Fig. 1.

The effect of loss of MutL on growth-dependent and adaptive reversion of an episomal Lac⁻ allele. The results are from fluctuation tests (25 cultures for wild type and 48 cultures for the mutL⁻ derivative). Growth-dependent mutation rates (Day 2, white) are Lac⁺ mutations per cell per generation, calculated from the proportion of cultures giving no Lac⁺ colonies 2 days after plating on lactose minimal medium. Adaptive mutation rates (Days 3–5, grey) are the mean Lac⁺ mutations per cell per day (±S.E.), calculated from the numbers of Lac⁺ colonies appearing on days 3 to 5 after plating on lactose minimal medium.