MODIFYING FACTORS OF THE CELLULAR CONCENTRATION OF PHOTOLYASE MOLECULES IN Saccharomyces cerevisiae—II. EFFECTS OF PREILLUMINATION WITH LIGHT FLASHES

Abstract

The effects of preillumination with photoreactivating light flashes before UV‐irradiation on the number of photoreactivable complexes consisting of UV‐induced DNA‐damages and active photolyase molecules (NPREact), on the fluence decrements, ΔDPRE, that are obtained from two UV‐survival curves without and with 1 flash photoreactivation and proportional to NPREact were determined in haploid Saccharomyces cerevisiae cells. ΔDPRE increased by preillumination from 0.115 Jm–2 to 0.460 Jm–2 and from 0.376 Jm–2 to 0.494 Jm–2 in cells in logarithmic growth phase and in stationary growth phase, respectively. ΔPRF in log‐cells that were preilluminated before and after resuspension in buffer at 40°C for 60 min was larger than ΔDPRE in log‐cells preilluminated only after resuspension in buffer.

Expression of the Yeast PHR1 Gene Is Induced by DNA-Damaging Agents

“…Previous studies have shown that growth of S. cerevisiae cells under photoreactivating light before exposure to 254nm radiation enhances cell survival as a result of an apparent increase in the number of active photolyase molecules per cell (22)(23)(24). In contrast, we were unable to detect induction of Phrl-3-galactosidase fusion protein after exposure of dark-grown cells to fluences of photoreactivating light as great as 3 J/m2, more than 104-fold greater than the smallest fluence of 254-nm light producing reproducible induction PHRI.…”

“…The only known function of the enzyme is DNA repair. It has been previously reported that photolyase activity increases in vivo in response to photoreactivating light and that at least a part of this increase requires protein synthesis, suggesting that enzyme activity or concentration is regulated (22)(23)(24). Whether this increase reflects transcriptional, translational, or posttranslational modifications has been unclear, in part because a method of quantifying the number of photolyase molecules independent of photoreactivating activity was not available.…”