Chemotaxis pathways of E. coli. The pathway consists of transmembrane chemoreceptors, which constantly undergo molecular transitions between their on (black) and off (white) conformations. Receptors signal to CheA, which subsequently autophosphorylates. The average receptor activity is modulated by ligand binding and unbinding, as well receptors methylation and demethylation. CheA phosphorylates the response regulator CheY, which diffuses through the cell and binds to the rotary motors. Upon CheY-P binding, motors switch from their default state of CCW rotation (i.e. running mode) to CW rotation (i.e. tumbling mode). In addition, CheY-P is dephosphorylated by its phosphatase CheZ. Receptor methylation is catalysed by CheR, which preferentially modifies inactive receptors. Receptor demethylation is catalysed by CheB, which is activated by phosphorylation, and modifies preferentially active receptors.