Figure 1 Structure of coronavirus virions. (A) Schematic diagram of virus structure; (B) Diagram of virion surface. (C1) Electron micrograph of virus particles of Transmissible gastroenteritis virus (TGEV) stained with uranyl acetate (C1) or sodium phosphotungstate (C2) showing the surface of the virus particles. The peplomers are better defined using sodium phosphotungstate. (D) Cryo-electron microscopic visualization of unstained TGEV in vitreous ice. The particles contain an internal structure inside the viral envelope and well extended peplomers. MEM, lipid membrane; S, spike protein; M, large membrane protein, E, small envelope protein; HE, hemagglutinin-esterase; N, nucleocapsid protein; CS, core-shell; NC, nucleocapsid. The bars represents 100 nm.
Figure 2 Representation of prototype Coronavirus genome. ORFs are represented by boxes. Numbers above boxes indicate the nRNA designation. The pseudoknots in ORF1 are aligned. The proteins encoded by the ORFs are indicated. The 5 leader sequence is depicted by a small black box; poly(A) tail is indicated by An; M, membrane protein; N, nucleocapsid protein; S, spike protein; HE, hemagglutinin-esterase protein; I, internal ORF; empty boxes, nonstructural proteins; L, leader; UTR, untranslated region.
Figure 3 Genetic structure in coronaviruses. The numbers above the bars indicate mRNA. The names in the bars indicate the protein encoded by the corresponding ORF. The structural protein genes are marked by various symbols while the non-structural protein genes are represented by unfilled boxes or by boxes with small letters. The Human coronavirus OC43 (HCoV-OC43) does not have the two nonstructural proteins encoded by gene 4. The translation products of ORFs Murine hepatitis virus (MHV) 5a, Bovine coronavirus (BCoV) 4a,b, Feline coronavirus (FCoV) and Canine coronavirus (CCoV) 3b have not been detected. The ORFs Porcine transmissible gastroenteritis virus (TGEV) 3b, FCoV and CCoV 3c, and HCV-229E 3a,b are homologous. The 3a ORFs of TGEV, FCV and CCoV are homologous. The ORFs TGEV 7 and the 7a of FCoV and CCoV are homologous. The ORF abbreviations are as in Figure 2. L, leader; , deleted.
Figure 4 Structural relationship between mRNAs and the genomic RNA of coronaviruses. Thick lines represent the translated sequences. Thinner lines, untranslated sequences. Numbers above the boxes indicate the mRNA designation. The names in the boxes indicate the proteins encoded by the corresponding genes. L, leader. Abbreviations are as in Figure 2.
Figure 5 Torovirus particle structure. (A) Schematic representation of the architecture of Equine torovirus (EqTV). The localization of the structural proteins and genome are indicated. S, spike protein; M, large membrane protein; HE, hemagglutinin-esterase; N, nucleocapsid protein. (B) Negative contrast electron micrograph of EqTV strain Berne particles. (C) Different forms of EqTV particles in ultrathin sections of EqTV-infected equine dermis cells. The bar represent 100 nm.
Figure 6 Genetic structure in toroviruses. The numbers above the bars indicate mRNA. The names within the bars indicate the protein encoded by the corresponding ORF. Poly(A) tails are indicated by An; M, membrane protein; N, nucleocapsid protein; S, spike protein; HE, hemagglutinin-esterase protein.
Figure 7 Structural relationship between mRNAs and the genomic RNA of toroviruses. Thick lines represent the translated sequences. Thinner lines, untranslated sequences. Numbers above the boxes indicate the mRNA designation. The names in the boxes indicate the proteins encoded by the corresponding genes. Abbreviations are as in Figure 2.
Figure 8 Phylogenetic relationship of the large-surface glycoprotein of coronaviruses and toroviruses. Amino acid sequences were aligned using the “clustal” method and phylogenetic trees were constructed using the neighbor-joining method. The analyses were done using the MegAlign module of the Lasergene software suite (DNASTAR). The phylogenies are rooted assuming a biological clock. (Siddell, 1995.)
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