Formation of an F′ Plasmid by Recombination between Imperfectly Repeated Chromosomal Rep Sequences: a Closer Look at an Old Friend (F′ 128 pro lac )

Plasmid F'(128) was formed by an exchange between chromosomal Rep sequences that placed lac near dinB between many pairs of Rep sequences. Plasmid F'(128) is critical for selection-enhanced lac reversion (adaptive mutation), which requires prior lac amplification. The structure of F'(128) supports the idea that amplification is initiated by Rep-Rep recombination and that general mutagenesis requires coamplification of dinB (error-prone polymerase) with lac.