Inhibition of quorum sensing by a Pseudomonas aeruginosa dksA homologue

The Pseudomonas aeruginosa las (lasR-lasI) and rhl (rhlR-rhlI) quorum-sensi ng systems regulate the expression of several virulence factors, including elastase and rhamnolipid. P. aeruginosa strain PR1-E4 is a lasR deletion mu tant that contains a second, undefined mutation which allows production of elastase and rhamnolipid despite a nonfunctional las system. We have previo usly shown that this strain accomplishes this by increasing the expression of the autoinducer synthase gene rhlI In this report, we show that the elas tolytic 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 re vealed that dksA was neither mutated nor less transcribed in mutant PR1-E4. When overexpressed, dksA also reduced rhamnolipid production by both mutan t PR1-E4 and the wild type, PAO1. Using Northern blot analysis and lacZ rep orter fusions, we show that dksA inhibits rhlI, rhlAB, and lasB transcripti on. Exogenous N-butyryl-L-homoserine lactone overcame the reduced expressio n of rhlI and restored rhlAB and lasB expression, as well as elastase produ ction. Our results suggest that the overproduction of the P. aeruginosa Dks A homologue inhibits quorum-sensing-dependent virulence factor production b y downregulating the transcription of the autoinducer synthase gene rhlI.