Abstract

Signal amplification and robustness against perturbations are common to most biological networks. We studied these features in chemotaxis on Escherichia coli, the most thoroughly studied model system for signal transduction. Despite its apparent simplicity, the chemotaxis pathway shows extremely high sensitivity, wide dynamic range and surprising robustness. To better understand these features, we use several fluorescence microscopy techniques, including fluorescence resonance energy transfer (FRET), to quantitatively characterize signal processing by the pathway in vivo. We showed that chemotactic stimuli are amplified and integrated by the chemoreceptor clusters, which work as large allosteric complexes. We also observed strong cell-to-cell variation in the levels of chemotaxis proteins, and investigated the effects of such variation on signal transduction. Combining theoretical and experimental analysis, we demonstrated that the pathway is robust to the observed gene expression noise and determined the network design features responsible for this robustness.