Searching for genes encoding quorum sensing components in peanut nodulating bradyrhizobia

NIEVAS, F.L.; BOGINO, P.C.; SORROCHE, F.G.; GIORDANO, W.F.

Mar del Plata

Congreso; VIII Congreso Argentino de Microbiología General; 2012

Quorum Sensing (QS) is a mechanism of bacterial communication depending on cell density. The canonical QS system in Gram negative bacteria consists of two proteins, an enzyme and a receptor, belonging to the family LuxI and LuxR, respectively. The LuxI synthase is involved in the production of acyl-homoserine lactone molecules (AHL), the signaling molecule, which recognize LuxR, a regulatory protein that binds targets genes thereby modifying the gene expression. Little is known about the mechanism of communication between peanut nodulanting bradyrhizobia, so the objective of this study was to detect the presence of luxI-luxR homologous genes in these rhizobia. In order to PCR-amplify lux homologous sequences in our rhizobia, we designed specific primers using putative luxI-luxR genes present in the genomes of the sequenced Bradyrhizobium sp. strains Btai1 and ORS278, which are able to nodulate non-leguminous plants. Amplification products using primers for luxR sequences, were obtained in all (6) strains. Additionally, when we used primers for luxI sequences, it was possible to detect amplification products in 3 strains. All amplicons were cloned, sequenced and subjected to bioinformatics analysis. The fragment sequences showed high identity (99 and 84 %) with nucleotide sequences of synthases and regulator genes of strains B. sp Btai1 and B. sp. ORS278 respectively. The deduced aminoacid sequences revealed high homologies (>92%) with synthases and regulator proteins of bacteria phylogenetically related Bradyrhizobium sp. These high homologies percentages indicate that the aminoacids sequences of these proteins would be highly conserved among Bradyrhizobium sp., suggesting that these proteins could regulate important physiological mechanisms. In silico analyses at the protein level allowed us to infer that the amplified sequences corresponded to the C-terminal portions. In the putative autoinductor synthase proteins (LuxI-type) we detected domains related to acetyl-transferase activity required for signal molecule synthesis, while the putative LuxR-type regulatory proteins presented an HTH DNA binding domain and the binding site for AHL autoinducers. The results obtained so far demonstrated for the first time the existence of QS putative genes in peanut nodulating rhizobia. We are currently exploring the role of QS in the context of free-living and symbiotic scenarios.