The fail-safe mechanism of post-transcriptional silencing of unspliced HAC1 mRNA

encodes a transcription factor that is the central effector of the unfolded protein response (UPR) in budding yeast. When the UPR is inactive, mRNA is stored as an unspliced isoform in the cytoplasm and no Hac1 protein is detectable. Intron removal is both necessary and sufficient to relieve the post-transcriptional silencing of mRNA, yet the precise mechanism by which the intron prevents Hac1 protein accumulation has remained elusive. Here, we show that a combination of inhibited translation initiation and accelerated protein degradation-both dependent on the intron-prevents the accumulation of Hac1 protein when the UPR is inactive. Functionally, both components of this fail-safe silencing mechanism are required to prevent ectopic production of Hac1 protein and concomitant activation of the UPR. Our results provide a mechanistic understanding of regulation and reveal a novel strategy for complete post-transcriptional silencing of a cytoplasmic mRNA.