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. Increases the synthesis of endoplasmic reticulum-resident proteins required for protein folding as well as components of the secretory pathway (By similarity). Homodimer. Splicing occurs by a non-spliceosomal, regulated splicing mechanism when UPR is induced. By the unfolded protein response pathway. Accumulation of unfolded proteins in the ER leads to splicing of the hac1 precursor mRNA to produce the mature form. Induced and active isoform. Probably not translated. Belongs to the bZIP family.