Fig. 2.

Fig. 2. Schematic representation of the genomic organization of SARS-CoV-2 and polyprotein 1ab (pp1ab). SARS-CoV-2 harbors a remarkably large RNA genome (~30 kilobases). It encodes non-structural proteins in the open reading frames 1a and 1b (ORF1a, ORF1b), structural proteins (designated as S, E, M, and N), and other accessory proteins. Two large open reading frames, ORF1a and ORF1b, localized at the 5'-end are translated into polyproteins pp1a and pp1ab. Translation of ORF1b is regulated by a -1 programmed ribosomal frameshift (PRF) signal located upstream of ORF1b generating the pp1ab. The ORF1b contains a replicase gene cluster, which consists of non-structural protein 12 (nsp12) (also known as RNAdependent RNA polymerase, RdRp), nsp13 (helicase, Hel), nsp14 (exoribonuclease/(guanine-N7)-methyltransferase, ExoN/ N7-MTase), nsp15 (nidoviral RNA uridylate-specific endoribonuclease, NendoU), and nsp16 (2′-O-methyltransferase, 2’-OMTase). The pp1ab is cleaved into 16 individual proteins by two viral proteases; papain-like protease (PL^pro) and 3C-like protease (3CL^pro, also known as the main protease).

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