X-ray structure of the GCN4 bZIP region in complex with its target DNA. The complex is viewed with its 2-fold axis vertical. The DNA, which is drawn in stick form with C magenta, N blue, O red, and P orange and with successive P atoms connected by orange rods, consists of a 19-bp segment with a single nucleotide overhang at each end and contains the protein's palindromic (except for the central base pair) 7-bp target sequence. The two identical protein subunits, shown in ribbon form, each contain a continuous 52-residue α helix. The C-terminal segments (yellow) of the two subunits associate in a parallel coiled coil (a leucine zipper), and their basic regions (green) smoothly diverge to each engage the DNA in its major groove at the target sequence half a helical turn apart. Click on the Show "Side Chains" button to see the Arg and Leu side chains in the basic regions (blue) and Leu and Val side chains in the C-terminal segments (brown). [Based on an X-ray structure by Stephen Harrison, Harvard University. PDBid 1YSA.]