Mutations at the human minisatellite MS32 integrated in yeast occur with high frequency in meiosis and involve complex recombination events
To study chemically induced DNA amplifications we used the haploid Saccharomyces cerevisiae strain TR(MS1)-1 carrying an integrated chromosomal copy of the human minisatellite, MS1. Chemicals with different mechanisms of action were tested in this strain: methyl methanesulphonate, ethylene oxide (EO), propylene oxide (PO), camptothecin, 2,3,7,8-tetrachlorodibenso-p-dioxin (TCDD) and reserpine. No increase in frequency of new MS1 length alleles was seen with any of the tested chemicals relative to the spontaneous frequency of approximately 30%. EO and TCDD induced changes in the amplification spectrum, i.e. the frequency distribution of MS1 length alleles longer than the original 1.42 kb allele. PO and camptothecin increased the frequency of plasmid “pop-out” events. It seems likely that several mechanisms e.g. unequal exchanges, replication slippage and loop formation leading to deletion of a ring of tandem repeats, are involved in the generation of new MS1 length alleles. A loop-forming deletion mechanism is supported by the tendency to multimodality shown in the deamplification (loss of repeat units) spectra, i.e. the frequency distribution of new MS1 length alleles shorter than the original allele. EO and TCDD induced “longer” MS1 length alleles as compared to the control. The frequent generation of new MS1 length alleles in this haploid yeast strain further demonstrates the instability of such sequences and their possible relevance to genetic toxicology and the mechanisms of induction of cancer as well as other diseases. This study is a first step towards the development of an assay for DNA amplification without the use of a selective agent.