Figure 3. Assembly and stability of the SARS-CoV-2 polymerase-helicase complex.

(A) Dwell-time distributions extracted from RTC elongation traces on a dsRNA template with 20 pN tension, which were obtained at the indicated nsp13-helicase concentrations (circles) and their respective dwell-time fits (solid lines).

(B) The dwell-time fit function consists of five probability density functions (PDFs): very fast, fast, slow, and very slow nucleotide addition (VFNA, FNA, SNA, and VSNA, respectively) pathways and long-lived pauses (LLPs). These PDFs are combined and fitted (black line) on the dwell-time distribution (circles). The insets at the top are snapshots of representative parts of the traces.

(C and D) Characteristic timescales and probabilities of the different NA pathways and LLP state versus nsp13-helicase concentration.

The circles with error bars in (A)–(D) and the solid lines with shaded areas in (C) and (D) represent the mean ± SEM of the individual fits and the RTC assembly-elongation dynamics model fits, respectively. VFNA and VSNA characteristic timescales were fixed to 0.2 and 4.8 s.

(E) Schematic of the RTC assembly model.

(F and G) The extracted free energy landscape (F) and corresponding fractional occupancies (G) versus nsp13-helicase concentration fitted on the dwell-time distributions with shaded areas as error bars, as in (C) and (D).

See also Figures S2E, S2F, S2K, S2L, S2Q, S2R, S3E–S3G, and S5J–S5M.