Figure 3.

LexA-repressed inhibitor(s) of Lac⁺ adaptive mutation. (A) Complete de-repression of the LexA regulon inhibits mutation. The lexA defective strain carries lexA51(Def) sulA211 (◊), lexA⁺ (□), and sulA211(○). The sulA mutation, required for viability of lexA51(Def) strains, also depresses mutation modestly (discussed in text). (B) The LexA-controlled inhibitor of adaptive mutation is not DinI. Both lexA51(Def) strains also carry sulA211. □, dinI⁺ lexA⁺; ▵, dinI∷kan lexA⁺; ○, sulA211; ■, lexA51(Def) sulA211 dinI∷kan; ◊, lexA51(Def) sulA211. (C) PsiB inhibits adaptive mutation in LexA de-repressed cells. All strains shown carry sulA211. Additional alleles carried are as follows: ○, lexA⁺; ●, lexA51(Def) psiB∷cat; ▵, psiB∷cat; ◊, lexA51(Def); ■, lexA51(Def) psiB∷cat dinI∷kan. Results are discussed in the text.