Lost in translation: codon optimization inactivates SARS-CoV-2 RdRp

RNA-dependent RNA polymerase (RdRp) is a primary target for antivirals. We report that Nsp12, a catalytic subunit of SARS-CoV-2 RdRp, produces an inactive enzyme when codon-optimized for bacterial expression. We also show that accessory subunits, NTPs, and translation by slow ribosomes partially rescue Nsp12. Our findings have implications for functional studies and identification of novel inhibitors of RdRp and for rational design of other biotechnologically and medically important expression systems.