In the absence of autoinducers, under conditions where bacteria are at low density, the quorum-sensing system acts to phosphorylate the response regulator LuxO, which allows LuxO to act as a repressor of the lux and other quorum sensing-controlled genes. Lenz et al. identified hfq, which encodes a protein involved in targeting small, regulatory RNAs (sRNAs) to specific messenger RNAs (mRNAs), in a screen for genes that suppressed a constitutively active LuxO mutant (luxO D47E). Vibrio harveyi and V. cholerae bioluminesce in the presence of autoinducer, and bacteria are dark in the absence of autoinducer or in the luxO D47E strain. Bacteria null for hfq or luxO are bright at both high and low cell densities, which suggests that the repressor pathway is inactive. Bacteria null for hfq and containing the luxO D47E mutation are also light, which suggests that Hfq is downstream of LuxO. The master regulators of quorum sensing are LuxR (V. harveyi) and HapR (V. cholerae) and the half-lives of the transcripts for these proteins are prolonged in luxO D47E, hfq double-mutant cells, which suggests that Hfq acts to destabilize these transcripts at low cell density. Sequence analysis of the V. cholerae genome revealed four loci (called qrr1-4 for quorum regulatory RNA) that may encode sRNAs, the expression of which might be stimulated by LuxO-P. Only Qrr4 was obviously regulated by LuxO and showed increased abundance in the luxO D47E cells. However, all four qrr loci needed to be deleted in order to cause bioluminescence at both high and low cell densities, which suggested that all four are involved in quorum-sensing repression. Indeed, overexpression of only qrr1 inhibited bioluminescence. Thus, LuxO repression appears to be mediated by the stimulation of production of sRNAs that are targeted to the transcripts for the master regulators, HapR and LuxR, by Hfq (see Gottesman for discussion).
D. H. Lenz, K. C. Mok, B. N. Lilley, R. V. Kulkarni, N. S. Wingreen, B. L. Bassler, The small RNA chaperone Hfq and multiple small RNAs control quorum sensing in Vibrio harveyi and Vibrio cholerae. Cell 118, 69-82 (2004). [Online Journal]
S. Gottesman, Small RNAs shed some light. Cell 118, 1-2 (2004). [Online Journal]