Parallel pathways for transduction of chemotactic signals in Escherichia coli

Abstract

Bacteria respond transiently to tactic stimuli¹, which indicates that cells adapt to new levels of stimulation. Receptors^2,3 send signals to a tumble regulator that controls the balance between directions of flagellar rotation⁴, creating a pattern of swimming and tumbling that results in taxis⁵. In Escherichia coli, mutations in tsr and tar affect receptor–regulator linkages, eliminating responses mediated by two different groups of receptors^6–9. Sensitivity to most sugars is not reduced by either mutation but observations that tsr–tar double mutants do not respond to spatial gradients of any compound led to the suggestion that the tsr and tar products together provide a necessary final step in transduction from all receptors^7,8. Another class of pleiotropic taxis mutations, trg^10–13, causes defective transduction from only two receptors, those for galactose and ribose. We were therefore interested in determining whether the trg function represented an early stage of transduction or whether it was analogous to the tsr and tar functions. We have examined the sensitivity of tsr and tar double mutants to temporal gradients, prompted by a report that such gradients of galactose or ribose evoke responses in those strains¹⁴. We report here that tsr–tar double mutants were sensitive to stimulation by compounds to which both single mutants respond but were drastically defective in adaptation to these stimuli. We suggest there are at least four parallel, single-component pathways for transduction of tactic signals, from the Tsr, Tar, Trg and enzyme II groups of receptors.