An rhl-like quorum-sensing system negatively regulates pyoluteorin production in Pseudomonas sp M18

Pseudomonas sp. M18, isolated from the watermelon rhizosphere, is antagonistic against a number of soil-borne pathogens. This strain produces an uncharacterized red pigment, pyoluteorin (Plt), and two N-acylhomoserine lactones (AHLs). A previously isolated red-pigment-defective mutant, M18-T510, contains an insert within a gene similar to rhll in P. aeruginosa PAO1. The M18 rhll gene product is responsible for the production of two AHL signals: N-butyryl-homoserine lactone and N-hexanoylhomoserine, lactone. Mutants defective in either rhll or rhlR showed enhanced PIt biosynthesis due to loss of transcriptional repression, which was mediated, at least in part, by suppressed expression of the activator PltR. A Plt-specific ABC transporter was also upregulated in the rhl mutants in a Plt-dependent manner. In comparison with the wild-type strain, the rhl mutants survived longer during stationary-phase growth.