Fig. 4

The PHP domain has a trinuclear zinc center. a X-ray emission scan of a DnaE1 crystal showing two distinct peaks at the sulfur and zinc emission energy. b ICP-OES analysis of purified DnaE1 protein confirms that only zinc is bound by the protein, in both wild-type and the catalytic mutant D226N. c, d Anomalous difference density maps before and after the zinc absorption edge uniquely identify all three metals in the PHP active as zinc. e Primer extension assays of a matched (AT) or mismatched (GT) DNA substrate reveals that the Mtb DnaE1 PHP-exonuclease is inhibited by magnesium. f In contrast, the E. coli exonuclease ε is activated by magnesium. The graphs on the right of the gels show the averaged amount of extended DNA from three independent experiments. Error bars indicate standard deviation. Uncropped gels are shown in Supplementary Fig. 8