Materials: Bz-m 3 C phosphoramidite monomer for the synthesis of RNA 1g was synthesized using reported procedures with modifications (supporting information). RP HPLC purified RNA 1a was purchased from IDT. Reagents for RNA synthesis and cartridges for RNA purification (Glen-Pak™ RNA purification cartridge and Glen Gel-Pak™ 2.5 Desalting Column) were purchased from Glen Research. SC2RC (RdRp/NSP7/NSP8 SARS-CoV-2 Complex) was purchased from BPS Bioscience. NTPs were purchased from Fisher Scientific. RNA loading dye was purchased from NEB. 10% Mini-PROTEAN ® TBE-Urea Gel and Mini-PROTEAN Tetra Cell were purchased from Bio-Rad. GelRed was purchased from Biotium.