AUTOINDUCER-MEDIATED REGULATION OF RHAMNOLIPID BIOSURFACTANT SYNTHESIS IN PSEUDOMONAS-AERUGINOSA

The opportunistic human pathogen Pseudomonas aeruginosa produces a var iety of virulence factors, including exotoxin A, elastase, alkaline pr otease, alginate, phospholipases, and extracellular rhamnolipids, The previously characterized rhlABR gene cluster encodes a regulatory prot ein (RhlR) and a rhamnosyltransferase (RhlAB), both of which are requi red for rhamnolipid synthesis. Another gene, rhlI, has now been identi fied downstream of the rhLABR gene cluster. The putative RhlI protein shares significant sequence similarity with bacterial autoinducer synt hetases of the LuxI type, A P. aeruginosa rhlI mutant strain carrying a disrupted rhlI gene was unable to produce rhamnolipids and lacked rh amnosyltransferase activity. Rhamnolipid synthesis was restored by int roducing a wild-type rhlI gene into such strains or, alternatively, by adding either the cell-free spent supernatant from a P. aeruginosa wi ld-type strain or synthetic N-acylhomoserine lactones. Half-maximal in duction of rhamnolipid synthesis in the rhlI mutant strain required 0. 5 mu M N-butyrylhomoserine factone or 10 mu M N-(3-oxohexanoyl)homoser ine lactone. The P. aeruginosa rhlA promoter was active in the heterol ogous host Pseudomonas putida when both the rhlR and rhlI genes were p resent or when the rhlR gene alone was supplied together with syntheti c N-acylhomoserine lactones. The RhlR-RhlI regulatory system was found to be essential for the production of elastase as well, and cross-com munication between the RhlR-RhlI rhamnolipid regulatory system and the LasR-LasI elastase regulatory system was demonstrated.