Date of Award

Spring 2006

Document Type

Dissertation - Restricted

Degree Name

Doctor of Philosophy (PhD)


Biological Sciences

First Advisor

Anderson, James T.

Second Advisor

Karrer, Kathleen

Third Advisor

Stuart, Rosemary


Cells are composed of several types of RNAs such as messenger RNA (mRNA), transfer RNA (tRNA), small nuclear and small nucleolar RNAs (sn and snoRNAs) and ribosomal RN As (rRNAs) that perform various cellular activities like protein synthesis. tRNA serves as an adaptor in protein synthesis by transferring a specific amino acid to the site of protein synthesis. A newly synthesized tRNA consists of a 5' leader and 3' trailer which are removed during processing to generate mature tRNA. During its biogenesis, aberrant tRNAs might be produced due to processing errors and if not degraded they might get incorporated in to the protein synthesis machinery forming abnormal proteins. Hence degradation mechanisms for RNAs have evolved to ensure normal functioning of cells. One of the striking features of a tRNA molecule is the presence of numerous post transcriptional modifications. Methylation of adenosine at position 58 is a post transcriptional modification in tRNA that is catalyzed by the enzyme complex Trm6p/Trm6lp in yeast, Saccharomyces cerevisiae. In TRM6 and TRM61 mutants, tRNAs lack m1A modification and this renders a unique tRNA, Initiator tRNA Met (tRNAiMet) unstable and targets it for degradation. I have characterized the molecular details of this novel degradation pathway of hypomethylated tRNAiMet. Genetic suppressor analysis of a trm6-504 mutant showed that RRP44, a subunit of a multiprotein RNA processing and degradation complex, exosome and TRF4, a member of nucleotidyltransferase family of proteins are required for the degradation of hypomethylated tRNAiMet. Deletion of another exosome subunit gene, RRP6, impaired degradation ofhypomethylated tRNAiMet proving strong evidence that the exosome is required for the degradation of tRNAte'. As Rrp6p is associated only with the nuclear form of the exosome, I conclude that bulk of the degradation ofhypomethylated tRNAiMet in trm6-504 cells occurs in the nucleus...



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