Physiological basis of copper tolerance of Saccharomyces cerevisiae nonsense-mediated mRNA decay mutants.

The eukaryotic nonsense-mediated mRNA decay pathway (NMD) is a specialized pathway that contributes to the recognition and rapid degradation of mRNA with premature termination codons. In addition to mRNAs containing premature termination codons, NMD degrades non-nonsense-containing, natural mRNAs. Approximately 5-10% of the total Saccharomyces cerevisiae transcriptome is affected when NMD is inactivated. The regulation of natural mRNAs by NMD has physiological consequences. However, the physiological outcomes associated with the degradation of specific natural mRNAs by NMD are not fully understood. Here, we examined the physiological consequences resulting from the NMD-mediated regulation of an mRNA involved in copper homeostasis, in an attempt to understand why nmd mutant strains are more tolerant of toxic copper levels than wild-type yeast strains. We found that wild-type (UPF1) and upf1Δ mutants accumulate similar amounts of total copper when grown in medium containing elevated levels of copper; however, the copper levels in the cytoplasm of wild-type yeast cells were higher than in the upf1Δ mutant. Copper tolerance by the upf1Δ mutant is dependent on the presence of CTR2. Deletion of CTR2 resulted in similar cytoplasmic copper levels in wild-type and upf1Δ mutant strains, regardless of the environmental copper levels. This suggests that CTR2 plays a role in regulating the level of copper in the cytoplasm. We also found that the upf1Δ mutant contained elevated copper levels in the vacuole relative to wild-type yeast cells, after both strains were exposed to elevated copper levels.