Figure 4. Two views of a protein binding DNA. A virus-encoded protein, Cro, is shown attached to a region of viral DNA. The panels show only the portion of the protein that interacts with the DNA. Cro consists of two polypeptides, arranged head-to-head (note the symmetry). The DNA is shown as a stick figure cartoon, with red-violet representing cytosine, blue-violet representing guanine, green representing thymine, and cyano representing adenine. White and yellow sticks represent oxygen and phosphate, respectively. In the left panel, the amino acids of the protein use the same color conventions as in Figure 3. In the right panel, only the backbone of the two polypeptide chains are shown, with a-helices in yellow. For each polypeptide, one helix of the helix-turn-helix motif is inserted in a major groove of the DNA.