CHROMATIN STRUCTURE MODULATES DNA-REPAIR BY PHOTOLYASE IN-VIVO

Yeast and many other organisms use nucleotide excision repair (NER) an d photolyase in the presence of light (photoreactivation) to repair cy clobutane pyrimidine dimers (CPDs), a major class of DNA lesions gener ated by UV light, To study the role of photoreactivation at the chroma tin level in vivo, we used yeast strains which contained minichromosom es (YRpTRURAP, YRpC51) with well-characterized chromatin structures, T he strains were either proficient (RAD1) or deficient (rad1 Delta) in NER. In contrast to NER, photolyase rapidly repairs CPDs in non-nucleo somal regions, including promoters of active genes (URA3, HIS3, DED1) and in linker DNA between nucleosomes, CPDs in nucleosomes are much mo re resistant to photoreactivation, These results demonstrate a direct role of chromatin in modulation of a DNA repair process and an importa nt role of photolyase in repair of damaged promoters with presumptive effects on gene regulation, In addition, photoreactivation provides an in vivo test for chromatin structure and stability, In active genes ( URA3, HIS3), photolyase repairs the non-transcribed strand faster than the transcribed strand and can match fast removal of lesions from the transcribed strand by NER (transcription-coupled repair). Thus, the c ombination of both repair pathways ensures efficient repair of active genes.