Polyadenylation dependent nuclear RNA surveillance in Saccharomyces cerevisiae
Abstract
Cells are composed of several RNAs such as messenger RNA (mRNA), transfer RNA (tRNA), small nuclear and small nucleolar RNAs (sn and snoRNAs) and ribosomal RNAs (rRNAs) that perform various cellular activities such as protein synthesis. tRNA serves as an adaptor in protein synthesis as it transfers a specific amino acid to the site of protein synthesis. A newly synthesized tRNA consists of a 5' leader and 3' trailor which are removed to generate mature tRNA. During 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. Thus 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 one of which is Methylation of adenosine at position 58 of tRNA catalyzed by the enzyme complex Trm6p/Trm61p in yeast, Saccharomyces cereviseae . In TRM6 and TRM61 mutants, tRNAs lack m1 A modification and this results in the degradation of a unique tRNA, Initiator tRNAMet (tRNA iMet ). 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, the exosome and TRF4 , a member of polβ nucleotidyl transferase family of proteins are required for the degradation of hypomethylated tRNAiMet which occurs in the nucleus. The in vivo substrate for polyadenylation and degradation is the precursor form of hypomethylated tRNAi Met which possesses the unprocessed 5' and 3' extensions, demonstrating that the integrity of tRNA is tested early during its biogenesis. Interestingly, Trf5p, homolog of Trf4p is required for efficient degradation of tRNA iMet . Nuclear tRNA surveillance is a global mechanism to degrade aberrant RNAs as mutant U6 SnRNA and 5S rRNA are polyadenylated and degraded by Trf4p and the exosome similar to hypomethylated tRNA iMet . This study has initiated a new area of study, RNA surveillance that involves polyadenylation by Trf4p and degradation by the exosome.
This paper has been withdrawn.