Cyclobutane pyrimidine dimer (CPD) photolyase repairs ultraviolet-B-induced CPDs in rice chloroplast and mitochondrial DNA

Plants use sunlight as energy for photosynthesis; however, plant DNA is exposed to the harmful effects of ultraviolet-B (UV-B) radiation (280-320 nm) in the process. UV-B radiation damages nuclear, chloroplast and mitochondrial DNA by the formation of cyclobutane pyrimidine dimers (CPDs), which are the primary UV-B-induced DNA lesions, and are a principal cause of UV-B-induced growth inhibition in plants. Repair of CPDs is therefore essential for plant survival while exposed to UV-B-containing sunlight. Nuclear repair of the UV-B-induced CPDs involves the photoreversal of CPDs, photoreactivation, which is mediated by CPD photolyase that monomerizes the CPDs in DNA by using the energy of near-UV and visible light (300-500 nm). To date, the CPD repair processes in plant chloroplasts and mitochondria remain poorly understood. Here, we report the photoreactivation of CPDs in chloroplast and mitochondrial DNA in rice. Biochemical and subcellular localization analyses using rice strains with different levels of CPD photolyase activity and transgenic rice strains showed that full-length CPD photolyase is encoded by a single gene, not a splice variant, and is expressed and targeted not only to nuclei but also to chloroplasts and mitochondria. The results indicate that rice may have evolved a CPD photolyase that functions in chloroplasts, mitochondria and nuclei, and that contains DNA to protect cells from the harmful effects of UV-B radiation.