The "cbh2 activating element," CAE,, consisting of two separate boxes (ATTGG = CCAAT and GTAATA, respectively) is essential for cellobiohydrolase ii gene expression in the filamentous fungus Hypcrea jecorina. Here we report that cell-free extracts from a cellulase-negative mutant form CAE-protein complexes with higher mobility and lower binding-strength compared to the wild type. EMSA analysis demonstrated an increased mobility of the GTAATA-binding protein complex and, supported by in vivo footprinting, a lowered binding strength of the HAP2/3/5 proteins. However, the hap2/hap3/hap5 genes of the mutant are unaltered and transcribed normally. A nucleotide fragment of the cbh1 promoter containing a (GG)CTAATA motif without an adjacent CCAAT box is also bound by cell-free extracts of Ii. jecorina, and the protein-DNA complex of the mutant shows the characteristic increase in mobility. We conclude that this mutant is defective in the functional formation of the CAE-protein complexes but not in their binding to the target sequences itself.