Transcriptional regulator that plays an essential role in the activation the type III secretion system (T3SS) operons (PubMed:1655713, PubMed:26317977). In addition, ExsA directly regulates the transcription of ImpA virulence factor that cooperatively inhibits the functions of host macrophages together with the T3SS (PubMed:31527124). In the absence of inducing signals such as low Ca(2+) or host cell contact, the T3SS/injectisome is expressed at a low basal level and exists in a quiescent state due to ExsA sequestration by ExsD in a 1:1 complex (PubMed:19369699, PubMed:20008065). Upon host cell contact, this interaction is disrupted by the anti-antiactivator protein ExsC leading to ExsA activation (PubMed:15225323). Homodimer (PubMed:26317977). Interacts with ExsD; this interaction inhibits ExsA activity (PubMed:19369699, PubMed:20008065). By cAMP-dependent DNA-binding protein Vfr that directly activates exsA transcription from a promoter located immediately upstream of exsA. Deletion mutants are defective in expression and secretion of type III secretion system-related exoproducts.