| Popis: |
Chemotactic responses of the bacterium Escherichia coli were elicited by iontophoretic ejection of charged compounds from micropipettes. Responses were measured using cells tethered by a single flagellum and following the direction of rotation of the cell body (with cells viewed from above the surface to which the flagellum is attached). Mean response latencies to addition of attractant or repellent were 0.1 to 0.2 seconds. Brief pulses of attractants and repellents were used to determine the impulse response for chemotaxis. In response to a brief pulse of attractant the counterclockwise (CCW) bias (fraction of time that the flagellar motor spent rotating CCW) increased rapidly to a peak, then fell below the prestimulus value, returning to baseline within 5 seconds -- the response was biphasic. Repellent impulse responses were similar but inverted. The attractant impulse response accurately predicted responses to step changes, linear ramp changes and sine wave changes in receptor occupancy. Mutants defective in the enzymes responsible for methylation (cheR) and demethylation (cheB), which do not adapt to attractant stimuli, gave monophasic impulse responses--only the initial peak was evident. Mutants defective in the cheZ gene product had responses that were slower than the wild-type by a factor of about 10. The responses of flagellar motors on filamentous cells also were studied. The reversals of two motors on the same cell were not correlated, but fluctuations in bias were correlated for motors less than l 0 microns apart. Responses of motors of filamentous cells to iontophoretic application of aspartate indicated that the internal chemotaxis signal travels about 2 microns in cells lacking the cheR and cheB gene products and about 6 microns in cells with a defective cheZ gene product. These data are consistent with a simple model involving destruction of a cellular chemotaxis signal by the cheZ gene product. |
