N‐(3‐hydroxy‐7‐cis‐tetradecenoyl)‐l‐homoserine lactone (3OH,C14:1‐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,C14:1‐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,C14:1‐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,C14:1‐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,C14:1‐HSL. AHLs produced by these other loci include N‐hexanoyl‐ and N‐octanoyl‐l‐homoserine lactones and, unexpectedly, N‐heptanoyl‐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,C14:1‐HSL produced. Mutation of cinI had little effect on growth or nodulation. However, plasmid transfer was affected, and the results obtained indicate that 3OH,C14:1‐HSL produced by either the donor or the recipient in mating experiments can stimulate transfer of pRL1JI.

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The rhi genes of Rhizobium leguminosarumbiovar viciae are expressed in the rhizosphere and play a role in the interaction with legumes, such as the pea. Previously (K. M. Gray, J. P. Pearson, J. A. Downie, B. E. A. Boboye, and E. P. Greenberg, J. Bacteriol. 178:372–376, 1996) therhiABC operon had been shown to be regulated by RhiR and to be induced by addedN-(3-hydroxy-7-cis-tetradecenoyl)-l-homoserine lactone (3OH,C14:1-HSL). Mutagenesis of a cosmid carrying the rhiABC and rhiR gene region identified a gene (rhiI) that affects the level of rhiAexpression. Mutation of rhiI slightly increased the number of nodules formed on the pea. The rhiI gene is (likerhiA) regulated by rhiR in a cell density-dependent manner. RhiI is similar to LuxI and other proteins involved in the synthesis of N-acyl-homoserine lactones (AHLs). Chemical analyses of spent culture supernatants demonstrated that RhiI produces N-(hexanoyl)-l-homoserine lactone (C6-HSL) andN-(octanoyl)-l-homoserine lactone (C8-HSL). Both of these AHLs induced rhiA-lacZand rhiI-lacZ expression on plasmids introduced into anAgrobacterium strain that produces no AHLs, showing thatrhiI is positively regulated by autoinduction. However, in this system no induction of rhiA or rhiI with 3OH,C14:1-HSL was observed. Analysis of the spent culture supernatant of the wild-type R. leguminosarum bv. viciae revealed that at least seven different AHLs are made. Mutation ofrhiI decreased the amounts of C6-HSL and C8-HSL but did not block their formation, and in this background the rhiI mutation did not significantly affect the expression levels of the rhiI gene orrhiABC genes or the accumulation of RhiA protein. These observations suggest that there are additional loci involved in AHL production in R. leguminosarum bv. viciae and that they affect rhiI and rhiABC expression. We postulate that the previously observed induction of rhiA by 3OH,C14:1-HSL may be due to an indirect effect caused by induction of other AHL production loci.

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We studied the growth and capacities for pesticides removal of bacterial strains isolated from the Laguna Grande, an oligotrophic lake at the South of Spain (Archidona, Málaga). Strains were isolated from water samples amended with 10 and 50 microg/ml of nine pesticides: organochlorinated insecticides (aldrin and lindane), organophosphorous insecticides (dimetoate, methyl-parathion and methidation), s-triazine herbicides (simazine and atrazine), fungicide (captan) and diflubenzuron (1-(-4-chlorophenyl)-3-(2,6-difluorobenzoyl urea), a chitinase inhibitor. The majority of the strains belonged to the genera Pseudomonas and Aeromonas and only 9% of the total of strains were Gram positive. From all the strains isolated, only 22 showed a wide growth range in all the pesticides tested and 4 of them were chosen for pesticide removal studies. The genetic identification of these strains showed their affiliation to Pseudomonas pseudoalcaligenes, Micrococcus luteus, Bacillus sp. and Exiguobacterium aurantiacum. These last two strains were those that showed the highest pesticide removal capacities and a high bacterial growth.

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