Pre-catalytic complex for ribonucleotide incorporation (incorporation-ternary). (A) Active site of the ternary complex of Pol λ DL with bound rAMPNPP. Contoured at 3σ, the simulated annealing Fo-Fc omit density map (blue) and the anomalous difference density map (orange) for both Mn2+ ions (purple), are shown. (B) Superposition of Pol λ DL with wt Pol λ. The structure of the ternary complex of Pol λ DL:rAMPNPP (green) is superimposed with ternary wt Pol λ:dUMPNPP complex [PDB ID code 2PFO, wheat, (23)]. The distance between the nucleophile at the primer terminus, 3′-ON and the α-phosphate (α-P) is shown as dashed line. (C) Active site of Pol λ DL:rAMPNPP showing interactions for 2′-OH, 3′-OH, β- and γ-phosphates of the bound nucleotide in black dashed lines. Interactions of Tyr505 and Arg517 with bases at the −1 site are also shown in black dashed lines. α-, β- and γ-phosphates on the nucleotide are indicated as α-P, β-P and γ-P, respectively.