(D–F) Ribbon diagrams for the complete cryo-EM structures. Shown are (D) the A/H7N9 TRX structure stalled by CMPcPP, (E) T1106-stalled and backtracked A/H7N9 structure, and (F) T1106-stalled and backtracked FluB structure. The polymerase color code is as follows: PA endonuclease (forest green), PA-C-terminal domain (green), PB1 (cyan), PB2-N (red), PB2-midlink domain (magenta), PB2-cap-binding domain (orange), PB2-627 domain (deep pink), and PB2-NLS domain (firebrick). The RNA is colored as in (A)–(C), with (D) the CMPcPP shown in red and (E and F) the observed backtracked nucleotides shown in purple-blue.
(G–I) Determined structures of the RNA moieties. Shown are (G) the A/H7N9 TRX structure stalled by CMPcPP (red) with template (orange), 18-mer capped product (slate blue), and 5′ end (violet); (H) T1106-stalled and backtracked A/H7N9 structure with singly incorporated T1106 (magenta), template (pale orange), 21-mer capped product (pale blue), and 5′ end (violet); and (I) the T1106-stalled and backtracked FluB structure with doubly incorporated T1106 (magenta), template (yellow), 21-mer capped product (blue), and 5′ end (violet). In each case, the conformation of the priming loop residues 632–660 in A/H7N9 (631–659 in FluB) are shown in cyan, fully extruded in (G) and (I), partially extruded in (H), and with the position of Tyr657 (Tyr656 in FluB) highlighted.
Stephen Cusack Research Group
The antiviral pseudo-base T705 and its de-fluoro analog T1106 mimic adenine or guanine and can be competitively incorporated into nascent RNA by viral RNA-dependent RNA polymerases. Although dispersed, single pseudo-base incorporation is mutagenic, consecutive incorporation causes polymerase stalling and chain termination. Using a template encoding single and then consecutive T1106 incorporation four nucleotides later, we obtained a cryogenic electron microscopy structure of stalled influenza A/H7N9 polymerase. This shows that the entire product-template duplex backtracks by 5 nt, bringing the singly incorporated T1106 to the +1 position, where it forms an unexpected T1106:U wobble base pair. Similar structures show that influenza B polymerase also backtracks after consecutive T1106 incorporation, regardless of whether prior single incorporation has occurred. These results give insight into the unusual mechanism of chain termination by pyrazinecarboxamide base analogs. Consecutive incorporation destabilizes the proximal end of the product-template duplex, promoting irreversible backtracking to a more energetically favourable overall configuration.
Kouba, T., et.al.: Direct observation of backtracking by influenza A and B polymerases upon consecutive incorporation of the nucleoside analog T1106 Cell Rep. 2023, 42, 111901, https://doi.org/10.1016/j.celrep.2022.111901