1.D Photolyase

　　Light is essential for life, yet ultraviolet (UV) light can be harmful due to its damaging photochemical reactions with biological molecules. For example, UV-induced chemical reactions between two adjacent pyrimidine bases within the same DNA strand produce two major photoproducts: 70-80 % are cyclobutane pyrimidine dimers (CPDs) and 20-30 % are pyrimidine-pyrimidone (6-4) photoproducts ((6-4) PPs). To maintain genetic integrity, organisms use self-defense machineries. Photolyases (PHR) are unique DNA repair enzymes, which light-dependently catalyze the conversion of these photoproducts to original pyrimidines. Two types of PHRs are known: CPD photolyase (CPD PHR), which repairs only CPDs, and (6-4) photolyase ((6-4) PHR), which specifically recognizes and repairs (6-4) PPs. Besides these PHRs, many higher organisms have PHR-like proteins termed cryptochromes (CRYs), controlling growth and flowering in plant and daily rhythms in animals. The PHRs and CRYs have a common choromophore, flavin adenine dinucleotide (FAD). Within the cryptochrome/photolyase family, the (6-4) PHRs show the highest sequence and structural similarities to mammalian circadian clock-related CRYs.
　　The (6-4) photolyase by itself showed significant volume changes after blue-light activation, indicating protein conformational changes distant from the flavin cofactor. A drastic diffusion change was observed only in the presence of both (6-4) photolyase and damaged DNA, and not for (6-4) photolyase alone or with undamaged DNA. Thus, we propose that this diffusion change reflects the rapid (50 s time constant) dissociation of the protein from the repaired DNA product. Conformational changes with such fast turnover would likely enable DNA repair photolyases to access the entire genome in cells.


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