(a) When the methyl-accepting chemotaxis proteins (MCPs) are highly methylated or unbound (yellow), CheA is activated by phosphorylation. Once activated, it phosphorylates the CheY and CheB response regulators. CheB-P demethylates the MCPs and the high level of CheY-P interacts with the flagellar motor, increasing the frequency of clockwise (CW) rotation, which causes the cell to tumble. (b) MCPs associated with a ligand (attractant) or less methylated (orange), maintain the CheA in a non-phosphorylated, inactive state. Consequently, the CheB methylesterase is not active and does not demethylate the MCPs. CheR methylation of the MCPs, decreases the sensitivity of these receptors. In addition, the CheY-P levels are reduced, leading to an increase in counterclockwise (CCW) flagellar rotation frequency, causing the cell to run. PE, periplasm; IM, inner membrane; Z, CheZ; W, CheW. The grey discontinuous arrows indicate non-occurring reactions (based on [11], [12]).