Robustness in bacterial chemotaxis

Networks of interacting proteins orchestrate the responses of living cells to a variety of external stimuli(1), but how sensitive is the functioning o f these protein networks to variations in their biochemical parameters? One possibility is chat to achieve appropriate function, the reaction rate con stants and enzyme concentrations need to be adjusted in a precise manner, a nd any deviation from these 'fine-tuned' values ruins the network's perform ance. An alternative possibility is that key properties of biochemical netw orks are robust(2); that is, they are insensitive to the precise values of the biochemical parameters. Here we address this issue in experiments using chemotaxis of Escherichia coli, one of the best-characterized sensory syst ems(3,4). We focus on how response and adaptation to attractant signals var y with systematic changes in the intracellular concentration of the compone nts of the chemotaxis network. We find that some properties, such as steady -state behaviour and adaptation time, show strong variations in response to varying protein concentrations. In contrast, the precision of adaptation i s robust and does not vary with the protein concentrations. This is consist ent with a recently proposed molecular mechanism for exact adaptation, wher e robustness is a direct consequence of the network's architecture(2).