CoV-er all the bases: Structural perspectives of SARS-CoV-2 RNA synthesis

The ongoing Covid-19 pandemic has spurred research in the biology of the nidovirus severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). Much focus has been on the viral RNA synthesis machinery due to its fundamental role in viral propagation. The central and essential enzyme of the RNA synthesis process, the RNA-dependent RNA polymerase (RdRp), functions in conjunction with a coterie of viral-encoded enzymes that mediate crucial nucleic acid transactions. Some of these enzymes share common features with other RNA viruses, while others play roles unique to nidoviruses or CoVs. The RdRps are proven targets for viral pathogens, and many of the other nucleic acid processing enzymes are promising targets. The purpose of this review is to summarize recent advances in our understanding of the mechanisms of RNA synthesis in CoVs. By reflecting on these studies, we hope to emphasize the remaining gaps in our knowledge. The recent onslaught of structural information related to SARS-CoV-2 RNA synthesis, in combination with previous structural, genetic and biochemical studies, have vastly improved our understanding of how CoVs replicate and process their genomic RNA. Structural biology not only provides a blueprint for understanding the function of the enzymes and cofactors in molecular detail, but also provides a basis for drug design and optimization. The concerted efforts of researchers around the world, in combination with the renewed urgency toward understanding this deadly family of viruses, may eventually yield new and improved antivirals that provide relief to the current global devastation.