A novel promoter driving the expression of the quorum-sensing PqsE protein shows substantial differences in Pseudomonas aeruginosa and Pseudomonas paraeruginosa

Pseudomonas aeruginosa is an important bacterial pathogen due to its production of different virulence factors, several of which, like rhamnolipids elastase and pyocyanin (PYO), are regulated by the quorum-sensing (QS) response, which is composed of three transcriptional regulators/autoinducer systems. At the top of the regulatory cascade is LasR/3-oxo-dodecanoyl homoserine lactone, which activates the expression of the RhlR/butanoyl homoserine lactone system and of PqsR. The latter, when coupled with its autoinducer 2-heptyl-3,4-dihydroxyquinoline (PQS), activates the transcription of the pqsABCDE operon, which encodes the enzymes responsible for the synthesis of 4-hydroxy-2-heptylquinoline which is the precursor of PQS. The Pqs system plays a key role in the production of PYO, mainly by regulating the expression of PqsE. The PqsE protein forms a protein complex with RhlR required to activate the transcription of the phz operons for PYO production. This hierarchical QS regulatory network is conserved in the recently described Pseudomonas paraeruginosa species, which produces the same virulence factors as P. aeruginosa. In this work, we characterize a promoter that is located directly upstream of pqsE in P. aeruginosa PAO1 and show that transcription from this promoter is repressed by the global regulator Vfr. By contrast, in P. paraeruginosa strains, whereas the -10 and -35 promoter sequences are conserved, the Vfr-binding site is not present, enabling the constitutive expression of the pqsE ORF independent of PqsR/PQS regulation. The implications for PYO production as a virulence-related trait in P. paraeruginosa are discussed.