luxI- and luxR-Homologous Genes of Rhizobium etli CNPAF512 Contribute to Synthesis of Autoinducer Molecules and Nodulation of Phaseolus vulgaris

Autoinduction plays an important role in intercellular communication among symbiotic and pathogenic gram-negative bacteria. We report here that a nitrogen-fixing symbiont of Phaseolus vulgaris, Rhizobium etli CNPAF512, produces at least seven different autoinducer molecules. One of them exhibits a growth-inhibitory effect like that of the bacteriocin small [N-(3R-hydroxy-7-cis-tetradecanoyl)-l-homoserine lactone]. At least two of the other autoinducers are synthesized by a LuxI-homologous autoinducer synthase. The corresponding luxI homologous gene (raiI) and a luxR homolog (raiR) have been identified and characterized. Enhanced expression of raiI is dependent on cell density and on the presence of one or more autoinducer molecules synthesized by R. etli CNPAF512. A raiI mutant was shown to release only three different autoinducer molecules; a raiR mutant releases four different autoinducer molecules. Examination of different mutants for nodulation of beans showed that raiI is involved in the restriction of nodule number, whereas nitrogen-fixing activity in terms of acetylene reduction per nodule was not affected.