Figure 11.

A model illustrating the regulation of T3SS1 genes. When ExsD is present in molar excess of ExsC, ExsD binds ExsA and blocks transcriptional promoter activity. T3SS1 genes are expressed when ExsC binds ExsD or if ExsA is synthesized to a molar concentration in excess of the ExsD concentration. The induction signal for ExsC has not been identified and there is no direct evidence whether or not the same or independent induction signal is required to express and synthesize ExsA (the latter can occur independently of ExsC; Fig. 4).