Hypomodified tRNA in evolutionarily distant yeasts can trigger rapid tRNA decay to activate the general amino acid control response, but with different consequences

Abstract

All tRNAs are extensively modified, and modification deficiency often results in growth defects in the budding yeast Saccharomyces cerevisiae and neurological or other disorders in humans. In S. cerevisiae, lack of any of several tRNA body modifications results in rapid tRNA decay (RTD) of certain mature tRNAs by the 5-3 exonucleases Rat1 and Xrn1. As tRNA quality control decay mechanisms are not extensively studied in other eukaryotes, we studied trm8{Delta} mutants in the evolutionarily distant fission yeast Schizosaccharomyces pombe, which lack 7-methylguanosine at G46 of tRNAs. We report here that S. pombe trm8{Delta} mutants are temperature sensitive primarily due to decay of tRNATyr(GUA) and that spontaneous mutations in the RAT1 ortholog dhp1+ restored temperature resistance and prevented tRNA decay, demonstrating conservation of the RTD pathway. We also report for the first time evidence linking the RTD and the general amino acid control (GAAC) pathways, which we show in both S. pombe and S. cerevisiae. In S. pombe trm8{Delta} mutants, spontaneous GAAC mutations restored temperature resistance and tRNA levels, and the temperature sensitivity of trm8{Delta} mutants was precisely linked to GAAC activation due to tRNATyr(GUA) decay. Similarly, in the well-studied S. cerevisiae trm8{Delta} trm4{Delta} RTD mutant, temperature sensitivity was closely linked to GAAC activation due to tRNAVal(AAC) decay; however, in S. cerevisiae, GAAC mutations increased tRNA decay and enhanced temperature sensitivity. Thus, these results demonstrate a conserved GAAC activation coincident with RTD in S. pombe and S. cerevisiae, but an opposite impact of the GAAC response in the two organisms. We speculate that the RTD pathway and its regulation of the GAAC pathway is widely conserved in eukaryotes, extending to other mutants affecting tRNA body modifications. Author SummarytRNA modifications are highly conserved and their lack frequently results in growth defects in the yeast Saccharomyces cerevisiae and neuorological disorders in humans. S. cerevsiaie has two tRNA quality control decay pathways that sense tRNAs lacking modifications in the main tRNA body. One of these, the rapid tRNA decay (RTD) pathway, targets mature tRNAs for 5-3 exonucleolytic decay by Rat1 and Xrn1. It is unknown if RTD is conserved in eukaryotes, and if it might explain phenotypes associated with body modification defects. Here we focus on trm8{Delta} mutants, lacking m7G46, in the evolutionarily distant yeast Schizosaccharomyces pombe. Loss of m7G causes temperature sensitivity and RTD in S. cerevisiae, microcephalic primordial dwarfism in humans, and defective stem cell renewal in mice. We show that S. pombe trm8{Delta} mutants are temperature sensitive due to tY(GUA) decay by Rat1, implying conservation of RTD among divergent eukaryotes. We also show that the onset of RTD triggers activation of the general amino acid control (GAAC) pathway in both S. pombe and S. cerevisiae, resulting in exacerbated decay in S. pombe and reduced decay in S. cerevisiae. We speculate that RTD and its regulation of the GAAC pathway will be widely conserved in eukaryotes including humans.