Quorum sensing employs a dual regulatory mechanism to repress T3SS gene expression

The type III secretion system (T3SS) is a needle-like complex used by numerous bacterial pathogens in host infection by directly injecting exotoxins into the host cell cytoplasm, leading to cell death. The T3SS is a known virulence factor in the shrimp pathogen Vibrio campbellii . The ∼40 genes comprising the V. campbellii T3SS are regulated by a network of transcription factors in response to changes in the cell’s environment: cell density (quorum sensing; QS), temperature, calcium, and host cell contact. Under positive environmental stimuli, the master T3SS transcription factor ExsA activates expression of the four structural T3SS operons required for needle formation. Previous studies identified a key role of the master QS transcription factor LuxR: repression of exsA transcription via DNA binding at the exsBA promoter. Here we uncovered a new regulatory role of LuxR: indirect post-translational repression of ExsA activity via direct transcriptional repression of the gene encoding the anti-anti-activator ExsC. In V. campbellii , ExsC is a positive regulator of T3SS transcription: high ExsC expression leads to full ExsA transcription activation of the T3SS structural promoters. LuxR binding at the exsC promoter represses transcription of exsC through disruption of ExsA binding. Our findings collectively show that V. campbellii responds to high cell density signals to shut down the expression of the T3SS. We postulate that this dual regulatory mechanism by LuxR enables both the rapid inactivation of existing ExsA protein and blocks its further synthesis, leading to a rapid shutdown of T3SS activity at high cell density.