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A study was conducted to examine the role of the rhi gene region of Rhizobium leguminosarum biovar viciae, which is expressed in the rhizosphere. Results show that N-acyl-homoserine-lacone production locus triggers the induction of rhi genes, rhiA, rhiB and rhiC, in the rhizosphere. The particular biochemical role of rhi genes has not been identified although they are evidences that indicate their influence on interaction with legumes.

N-(3-hydroxy-7-cis-tetradecenoyl)-L-homoserine lactone (3OH,C[sub 14:1]-HSL) is a quorum-sensing signalling molecule produced by Rhizobium leguminosarum. It is unusual in that it inhibits the growth of several strains of R. leguminosarum and was previously known as ‘small bacteriocin’. The cinRI locus responsible for the production of 3OH,C[sub 14:1]-HSL has been characterized; it is predicted to be on the chromosome, based on DNA hybridization. The cinR and cinI genes are in different transcriptional units, separated by a predicted transcription terminator. CinR regulates cinI expression to a very high level in a cell-density dependent manner, and cinI expression is positively autoregulated by 3OH,C[sub 14:1]-HSL, the only identified N-acyl homoserine lactone (AHL) produced by CinI. No other AHLs were identified that strongly induced cinI expression. Mutation of cinI or cinR abolishes the production of 3OH,C[sub 14:1]HSL and also reduces the production of several other AHLs. This is thought to result from the expression of three other AHL production loci being affected by the absence of 3OH,C[sub 14:1]-HSL. AHLs produced by these other loci include N-hexanoyl- and N-octanoyI-Lhomoserine lactones and, unexpectedly, N-heptanoyI-L-homoserine lactone (C7-HSL). The expression of the 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, cinR and cinI appear to be at the top of a regulatory cascade or network that influences several AHL-regulated quorum-sensing loci. The expression of cinI-lacZ fusions is significantly reduced (but not abolished) when the symbiosis plasmid pRL1JI is present, resulting in a reduction in the level of 3OH,C[sub 14:1]-HSL produced. Mutation of cinI had little effect on growth or nodulation. However, plasmid transfer was affected, and the results obtained... [ABSTRACT FROM AUTHOR]