Polar accumulation of the metabolic sensory histidine kinases DcuS and CitA in Escherichia coli

Signal transduction in prokaryotes is frequently accomplished by two-component regulatory systems in which a histidine protein kinase is the sensory component. Many of these sensory kinases control metabolic processes that do not show an obvious requirement for inhomogeneous distribution within bacterial cells. Here, the sensory kinases DcuS and CitA, two histidine kinases of Escherichia coli, were investigated. Both are membrane-integral and involved in the regulation of carboxylate metabolism. The two-component sensors were fused with yellow fluorescent protein (YFP) and live images of immobilized cells were obtained by confocal laser fluorescence microscopy. The fluorescence of the fusion proteins was concentrated at the poles of the cells, indicating polar accumulation of the sensory kinases. For quantitative evaluation, line profiles of the imaged fluorescence intensities were generated; these revealed that the fluorescence intensity of the polar bright spots was 2.3-8.5 times higher than that of the cytoplasm. With respect to the cylindrical part of the membrane, the values were lower by about 40%. The polar accumulation was comparable to that of methyl-accepting chemotaxis proteins (MCPs) and MCP-related proteins. The degree of DcuS-YFP localization was independent of the presence of MCP and the expression level of dcuS-yfp (or DcuS concentration). The presence of effector (fumarate or citrate, respectively) increased the polar accumulation by more than 20%. Cell fractionation demonstrated that polar accumulation was not related to inclusion body formation. Therefore, sensory kinases DcuS and CitA, which regulate metabolic processes without obvious polar function, exhibit polar accumulation.