Bacteria communicate with each other to coordinate expression of specific genes in a cell density-dependent fashion, a phenomenon called quorum sensing and response. Although we know that quorum sensing via acyl-homoserine lactone (HSL) signals controls expression of several virulence genes in the human pathogen Pseudomonas aeruginosa, the number and types of genes controlled by quorum sensing have not been studied systematically. We have constructed a library of random insertions in the chromosome of a P. aeruginosa acyl-HSL synthesis mutant by using a transposon containing a promoterless lacZ. This library was screened for acyl-HSL induction of lacZ. Thirty-nine quorum sensing-regulated genes were identified. The genes were organized into classes depending on the pattern of regulation. About half of the genes appear to be in seven operons, some seem organized in large patches on the genome. Many of the quorum sensing-regulated genes code for putative virulence factors or production of secondary metabolites. Many of the genes identified showed a high level of induction by acyl-HSL signaling. Many host-associated bacteria use chemical signals to monitor their own species population density and control expression of specific genes in response to population density. This type of gene regulation is termed quorum sensing (1). Several Gram-negative bacteria use acylated homoserine lactone (HSL) signals in quorum sensing. Quorum sensing in Pseudomonas aeruginosa, an opportunistic human pathogen responsible for persistent and often incurable infections in immunocompromised people and individuals with cystic fibrosis, has been well studied. The sequence of the P. aeruginosa genome is available publicly. Expression of a number of extracellular virulence factors produced by P. aeruginosa is controlled by quorum sensing (for recent reviews see refs. 2 and 3).Two quorum sensing systems, the las and rhl systems, have been identified in P. aeruginosa. In the las quorum sensing system, the lasI gene product directs the formation of the diffusible extracellular signal, N-(3-oxododecanoyl)-L-HSL (3OC 12 -HSL) (4), which interacts with LasR (5, 6) to activate a number of virulence genes, including lasB, lasA, apr, toxA, and lasI itself (6-10). Synthesis of the siderophore pyoverdine also is activated by the las system (11). P. aeruginosa strains lacking a functional LasR are avirulent in animal models (12). Although 3OC 12 -HSL is diffusible, it appears to partition into cell membranes, and P. aeruginosa efflux pumps aid in the movement of this signal to the external environment (13,14).The rhlI product catalyzes the synthesis of N-butyryl-L-HSL (C 4 -HSL) (15,16). This diffusible signal (14) in conjunction with RhlR activates expression of the rhlAB rhamnolipid synthesis genes, rhlI, and to some extent lasB (17)(18)(19)(20). Other virulence factors and secondary metabolites, including pyocyanin, cyanide, and chitinase, are positively regulated by the rhl system (18, 21), although direct transcriptional regulation of the genes in...

The opportunistic pathogenic bacterium Pseudomonas aeruginosa uses quorum-sensing signaling systems as global regulators of virulence genes. There are two quorum-sensing signal receptor and signal generator pairs, LasR-LasI and RhlR-RhlI. The recently completed P. aeruginosa genome-sequencing project revealed a gene coding for a homolog of the signal receptors, LasR and RhlR. Here we describe a role for this gene, which we call qscR. The qscR gene product governs the timing of quorum-sensing-controlled gene expression and it dampens virulence in an insect model. We present evidence that suggests the primary role of QscR is repression of lasI. A qscR mutant produces the LasI-generated signal prematurely, and this results in premature transcription of a number of quorum-sensing-regulated genes. When fed to Drosophila melanogaster, the qscR mutant kills the animals more rapidly than the parental P. aeruginosa. The repression of lasI by QscR could serve to ensure that quorum-sensing-controlled genes are not activated in environments where they are not useful.