BTEB is a small GC box-binding protein containing three contiguous zinc finger structures in the C-terminal region of the molecule, and activates or represses the transcription of genes with the GC box sequence in the promoter, depending on the repetitiveness of the GC box sequence [Imataka et al. (1992) EMBO J. 11, 3663-3671]. We have analyzed functional domains of BTEB in a transient expression system using Y-1 cells (a mouse adrenal cortex cell line). BTEB contained two regions responsible for the transcriptional activation. These regions showed a sequence similarity to each other and appeared to enhance the transcription independently. The two sequences were rich in hydrophobic amino acids and showed no similarity to the sequences of transactivation domains so far elucidated. The DNA-binding properties of BTEB were also analyzed by means of the gel mobility shift assay. Three contiguous zinc finger motifs were needed for the binding activity. Furthermore, it was revealed that a short basic region immediately N-terminal to the zinc finger motifs was required for the DNA binding.