Figure 8.

Exceptional hypermutagenized revertant. One stable lac⁺ revertant (S30) accumulated 133 mutations—105 chromosomal and 28 plasmid. The small central circle represents the F’lac plasmid drawn to scale with the chromosome (outer circle). Colored sectors indicate portions of the plasmid derived from the parent F plasmid (blue) and the added bacterial segment (red). The white arrow designates the tra operon. Chi sites that could activate RecBCD are indicated outside the plasmid map. Note that the F-derived segment includes no chi sites. This strain acquired transfer deficiencies (tra) after reversion, and, therefore, escaped the backcrosses that usually separate the revertant lac⁺ allele from deleterious or lethal associated mutations in the Cairns-Foster system.