| Summary: | <i>HAC1</i> encodes a key transcription factor that transmits the unfolded protein response (UPR) from the endoplasmic reticulum (ER) to the nucleus and regulates downstream UPR genes in <i>Saccharomyces cerevisiae</i>. In response to the accumulation of unfolded proteins in the ER, Ire1p oligomers splice <i>HAC1</i> pre-mRNA (<i>HAC1</i><sup>u</sup>) via a non-conventional process and allow the spliced <i>HAC1</i> (<i>HAC1<sup>i</sup></i>) to be translated efficiently. However, leaky splicing and translation of <i>HAC1</i><sup>u</sup> may occur in non-UPR cells to induce undesirable UPR. To control accidental UPR activation, multiple fail-safe mechanisms have been proposed to prevent leaky <i>HAC1</i> splicing and translation and to facilitate rapid degradation of translated Hac1<sup>u</sup>p and Hac1<sup>i</sup>p. Among proposed regulatory mechanisms is a degron sequence encoded at the 5′ end of the <i>HAC1</i> intron that silences Hac1<sup>u</sup>p expression. To investigate the necessity of an intron-encoded degron sequence that specifically targets Hac1<sup>u</sup>p for degradation, we employed publicly available transcriptomic data to quantify leaky <i>HAC1</i> splicing and translation in UPR-induced and non-UPR cells. As expected, we found that <i>HAC1<sup>u</sup></i> is only efficiently spliced into <i>HAC1<sup>i</sup></i> and efficiently translated into Hac1<sup>i</sup>p in UPR-induced cells. However, our analysis of ribosome profiling data confirmed frequent occurrence of leaky translation of <i>HAC1<sup>u</sup></i> regardless of UPR induction, demonstrating the inability of translation fail-safe to completely inhibit Hac1<sup>u</sup>p production. Additionally, among 32 yeast <i>HAC1</i> surveyed, the degron sequence is highly conserved by <i>Saccharomyces</i> yeast but is poorly conserved by all other yeast species. Nevertheless, the degron sequence is the most conserved <i>HAC1</i> intron segment in yeasts. These results suggest that the degron sequence may indeed play an important role in mitigating the accumulation of Hac1<sup>u</sup>p to prevent accidental UPR activation in the <i>Saccharomyces</i> yeast.
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