Title: Ribosomal heterogeneity and translational regulation in Saccharomyces cerevisiae
Description: RNA localization plays an important role in the establishment of cell polarity and in development. During transport, localized mRNAs are translationally repressed, with translation only occurring at their final destinations. In order to better understand the mechanisms of regulation used in mRNA localization, I analyzed factors required for the regulation of ASH1 , a localized mRNA in the yeast Saccharomyces cerevisiae . I applied a variety of genome-wide approaches to Loc1, Puf6, and Hek2, three proteins required for the translational regulation of ASH1 mRNA, in order to identify other mRNA targets and to determine the effect these factors have on their targets. These analyses showed that the factors share many genomic binding sites and are recruited to their mRNA targets co-transcriptionally. Further studies with Loc1 demonstrated that its novel mRNA targets are subjected to translational repression. Together these results demonstrate that the factors required for the regulation of localized translation are also responsible for the translational repression of other populations of cellular transcripts. Moreover, my data show that these factors are recruited co-transcriptionally, demonstrating widespread coupling between the processes of transcription and translation in yeast.

The requirement for Loc1 in ribosomal assembly, as well as its shared requirement with certain duplicated ribosomal protein genes in bud-site selection, led me to ask whether Loc1 acts with specific ribosomal proteins to elicit the translational repression of ASH1 mRNA. Studies of ASH1 localization demonstrated that both Loc1 and a subset of duplicated ribosomal protein genes are required for ASH1 to exhibit wild-type association with the bud tip. Further studies of duplicated ribosomal protein genes showed functional divergence among all pairs of duplicated ribosomal protein genes, with paralog-specific effects on cellular mRNA abundances, differential requirements for certain processes, and paralog-specific localizations and assembly requirements. Together these results suggest that different combinations of ribosomal proteins generate ribosomes with unique cellular roles. Finally, computational and statistical analyses of genomic datasets identified a specific subset of ribosomal protein genes that may be subjected to transcriptional regulation by the MBF complex during DNA replication.