A DNA segment conferring stable maintenance on R6K gamma-origin core replicons.

The plasmid R6K gamma origin consists of two adjacent modules, the enhancer and the core, and requires R6K initiator protein pi for replication. While the core alone can replicate at a low level of wild-type pi protein, we show here that host cells do not stably maintain core plasmids. The presence of the enhancer segment confers stable inheritance on core plasmids without a significant change in average plasmid copy number. Deletions and site-directed mutagenesis indicated that the stability of core plasmids is not mediated by binding sites or consensus sequences in the enhancer for DnaA, pi protein, gyrase, Fis, or Dcm methylase. Proper segregation of core plasmids requires only the R6K stb or stability-related region, which includes the 20-bp segment of the 100-bp enhancer adjacent to the core. The use of the pi 116 mutant protein, which increases plasmid copy number fourfold, does not stabilize core plasmids lacking the enhancer. We also show that at an elevated level of wild-type pi, the gamma-origin plasmid is unstable, even in the presence of the enhancer. We discuss the differences and similarities between the R6K stability system and those found in other plasmids.