Superposition of the all-DNA structure (magenta) with the structure containing ribonucleotides at 0 and −1 position (green). (A) Schematic indicating the positions of the two ribonucleotides. The red arrow indicates that incorporation is problematic here (Fig. 1D3, 3′-rG). (B) Superposition of template nucleotides at positions 0 to −3, Tyr391, and Tyr567. A red sphere indicates the position of an additional water molecule. (C) Simulated annealing Fo-Fc maps contoured at 3 σ for the ribonucleotides at template positions 0 and −1. (D) Overlay with the all-DNA structure, showing the ribonucleotide at the −1 position, as well as Tyr391, Tyr567, and the additional water molecule. (E) Overlay of the ribonucleotides at positions 0 and −1 with the all-DNA structure.