Effect of a water soluble derivative of alpha-tocopherol on radiation response of Saccharomyces cerevisiae.

The radioprotection conferred by a highly water soluble glucose derivative of alpha-tocopherol, namely, 2-(alpha-D-glucopyranosyl) methyl-2,5,7,8-tetramethylchroman-6-ol (TMG) in Saccharomyces cerevisiae was studied. Cells grown in standard YEPD-agar medium and irradiated in the presence of TMG showed a concentration dependent higher survival up to 10 mM of TMG in comparison to cells irradiated in distilled water. Treatment of TMG to cells given either before or immediately after irradiation but not during irradiation, had no effect on their radiation response. S. cerevisiae strain LP1383 (rad52) which is defective in recombination repair showed enhanced radioresistance only when subjected to irradiation in presence of TMG. Cells of rad52 strain grown in the medium containing TMG showed a radiation response similar to that of cells grown in the medium without TMG. The nature of TMG dependent enhanced radioresistance was studied by scoring the mutations in the strain D-7, which behaved like wild type strain in complete medium, at trp and ilv loci. Our study indicated that TMG confers radioresistance in S. cerevisiae possibly by two mechanisms viz. (i), by eliminating radiation induced reactive free radicals when the irradiation is carried out in the presence of TMG and (ii), by activating an error prone repair process involving RAD52 gene, when the cells are grown in the medium containing TMG.

Effect of environmental stress on radiation response of Saccharomyces cerevisiae.

We have investigated the effect of pH shock and oxidative stress (H2O2 effect) both separately and together on the response of Saccharomyces cerevisiae exposed to UV and gamma radiations for one hour. Exposure to these environmental stresses resulted in S. cerevisiae cells acquiring resistance to UV radiation. Presence of cycloheximide (a known protein synthesis inhibitor) during stress inhibited the acquired UV resistance. The increased UV resistance is apparently mediated through nucleotide excision repair as the stress exposure to rad3 mutants (defective in nucleotide excision repair) do not have any effect on UV response. Both types of stresses used probably follow the same path of induction of radioresistance as the effect of both of them is nonadditive. In the strains used in our study stress exposure does not have any significant effect on gamma radiation response.

Presence of RNA from yeast inhibits the photoreactivation of UV-irradiated DNA by Phr A photolyase from Escherichia coli.

Photoreactivation of UV-irradiated DSNA with phr A photolyase from Escherichia coli was studied in the presence of yeast RNA. Mixing of RNA with UV-irradiated DNA before its treatment with photolyase inhibited the photoreactivation of DNA. Denatured (by sonication) RNA was found to be more effective in blocking photolyase action. Agarose gel electrophoresis experiments suggest that this inhibition of photoreactivation is due to interference in the binding of photolyase with UV-irradiated DNA by yeast RNA.