The photorepair and photoisomerization of DNA lesions in etiolated cucumber cotyledons after irradiation by UV-B depends on wavelength

To examine the light-dependent repair of DNA photo-lesions, such as, cyclobutane pyrimidine dimers (CPDs) and pyrimidine-(6-4)-pyrimidone photoproducts [(6-4)photoproducts], we incubated cucumber cotyledons under monochromatic light (325-500 nm) after they had been exposed to UV-B irradiation. Irradiation at wavelengths between 375 and 425 nm was the most efficient for photorepair of CPDs. The dependence on wavelength for the removal of (6-4)photoproducts differed from that for the photorepair of CPDs. The (6-4)photoproducts were most efficiently removed upon irradiation at 325 nm with a second peak at 400 to 425 nm. An immunological assay using antibodies specific to Dewar photoproducts revealed that irradiation at 325 nm induced the photoisomerization of (6-4)photoproducts to Dewar photoproducts. Thus, the most efficient wavelength for the photorepair of (6-4)photoproducts was between 400 and 425 nm. It seems likely that processes similar to those involved in photorepair of CPDs might be involved in the photorepair of (6-4)photoproducts.