Abstract
Pseudomonas aeruginosa is a ubiquitous Gram-negative γ-proteobacterium capable of causing disease in plants, animals and humans (Cao et al., 2001a). Different extracellular virulence factors, including proteases, a toxin and secondary metabolites, are produced both in a cell-density-dependent manner, via cell-to-cell communication or quorum sensing (QS) (Withers et al., 2001; Smith & Iglewski, 2003a), and in a growth-phase-dependent manner (Diggle et al., 2003). Also, iron limitation as encountered in the host induces the production of siderophores and virulence factors (Lamont et al., 2002). Understanding how these different regulatory networks interact is vital in order to be able to model and predict bacterial behaviour in novel conditions.
Typically, QS in Gram-negative bacteria involves the production of N-acyl-homoserine lactones (AHLs), which circulate in and out of the cell where they associate with a regulator protein of the so-called LuxR family (Fuqua et al., 1996). In P. aeruginosa, two different AHL synthases produce N-(3-oxododecanoyl)-L-homoserine lactone (3-oxo-C12-HSL), the product of the LasI synthase, and N-butanoyl-L-homoserine lactone (C4-HSL), the product of the RhlI synthase (de Kievit & Iglewski, 2000; Cámara et al., 2002). The cognate LuxR regulators are LasR for 3-oxo-C12-HSL and RhlR for C4-HSL. The LasILasR system exerts a control on the RhlIRhlR system, imposing a hierarchy in the QS regulation (de Kievit & Iglewski, 2000; Cámara et al., 2002). The regulator in association with its cognate AHL binds to so-called lux boxes' (called las boxes in the case of P. aeruginosa) upstream of the promoters of genes regulated by QS (Whiteley & Greenberg, 2001). A third, entirely different, signal molecule, 2-heptyl-3-hydroxy-4(1H)-quinolone, termed the Pseudomonas quinolone signal (PQS), and which closely resembles the 4-quinolone family of synthetic antimicrobials, has also been shown to regulate the expression of some virulence factors, such as the production of the redox active phenazine compound pyocyanin (Pesci et al., 1999; McKnight et al., 2000; Gallagher et al., 2002; D'Argenio et al., 2002; Diggle et al., 2003; Déziel et al., 2004). PQS functions as a regulatory link between the las and rhl AHL-dependent QS systems and is needed for the expression of some rhl-regulated genes (Diggle et al., 2003; Déziel et al., 2004).
Another level of complexity is added by the existence of other regulators influencing the QS regulatory circuit, such as the global Vfr and GacA regulators (Albus et al., 1997; Reimmann et al., 1997), the RsaL, QscR (a third LuxR-type regulator) and MvaT repressors (de Kievit et al., 1999; Chugani et al., 2001; Diggle et al., 2002) and the MfvR (PqsR) regulator (Diggle et al., 2003; Déziel et al., 2004). Moreover, the sigma factor RpoS has recently been shown to influence the transcription of a large proportion of QS-regulated genes (Schuster et al., 2004). Three different microarray-based transcriptome analyses of all P. aeruginosa open reading frames (n=5570) have shown that between 163 and 388 genes could be QS-regulated (Schuster et al., 2003; Wagner et al., 2003; Hentzer et al., 2003). In this issue, Juhas et al. (2004) describe a fourth LuxR regulator, which they term VqsR (virulence and quorum-sensing regulator). Although vqsR gene expression is low when the cells are grown in LB medium, its inactivation negates the production of AHLs. The same authors also demonstrate that vqsR expression is enhanced by the presence of H2O2 or serum in the medium.
VqsR and QS
A transcriptome analysis revealed that a high proportion of the genes regulated by QS in P. aeruginosa are repressed in a vqsR mutant. Table 1 lists 56 genes down-regulated in a vqsR mutant in the presence of serum or H2O2 and also found as being QS-regulated in at least two of the above-mentioned transcriptome analyses. Interestingly, the vast majority of these genes (n=44) belong to the category induced by human serum, four belong to the category of genes induced by H2O2, while eight are detected under both conditions.