Figure 2.

(a) Salt plots for Drosophila Antennapedia (Antp) and its truncated form (desAntp) lacking the N-terminal tail (–RKRGRQ–) that binds in the minor groove. Identical slopes indicate that the same number of counterions are displaced, i.e. the N-tail does not make electrostatic contacts with the DNA (as seen in the structure of the complex). (b) Two salt plots for the Engrailed homeodomain binding to its target DNA sequence, one using KCl (blue), the other using KF (red). The reduced slope using fluoride shows that fewer counterions are displaced than in chloride solution. Data taken from Ref. (13).