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Transmissible gastroenteritis virus (TGEV) causes gastroenteritis in pigs of all ages but has a high mortality in neonatal piglets. In piglets, under two weeks of age, the first clinical sign is usually vomiting 18–24 h after infection rapidly followed by a diarrhoea, resulting in loss of weight and dehydration; death usually occurs after 2–5 days (1). Like all the other coronaviruses TGEV proteins are expressed from a ‘nested’ set of subgenomic mRNAs which have common 3′ termini but different 5′ extensions. The region of each mRNA responsible for the expression of a protein appears to correspond to the 5′-terminal region that is absent on the preceding smaller species. Mouse hepatitis virus (MHV) and infectious bronchitis virus (IBV) mRNA species contain identical short non-coding sequences at their 5′ ends, specific to each virus, which appear to be joined to the sequences encoding the virual genes by discontinuous transcription. A consensus sequence identified upstream of each gene/ORF may act as a binding site for the RNA polymerase-leader complex (2, 3, 4, 5, 6). It has been previously postulated that a heptameric sequence , ACTAAC (7) or a hexameric sequence, CTAAAC (8, 9, 10) may be involved in the binding of the TGEV RNA polymerase-leader complex. The TGEV virion contains three major structural polypeptides; a surface glycoprotein (spike or peplomer protein) with a monomeric Mr 200000, a glycosylated integral membrane protein observed as a series of polypeptides of Mr 28000–31000 and a basic phosphorylated protein (the nucleoprotein) of Mr 47000 associated with the viral genomic RNA (11). |
