The second replicator region of the native plasmid pTAV1 of Paracoccus versutus has been identified thus proving the composite nature of this replicon. The minimal replicon designated pTAV320 (4.3 kb) was cloned and sequenced. pTAV320 encodes three putative proteins--RepA, RepB and RepC. This replicator region shows strong structural and functional similarity to repABC-type replicons found in several Agrobacterium and Rhizobium plasmids. The origin of replication appears to be localized within the coding sequence of the repC gene. RepC was shown to be essential for replication. RepA and RepB were necessary for stable maintenance of the plasmid, which implies a role in active partitioning. The presence of the complete sequence of pTAV320 (in its non-replicative form) could stabilize in cis pTAV202, a mini-replicon derived from the other pTAV1 replicator region. Deletions introduced into the repC gene abolished the 'stabilizing' activity of pTAV320, suggesting that the centromere-like sequence, necessary for partitioning, might be localized within this gene. The two replicator regions of pTAV1 (pTAV320 and pTAV202) expressed incompatibility towards the parental plasmid but were compatible in trans in P. versutus cells. The pTAV320 replicon can be maintained in several Paracoccus, Agrobacterium, Rhizobium and Rhodobacter strains in addition to P. versutus.