Figure 4.

The 3′ exonuclease active site of the D355A/E357A mt-KF fragment with the bound oligo(dT). Only the three 3′ terminal residues are defined in the electron density maps. The enzyme is shown in a surface representation, the d(T)₃ fragment is shown as a stick model in yellow, and atoms of phosphate groups are colored red and magenta for oxygen and phosphorus, respectively. Protein side chains involved in the binding of Zn²⁺ (site A) and Mg²⁺ (site B) are highlighted.