FIG. 5.

SOS induction following loss of the EcoRI RM gene complex. Isogenic rec⁺ and recBC strains with a sfiA::lacZ promoter fusion (BIK3920 and BIK3921) carrying pIK172 (pSC101Ts, EcoRI r⁺ m⁺, Ap^r) or pIK173 (its r⁻ version) were aerated at 30°C in L broth with antibiotics and grown to an optical density at 660 nm (OD₆₆₀) of 0.3. Then the antibiotics were removed, and the culture was transferred to 42°C with aeration. The culture was diluted every time its OD₆₆₀ reached 0.3. The beta-galactosidase activity was measured as described previously (41). Enzyme concentrations (in units per milliliter) were calculated from the following formula: 1,000 × (OD₄₂₀ − 1.75 × OD₅₅₀)/t× v× OD₆₀₀, where t is the duration of the reaction in minutes and v is the volume of culture in milliliters. The results plotted were from two independent experiments.