Properties of Mitotic and Meiotic Recombination in the Tandemly-Repeated CUP1 Gene Cluster in the Yeast Saccharomyces cerevisiae .

In the yeast Saccharomyces cerevisiae , the genes encoding the metallothionein protein Cup1 are located in a tandem array on chromosome VIII. Using a diploid strain that is heterozygous for an insertion of a selectable marker ( URA3 ) within this tandem array, and heterozygous for markers flanking the array, we measured interhomolog recombination and intra/sister chromatid exchange in the CUP1 locus. The rate of intra/sister chromatid recombination exceeded the rate of interhomolog recombination by >10-fold. Loss of the Rad51 and Rad52 proteins, required for most interhomolog recombination, led to a relatively small reduction of recombination in the CUP1 array. Although interhomolog mitotic recombination in the CUP1 locus is elevated relative to the average genomic region, we found that interhomolog meiotic recombination in the array is reduced compared to most regions. Lastly, we showed that high levels of copper (previously shown to elevate CUP1 transcription) lead to a substantial elevation in rate of both interhomolog and intra/sister chromatid recombination in the CUP1 array; recombination events that delete the URA3 insertion from the CUP1 array occur at a rate of >10-3 /division in unselected cells. This rate is almost three orders of magnitude higher than observed for mitotic recombination events involving single-copy genes. In summary, our study shows that some of the basic properties of recombination differ considerably between single-copy and tandemly-repeated genes.