Novel F prime factors able to replicate in Escherichia coli Hfr strains.

A novel type of F' plasmid that can replicate extrachromosomally in cells of Hfr strains has been isolated. Genetic analysis of these plasmids indicates that all carry the dnaA-bglA region of the E. coli chromosome as well as ilv+ used as a selective marker. It is suggested that a specific site, designated poh+ (permissive on Hfr), is located in this region, and is essential for these plasmids to replicate in Hfr cells. The most interesting explanation, among others, would be that the poh+ site represents the replication origin of the bacterial chromosome.