NUCLEAR ACCUMULATION OF MATURE tRNA: CHARACTERIZATION OF GENETIC FACTORS AND NUTRIENTS THAT AFFECT tRNA CELLULAR DISTRIBUTION
Open Access
- Author:
- Whitney, Michael L
- Graduate Program:
- Genetics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- March 06, 2007
- Committee Members:
- Ralph Lauren Keil, Committee Chair/Co-Chair
Anita Klein Hopper, Committee Member
Scot R Kimball, Committee Member
David Joseph Spector, Committee Member
Maricarmen Planas Silva, Committee Member - Keywords:
- tRNA nuclear import
transfer RNA
: glucose starvation
yeast - Abstract:
- Nuclear export of tRNA is an essential process and long believed to be a unidirectional event. However, recent evidence indicates that mature, cytoplasmic tRNA accumulates in the nucleus of Saccharomyces cerevisiae in response to amino acid starvation or when nuclear export of tRNA is defective. The mechanism and regulation by which tRNA returns to the nucleus is unclear. To investigate whether this phenomenon occurs as a general response to nutrient starvation we deprived cells of glucose. Employing fluorescence in situ hybridization (FISH) we were able to assay the localization of tRNA. During glucose starvation, cytosolic tRNAs accumulated in the nucleus. The accrual of mature tRNA in the nucleus was rapid (within 10 minutes) in response to nutrient starvation and completely reversible in an equally prompt manner. This process did not require new transcription and was independent of the aminoacylation status of tRNA. Investigation into the signal transduction pathways responsible for altering the cellular localization of tRNA revealed the TOR pathway is required for nuclear accumulation of tRNA in response to amino acid starvation, but not glucose starvation. Cells with constitutively low levels of cAMP–dependent protein kinase (PKA) activity did not accumulate tRNA in the nucleus in response to amino acid or glucose deprivation. As reported with amino acid starvation, the karyopherin Mtr10 was also required for nuclear accumulation of tRNA in response to glucose starvation. Examination of the mechanism by which various multi-copy suppressors are able to suppress defects in tRNA export revealed novel factors involved in cellular distribution of mature tRNA. SOL1 and SOL2 were originally identified as multi-copy suppressors of the loss of tRNA-mediated nonsense suppression phenotype of a mutation in a known tRNA exporter, LOS1. We show that SOL1 and SOL2 did not restore tRNA nuclear export or processing in los1 cells. However, SOL1 and SOL2 are required for normal cellular distribution of mature tRNA and tRNA-mediated nonsense suppression. Similarly, SBP1 was found to suppress the nuclear export defect of a mutation in the tyrosyl-tRNA synthetase gene (TYS1). However, SBP1 did not affect tRNA charging or processing, but was required for proper nucleo-cytoplasmic distribution of mature tRNA in fed cells. Together these findings define nutrients and novel factors involved in the cellular localization of tRNA. Regulation of the cellular location of tRNA may provide the cell with a novel mechanism to rapidly and reversibly alter gene expression.