The bZIP DNA-binding proteins are characterized by a 50-amino-acid DNA binding and dimerization motif, consisting of a highly basic DNA-binding region ('b') followed by a leucine zipper dimerization region ('ZIP'). The best characterized bZIP DNA-binding protein is GCN4, a yeast transcriptional activator. GCN4 binds to a 9-base-pair two-fold-symmetric DNA site, 5'-A-4T-3G-2A-1C0T+1C+2A+3T+4-3' (refs 7-10). A detailed model known as the 'induced helical fork' model has been proposed for the structure of the GCN4-DNA complex. Using a site-specific bromouracil-mediated photocrosslinking method, we show here that the alanine at position 238 of GCN4 contacts, or is close to, the thymine 5-methyl of A.T at position +3 of the DNA site in the GCN4-DNA complex. Our results strongly support the induced helical fork model. Our site-specific bromouracil-mediated photocrosslinking method requires no prior information regarding the structure of the protein or the structure of the protein-DNA complex and should be generalizable to DNA-binding proteins that interact with the DNA major groove.