Protein phosphorylation and regulation of adaptive responses in bacteria.

Networks of interacting proteins orchestrate the responses of living cells to a variety of external stimuli, but how sensitive is the functioning of these protein networks to variations in their biochemical parameters? One possibility is that to achieve appropriate function, the reaction rate constants and enzyme concentrations need to be adjusted in a precise manner, and any deviation from these 'fine-tuned' values ruins the network's performance An alternative possibility is that key properties of biochemical networks are robust; 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 systems We focus on how response and adaptation to attractant signals vary with systematic changes in the intracellular concentration of the components 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 is robust and does not vary with the protein concentrations This is consistent with a recently proposed molecular mechanism for exact adaptation, where robustness is a direct consequence of the network's architecture

Robustness in bacterial chemotaxis

http://www.damtp.cam.ac.uk/user/lauga/papers/12.pdf

Swimming in circles: motion of bacteria near solid boundaries

Cells display a remarkable ability to respond to small fluctuations in their surroundings. In simple microbial systems, information from sensory receptors feeds into a circuitry of regulatory proteins that transfer high energy phosphoryl groups from histidine to aspartate side chains. This phosphotransfer network couples environmental signals to an array of response elements that control cell motility and regulate gene expression.

Signal transduction in bacteria.

When chemotactic strains of the bacterium Escherichia coli are inoculated on semi-solid agar containing mixtures of amino acids or sugars, the cells swarm outwards in a series of concentric rings: they respond to spatial gradients of attractants generated by uptake and catabolism. Cells also drift up gradients generated artificially, for example by diffusion from the tip of a capillary tube or by mixing. Here we describe conditions under which cells aggregate in response to gradients of attractant which they excrete themselves. When cells are grown in semi-solid agar on intermediates of the tricarboxylic acid cycle, they form symmetrical arrays of spots or stripes that arise sequentially. When cells in a thin layer of liquid culture are exposed to these compounds, spots appear synchronously, more randomly arrayed. In either case, the patterns are stationary. The attractant is a chemical sensed by the aspartate receptor. Its excretion can be triggered by oxidative stress. As oxygen is limiting at high cell densities, aggregation might serve as a mechanism for collective defence.

Complex patterns formed by motile cells of Escherichia coli

Strains missing several genes required for chemotaxis toward amino acids, peptides, and certain sugars were tethered and their rotational behavior was analyzed. Null strains (called gutted) were deleted for genes that code for the transducers Tsr, Tar, Tap, and Trg and for the cytoplasmic proteins CheA, CheW, CheR, CheB, CheY, and CheZ. Motor switch components were wild type, flaAII(cheC), or flaBII(cheV). Gutted cells with wild-type motors spun exclusively counterclockwise, while those with mutant motors changed their directions of rotation. CheY reduced the bias (the fraction of time that cells spun counterclockwise) in either case. CheZ offset the effect of CheY to an extent that varied with switch allele but did not change the bias when tested alone. Transducers also increased the bias in the presence of CheY but not when tested alone. However, cells containing transducers and CheY failed to respond to attractants or repellents normally detected in the periplasm. This sensitivity was restored by addition of CheA and CheW. Thus, CheY both enhances clockwise rotation and couples the transducers to the flagella. CheZ acts, at the level of the motor, as a CheY antagonist. CheA or CheW or both are required to complete the signal pathway. A model is presented that explains these results and is consistent with other data found in the literature.

Reconstitution of signaling in bacterial chemotaxis.

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Reconstitution of signaling in bacterial chemotaxis.