PROJECT SUMMARY The nonsense-mediated mRNA decay (NMD) pathway is a specialized pathway that contributes to the recognition and rapid degradation of mRNA with premature termination codons. This prevents the production of non-functional, potentially harmful truncated proteins. NMD affects the expression of a number of human genetic diseases by modulating the expression of genes carrying nonsense mutations. NMD also regulates the expression of specific genes by degrading natural mRNAs. Regulation of natural mRNAs by NMD has been identified in numerous organisms ranging from yeast to humans. However, the extent and reason for the targeting of most of these natural mRNAs is generally unknown and could be a way to support a proper cellular response to changing environmental conditions. The objective of this study is to investigate the role NMD plays in copper homeostasis. We hypothesize that mRNAs involved in copper homeostasis and are sensitive to NMD are regulated by the pathway through similar features. In addition, we postulate that the regulation of these mRNAs by NMD is responsive to environmental conditions. This hypothesis is formulated based on our preliminary observations showing that four mRNAs involved in copper homeostasis have identical NMD-targeting features. Additionally, regulation of one of the mRNAs by NMD is responsive to environment copper levels. This research will determine the extent to which functionally related mRNAs are regulated by NMD due to similar features. In addition, it will demonstrate the extent to which this regulation is responsive to environmental conditions. We plan to test our central hypothesis and accomplish the overall objective of this project by pursuing the following specific aims. In Aim 1 we will determine the features/factors that target mRNAs involved in copper homeostasis to NMD. In Aim 2 we will determine the environmental impact on the regulation of these mRNAs by NMD and the physiological consequences resulting from this regulation The contributions of this research is that it will demonstrate the extent to which natural mRNAs from the same functional group are regulated by NMD and the influence the environment has on this regulation. Successful completion of these studies would demonstrate for the first time the effect environmental conditions have on the regulation of functionally related mRNAs by NMD. This knowledge will allow the categorization of cellular processes regulated NMD, and lead to a more complete understanding of gene regulation in the genetically tractable organism S. cerevisiae. It is important to understand natural mRNA regulation by NMD given that these mRNAs are found in multiple organisms including humans and the strategies used by S. cerevisiae to regulate functionally related mRNAs could be utilized in other systems and under other conditions. Furthermore, The NMD pathway in humans is being targeted to treat genetic diseases caused by genes that contain nonsense codons.