Does Require Specific Post-Translational Silencing against Leaky Translation of Hac1up?

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 . In response to the accumulation of unfolded proteins in the ER, Ire1p oligomers splice pre-mRNA () via a non-conventional process and allow the spliced () to be translated efficiently. However, leaky splicing and translation of 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 splicing and translation and to facilitate rapid degradation of translated Hac1p and Hac1p. Among proposed regulatory mechanisms is a degron sequence encoded at the 5' end of the intron that silences Hac1p expression. To investigate the necessity of an intron-encoded degron sequence that specifically targets Hac1p for degradation, we employed publicly available transcriptomic data to quantify leaky splicing and translation in UPR-induced and non-UPR cells. As expected, we found that is only efficiently spliced into and efficiently translated into Hac1p in UPR-induced cells. However, our analysis of ribosome profiling data confirmed frequent occurrence of leaky translation of regardless of UPR induction, demonstrating the inability of translation fail-safe to completely inhibit Hac1p production. Additionally, among 32 yeast surveyed, the degron sequence is highly conserved by yeast but is poorly conserved by all other yeast species. Nevertheless, the degron sequence is the most conserved intron segment in yeasts. These results suggest that the degron sequence may indeed play an important role in mitigating the accumulation of Hac1p to prevent accidental UPR activation in the yeast.