Inhibition of spontaneous mutagenesis in yeast cultures by selenite, selenate and selenide.

Sodium selenite (1--15 mumol/plate) was found to completely suppress spontaneous mutagenesis at 2 independent loci in both wild (YO-300-IC) and mutator (mut 1-1, mut 2-2, mut 3-1, mut 4-1, mut 5-2, mut 6-1, mut 8-1, mut 9-1 and mut 10-1) isogenic strains of Saccharomyces cerevisiae. The 2 loci which where studied were his 1-7, a missense mutation, and lys 1-1, a super-suppressible mutant of the amber variety. The degree of suppression of spontaneous reversion to prototrophy at these 2 loci depended on the concentration of sodium selenite present, the strain of yeast being studied, and the loci being studied. Greater concentrations of sodium selenite (up to 30-fold higher) were required to suppress the frequency of spontaneous reversion at the histidine locus compared to quantities necessary to elicit a similar inhibition of lysine spontaneous reversion rates. The 2 loci also responded differently to the presence of 2 other inorganic selenium derivatives. Spontaneous mutagenesis at the lysine locus for strain YO-800-1C (mu 1-1) was completely inhibited by sodium selenide at 3 mumol/plate with complete suppression of histidine reversion occurring at 30 mumol/plate. Sodium selenate suppressed the spontaneous mutagenesis at the lysine, but not the histidine locus. These results indicate that environmentally added components can have a significant effect on the genetically controlled predisposition of an organism to mutagenesis and suggest the complexity of such interactions.