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Background   Rhizobium grahamii  belongs to a new phylogenetic group of rhizobia together with  Rhizobium mesoamericanum  and other species.  R. grahamii  has a broad-host-range that includes  Leucaena leucocephala  and  Phaseolus vulgaris , although it is a poor competitor for  P. vulgaris  nodulation in the presence of  Rhizobium etli  or  Rhizobium phaseoli  strains. This work analyzed the genome sequence and transfer properties of  R. grahamii  plasmids.    Results  Genome sequence was obtained from  R. grahamii  CCGE502 type strain isolated from  Dalea leporina  in Mexico. The CCGE502 genome comprises one chromosome and two extrachromosomal replicons (ERs), pRgrCCGE502a and pRgrCCGE502b. Additionally, a plasmid integrated in the CCGE502 chromosome was found. The genomic comparison of ERs from this group showed that gene content is more variable than average nucleotide identity (ANI). Well conserved  nod  and  nif  genes were found in  R. grahamii  and  R. mesoamericanum  with some differences.  R. phaseoli  Ch24-10 genes expressed in bacterial cells in roots were found to be conserved in pRgrCCGE502b. Regarding conjugative transfer we were unable to transfer the  R. grahamii  CCGE502 symbiotic plasmid and its megaplasmid to other rhizobial hosts but we could transfer the symbiotic plasmid to  Agrobacterium tumefaciens  with transfer dependent on homoserine lactones.    Conclusion  Variable degrees of nucleotide identity and gene content conservation were found among the different  R. grahamii  CCGE502 replicons in comparison to  R. mesoamericanum  genomes. The extrachromosomal replicons from  R. grahamii  were more similar to those found in phylogenetically related  Rhizobium  species. However, limited similarities of  R. grahamii  CCGE502 symbiotic plasmid and megaplasmid were observed in other more distant  Rhizobium  species. The set of conserved genes in  R. grahamii  comprises some of those that are highly expressed in  R. phaseoli  on plant roots, suggesting that they play an important role in root colonization.

Characterization of Rhizobium grahamii extrachromosomal replicons and their transfer among rhizobia