Adjusting for Stochasticity?

Science Signaling  17 Apr 2012:
Vol. 5, Issue 220, pp. ec111
DOI: 10.1126/scisignal.2003140

Reversible receptor methylation serves as the main mechanism enabling the rapid adaptation of bacterial chemotaxis machinery to allow directional movement through a gradient (see Hazelbauer). Yuan et al. describe a longer-acting mechanism, detectable in bacteria genetically deficient in the enzymes responsible for reversible receptor methylation, for adaptation of the bacterial chemotaxis machinery to changes in the concentration of the phosphorylated protein CheY-P, which binds to the flagellar motor protein FliM to trigger a change in the rotation of the motor. Receptor activation reduces the receptor’s kinase activity, which reduces the amount of CheY-P, causing the bacteria to switch from a tumbling movement when CheY-P is bound to FliM to a running, directional movement when FliM is not bound to CheY-P. Yuan et al. showed that cells defective in the receptor-level adaptation machinery exhibited partial adaptation to a chemoattractant stimulus, and mathematical modeling suggested that this may arise through a change in the abundance of FliM. The authors tested this prediction using total internal reflection fluorescence (TIRF) microscopy and fluorescent protein–tagged FliM and showed that the number of FliM molecules increased by ~25% in response to conditions that produced partial adaptation. This slow mechanism of adaptation does not contribute to the rapid adaptation observed in wild-type bacteria encountering a chemoattractant gradient but may match the motor proteins with intrinsic cell-cell differences in the output of the chemotaxis signaling machinery to ensure that stochastic differences among the cells do not compromise chemotaxis behavior.

J. Yuan, R. W. Branch, B. G. Hosu, H. C. Berg, Adaptation at the output of the chemotaxis signalling pathway. Nature 484, 233–236 (2012). [Online Journal]

G. L. Hazelbauer, Adaptation by target remodelling. Nature 484, 173–175 (2012). [Online Journal]