Messenger RNA degradation is crucial for the regulation of eukaryotic gene expression. It not only modulates the basal mRNA levels but also functions as a quality control system, thereby controlling the availability of mRNA for protein synthesis. In Saccharomyces cerevisiae, the first and the rate-limiting step in the process of mRNA degradation is the shortening of the poly(A) tail by deadenylation complex. After the poly(A) tail shortens, mRNA can be degraded either through the major 5' to 3' decapping dependent or the 3' to 5' exosome-mediated degradation pathway. In this thesis, we show some of the means by which mRNA decay factors can modulate gene expression. First, Pat1 is a major cytoplasmic mRNA decay factor that can enter the nucleus and nucleo-cytoplasmically shuttle.  Recent evidence suggested several possible nuclear roles for Pat1. We analyzed them and showed that Pat1 might not function in pre-mRNA decay or pre-mRNA splicing, but it is required for normal rRNA processing and transcriptional elongation. We show that the mRNA levels of the genes related to ribosome biogenesis are dysregulated in the strain lacking Pat1, a possible cause of the defective pre-rRNA processing. In conclusion, we theorize that Pat1 might regulate gene expression both at the level of transcription and mRNA decay. Second, Edc3 and Lsm4 are mRNA decapping activators and mRNA decay factors that function in the assembly of RNA granules termed P bodies. Mutations in mRNA degradation factors stabilize mRNA genome-wide or stabilize individual mRNAs. We demonstrated that paradoxically, deletion of Edc3 together with the glutamine/asparagine-rich domain of Lsm4 led to a decrease in mRNA stability. We believe that the decapping activator Edc3 and the glutamine/asparagine-rich domain of Lsm4 functions together, to modify mRNA decay pathway by altering cellular mRNA decay protein abundance or changing the mRNP composition or by regulating P bodies, to enhance mRNA stability. Finally, mRNA decay was recently suggested to occur on translating ribosomes or within P bodies. We showed that mRNA degradation factors associate with large structures in sucrose density gradients and this association is resistant to salt and sensitive to detergent. In flotation assay, mRNA decay factors had buoyancy consistent with membrane association, and this association is independent of stress, translation, P body formation or RNA. We believe that such localization of mRNA degradation to membranes may have important implications in gene expression. In conclusion, this thesis adds to the increasing evidence of the importance of the mRNA degradation factors in the gene expression.

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