Figure 1: Genomes and structures of SARS-CoV and MERS-CoV.
From: Coronaviruses — drug discovery and therapeutic options
The typical coronavirus (CoV) genome is a single-stranded, non-segmented RNA genome, which is approximately 26–32 kb. It contains 5′-methylated caps and 3′-polyadenylated tails and is arranged in the order of 5′, replicase genes, genes encoding structural proteins (spike glycoprotein (S), envelope protein (E), membrane protein (M) and nucleocapsid protein (N)), polyadenylated tail and then the 3′ end. The partially overlapping 5′-terminal open reading frame 1a/b (ORF1a/b) is within the 5′ two-thirds of the CoV genome and encodes the large replicase polyprotein 1a (pp1a) and pp1ab. These polyproteins are cleaved by papain-like cysteine protease (PLpro) and 3C-like serine protease (3CLpro) to produce non-structural proteins, including RNA-dependent RNA polymerase (RdRp) and helicase (Hel), which are important enzymes involved in the transcription and replication of CoVs. The 3′ one-third of the CoV genome encodes the structural proteins (S, E, M and N), which are essential for virus–cell-receptor binding and virion assembly, and other non-structural proteins and accessory proteins that may have immunomodulatory effects297. Particle image from Ref. 296, Nature Publishing Group. MERS, Middle East respiratory syndrome; SARS, severe acute respiratory syndrome.
