Type IV pili are important virulence factors for many pathogens, including
Pseudomonas aeruginosa. Transcription of the major pilin gene— pilA—is controlled by the PilS-PilR two-component system in response to unknown signals. The absence of a periplasmic sensing domain suggested that PilS may sense an intramembrane signal, possibly PilA. We suggest that direct interactions between PilA and PilS in the inner membrane reduce pilAtranscription when PilA levels are high. Overexpression in transof PilA proteins with diverse and/or truncated C termini decreased native pilAtranscription, suggesting that the highly conserved N terminus of PilA was the regulatory signal. Point mutations in PilA or PilS that disrupted their interaction prevented autoregulation of pilAtranscription. A subset of PilA point mutants retained the ability to interact with PilS but could no longer decrease pilAtranscription, suggesting that interaction between the pilin and sensor kinase is necessary but not sufficient for pilAautoregulation. Furthermore, PilS’s phosphatase motif was required for the autoregulation of pilAtranscription, suggesting that under conditions where PilA is abundant, the PilA–PilS interaction promotes PilR dephosphorylation and thus down-regulation of further pilAtranscription. These data reveal a clever bacterial inventory control strategy in which the major subunit of an important P. aeruginosavirulence factor controls its own expression.