Figure 6.

DNA synthesis reaction does not occur in the presence of dTMPPnP. Omit F_o – F_c maps are displayed at a contour of 3.3 for the PolIV_DNA:dTMPPnP complex before (A) and 60 min after the addition of Mg²⁺ (B). The electron density maps show that although the 3′-OH is activated, the DNA synthesis reaction does not proceed further. These observations are in line with the inference that cleavage of the pyrophosphate moiety is critical for the irreversible completion of the dNTP incorporation reaction.