the methylation sites to specifically render them non-methylatable. These Tar mutants were then tested for their ability to mediate chemotaxis on soft agar plates. Furthermore, adaptation kinetics of Tar mutants were analyzed by in vivo FRET microscopy and wild-type Tar was investigated by mass spectrometrical analysis, which allows to follow the order and kinetics of methylation at individual modification sites during the adaptation process. We found that the receptor methylation rate following addition of attractant differs for the individual methylation sites with methylation site 2 being fastest, followed by sites 1 and 3, and site 4 having the slowest rate of methylation. Demethylation upon removal of attractant occurs first at methylation site 3, followed by sites 2 and 1. Furthermore, we discovered that specific methylation sites are responsible for different features of chemotaxis and adaptation. Methylation site 1 mainly contributes to the adaptation precision and the methylation rate, whereas methylation site 2 is important for the methylation rate as well as for the demethylation rate. Methylation site 3 is responsible for the chemotaxis and the demethylation rate and methylation site 4 mainly contributes to the methylation rate. In summary, the results of the present study provide new insights into the molecular details of the adaptation process in E. coli chemotaxis and the subtle interplay of individual methylation sites in the regulation of chemotactic behavior

LES PARENTS,L'ECHOGRAPHISTE,LE PSYCHIATRE (MISE EN PERSPECTIVE DE QUELQUES ASPECTS PSYCHODYNAMIQUES DE L'ACCESSION A LA PARENTALITE A TRAVERS L'EXPERIENCE DE L'ECHOGRAPHIE DE LA GROSSESSE)

ANGERS-BU Médecine-Pharmacie (490072105) / SudocPARIS-BIUM (751062103) / SudocSudocFranceF

Evolution règlementaire dans le cadre de la désinfection des endoscopes souples et enquête sur les pratiques au CHRU de Lille

LILLE2-BU Santé-Recherche (593502101) / SudocSudocFranceF

Universal Response-Adaptation Relation in Bacterial Chemotaxis

The bacterial strategy of chemotaxis relies on temporal comparisons of chemical concentrations, where the probability of maintaining the current direction of swimming is modulated by changes in stimulation experienced during the recent past. A short-term memory required for such comparisons is provided by the adaptation system, which operates through the activity-dependent methylation of chemotaxis receptors. Previous theoretical studies have suggested that efficient navigation in gradients requires a well-defined adaptation rate, because the memory time scale needs to match the duration of straight runs made by bacteria. Here we demonstrate that the chemotaxis pathway of Escherichia coli does indeed exhibit a universal relation between the response magnitude and adaptation time which does not depend on the type of chemical ligand. Our results suggest that this alignment of adaptation rates for different ligands is achieved through cooperative interactions among chemoreceptors rather than through fine-tuning of methylation rates for individual receptors. This observation illustrates a yet-unrecognized function of receptor clustering in bacterial chemotaxis

Importance of Multiple Methylation Sites in Escherichia coli Chemotaxis.

Bacteria navigate within inhomogeneous environments by temporally comparing concentrations of chemoeffectors over the course of a few seconds and biasing their rate of reorientations accordingly, thereby drifting towards more favorable conditions. This navigation requires a short-term memory achieved through the sequential methylations and demethylations of several specific glutamate residues on the chemotaxis receptors, which progressively adjusts the receptors' activity to track the levels of stimulation encountered by the cell with a delay. Such adaptation also tunes the receptors' sensitivity according to the background ligand concentration, enabling the cells to respond to fractional rather than absolute concentration changes, i.e. to perform logarithmic sensing. Despite the adaptation system being principally well understood, the need for a specific number of methylation sites remains relatively unclear. Here we systematically substituted the four glutamate residues of the Tar receptor of Escherichia coli by non-methylated alanine, creating a set of 16 modified receptors with a varying number of available methylation sites and explored the effect of these substitutions on the performance of the chemotaxis system. Alanine substitutions were found to desensitize the receptors, similarly but to a lesser extent than glutamate methylation, and to affect the methylation and demethylation rates of the remaining sites in a site-specific manner. Each substitution reduces the dynamic range of chemotaxis, by one order of magnitude on average. The substitution of up to two sites could be partly compensated by the adaptation system, but the full set of methylation sites was necessary to achieve efficient logarithmic sensing