The opportunistic pathogen Pseudomonas aeruginosa has two acyl-homoserine lactone (acyl-HSL) signalling systems, LasR-I and RhlR-I. LasI catalyses the synthesis of N-3-oxododecanoyl homoserine lactone (3OC12) and LasR is a transcription factor that requires 3OC12 as a ligand. RhlI catalyses the synthesis of N-butanoyl homoserine lactone (C4) and RhlR is a transcription factor that responds to C4. LasR and RhlR control the transcription of hundreds of P. aeruginosa genes. There is a third P. aeruginosa LasR-RhlR homologue encoded by qscR for which there is no cognate acyl-HSL synthase gene. To test the hypothesis that QscR functions by direct control of specific promoters in an acyl-HSL-dependent manner we purified QscR and characterized QscR activity in vitro. We also studied QscR activity in recombinant Escherichia coli. QscR binds to promoters that have elements similar in sequence to those found in LasR- or RhlR-dependent promoters but QscR does not bind to the LasR- or RhlR-specific promoters we examined. QscR binding to DNA requires 3OC12, but QscR exhibits a relaxed acyl-HSL specificity compared with the 3OC12-cognate signal receptor LasR. Our results support the hypothesis that there is a specific QscR-dependent regulon. We show that QscR controls genes in this regulon directly and that regulation is dependent on an acyl-HSL produced by LasI. Because of its relaxed signal specificity QscR may also respond to acyl-HSLs made by other bacteria in mixed bacterial communities.