The Transcriptional Co-activator ADA5 Is Required for HAC1 mRNA Processing in Vivo

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates signaling pathways to induce transcription of a number of genes encoding ER protein chaperones and-folding catalysts. In Saccharomyces cerevisiae this transcriptional induction is mediated by an increase in the synthesis of the transcription factor Hac1p. The transmembrane receptor Ire1p/Ern1p containing a Ser/Thr protein kinase and endoribonuclease activity transmits the unfolded protein response (UPR) from the ER to the nucleus. Activation of Ire1p kinase induces its endoribonuclease activity to cleave unspliced HAC1 mRNA and generate exon fragments that are subsequently ligated by tRNA ligase (RLG1). Whereas unspliced HAC1 mRNA is poorly translated, spliced HAC1 mRNA is efficiently translated. Subunits of the yeast transcriptional co-activator complex SAGA also play a role in the UPR. Deletion of GCN5,ADA2, or ADA3 reduces, and deletion ofADA5 completely abolishes, the UPR. AlthoughHAC1 mRNA requires only Ire1p and Rlg1p in vitro, we demonstrate that ADA5 is required for theIRE1/RLG1-dependent splicing reaction ofHAC1 mRNA in vivo. In addition, Ada5p interacts with Ire1p. These results suggest that subcomponents of transcriptional co-activator complexes may be involved in RNA processing events.