The Pseudomonas Quinolone Signal Regulates rhl Quorum Sensing in Pseudomonas aeruginosa

Abstract

The opportunistic pathogen Pseudomonas aeruginosa uses intercellular signals to control the density-dependent expression of many virulence factors. The las and rhl quorum-sensing systems function, respectively, through the autoinducers N-(3-oxododecanoyl)-L-homoserine lactone and N-butyryl-L-homoserine lactone (C 4 -HSL), which are known to positively regulate the transcription of the elastase-encoding gene, lasB. Recently, we reported that a second type of intercellular signal is involved in lasB induction. This signal was identified as 2-heptyl-3-hydroxy-4-quinolone and designated the Pseudomonas quinolone signal (PQS). PQS was determined to be part of the quorum-sensing hierarchy since its production and bioactivity depended on the las and rhl quorum-sensing systems, respectively. In order to define the role of PQS in the P. aeruginosa quorumsensing cascade, lacZ gene fusions were used to determine the effect of PQS on the transcription of the quorum-sensing system genes lasR, lasI, rhlR, and rhlI. We found that in P. aeruginosa, PQS caused a major induction of rhlI-lacZ and had lesser effects on the transcription of lasR-lacZ and rhlR-lacZ. We also observed that the transcription of both rhlI-lacZ and lasB-lacZ was cooperatively effected by C 4 -HSL and PQS. Additionally, we present data indicating that PQS was not produced maximally until cultures reached the late stationary phase of growth. Taken together, our results imply that PQS acts as a link between the las and rhl quorum-sensing systems and that this signal is not involved in sensing cell density.Pseudomonas aeruginosa is a ubiquitous environmental organism capable of infecting a wide variety of animals, plants, and insects. As a human pathogen, this bacterium is a major source of opportunistic infections in both immunocompromised individuals and cystic fibrosis patients. P. aeruginosa is now the leading source of gram-negative nosocomial infections (25) and causes chronic lung infections in approximately 90% of cystic fibrosis patients (7). The ability of this organism to cause devastating infections stems from the production of an arsenal of virulence factors, several of which are controlled according to cell density through an elegant mechanism known as quorum sensing. In gram-negative bacteria, most quorumsensing systems consist of a LuxR-type transcriptional activator (R protein) and an acylated homoserine lactone signal molecule (autoinducer) (see reference 4 for a review). When a bacterial culture is at a low cell density, basal levels of autoinducer and R protein are produced. As a population grows, the concentration of autoinducer increases with cell density until it reaches a threshold concentration where it binds to and thereby activates an R protein. Activated R protein then activates specific genes, causing cell density-dependent gene expression.In P. aeruginosa, there are at least two quorum-sensing systems, las and rhl, which control the expression of numerous genes (see reference 21 for a review). The las quorum-sensing system consis...

Anaerobiosis-Induced Loss of Cytotoxicity Is Due to Inactivation of Quorum Sensing in Pseudomonas aeruginosa

“…Although the LasI/R QS system plays a dominant role in the complex QS hierarchy in P. aeruginosa (34), three QS systems, namely, LasI/R, RhlI/R, and PQS, all participate in a complex signaling network to regulate lasB gene expression (29). Recently, it was reported that production of PQS is completely abrogated during anaerobic growth of P. aeruginosa (43,51).…”

“…P. aeruginosa QS is also regulated by another system, which involves potentiation of transcriptional activation by MvfR (also known as PqsR) upon binding of a Pseudomonas quinolone signal (PQS) (55). The PQS/MvfR complex actively participates in the intertwined P. aeruginosa QS network, and accumulating evidence now suggests that PQS-mediated QS is absolutely required for the uninterrupted production of elastase (29,36).…”

Quorum sensing inhibitors: a bargain of effects

“…However, a lasR mutant is still able to produce PQS, although at a later stage in growth (Diggle et al, 2003). The PQS signal, if supplemented exogenously, is capable of restoring production of elastase encoded by lasB, which is normally LasR dependent (McKnight et al, 2000;Gallagher et al, 2002;Pesci et al, 1999). Due to their central role in expression of multiple virulence traits in P. aeruginosa and the fact that they function by means of low-molecular-mass extracellular signal molecules, the QS systems are clearly potential antibacterial drug targets.…”

Influence of the regulatory protein RsmA on cellular functions in Pseudomonas aeruginosa PAO1, as revealed by transcriptome analysis

“…LasR has been shown to regulate PQS production, and RhlR is required for PQS activity (Pesci et al, 1999). PQS has been shown to induce expression of lasB, the gene encoding LasB elastase, and rhlI (McKnight et al, 2000;Pesci et al, 1999). PQS has also been implicated in the regulation of the secondary metabolites pyocyanin and hydrogen cyanide (Gallagher et al, 2002).…”