This study aims to investigate the effect of different wavelengths (254, 266, 280 and 365 nm) in polychromatic medium-pressure (MP) UV radiation on the ability of photolyases in repairing dimers and discusses its impact on subsequent photoreactivation. Photolyase was exposed to various doses and irradiances of the UV wavelengths and the dimer repair abilities of the irradiated photolyase were determined via a spectrophotometric assay. At wavelengths below 300 nm, dimer repair rates were not influenced by the UV irradiation between 0.03 and 0.10 mW cm−2. For 365 nm, photolyase exhibited enhanced dimer repair at 0.05 mW cm−2 and then reduced dimer repair with increasing irradiance. In addition, photolyase was found to have decreasing dimer repair rates when exposed to increasing UV doses at all tested wavelengths. Lower photoreactivation levels after MP UV disinfection as compared to low-pressure (LP) UV disinfection was not attributable to a single wavelength in the polychromatic radiation, but is possibly due to the simultaneous exposure of photolyase to a broad spectrum of radiation, which led to a reduction in the dimer repair ability of photolyase. This study is the first to report the direct effects of UV radiation on photolyase enzyme. The data in the study provide some evidence for the mechanism for which MP UV disinfection suppresses photoreactivation in Escherichia coli, which has only been speculated on so far. The knowledge from this study will provide a basis upon which to investigate other enzymes involved in the repair of UV damage to DNA.

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