A gene required for the novel activation of a class II DNA photolyase in Chlamydomonas

DNA photolyases catalyze the blue light-dependent repair of UV light-induce d damage in DNA. DNA photolyases are specific for either cyclobutane-type p yrimidine dimers or (6-4) photoproducts. PHR2 is a gene that in Chlamydomon as reinhardtii encodes a class II DNA photolyase which catalyzes the photor epair of cyclobutane-type pyrimidine dimers. Based on amino acid sequence a nalysis of PHR2, which indicates the presence of a chloroplast targeting se quence, PHR2 was predicted to encode the chloroplast photolyase of Chlamydo monas. Using a sensitive gene-specific in vivo repair assay, we found that overexpression of PHR2 in Chlamydomonas results in targeting of the protein to not only the chloroplast, but also to the nucleus. Overexpression of PH R2 photolyase in a photoreactivation-deficient mutant, phr1, results in a l argely inactive product. The phr1 mutant was found to be deficient in both photorepair of a chloroplast gene, rbcL, and a nuclear gene, rDNA. These re sults suggest that PHR2 is the structural gene for the photolyase targeted to both the chloroplast and the nucleus, and that the PHR1 gene product is necessary for full activity of PHR2 protein. To our knowledge, the requirem ent for a second gene for full activity of a DNA photolyase is novel.