Fig. 2.

Structure and genome organization of coronaviruses: A. Schematic diagram of virus structure showing the envelope, the core and the nucleoprotein structure. S, spike protein; M and M′, M proteins with the amino-terminus facing the external surface of the virion and the carboxy-terminus towards the inside or the outside face of the virion, respectively; E, small envelope protein; N, nucleocapsid protein; NC, nucleocapsid. Some coronaviruses of group 2 have an additional protein, the haemagglutinin-esterase (HE) (not shown). B. Representation of a prototype TGEV coronavirus genome and subgenomic RNAs. Beneath the top bar a set of positive- and negative-sense mRNA species synthesized in infected cells is shown. The protein products obtained from each positive-sense RNA are indicated. Two products, polyproteins 1a and 1b, are translated from the genomic RNA by a ribosomal frameshifting mechanism. All other proteins are translated from the first open reading frame of each functionally monocistronic subgenomic RNA (dark lines). Poly(A) and Poly(U) tails are indicated by AAA or UUU. S, spike protein; E, envelope protein; M, membrane protein; N, nucleocapsid protein.