Translational attenuation mediated by an mRNA intron.

The unfolded protein response (UPR) is an intracellular signaling pathway that is activated by the accumulation of unfolded proteins in the endoplasmic reticulum (ER). The UPR results in an increase in transcription of ER-resident proteins that facilitate protein folding in the ER. A key regulatory step in UPR activation is the regulated splicing of HAC1 mRNA, which encodes Hac1p, a transcription factor dedicated to this pathway. Hac1p can be detected only when the spliced form of HAC1 mRNA (termed HAC1i mRNA, for induced) is produced; this was surprising because the unspliced HAC1u mRNA (HAC1u for uninduced) is equally stable in cells.