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.

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A series of homozygous Saccharomyces-strains of different ploidy (haploid to tetraploid) has been bred. Except for the mating type alleles in the tri- and tetraploid strain the genomes of these strains are completely identical. The inactivation of these strains by UV, organic peroxides, X-rays (50 kV), 210Po-α-rays, accelerated He-, C- and O-nuclei was studied by avaluation of the ability to form macroscopic colonies. These different kinds of ionizing radiations cover a wide range of ionization density (LET). The triploid strain proved to be the most resistent one for each kind of radiation inactivation. A comparison of the RBE-values of the ionizing radiations showed a maximum at the LET-value of 1360 MeV cm2 g-1 (210Po-a-rays). It could be shown for diploid strains that a heterozygous condition in only the mating type alleles produces a remarkable resistance for X- and 210Po-α-rays. This effect, however, is absent in UV-inactivation.

The effects of light-illumination in the absence or presence of cycloheximide (CHX) on the number of photoreactivating enzyme molecules per haploid cell of Saccharomyces cerevisiae (NPRE) were investigated. NPRE increased, when the cells were held in buffer in light in the absence of CHX for 24 or 48 h before UV-irradiation (buffer-holding). The increase of NPRE was clearly observed in cells, whose NPRE before bufferholding was small, that is, in stationary growth phase cells grown at 37 degrees C and in logarithmic growing cells incubated at 30 degrees C. In the case of buffer-holding in light in the presence of CHX or in the dark in the absence or presence of CHX, the increase of NPRE was small or not observed.

Abstract— The effects of temperature and light on the cellular concentration of photoreactivating enzyme (PRE) molecules in haploid Saccharomyces cerevisiae cells were investigated. (1) Temperature effect: The number of active PRE molecules per cell (NPRE) in cells grown at 37°C was about 13% of that grown at 23°C, although the amount of proteins per cell remained the same. (2) Light effect: NPRE in cells grown in light was about 2.8 times larger than that grown in the dark. The value of NPRE in cells grown in the light decreased more rapidly during holding in buffer in the dark than in the light. The NPRE decrease during holding in buffer in the dark was more rapid in cells grown in the light than grown in the dark. A comparable decrease was observed after holding in buffer in the presence of cycloheximide. (3) In cells harboring a plasmid containing the gene PHR1, NPRE was larger in cells grown at 23 than at 30°C.