Transcription factor (By similarity). Plays a role in the unfolded protein response (UPR), perhaps mainly during constitutive endoplasmic reticulum (ER) stress, by activating transcription of genes involved in the UPR (PubMed:16184190). Plays a role in modulating lifespan, acting by positively regulating expression of calcium-binding chaperone crt-1, thereby influencing ER calcium homeostasis (PubMed:25186652, PubMed:32905769). By activating the UPR pathway, confers adaptive protection to subsequent exposure to hypoxia (PubMed:20733002). Involved in protection against proteotoxicity, probably acting via the UPR (PubMed:31570707). Probably acts in the UPR in parallel with the ire-1-xbp-1 and pek-1 pathways (PubMed:16184190). May be regulated by endopeptidase S2P-mediated proteolytic cleavage (PubMed:16184190). Expression is repressed by micro-RNA mir-124 (at protein level). RNAi-mediated knockdown causes lifespan extension (PubMed:25186652). Knockdown causes no obvious other phenotypes, but in an ire-1 mutant background causes sluggish movement, arrested development at the L2 larval stage, and lethality soon thereafter; larvae have intestinal degeneration and develop many vacuoles in the intestinal cells (PubMed:16184190). In combination with RNAi-mediated knockdown of xbp-1, causes lethality early in larval development (PubMed:16184190). Belongs to the bZIP family. ATF subfamily.