One group of transcription factors is defined by a basidleucine zipper (bZIP) motif. This bipartite DNA binding structure consists of a region enriched in basic amino acids (basic region) adjacent to a leucine zipper that is characterized by several leucine residues regularly spaced at seven-amino acid intervals. Whereas the basic region directly contacts the DNA, the leucine zipper mediates homodimerization and heterodimerization of protein monomers through a parallel interaction of the hydrophobic dimerization interfaces of two a-helices, resulting in a coiled-coil structure. The Arabidopsis bZlP family of G-box binding factors (GBF1,GBF2, and GBF3) interact with the palindromic G-box motif(CCACGTGG) found in several plant promoters.