THE OTHER FUNCTION OF DNA PHOTOLYASE - STIMULATION OF EXCISION-REPAIROF CHEMICAL-DAMAGE TO DNA

DNA photolyase is a light-dependent DNA repair enzyme. It binds to cyc lobutane pyrimidine dimers (Pyr[]Pyr) in DNA and upon excitation with a blue light photon splits the cyclobutane ring and restores the pyrim idines to native forms. The enzyme is specific for pyrimidine dimers, and it is not known to catalyze any other reaction either in ground or in excited state. However, when photolyase binds to Pyr[]Pyr but cann ot catalyze repair because of lack of photoreactivating light, it stil l aids DNA repair by stimulating the nucleotide excision repair system . Recently, it was found that yeast photolyase binds to other lesions in DNA. In particular, the binding to cisplatin damaged DNA was highly specific. However, in vivo experiments revealed that this binding, in contrast to Pyr[]Pyr binding, did not stimulate but actually inhibite d the removal of cisplatin damage by excision repair and hence photoly ase sensitized cells to killing by cisplatin. In the present study, it is demonstrated that Escherichia coli DNA photolyase binds specifical ly to cisplatin 1,2-d(GpG) intrastrand cross-link and stimulates the r emoval of the lesion by E. coli excision nuclease in vitro. In agreeme nt with the in vitro data, in vivo experiments revealed that photolyas e makes cells more resistant to cisplatin killing.