Enhanced effectiveness of copper ion buffering by CUP1 metallothionein compared with CRS5 metallothionein in Saccharomyces cerevisiae

The bakers' yeast Saccharomyces cerevisiae contains a metallothionein (MT) gene family comprised of the amplified CUP1 locus and the single copy CRS5 gene, We demonstrate that CUP1 plays the dominant role in copper detoxification, A single copy of CUP1 was far more effective in conferring copper resistance than was CRS5. The CUP1 promoter contributes to this resistance; in a promoter exchange experiment, the Crs5 MT conferred strong copper resistance when its expression was driven by the CUP1 promoter, and conversely, the CRS5 promoter reduced the effectiveness of Cup1 MT, Unlike CUP1, the CRS5 promoter appears to be refractory to high concentrations of copper, The CUP1 coding sequences also contribute to copper tolerance, presumably reflecting the enhanced binding avidity of Cup1 MT for Cu(I) ions, In studies with the bathocuproine Cu(I) chelator, the Cu(I) ions bound to Crs5 were kinetically more labile than the Cu(I) binding to Cup1. Our findings are consistent with the assembly of Crs5 into two metal-binding clusters, similar to mammalian MTs, but unlike Cup1. Overall, the striking differences in gene structure regulation, and function of CUP1 and CRS5 are remarkably reminiscent of the MTI and MTII genes of the pathogenic yeast Candida glabrata.