Requirement of the N-terminal region of nonstructural protein 1 in cis for SARS-CoV-2 defective RNA replication.

SARS-CoV-2 belongs to the family and carries a single-stranded positive-sense RNA genome. During coronavirus (CoV) replication, defective or defective interfering RNAs that lack a large portion of the genome often emerge. These defective RNAs typically carry the necessary RNA elements that are required for replication and packaging. We identified the minimum requirement of the 5' proximal region necessary for viral RNA replication by using artificially generated SARS-CoV-2 minigenomes. The minigenomes consist of the 5'-proximal region, an open reading frame (ORF) that encodes a fusion protein consisting of the N-terminal of viral NSP1 and a reporter gene, and the 3' untranslated region of the SARS-CoV-2 genome. We used a modified SARS-CoV-2 variant to support replication of the minigenomes. A minigenome carrying the 5' proximal 634 nucleotides replicated, whereas those carrying shorter than 634 nucleotides did not, demonstrating that the entire 265 nt-long 5' untranslated region and N-terminal portion of the NSP1 coding region are required for the minigenome replication. Minigenome RNAs carrying a specific amino acid substitution or frame shift insertions in the partial NSP1 coding sequence abrogated minigenome replication. Introduction of synonymous mutations in the minigenome RNAs also affected the replication efficiency of the minigenomes. These data suggest that the expression of the N-terminal portion of NSP1 and the primary sequence of the 5' proximal 634 nucleotides are important for minigenome replication.IMPORTANCESARS-CoV-2, the causative agent of COVID-19, is highly transmissible and continues to have a significant impact on public health and the global economy. While several vaccines mitigate the severe consequences of SARS-CoV-2 infection, mutant viruses with reduced reactivity to current vaccines continue to emerge and circulate. This study aimed to identify the minimal 5' proximal region of SARS-CoV-2 genomic RNA required for SARS-CoV-2 defective RNA replication and investigate the importance of an ORF encoded in these defective RNAs. Identifying cis-acting replication signals of SARS-CoV-2 genomic RNA is critical for the development of antivirals that target these signals. Additionally, replication-competent defective RNAs can serve as therapeutic reagents to interfere with SARS-CoV-2 replication. Our findings provide valuable insights into the mechanisms of SARS-CoV-2 RNA replication and the development of reagents that suppress SARS-CoV-2 replication.