Inhibition of quorum sensing by a Pseudomonas aeruginosa dksA homologue

The Pseudomonas aeruginosa las (lasR-lasI) and rhl (rhlR-rhlI) quorum-sensing systems regulate the expression of several virulence factors, including elastase and rhamnolipid. P. aeruginosa strain PR1-E4 is a lasR deletion mutant that contains a second, undefined mutation which allows production of elastase and rhamnolipid despite a nonfunctional las system. We have previously shown that this strain accomplishes this by increasing the expression of the autoinducer synthase gene rhlI. In this report, we show that the elastolytic phenotype of mutant PR1-E4 can be complemented with a P. aeruginosa homologue of the Escherichia coli dnaK mutation suppressor gene dksA. When supplied in trans on a multicopy plasmid, this gene completely suppressed elastase production by mutant PR1-E4. Cloning and Northern blot analysis revealed that dksA was neither mutated nor less transcribed in mutant PR1-E4. When overexpressed, dksA also reduced rhamnolipid production by both mutant PR1-E4 and the wild type, PAO1. Using Northern blot analysis and lacZ reporter fusions, we show that dksA inhibits rhlI, rhlAB, and lasB transcription. Exogenous N-butyryl-L-homoserine lactone overcame the reduced expression of rhlI and restored rhlAB and lasB expression, as well as elastase production. Our results suggest that the overproduction of the P. aeruginosa DksA homologue inhibits quorum-sensing-dependent virulence factor production by downregulating the transcription of the autoinducer synthase gene rhlI.