Sequencing and analysis of pLPU83a, an accessory plasmid of a broad-host-range Rhizobium sp. LPU83

TORRES TEJERIZO, G. A.; DEL PAPA, M.F.; DRAGHI, W. O.; LOZANO, M. J.; GIUSTI, M. A.; MARTINI, C; SALAS, M, E.; SALTO, I. P.; WIBBERG, D.; SZCZEPANOWSKI, R.; WEIDNER, S.; SCHLÜTER, A.; PUHLER, A.; LAGARES, A.; PISTORIO, M.

Bariloche

Congreso; International Plasmid Biology Conference 2010; 2010

International Society for Plasmid Biology and other Mobile Genetic Elements

Bacteria belonging to the genera Rhizobium, Sinorhizobium, Azorhizobium, and Mesorhizobium grow in the soil in free-living conditions, and in symbiosis associated with the root of legumes as nitrogen-fixing organisms. A general feature of rhizobia is the presence of large amount of plasmid that DNA in their genomes. Rhizobia could borne plasmids whose role is not currently understood, referred as accessory plasmids. Conjugal transfer of rhizobial plasmids has been well documented since decades ago. In our laboratory we have characterized acid-tolerant isolates of rhizobia that have an extended host range for nodulating alfalfa, the common bean, and Leucena leucocephala (1). Such isolates have been shown to be closely related to the previously described strain R. sp. Or191, isolated from an acid soil in Oregon, USA (2). The genetic structure of the Or-like strains is particularly relevant since they have a chromosome related to the bean-, pea-, and clover-nodulating rhizobia; and they contain plasmid-encoded symbiotic genes related to those present in the Medicago symbionts (3). We were interested in the characterization of the plasmid gene pool present in Medicago-nodulating rhizobia in order to ascertain the plasmid-related evolutionary relationships within this group of rhizobia. We have previously analyzed the conjugation and symbiotic characteristics of the plasmids of R. sp. LPU83. To make a deeper characterization a sequencing approach was carried out and the final assembly results in a 148 kbp contig for plasmid pLPU83a. 157 predicted coding sequences were annotated and categorized by functional class by the use of Clusters of Orthologous Groups (COG). With this classification, genes for DNA replication, recombination and repair, Intracellular trafficking and secretion and Cell wall/membrane/envelop biogenesis, were well represented. A detailed analysis revels that a cluster of genes involved in the Mpf/Dtr similar to the one presented in pRL8 of Rhizobium leguminosarum bv. viciae is presented in pLPU83a, where the absence of traI is remarkable. Anyway, an ortholog of traI is presented faraway of this gene cluster. The characterization of the Or-like plasmids will be valuable in the elucidation of mechanisms involved in the evolution and diversification of the alfalfa- and bean-nodulating rhizobia.