Arch Viro1135 (1994) Tl IkT Virology Division ews JI./, todate it has only been looked for and demonstrated in IBV and TGEV 13, 20 (Fig. 1). Replication is within the cytoplasm and five or more subgenomic mRNAs are generated, these forming a 3 coterminal nested set (Fig. 2). Table 1. Species within the genera Coronavirus, Torovirus and Arterivirus Coronavirus Avian infectious bronchitis virus (IBV) Feline coronavirus (feline infectious peritonitis virus, FIPV) (feline enteric coronavirns, FECV) Canine coronavirus (CCV) Porcine transmissible gasuoenteritis virus (TGEV) Human coronavirus 229E (HCV 229E) Porcine epidemic diarrhoea virus (PEDV) Murine hepatitis virus (MHV) Bovine coronavirus (BCV) a Human coronavirus OC43 (HCV OC43) Turkey eoronavirus (TCV) Porcine haemagglutinating encephalomyelitis virus (HEV) Rat coronavirns (RCV; sialodacyroadenitis virus, SDAV) Rabbit coronavirns (RbCV) Torovirus Berne virus (BEV; equine) Breda virus (BRV; bovine) Arterivirus Equine arteritis virus (EAV) Lactate dehydrogenase-elevating virus (LDV; murine) Simian haemorrhagic fever virus (SHFV) Porcine reproductive and respiratory syndrome virus (PRRSV) (Swine infertility and respiratory syndrome virus, SIRSV) These species contain a gene encoding a haemagglutinin-esterase glycoprotein (HE) ,rein (S) CO small .g ral membrane pro protein (M) haemagglutinin ase glycoprotein (HE) a t-= 120 nm =I torovin ;in (S) ral membrane ,rotein (M) nucleo( (N+R :e b Ii= 120 nm =ll arterivirus nucleocapsidl (N+RNA) /large glycoprotein (GL) small glycoprotein (Gs) _ integral membrane I . protein(M) envelope 60 nm l Fig. 1. Diagrammatic representation of the virions of (a) a coronavirus, (b) a torovirus and (c) an arterivirus. A sub-set of the coronaviruses contain an haemagglutinin-esterase glycoprotein which is not shown (see Table 1). The sM protein has not been identified in all coronaviruses and its exact association with the virion envelope is unclear 230 Virology Division News A leader sequence of 60-70 or so nucleotides, corresponding to sequence at the 5 terminus of the genome, is present at the 5 end of all mRNas. Only the unique genes in the 5 unique of each mRNA are translated. The genes for the structural proteins are located in the 3 portion of the genome, in the order 5-S-M-N-3 29, 30, 36 (Fig. 2). The era of cloning and sequencing revealed that coronavirus genomes comprised about 30.000 nucleotides, the 5-most gene, number 1, comprising approximately 20.000 nucleotides and encoding the putative RNA-dependent RNA polymerase 2, 3, 14, 19. Torovirus replication fits the Coronaviridae family picture The broad picture painted above applies in general to the Torovirus genus, of which Berne virus is the most thoroughly studied member at the molecular level 31 (Table 1). In addition to many of the characteristics already described above for coronaviruses, gene 1 of both genera comprise two overlapping open reading frames (ORFs), la and tb (Fig. 2), ribosomal frame-shifting being involved in the translation of the second ORF 4, 32. The carboxy-terminal half of the S protein has a coiled-coil structure and the M protein has three membrane-spanning regions 31, 35. In consequence, the Coronaviridae Study Group proposed to the ICTV Executive Committee that the genus Torovirus should join Coronavirus in the Coronaviridae. This proposal was accepted at the mid-term meeting of the Executive Committee in April 1992 6 and was ratified at the IXth International coronavirus 1 REPLtCASE HE S M N REPLICASE S M N ORF lb I LJ LJ A REPLICASE G S GLM N torovirus BEV 25 kb ? arterivirus EAV i t3 kb I Replication g Transcr/)t/on I I (EA V) l mRNA A 1 A 2 A 3 A., 4 II A0 5 A 6 tA 7 Fig. 2. Genome organization and mRNA arrangement of a coronavirus (murine hepatitis virus, MHV), a torovirus (Berne virus, BEV) and an arterivirus (equine arteritis virus, EAV). Gene 1 comprises two large ORFs, la and lb. Some of the smaller genes, e.g. gene 5 of MttV, comprise more than one ORF. Not all members of one genus have an identical gene complement. For example, the genes containing ORFs 2 and 2a (which encodes the HE protein) of MHV are absent in some coronaviruses. The HE gene is present as a pseudogene in some strains of MHV. The black box at the 5 terminus of the genome of coronavirnses and eriviruses is a leader sequence which is also present at the 5 end of the mRNAs. The nested-set of mRNAs shown corresponds to EAV although a similar arrangement is a characteristic of all three genera Virology Division News Table 2. Comparison of some features of coronaviruses, toroviruses and arteriviruses 231 Feature Coronavirus Torovirus Arterivirus Enveloped Nucleocapsid Positive ssRNA with poly (A) tail Genome size (kb) 3 co-terminal nested set of mRNAs Leader on mRNAs Genome organization Ribosomal frame-shifting in pol gene Some (limited) amino acid identity in pol Prominent spikes Presence of coiled-coil