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Background  Bean-nodulating  Rhizobium etli  originated in Mesoamerica, while soybean-nodulating  Sinorhizobium fredii  evolved in East Asia.  S. fredii  strains, such as GR64, have been isolated from bean nodules in Spain, suggesting the occurrence of conjugative transfer events between introduced and native strains. In  R. etli  CFN42, transfer of the symbiotic plasmid (pRet42d) requires cointegration with the endogenous self-transmissible plasmid pRet42a. Aiming at further understanding the generation of diversity among bean nodulating strains, we analyzed the plasmids of  S. fredii  GR64: pSfr64a and pSfr64b (symbiotic plasmid).    Results  The conjugative transfer of the plasmids of strain GR64 was analyzed. Plasmid pSfr64a was self-transmissible, and required for transfer of the symbiotic plasmid. We sequenced pSfr64a, finding 166 ORFs. pSfr64a showed three large segments of different evolutionary origins; the first one presented 38 ORFs that were highly similar to genes located on the chromosome of  Sinorhizobium  strain NGR234; the second one harbored 51 ORFs with highest similarity to genes from pRet42d, including the replication, but not the symbiosis genes. Accordingly, pSfr64a was incompatible with the  R. etli  CFN42 symbiotic plasmid, but did not contribute to symbiosis. The third segment contained 36 ORFs with highest similarity to genes localized on pRet42a, 20 of them involved in conjugative transfer. Plasmid pRet42a was unable to substitute pSfr64a for induction of pSym transfer, and its own transfer was significantly diminished in GR64 background. The symbiotic plasmid pSfr64b was found to differ from typical  R. etli  symbiotic plasmids.    Conclusions   S. fredii  GR64 contains a chimeric transmissible plasmid, with segments from two  R. etli  plasmids and a  S. fredii  chromosome, and a symbiotic plasmid different from the one usually found in  R. etli  bv phaseoli. We infer that these plasmids originated through the transfer of a symbiotic-conjugative-plasmid cointegrate from  R. etli  to a  S. fredii  strain, and at least two recombination events among the  R. etli  plasmids and the  S. fredii  genome. As in  R. etli  CFN42, the  S. fredii  GR64 transmissible plasmid is required for the conjugative transfer of the symbiotic plasmid. In spite of the similarity in the conjugation related genes, the transfer process of these plasmids shows a host-specific behaviour.

bmc microbiology

The conjugative plasmid of a bean-nodulating Sinorhizobium fredii strain is assembled from sequences of two Rhizobium plasmids and the chromosome of a Sinorhizobiumstrain