You would expect to see DNA binding proteins with the ability to bind to the outside of the DNA double helix, the phosphate backbone. Since the backbone is negatively charged, the amino acids involved in binding should have positively charged R groups, amino acids such as lysine.
Work Step by Step
The key here is to remember that the DNA double helix has a strong negative charge outside, and recall that opposite charges attract. Then, review the amino acids and see which have an opposite, positive, charge on their R groups.