Isolation of plasmid deletion mutants and study of their instability

We describe a method which allows isolation of deletions within hybrid plasmids. It is based on the fact that the tetracycline resistance (Tc R) gene of pBR322 can be inactivated by inserting foreign DNA into its HindIII site, and that the easily selectable Tc R mutants of such plasmids are generally (>90%) due to deletions of certain hybrid plasmid sequences. We have found that Tc R mutants are usually maintained within the cell recombined with the parental Tc S plasmids. Such heterodimers dissociate in both Rec + and in recA hosts. Parental rather than mutant plasmids are then retained by the host cell.