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mRNA Import into Yeast Nuclei is Controlled by Components of Cytoplasmic P-bodiesIn eukaryotes, the regulation of mRNA translation and decay provides a mechanism which can be finely tuned to control gene expression. This ability to control the life cycle of an mRNA begins with the control of mRNA export from the nucleus and extends to the processes which regulate the degradation of the message. In my work, summarized below, I describe how some of the proteins involved in cytoplasmic decay regulate many aspects of the control of mRNA and also describe a novel regulatory mechanism involving the relocation of cytoplasmic mRNA back into the nucleus of the cell.Firstly, I have identified that the protein Pat1, which has been shown to be critical for translational repression and activation of decapping, consists of essentially 3 major domains. By means of a deletional functional analysis, I show that two of these domains are the primary functional domains responsible for all of the currently ascribed function of Pat1. One domain promotes translational repression and P-body assembly, while the second domain promotes mRNA decapping after assembly of the mRNA into a P-body mRNP. Along with the first evidence that Pat1 binds to RNA, we also determine numerous domain-specific interactions with mRNA decapping factors.In eukaryotic cells mRNAs are produced in the nucleus followed by what is thought to be unidirectional export to the cytoplasm. In the cytosol, mRNAs either associate with ribosomes for translation or can be found in cytoplasmic RNP granules, termed P-bodies, when they are translationally repressed. I have now demonstrated that yeast mRNAs can be re-imported into the nucleus. Import of mRNAs into the nucleus is in competition with translation and increased in strains lacking specific components of cytoplasmic processing bodies, which also exhibit nuclear-cytoplasmic shuttling. This indicates that one function of cytoplasmic granules is to limit the import of cytoplasmic mRNAs back into the nucleus. These results demonstrate a novel pathway for mRNA import into the nucleus and suggest distinct pathways of mRNA export of nascent mRNAs and imported mRNAs.