The regulatory locus cinRI in Rhizobium leguminosarum controls a network of quorum-sensing loci

N-(3-hydroxy-7-cis-tetradecenoyl)-L-homoserine lactone (3OH,C-14:1-HSL) is a quorum-sensing signalling molecule produced by Rhizobium leguminosarum. I t is unusual in that it inhibits the growth of several strains of R. legumi nosarum and was previously known as 'small bacteriocin'. The cinRI locus re sponsible for the production of 3OH,C-14:1-HSL has been characterized; it i s predicted to be on the chromosome, based on DNA hybridization. The cinR a nd cinI genes are in different transcriptional units, separated by a predic ted transcription terminator. CinR regulates cinI expression to a very high level in a cell-density dependent manner, and cinI expression is positivel y autoregulated by 3OH,C-14:1-HSL, the only identified N-acyl homoserine la ctone (AHL) produced by CinI. No other AHLs were identified that strongly i nduced cinI expression. Mutation of cinI or cinR abolishes the production o f 3OH,C-14:1-HSL and also reduces the production of several other AHLs. Thi s is thought to result from the expression of three other AHL production lo ci being affected by the absence of 3OH,C-14:1-HSL. AHLs produced by these other loci include N-hexanoyl- and N-octanoyl-L-homoserine lactones and, un expectedly, N-heptanoyl-L-homoserine lactone (C-7-HSL). The expression of t he rhiI gene on the symbiotic plasmid is greatly reduced in a cinI mutant, and the major regulatory effect appears to be mediated at least in part as a result of an effect on expression of RhiR, the regulator of rhiI. Thus, c inR and cinI appear to be at the top of a regulatory cascade or network tha t influences several AHL-regulated quorum-sensing loci. The expression of c inI-lacZ fusions is significantly reduced (but not abolished) when the symb iosis plasmid pRL1JI is present, resulting in a reduction in the level of 3 OH,C-14:1-HSL produced. Mutation of cinI had little effect on growth or nod ulation. However, plasmid transfer was affected, and the results obtained i ndicate that 3OH,C-14:1-HSL produced by either the donor or the recipient i n mating experiments can stimulate transfer of pRL1JI.