Induction and inhibition of cyclobutane pyrimidine dimer photolyase in etiolated cucumber (Cucumis sativus) cotyledons after ultraviolet irradiation depends on wavelength

Under polychromatic ultraviolet (UV) irradiation (maximum energy at 327 nm) the activity of DNA photolyase specific to cyclobutane pyrimidine dimers (CPDs), CPD photolyase, increased by an amount which depended on UV irradiance, and the level of CPD photolyase gene (CsPHR) transcripts temporarily increased before the activity reached a constant level. UV light (> 20 nm) was more effective than visible light at increasing CPD photolyase activity. In contrast, monochromatic UV irradiation at wavelengths < 300 nm increased the level of CsPHR transcripts similarly to irradiation at wavelengths > 20 nm, but reduced CPD photolyase activity compared with the dark control. Exposure of a CPD photolyase solution to UV-C (254 nm) reduced enzyme activity and induced accumulation of H2O2. Addition of H2O2 to the enzyme solution also inactivated CPD photolyase activity. These results suggest the possibility that reactive oxygen species participate in the inactivation of CPD photolyase in cotyledons exposed to UV irradiation of < 300 nm.