Comparative characterization of repABC-type replicons of Paracoccus pantotrophus composite plasmids

The repABC replicons have an unusual structure, since they carry genes coding for partitioning (repA, repB) and replication (repC) proteins, which are organized in an operon. So far, the presence of these compact bi-functional modules has been reported only in the megaplasmids of the Rhizobiaceae and within the plasmid pTAV1 (107kb) of Paracoccus versutus. We studied the distribution of repABC-type replicons within bacteria belonging to the genus Paracoccus. We found that repABC replicons occur only in the group of pTAV1-like plasmids: pKLW1, pHG16-a, pWKS2, and pPAN1, harbored by different strains of Paracoccus pantotrophus. A partial sequencing approach followed by phylogenetic analysis revealed that these replicons constitute a distinct evolutionary branch of repABC replicons. Incompatibility studies showed that they represent two incompatibility groups designated IncABC1 (pTAV1, pKLW1, and pHG16-a) and IncABC2 (pPAN1). Sequence comparison using available databases allowed the identification, within plasmid pRS241d of Rhodobacter sphaeroides 2.4.1, of an additional sequence highly homologous to the paracoccal repABC replicons, which has been included in comparative analyses.