Description of a novel plasmid replicative origin from a genetically distinct family of conjugative plasmids associated with phytosphere microflora.

A novel replicative origin (oriV) from a conjugative, mercury resistance plasmid (pQBR11, 304 kbp) has been cloned and sequenced. Homology to the pQBR11 oriV-containing 3.55 kbp BamHI fragment (pCV1200) was restricted to one of five genetically distinct classes (group I) of narrow host range, mega-plasmids that persist as a genetic component of the pseudomonad community indigenous to the microflora of sugar beet. The oriV of pQBR11 was located within a unique sequence of 300 bp which initiated the replication of pUC derived suicide vectors in Pseudomonas putida UWC1. The limited size of the DNA sequence required to initiate replication, and the presence of two 15/16 bp directly repeated motifs, indicate that this group of mega-plasmids contain a single origin of replication, which initiates replication via a host-polymerase dependent rolling circle mechanism.