Analysis of transcripts of 75 genes encoding putative basic leucine zipper (bZIP) transcription factors in the Arabidopsis genome identified AtbZIP60 , which was induced by tunicamycin. AtbZIP60 encodes a predicted protein of 295 aa with a putative transmembrane domain near its C terminus after a bZIP domain. A truncated form of AtbZIP60 without a transmembrane domain (AtbZIP60ΔC) fused with GFP localized to the nucleus, suggesting translocation of native protein to the nucleus by release from the membrane. AtbZIP60 was also induced by DTT and azetidine-2-carboxylate, which induce the endoplasmic reticulum (ER) stress response (also called the unfolded protein response). Expression of AtbZIP60ΔC clearly activated any of three BiP and two calnexin promoters in a dual luciferase assay using protoplasts of cultured cells. The induction was considered to be through cis-elements plant-specific unfolded protein response element and ER stress-response element. Interestingly, AtbZIP60ΔC also appeared to induce the expression of AtbZIP60 through an ER stress-response element-like sequence in the promoter of AtbZIP60 . These characteristics of AtbZIP60 imply a signal transduction pathway of the ER stress response unique to plants.