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.