Fig. 1.

SARS-CoV-2 backtrack complex. (A) RNA scaffolds: (Top) RTC scaffold (14); (Bottom) backtrack complex scaffolds (BTC₃ and BTC₅). (B) A native gel electrophoretic mobility shift assay reveals that holo-RdRp requires nsp13(ADP-AlF₃) to bind the BTC scaffolds efficiently. (C) Cryo-EM structures of SARS-CoV-2 BTCs. Shown is the transparent cryo-EM density [local-resolution-filtered (47)] with the refined models superimposed (SI Appendix, Table S1). The models and density are colored according to the key. Two major BTCs were observed (SI Appendix, Fig. S2), one containing one nsp13 protomer (nsp13₁-BTC₅), and one containing two nsp13 promoters (nsp13₂-BTC₅). We designate the nsp13 promoter common to both structures nsp13.1 and the other nsp13.2 (14). The cyan spheres denote the path of the single-stranded t-RNA 5′ segment, some of which is engaged with nsp13.1 in both structures.