Fig. 2.

Coronaviruses employ a variety of mechanisms to synthesize their viral RNAs. A–I Schematic diagram depicting the mechanism of RNA synthesis for each group of coronavirus transcripts shown in Fig. 1. For detailed explanations regarding the mechanisms, please see the text. ncTRS, noncanonical TRS-B; cTRS, canonical TRS-B. L, leader; c, canonical; nc, noncanonical; TRS, transcription regulatory sequence; TRS-L, leader TRS, ncTRS, noncanonical TRS; cTRS, canonical TRS. RdRp, RNA-dependent RNA polymerase; sgm, subgenomic mRNA; DVG, defective viral genome; ΔL_cTRS_sgm, leader-less sgmRNA derived from canonical TRS; L_ncTRS_sgm, sgmRNA derived from noncanonical TRS; ΔL_ncTRS_sgm, leader-less sgmRNA derived from noncanonical TRS; 5′3′DVG, DVG with sequence elements from 3′ UTR and 5′ UTR; Δ5′DVG, DVG with a sequence element from 3′ UTR; Δ3′DVG, DVG with a sequence element from 5′ UTR; Δ5′3′DVG, DVG lacking sequence elements from 3′ UTR and 5′ UTR