Cellular Stoichiometry of Chemotaxis Proteins in

Chemotaxis systems enable microbes to sense their immediate environment, moving toward beneficial stimuli and away from those that are harmful. In an effort to better understand the chemotaxis system of , a symbiont of the legume alfalfa, the cellular stoichiometries of all ten chemotaxis proteins in were determined. A combination of quantitative immunoblot and mass spectrometry revealed that the protein stoichiometries in varied greatly from those in and To compare protein ratios to other systems, values were normalized to the central kinase CheA. All chemotaxis proteins exhibited increased ratios to various degrees. The 10-fold higher molar ratio of adaptor proteins CheW1 and CheW2 to CheA might result in the formation of rings in the chemotaxis array that consist of only CheW instead of CheA and CheW in a 1:1 ratio. We hypothesize that the higher ratio of CheA to the main response regulator CheY2 is a consequence of the speed-variable motor in , instead of a switch-type motor. Similarly, proteins involved in signal termination are far more abundant in , which utilizes a phosphate sink mechanism based on CheA retrophosphorylation to inactivate the motor response regulator versus CheZ-catalyzed dephosphorylation as in and Finally, the abundance of CheB and CheR, which regulate chemoreceptor methylation, was increased compared to CheA, indicative of variations in the adaptation system of Collectively, these results mark significant differences in the composition of bacterial chemotaxis systems. The symbiotic soil bacterium contributes greatly to host-plant growth by fixing atmospheric nitrogen. The provision of nitrogen as ammonium by leads to increased biomass production of its legume host alfalfa and diminishes the use of environmentally harmful chemical fertilizers. To better understand the role of chemotaxis in host-microbe interaction, a comprehensive catalogue of the bacterial chemotaxis system is vital, including its composition, function, and regulation. The stoichiometry of chemotaxis proteins in has very few similarities to the systems in and In addition, total amounts of proteins are significantly lower. exhibits a chemotaxis system distinct from known models by incorporating new proteins as exemplified by the phosphate sink mechanism.