Final answer:
Photoreactivation, a DNA repair mechanism in E. coli that involves the enzyme photolyase and reverses UV-induced thymine dimers, would be inhibited if cells kept in the dark after irradiation as the enzyme requires blue/UV light for activation.
Step-by-step explanation:
If E. coli are irradiated and then kept in the dark, the DNA repair mechanism that would be inhibited is photoreactivation. This process is specifically for repairing thymine dimers, which are formed when UV light causes adjacent thymine bases to bond abnormally. Photoreactivation involves the enzyme photolyase, which requires light in the blue/UV spectrum (300-500 nm wavelength) to be activated and reverse the damage. In case of keeping E. coli in the dark post-irradiation, the photolyase enzyme would not get the light needed to facilitate the repair of the thymine dimers, thus inhibiting the photoreactivation repair process.