Identification of a maintenance system on rolling circle replicating plasmid pVT736-1

Abstract

Distribution of plasmid molecules to the two daughter cells at cell division is of major importance for their stable inheritance. Several mechanisms that control equipartitioning of low-copy-number plasmids have been described in molecular terms. However, no homologous or analogous systems have been identified for intermediate or high-copy-number plasmids, including rolling circle replicating (RCR) plasmids. It has been suggested that distribution of such plasmids at cell division relies solely on random segregation. Plasmid pVT736-1 is a 2kb RCR plasmid that was isolated from the Gram-negative capnophilic coccobacillus Actinobacillus actinomycetemcomitans. The plasmid contains a DNA region of approximately 0.8 kb that is associated with its segregational stability. An operon that consists of two genes (orf3 and orf2) is followed by a putative cis-acting site that contains an integration host factor (IHF) binding site, flanked by several repeats. Mutations in orf2 resulted in plasmid instability. In addition, this DNA region was able to stabilize partially a heterologous replicon, p15A. Homologues or analogues of the pVT736-1 stabilization system have been detected on numerous plasmid and bacterial genomes.