| The behavior of enteric bacteria is determined by the direction of rotation of the flagella that propel the cell. Flagellar rotation is controlled by inner membrane receptors that sense changes in the concentration of attractants and repellents, and communicate with the flagellar motors by controlling a cascade of phosphorylation events catalyzed by cytoplasmic chemotaxis proteins. The sensitivity of the receptors is regulated by two proteins, CheR and CheB, that reversibly methylate and demethylate the receptors in response to ligand binding.;This dissertation presents research undertaken to investigate the role of receptor methylation in chemotaxis. Chapter 1 proposes a mechanistic model for the influence of methylation on receptor structure. In this model, the two methylated domains of a receptor are located in ;Chapter 2 presents evidence that CheA phosphorylates CheB in its N-terminal domain; this reaction increases CheB methylesterase activity over tenfold. Because CheA kinase activity is controlled by the receptors, this reaction provides a feedback loop between the signaling state of the receptors and their methylation level.;Chapter 3 investigates the importance of this feedback loop for adaptation and chemotaxis through behavioral studies of a mutant strain expressing a truncated, nonphosphorylated, but active CheB protein. While adaptation to stimuli appears normal in this mutant strain, chemotaxis is severely impaired, suggesting that CheB has an additional function in excitation that is dependent on its N-terminal domain. It is hypothesized that this function is a receptor-regulated phosphatase activity that allows for additional control of P-CheA levels.;The Appendix presents an algorithm that evaluates the coiled-coil forming potential of a protein from its amino-acid sequence. This algorithm was used for building the model presented in Chapter 1. |
