Signal transduction in bacterial chemotaxis is arguably the best understood of the two-component signalling system first described by Ausubel (Nixon et al. 1986). Therefore, the understanding of the underlying mechanisms will have a great impact in many other examples of response to environmental signals. A long term goal of this application is characterization of the physical interactions and submolecular architecture of the chemotaxis components. The physical protein-protein interactions are being identified by the isolation of complexes of the chemotaxis proteins. These complexes are being isolated by antibody precipitation and protein affinity chromatography. The role of these complexes in the signalling process will be assessed by examining the composition of these complexes in different mutant backgrounds and different signalling states. Changes in biochemical properties such as phosphorylation will also be examined with respect to changes in the composition of these complexes. The elucidation of the submolecular architecture of these proteins will continue by using a combination of genetic selection, complex isolation and structural chemistry to map sites of interaction with other proteins and sites of function. This novel combination of molecular genetics and structural chemistry will produce testable hypotheses of protein structure and function which could not have been predicted from a purely structural or genetic approach. The other long term goal of this application is characterization of the growth-phase regulated expression of the flagellar regulon cascade. In particular, the roles of the positive trans-acting regulators, FlhD and FlhC, will be studied and their potential involvement in other postexponentially regulated phenomena will be further explored.