The QscR quorum-sensing regulon of Pseudomonas aeruginosa: an orphan claims its identity.

Quorum sensing is a process by which bacteria release and subsequently respond to signal molecules, as a mechanism for sensing population density (4). Acylated homoserine lactones (AHLs) are well-studied quorum-sensing signals among proteobacteria and are most commonly synthesized by enzymes of the LuxI family (3). AHLs are usually recognized by members of the LuxR family of transcription factors, often encoded adjacent to their corresponding LuxI-type AHL synthases. Proteobacterial genome sequencing has revealed the existence of many more LuxR homologues than were known or suspected to exist, frequently in significant excess over the number of recognized AHL synthases in the genome. Many of these regulatory genes are orphans, retaining key attributes of betterstudied luxR homologues but not linked to or associated with an AHL synthase gene. Some of these orphans are substantially different in size from characterized LuxR homologues, with large truncations or additional sequences. Of those typically sized LuxR-type proteins with end-to-end similarity, many lack one or more conserved amino acid residues known to be critical to the function of most LuxR-type proteins (3). These imperfect LuxR homologues might function to recognize AHLs by an alternate mechanism, provide ligand-independent activity, act as dominant-negative inhibitors, or even detect alternate small molecules. In other cases, all or most of the critical residues are present. These more highly conserved LuxR-type orphans may respond to endogenously synthesized AHL(s), generated by an otherwise unassociated AHL synthase of the same microbe or possibly signals from different microbes. A study from Lequette et al., published in this issue of the Journal of Bacteriology, establishes the role of an intriguing orphan LuxR homologue called QscR (quorum-sensing control repressor) in a previously unrecognized regulatory pathway within the larger AHL quorum-sensing network of the opportunistic human pathogen Pseudomonas aeruginosa (12). This work provides important new insights into the activity of the orphan QscR protein within this complex control system and also illustrates an impressive application of DNA microarray technology to address specific hypotheses at a genomic scale. Quorum sensing in Pseudomonas aeruginosa. AHL quorum sensing in P. aeruginosa is a complex, multisignal, global regulatory network with control over diverse target functions including virulence factors, exoenzymes, motility, nutrient acquisition, and biofilm formation (9). Two LuxI-type proteins encoded at separate sites within the P. aeruginosa PAO1 genome, LasI and RhlI, direct the synthesis of N-3-oxo-dodecanoyl-Lhomoserine lactone (3O-C12-HSL) and N-butyryl-L-homoserine lactone (C4-HSL), respectively (15, 16). LasR is a LuxR-type transcription factor encoded adjacent to lasI and in response to 3O-C12-HSL controls many target functions such as elastases, toxins, and other virulence factors. LasR also activates expression of the rhlR gene (Fig. 1) (see reference 5). RhlR is a second LuxR-type protein, encoded adjacent to rhlI and responsive to C4-HSL (14). RhlR also has numerous genomic targets, including rhamnolipid biosynthesis and siderophore production, many of which overlap to various degrees with the LasR regulon but others of which are clearly discrete (19, 20). LasR and RhlR are also linked to wider signaling pathways, including synthesis of the Pseudomonas quinolone signal and related hydroxyalkylquinolones (22). Transcription profiling suggests that the Las and Rhl pathways influence the expression of 3 to 11% of the P. aeruginosa genome (160 to 650 genes) (see references 7, 19, and 23). Definition of QscR as a quorum-sensing inhibitor. The P. aeruginosa PAO1 sequence revealed the existence of multiple LuxR-type proteins, beyond LasR and RhlR (www.pseudomonas .com). Only one of these translation products, defined as QscR (quorum-sensing control repressor, PA1898), exhibits full conservation with AHL-responsive LuxR homologues. Initial genetic studies of QscR suggested that this protein functions to modulate the activity of the Las and Rhl regulons (2). The qscR gene is not adjacent to an AHL synthase gene but is immediately upstream of phenazine pigment biosynthetic genes (phzA2 to -G2). Null mutants in qscR form blue-pigmented colonies, indicative of phenazine overproduction, and aberrantly express two separate phenazine biosynthetic operons (phzA to -G and phzA2 to -G2) (2, 10, 13). Additional quorumsensing-controlled genes were also expressed early and more strongly in the qscR mutant. Consistent with this finding, there were elevated levels of the 3O-C12-HSL and C4-HSL signals in early-stage cultures and early expression of lasI and rhlI (2). It seemed that QscR was limiting the activity of the LasR and RhlR regulators, although the mechanism(s) by which this occurred was unclear. Studies of QscR expressed in Escherichia coli contributed to the impression of this protein as a Las-Rhl antagonist. Coexpression of QscR with either LasR or RhlR in the absence of AHL, followed by in vivo chemical cross-linking, identified * Mailing address: Department of Biology, 1001 E. 3rd St., Jordan Hall 142, Indiana University, Bloomington, IN 47405. Phone: (812) 856-6005. Fax: (812) 855-6705. E-mail: [email protected].