The transcription factor CHOP/GADD153 gene is induced by cellular stress and is involved in mediating apoptosis. We report the identification of a conserved region in the promoter of the CHOP gene responsible for its inducibility by endoplasmic reticulum (ER) stress. Deletion mutants of the human CHOP promoter identify a region comprising nucleotides -75 to -104 required for both constitutive and ER-stress-inducible expression. This region of the promoter, the ER-stress element (ERSE) is sufficient to confer both increased basal activity and ER-stress inducibility to an otherwise inactive heterologous promoter. The CHOP ERSE is a novel variant of the ERSE as it contains two different functional domains, and a GA- instead of GC-rich intervening sequence. The CCAAT-box domain occupied by the constitutive transcriptional activator nuclear factor Y (NFY) is required for constitutive activation whereas the variant GCACG 'inducible' domain uniquely mediates ER-stress inducibility. By UV-crosslinking analysis NFY makes contact not only with the constitutive activator CCAAT box but also with the inducible GCACG domain. Deletions and nucleotide substitutions in the CCAAT box as well as its replacement by an SP1 site failed to support ER inducibility. These findings support the notion that NFY is not only required for constitutive activation of CHOP gene transcription, but is also an active and essential element for the assembly of an ER-stress-inducible enhanceosome that activates CHOP gene expression in response to cellular stress.