Mouse hepatitis virus (MHV) genomic and subgenomic RNA organization. MHV and other coronaviruses, including SARS-CoV-2, produce a genomic transcript that serves as a protein-producing template with an extremely long 3′UTR. A ribosomal frameshifting element within this transcript gives rise to a fusion product with the 5′-most ORF. During the viral life cycle, additional subgenomic RNAs are produced, some of which likewise bear what is presumably a long 3′UTR since only the 5′-most ORF is assumed to be translated. The same leader sequence is present on all genomic and subgenomic RNAs and is attached by a unique template-switching mechanism.
