One group of transcription factors is defined by a basic leucine 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. The in-dividual base pairs are numbered encompassing the G-box from -4 to +4 (numbering from 5' to 3'). The DNA binding specificity of GBF1 is strongly influenced by the nature of the nucleotides (positions -4, -3, +3, and +4) flanking the ACGT core sequence. For example, sequences that carry a TG at positions -4 and -3 and a T or G residue at position +4 are as efficiently bound by GBFl as the palindromic G-box.