Pseudomonas aeruginosa AlgR represses the Rhl quorum-sensing system in a biofilm-specific manner

AlgR controls numerous virulence factors in Pseudomonas aeruginosa, including alginate, hydrogen cyanide production, and type 1V pilus-mediated twitching motility. In this study, the role of AlgR in biofiims was examined in continuous-flow and static biofilm assays. Strain PSL317 ([DELTA]algR) produced one-third the biofilm biomass of wild-type strain PAP1. Complementation with algR, but notfimTU-pilVWXY1Y2E, restored PSL317 to the wild-type biofilm phenotype. Comparisons of the transcriptional profiles of biofilm-grown PAP1 and PSL317 revealed that a number of quorum-sensing genes were upregulated in the algR deletion strain. Measurement of rhlA::lacZ and rhlI::lacZ promoter fusions confirmed the transcriptional profiling data when PSL317 was grown as a hiofiim, but not planktonically. Increased amounts of rhamnolipids and N-butyryl homoserine iactone were detected in the biofilm effluent but not the planktonic superuatants of the algR mutant. Additionally, AlgR specifically bound to the rhlA and rhlI promoters in mobility shift assays. Moreover, PAP1 containing a chromosomal mutated AlgR binding site in its rhlI promoter formed biofilms and produced increased amounts of rhamnolipids similarly to the algR deletion strain. These observations indicate that AlgR specifically represses the Rhl quorum-sensing system during biofilm growth and that such repression is necessary for normal biofilm development. These data also suggest that AlgR may control transcription in a contact-dependent or biofilm-specific manner.