Pervasive generation of non-canonical subgenomic RNAs by SARS-CoV-2

BackgroundSARS-CoV-2, a positive-sense RNA virus in the family Coronaviridae, has caused a worldwide pandemic of coronavirus disease 2019 or COVID-19 Coronaviruses generate a tiered series of subgenomic RNAs (sgRNAs) through a process involving homology between transcriptional regulatory sequences (TRS) located after the leader sequence in the 5 UTR (the TRS-L) and TRS located near the start of structural and accessory proteins (TRS-B) near the 3 end of the genome. In addition to the canonical sgRNAs generated by SARS-CoV-2, non-canonical sgRNAs (nc-sgRNAs) have been reported. However, the consistency of these nc-sgRNAs across viral isolates and infection conditions is unknown. The comprehensive definition of SARS-CoV-2 RNA products is a key step in understanding SARS-CoV-2 pathogenesis. MethodsHere, we report an integrative analysis of eight independent SARS-CoV-2 transcriptomes generated using three sequencing strategies, five host systems, and seven viral isolates. Read-mapping to the SARS-CoV-2 genome was used to determine the 5 and 3 coordinates of all identified junctions in viral RNAs identified in these samples. ResultsUsing junctional abundances, we show nc-sgRNAs make up as much as 33% of total sgRNAs in vitro, are largely consistent in abundance across independent transcriptomes, and increase in abundance over time during in vitro infection. By assessing the homology between sequences flanking the 5 and 3 junction points, we show that nc-sgRNAs are not associated with TRS-like homology. By incorporating read coverage information, we find strong evidence for subgenomic RNAs that contain only 5 regions of ORF1a. Finally, we show that non-canonical junctions change the landscape of viral open reading frames. ConclusionsWe identify canonical and non-canonical junctions in SARS-CoV-2 sgRNAs and show that these RNA products are consistently generated across many independent viral isolates and sequencing approaches. These analyses highlight the diverse transcriptional activity of SARS-CoV-2 and offer important insights into SARS-CoV-2 biology.