Topoisomerase deficiencies subtly enhance global genomic repair of ultraviolet-induced DNA damage in Saccharomyces cerevisiae.

Genetic integrity depends upon the precision of all pathways that manipulate DNA. DNA repair mechanisms prevent mutations and aberrant recombination events by removing DNA damage. DNA topoisomerases maintain favorable nucleic acid topology for replication, transcription, and chromosome segregation. However, topoisomerases can also become trapped on DNA at sites of damage, and thereby, might alter the efficiency of DNA repair. The activities of the three nuclear DNA topoisomerases (Top1, Top2, and Top3) in the yeast Saccharomyces cerevisiae were examined for their influence upon the nucleotide excision repair (NER) of DNA damage induced by ultraviolet (UV) irradiation. A 10-20% increase in the global genomic repair (GGR) of cyclobutane pyrimidine dimers (CPDs) was observed with impaired Top1 or Top2 function. The GGR of 6-4 photoproducts (6-4PPs) and the strand-specific removal of CPDs from the yeast RPB2 gene were unaffected by the loss of topoisomerase activity. Even though the deletion of TOP3 conferred UV sensitivity, neither the GGR nor the strand-specific repair of UV-induced DNA damage was compromised in top3Delta yeast. Top1 and Top2 in DNA complexes near CPDs may inhibit GGR recognition of these lesions and produce protein-linked DNA breaks, resulting in CPD repair by an alternate pathway. While the physiological role of topoisomerase association with DNA damage has yet to be determined, these enzymes do not play a direct role in the NER pathways for removing UV-induced lesions in yeast.