Mechanism of Pseudomonas aeruginosa RhlR transcriptional regulation of the rhlAB promoter

Pseudomonas aeruginosa contains two transcription regulators (LasR and RhlR) that, when complexed with their specific autoinducers (3-oxo-dodecanoyl-homoserine lactone and butanoyl-homoserine lactone, respectively) activate transcription of different virulence-associated traits. We studied the RhlR-dependent transcriptional regulation of the rhlAB operon encoding rhamnosyltransferase 1, an enzyme involved in the synthesis of the surfactant monorhamnolipid, and showed that RhlR binds to a specific sequence in the rhlAB regulatory region, both in the presence and in the absence of its autoinducer. Our data suggest that in the former case it activates transcription, whereas in the latter it acts as a transcriptional repressor of this promoter. RhlR seems to repress the transcription of other quorum-sensing-regulated genes; thus, RhlR repressor activity might be of importance in the finely regulated expression of P. aeruginosa virulence-associated traits.