UV survival and SOS induction assays. Plasmids pCA24N, pRecA-Htg, and pRecA-GFP were introduced in the strain GW1030 (dinB::lacZ)  and its derivative ΔrecA  by transformation. Norfloxacin-mediated SOS induction of the dinB::lacZ was assayed by the disk-plate method as described . (A) Survival following UV irradiation of wild-type MG1655 recA+ (pCA24N) (1), JMG1655 ΔrecA (pCA24N) (2), JMG1655 ΔrecA (pRecA-Htg) (3) and JMG1655 ΔrecA (pRecA-GFP) (4) strains. UV doses in J/m2 are indicated. (B) The effect of RecA-Htg and RecA-GFP proteins on the norfloxacin-mediated SOS induction of a dinB::lacZ gene fusion and on sensitivity to norfloxacin. Plates on the left side show the effect of norfloxacin on the wild-type strain GW1030 containing either pCA24N, pRecA-Htg or pRecA-GFP plasmids. Plates on the right side show the effect of norfloxacin on the GW1030ΔrecA strain containing the indicated plasmids. The intensity of the gray border around the inhibition halo reflects the level of dinB transcription .