Figure 24-33a

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X-ray structure of the 69-residue N-terminal domain of 434 phage repressor in complex with a 20-bp dsDNA containing its target sequence. The complex is viewed with the homodimeric protein's 2-fold axis horizontal. The protein is drawn in ribbon form with one of its two identical subunits blue and the other pink and with their helix–turn–helix (HTH) motifs more deeply colored. The DNA is drawn in stick form with C gray, N blue, O red, and P orange, and with successive P atoms in the same chain connected by orange rods. Click the "Show HTH Motif" button to more closely view the interactions between the HTH motif and the DNA. Here the HTH motif is shown in worm form with its helices dark blue and its turn light blue, and the HTH side chains that interact with the DNA are drawn in stick form with C cyan, N blue, and O red. Clicking on the "Show HTH Motif" button a second time shows the hydrogen bonds between the HTH motif and the DNA (dashed black lines) together with their lengths. Based on an X-ray structure by Aneel Aggarwal, John Anderson, and Stephen Harrison, Harvard University. PDBid 2OR1.]