Also, the occurrence of frequent genomic rearrangements in rhizobial species has been amply documented [19, 20, 25, 28]. Integrating these data, we propose that the R. etli plasmids were transferred to a S. fredii strain and recombination events among the plasmids, the chromosome, and possibly another endogenous S. fredii plasmid, led to the generation Ralimetinib supplier of plasmids pSfr64a and pSfr64b. This would indicate that pSfr64a is an evolutionary

“”new”" plasmid of chimeric origin, that was generated after R. etli strains arrived to Europe, following the discovery of America, when bean seeds coated with bacteria were most likely introduced to that continent [29]. It is noteworthy that pSfr64a, in spite of carrying a large segment of chromosomal origin, would not be considered as a secondary chromosome, as it can be cured without affecting the saprophytic phenotype of the strain (data not shown). It is possible that ATM Kinase Inhibitor clinical trial such a plasmid is an “”intermediate”" in the formation of secondary chromosomes. Other plasmids with a structure similar to that of pSfr64a, have yet to be described. The finding of such a plasmid in a natural environment may be a living example of a pathway that allows shuffling

of the repABC genes, which has been proposed as a strategy to explain the plasmid diversity of Rhizobium [26]. Also, the fact that the repABC genes are located adjacent to the transfer region that is similar to that of pRet42a, and separate from the other sequences that are similar to the R. etli pSym, highlights the impact of evolutionary forces leading to this arrangement, which is highly conserved in many plasmids, and must have evolved in a relatively

short time period. Strain NGR234 was isolated in 1965 by M. J. Trinick, from Lablab purpureus nodules in Papua New Guinea [11]. The complete genome of strain NGR234 has been sequenced [30]. Tau-protein kinase Very recently, the classification of NGR234 was changed from Rhizobium sp to Sinorhizobium fredii. However, no genomic sequence of a type strain of S. fredii is available at present. Genome analysis of other S. fredii strains, both, typical and bean-nodulating, would help to define if the sequence migrated to a plasmid in a S. fredii ancestor, or in a more recent event. The segment containing sequences similar to the R. etli transmissible plasmid pRet42a includes the genes involved in conjugative transfer. Conjugative transfer of Agrobacterium tumefaciens pTi and other rhizobial plasmids is subject to quorum-sensing regulation [3, 4, 31]. In pRet42a, transcription of tra and trb genes is activated by the autoinducer TraI and the transcriptional regulators TraR and CinR. The repressor encoded by traM is not active [5]. Plasmid pSfr64a contains similar regulatory genes, indicating that its transfer is probably regulated by quorum-sensing.