A third type of conjugative plasmids has been recently proposed, represented by the largest plasmids of R. leguminosarum bv viciae strains [3]. Some plasmids are mobilizable in the presence of transmissible plasmids, either by cointegration (conduction) [7], or by classical (trans) helper mechanisms [8, 9]. Specifically in the bean nodulating type strain Rhizobium etli CFN42, we have previously shown that it contains a quorum-sensing regulated self-transmissible

plasmid (pRet42a) [5], and that transfer of the symbiotic plasmid (pRet42d) occurs only in the presence of pRet42a. check details The event requires cointegration of both replicons. This may be achieved through IntA-dependent site-specific recombination between

attA and attD sites, or through RecA-dependent homologous recombination among large sequence segments shared between the replicons. The cointegrate is able to transfer, using the pRet42a-encoded machinery. In the transconjugants, the cointegrate is usually resolved to regenerate the wild-type plasmids, but in a few cases, resolution of the cointegrate leads to the formation of recombinant plasmids that contain NSC 683864 segments of each plasmid, pRet42a and Roscovitine order pRet42d [7]. Mesoamerica has been identified as the place of origin of bean plants and Rhizobium etli bacteria [10], while soybean and its nodulating bacteria (Sinorhizobium fredii) originated in East Asia [11]. In the early XVIth century, common beans and their symbionts were transported to Europe and other parts of the world. A survey of bean-nodulating strains in Granada, Spain, showed the presence of strains belonging to five different species: R. etli, R. gallicum, IMP dehydrogenase R. giardinii, R. leguminosarum and S. fredii [12]. The usual host of Sinorhizobium fredii strains is soybean (Glycine soja), not common bean (Phaseolus vulgaris). Nevertheless, the bean-nodulating strains classified as S. fredii, were unable to nodulate cvs. Williams or Peking of Glycine max. Hybridization of

digested genomic DNA with nodB and nifH genes from R. etli, showed a very weak signal [12]. R. etli bv phaseoli symbiotic plasmids (pSyms) are characterized by the presence of three copies of nifH. The bean-nodulating S. fredii strains showed only one copy of this gene [12]. While conjugative transfer may explain the acquisition of new symbiotic features by strains belonging to diverse species, the relationship between R. etli and bean-nodulating S. fredii is not so easily established. In order to gain further insight into the mechanisms and pathways leading to the generation of new rhizobial strains, in this work we present the analysis of the bean-nodulating S. fredii strain GR64, isolated from the soil in Granada.