a, X-ray electron density for the primer-template duplex RNA in the crystal structure of the mixed initiation state with 15-mer capped primer. Fo-Fc omit map contoured at 3 σ. b, c and d, CryoEM density for the 5′ vRNA hook and primer-product-template duplex. Fit of the 5′ vRNA hook A1-G12 (b) and product-template duplex (c) into the cryoEM density of the elongation state filtered at 3.2 Å resolution. d, Verification of base pair identity of the product-template duplex using the cryoEM density. e, CTP electron density. Fo − Fc omit map electron density, contoured at 3σ, for the soaked CTP in the crystal structure of the mixed initiation state with the 14-mer capped primer.