Fig. 3.

A rpoS mutation decreases adaptive mutation.

A. The accumulation of Lac⁺ mutations in Lac⁻ cells incubated on lactose minimal medium. Circles, recG = FC526; triangles, recG rpoS = PFG36. Data are the cumulative number of Lac⁺ colonies appearing from days 3–5; each point is the mean of four cultures ± SEM (some error bars are smaller than the symbols).

B. The survival of Lac⁻ cells while incubating on lactose minimal medium. Circles, recG = FC526; triangles, recG rpoS::Cm = PFG36. Because 10-fold more PFG36 cells were plated than FC526 cells, the numbers of FC526 cells on each day were multiplied by 10; then the results for both strains were normalized to the value for FC526 on day 0. Because it takes two days for a Lac⁺ revertant to make a visible colony, the values shown in B correspond to the points two days later in the Lac⁺ curves in A.