Recircularization and Autonomous Replication of a Sheared R-Factor DNA Segment in Escherichia coli Transformants

Controlled shearing of R-factor DNA leads to formation of fragments carrying an antibiotic resistance gene present on, but not expressed by, the intact R-factor. Transformation of CaCl(2)-treated E. coli by such fragments yields an autonomously replicating tetracycline-resistance plasmid (Tc6-5) that contains only a small fraction of the genome of the parent R-factor, and lacks both its fertility functions and its other drug-resistance determinants. Although the Tc6-5 plasmid is not self-transmissible, it can interact and/or recombine with conjugally-proficient plasmids that promote its transfer to other bacteria.