tRNA family-specific, stress-specific changes in tRNA intron turnover
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Date
2020-05
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The Ohio State University
Abstract
Transfer ribonucleic acids (tRNAs) are abundant molecules, comprising ~15% of cellular RNAs. Although the major biological role for tRNAs is to bring amino acids to the ribosome during protein synthesis, they also play many secondary roles. Defects in pre-tRNA biogenesis and processing cause numerous disorders, from neurodegenerative diseases to cancer. In eukaryotes, a subset of tRNA-encoding genes contain non-coding introns that must be removed in post-transcriptional tRNA processing; in S. cerevisiae, these account for 20% of pre-tRNAs. Through an unbiased screen of the yeast genome, my lab identified two proteins required for the turnover of the intron derived from tRNAIleUAU, the tRNA employed in the screen (Wu et. al. 2015). Wu and Hopper showed that the free tRNAIleUAU intron is first phosphorylated on the 5' end by the tRNA ligase/kinase Rlg1, then degraded in the 5' to 3' direction by the exonuclease Xrn1. Rlg1 also ligates the mature tRNA halves (Wu and Hopper 2014). For the remaining intron-containing tRNAs, intron turnover is only Rlg1-dependent, only Xrn1-dependent, or neither dependent on Rlg1 nor Xrn1 (Bao and Hopper, unpublished data). The existence of multiple pathways for intron turnover provides evidence that this is an important process. Taken with the conservation of tRNA introns from Archaea to humans, there seems to be an evolutionary selection for the presence of them in cells. If so, levels of tRNA introns may be regulated in response to various environmental conditions as a mechanism to handle cellular stress. I have tested the effects of various environmental stress conditions including oxidative stress, glucose deprivation, and heat shock on tRNA intron levels in yeast, as assessed by Northern blot analysis. My data show that the free tRNATrpCCA intron accumulates immediately and significantly upon exposure to hydrogen peroxide stress. Likewise, I observe that tRNAProUGG accumulates immediately upon exposure to 42°C heat shock. Conversely, I report that the free tRNAIleUAU intron is degraded more rapidly when yeast cells are starved for glucose. My data suggests that the accumulation or accelerated degradation of free tRNA introns is specific to the family of free tRNA intron, as well as specific to the stress condition, suggesting that tRNA introns may play an important role in the cellular response to stress.
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Keywords
tRNA, introns, turnover, stress