Determination of the number of photoreactivating enzyme molecules per haploid saccharomyces cell

Two haploid radiation-sensitive mutants of Saccharomyces were studied to investigate the formation of complex between photoreactivating-enzyme and substrate after ultra-violet irradiation. Using photo-flashes, the time necessary for maximum complex formation has been determined. Within 1 min, 70 per cent of the complexes have been formed. To determine the number of photoreactivating enzyme molecules per cell, the maximum dose decrement obtained after one photo-flash was determined and corrected for the effects of non-photoreactivable lesions. The corrected maximum dose decrement was found to be identical for both strains (8·5 erg mm^-2). The number of photoreactivating-enzyme molecules involved in the photorepair of nuclear DNA damage was calculated as 272 ± 27.