Fig. 4. Structure of Pol I EC in the backtracked state.

a Schematic diagram of the transcription scaffold used in assembly of Pol I EC^bt. The dA⁻¹ in template strand was changed to dT⁻¹, generating mismatched base pair of DNA–RNA hybrid. The U⁻¹ and C⁻² of the RNA were cleaved in the assembled EC^bt. b Cryo-EM map of EC^bt shows that the C-ribbon of RPA12 is inserted into the active site. c Structure comparison shows that the cleft of EC^bt (orange) is wider than that of EC^post (gray). d Left panel shows the close-up view of the tip of RPA12 C-ribbon in the active site and its interaction with the gating tyrosine and 3′ end RNA. Residue Y687 in EC^post is shown in sticks and colored in marine. Conformational difference in residue Y687 in the two states is indicated with black arrow. Right panel shows the same view of yPol II EC^bt without TFIIS (gray; PDB: 3GTJ)⁴². Residue Y687 in hPol I EC^post is shown (marine) for comparison. e Comparison of the DNA–RNA hybrid in hPol I EC^bt (orange) and EC^post (gray). f Cryo-EM map and structural model of catalytic center of the Pol I EC^bt. Cryo-EM map is shown in transparent surface.