Figure 6. Model for regulation of Che₃ signal transduction pathway.

(A) In the absence of unknown signals, CheA₃ is deactivated; CheS₃ autophosphorylates and transfers phosphates to its cognate response regulator CheY₃; activated CheY₃ then interacts with downstream components to repress cyst formation. (B) In the presence of an unknown signal (denoted by a red star), CheA₃ autophosphorylation is activated; His-phosphorylated CheA₃ constantly transfers the phosphates to its C-terminal REC domain, which serves as a phosphate sink. CheA₃∼P also phosphorylates the REC1 domain of CheS₃, inhibiting CheS₃ kinase activity and CheY₃ remains unphosphorylated. Cyst formation is therefore derepressed without activated CheY₃. The thickness of the arrows represents the level of phosphate flow.