Figure 1.

Cairns-Foster tester strain and its reversion behavior. (A) Genotype of the E. coli tester strain (TT27001). The lac region is deleted from the chromosome and is carried by a conjugative F’lac plasmid, which is able to transfer from one cell to another. Most tester strains have 1–2 plasmid copies, but 10⁵ of the plated 10⁸ cells have >10 plasmid copies. These cells arise before plating, and initiate all of the revertant colonies that develop under selection (Sano et al. 2014). (B) Course of a reversion experiment, during which the plated 10⁸ lac mutant cells do not grow, but give rise to ∼50 stable Lac⁺ colonies per plate, which accumulate linearly over the course of 5 days, and are derived from the 10⁵ initiator cells. Of the revertant colonies appearing on day 5, 90% are stably Lac⁺, and carry a point mutation that corrects the mutant lac allele, while 10% are unstably Lac⁺, and owe their growth to a tandem amplification of the mutant lac allele, which has low residual function. The dashed blue line indicates the lawn population of 10⁸ plated cells that do not divide under selection.