The effect of spontaneous mutations on CUP1 copy number and copper tolerance in Saccharomyces cerevisiae

Genomic regions containing tandem duplications may be subject to particularly high rates of copy number mutations due to recombinational repair of DNA damage. Consequently, changes in copy number may be among the most accessible beneficial mutations during evolution in a new environment. In the budding yeast Saccharomyces cerevisiae , the gene CUP1 occurs in multiple tandem copies and encodes a metallothionein protein that is relevant for copper homeostasis. We examined CUP1 copy number in 220 mutation accumulation lines and their ancestors to quantify spontaneous genetic change at this locus. We also measured copper tolerance in the same strains to understand the phenotypic effects of CUP1 copy number change and other mutations. We found that mutations in CUP1 copy number occurred rapidly, with a bias towards the loss of copies. While copper tolerance also declined due to spontaneous mutations, this change was less rapid overall and was only partly driven by CUP1 copy number. Thus, while this locus is highly susceptible to mutation, such mutations may provide an accessible but insufficient evolutionary path towards increased copper resistance.