Multiple Sources Of Signaling Noise In Bacterial Chemotaxis Network

Due to their relatively small size and a limited number of components, cells are intrinsically subject to stochastic fluctuations. Whereas stochasticity in gene expression has been extensively investigated, much less is known about posttranslational noise arising from activity fluctuations within protein networks. The pathway controlling chemotaxis of Escherichia coli provides one of the few examples where signaling noise has been previously deduced from cellular behavior. Here we use direct single-cell FRET measurements of the pathway activity to directly confirm the existence of signaling noise in chemotaxis and to characterize its determinants. Our analysis confirms previously proposed role of chemoreceptor methylation enzymes as major contributors to the pathway noise. However, it also demonstrates that allosteric interactions and slow receptor rearrangements within clusters of chemoreceptors contribute largely to activity fluctuations. Resulting mathematical description of activity fluctuations illustrates the inherent relation between the noise in the signaling system and its sensitivity to perturbations.