Figure 1.

Schematic presentation of the genome of the SARS coronavirus. Occupying two‐thirds of the genome from the 5′ end, open‐reading frame 1 (ORF1) encodes two large polyproteins, pp1a and, through ribosomal frameshifting during translation, pp1ab. These polyproteins are processed into mature Nsps by the two proteases discussed here (indicated in yellow). The main protease (M^pro, also called 3C‐like protease, 3CL ^pro) is Nsp5, whereas the papain‐like protease (PL ^pro) is a part of Nsp3. The PL ^pro performs three cleavage reactions (red arrows) to release Nsp1, Nsp2 and Nsp3 (red), whereas the M^pro cleaves the polyprotein at 11 sites (cyan arrows) to release Nsp4–Nsp16 (cyan). The 3′‐terminal third of the genome codes for structural and accessory proteins.