Transcriptional activator that acts in the unfolded protein response (UPR) pathway. Acts during endoplasmic reticulum (ER) stress by activating UPR target genes via direct binding to the UPR element (UPRE) (5'-GGAACTGGACAGCGTGTCGAAA-3'). Activates expression of ER chaperones ERP72 and PDI. Binds DNA as a dimer. By ER stress-inducing agents tunicamycin, DTT, the calcium ionophore A23187, antimycin, monensin and H(2)O(2). Belongs to the bZIP family.