QscR is a quorum-sensing (QS) signal receptor that controls expression of virulence genes in the prevalent opportunistic pathogen, Pseudomonas aeruginosa. Unlike the previously reported LuxR-type QS receptor proteins, that is, LasR and TraR, QscR can be obtained as an apo-protein that can reversibly form an active complex in vitro with its cognate signal molecule, 3-oxododecanoyl-homoserine lactone (3OC12-HSL), and subsequently bind to target promoter DNA sequences. To search for potential QS inhibitors, an in vitro gel retardation assay was developed using the purified QscR. Both the in vitro assay and the in vivo cell-based assay using QscR-overproducing recombinant strains were applied in the screening process. Furanones were chosen for testing the activity as QS inhibitors because they have been reported to strongly inhibit expression of QS-related genes in Agrobacterium tumefaciens. Among more than a hundred furanones tested, three compounds showed strong and dose-dependent inhibitory effects on QscR in both assays. One compound in particular, designated as F2, could completely inhibit the 3OC12-HSL-dependent QscR activity in vitro at a concentration of 50-fold molar excess over 3OC12-HSL. However, with the furanones F3 and F4, which are structurally similar to F2 but with a nitro group instead of the amine moiety, significantly decreased activities were observed. These results suggest that (i) the in vitro assay is a sensitive and reliable tool for screening QS inhibitors, and (ii) furanones are potentially important QS inhibitors for many LuxR-type receptor proteins. Biotechnol. Bioeng. 2010;106: 119-126. (C) 2010 Wiley Periodicals, Inc.