Alternative excision repair pathway of UV-damaged DNA in Schizosaccharomyces pombe operates both in nucleus and in mitochondria

The fission yeast, Schizosaccharomyces pombe, possesses a UV-damaged DNA en donuclease-dependent excision repair (UVER) pathway in addition to nucleoti de excision repair pathway for W-induced DNA damage. We examined cyclobutan e pyrimidine dimer removal from the myo2 locus on the nuclear genome and th e coI locus on the mitochondrial genome by the two repair pathways. While n ucleotide excision repair repairs damage only on the nuclear genome, UVER e fficiently removes cyclobutane pyrimidine dimers on both nuclear and mitoch ondrial genomes. The ectopically expressed mild type UV-damaged DNA endonuc lease was localized to both nucleus and mitochondria, while modifications o f N-terminal methionine codons restricted its localization to either of two organelles, suggesting an alternative usage of multiple translation initia tion sites for targeting the protein to different organelles. By introducin g the same mutations into the chromosomal copy of the uvde(+) gene, we sele ctively inactivated UVER in either the nucleus or the mitochondria. The res ults of UV survival experiments indicate that although UVER efficiently rem oves damage on the mitochondrial genome, UVER in the mitochondria hardly co ntributes to UV resistance of S. pombe cells. We suggest a possible UVER fu nction in mitochondria as a backup system for other UV damage tolerance mec hanisms.