Transcriptional activator involved in the unfolded protein response (UPR) pathway. Recognizes and binds to the UPR element (UPRE) in the promoter of UPR-regulated genes such as KAR2, PDI1, EUG1 and FKB2. Increases the synthesis of endoplasmic reticulum-resident proteins required for protein folding as well as components of the secretory pathway. Homodimer. Splicing occurs by a non-spliceosomal, regulated splicing mechanism. When UPR is induced, the mRNA is cleaved by bifunctional transmembrane kinase/endoribonuclease IRE1 and the exons are joined by tRNA ligase TRL1. By the unfolded protein response pathway. Accumulation of unfolded proteins in the ER leads to activation of IRE1, which initiates splicing of the untranslated HAC1 precursor mRNA to produce the mature form. Present with 8970 molecules/cell in log phase SD medium. Induced and active isoform. Not translated. The unspliced HAC1 mRNA is stable, located in the cytoplasm, and is associated with polyribosomes, yet does not produce protein. Translational attenuation is due to basepairing of the intron with the 5'-UTR of the mRNA. Belongs to the bZIP family.