Figure 5.

Use of sequence markers. (a) Excitation scan at the Se K edge from a SeMet-labelled Pol I crystal indicating the presence of Se in the sample. Peaks corresponding to Zn Kβ, Se Kβ and Se Kα are also labelled. (b) The anomalous difference map localizing Se peaks is shown in violet. (c) Anomalous difference map (blue) from a lead-derivative crystal showing the location of the Pol I active site. The primary Mg²⁺ ion is coordinated by a triad of Asp residues in subunit A190. The putative position of the secondary Mg²⁺ ion is shown in purple next to the Asp residues in subunit A135.