The exceptionally high rate of spontaneous mutations in the polymerase delta proofreading exonuclease-deficient Saccharomyces cerevisiae strain starved for adenine.

BACKGROUND: Mutagenesis induced in the yeast Saccharomyces cerevisiae by starvation for nutrilites is a well-documented phenomenon of an unknown mechanism. We have previously shown that the polymerase delta proofreading activity controls spontaneous mutagenesis in cells starved for histidine. To obtain further information, we compared the effect of adenine starvation on mutagenesis in wild-type cells and, in cells lacking the proofreading activity of polymerase delta (phenotype Exo-, mutation pol3-01). RESULTS: Ade+ revertants accumulated at a very high rate on adenine-free plates so that their frequency on day 16 after plating was 1.5 x 10(-4) for wild-type and 1.0 x 10(-2) for the Exo- strain. In the Exo- strain, all revertants arising under adenine starvation are suppressors of the original mutation, most possessed additional nutritional requirements, and 50% of them were temperature sensitive. CONCLUSIONS: Adenine starvation is highly mutagenic in yeast. The deficiency in the polymerase delta proofreading activity in strains with the pol3-01 mutation leads to a further 66-fold increase of the rate of mutations. Our data suggest that adenine starvation induces genome-wide hyper-mutagenesis in the Exo- strain.

High rate of starvation-associated mutagenesis in Ung(-) yeast caused by the overproduction of human activation-induced deaminase.

We examined the role of Saccharomyces cerevisiae uracil DNA glycosylase in the suppression of mutagenesis in non-dividing, adenine-starved cells expressing human activation-induced deaminase (AID) gene. Our aim was to further understand the mechanisms preventing starvation-associated mutagenesis in yeast and to explore the consequences of AID gene expression in non-proliferating eukaryotic cells. Genetic control of starvation-induced mutagenesis in many aspects is similar to the control of spontaneous logarithmic phase mutagenesis. Low DNA polymerase fidelity, defects of mismatch repair or post-replication repair lead to the elevation of mutagenesis. Less is known about the role of uracil in DNA. In yeast, the UNG1 gene codes for a uracil DNA glycosylase, which removes uracil from DNA, thus preventing an accumulation of mutations. The UNG1 gene is constitutively expressed at low levels throughout the cell cycle and peaks in late G1/early S phase. We have shown that the wild-type UNG1 allele protects from AID-induced mutations in starved cells to the same extent as it does in logarithmic growth phase cells. This finding implies that the first step in uracil removal by base excision repair (BER) is similar in these two conditions and provides the first data for understanding the role of BER in starvation-associated mutagenesis.