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Coronaviruses are large, enveloped RNA viruses. The human coronaviruses cause common colds, may cause lower respiratory tract infections, and have been implicated in gastroenteritis in infants. A novel coronavirus was identified as the cause of a worldwide outbreak of a severe acute respiratory syndrome (SARS) in 2003. Animal coronaviruses cause diseases of economic importance in domestic animals. Coronaviruses of lower animals establish persistent infections in their natural hosts. The human viruses are difficult to culture and therefore are more poorly characterized.

Important properties of the coronaviruses are listed in Table 41-1.

Table 41–1 Important Properties of Coronaviruses

Structure and Composition

Coronaviruses are enveloped, 120- to 160-nm particles that contain an unsegmented genome of single-stranded positive-sense RNA (27–32 kb), the largest genome among RNA viruses. The genomes are polyadenylated at the 3' end. Isolated genomic RNA is infectious. The helical nucleocapsid is 9–11 nm in diameter. There are 20-nm-long club- or petal-shaped projections that are widely spaced on the outer surface of the envelope, suggestive of a solar corona (Figure 41-1). The viral structural proteins include a 50–60 kDa phosphorylated nucleocapsid (N) protein, a 20–35 kDa membrane (M) glycoprotein that serves as a matrix protein embedded in the envelope lipid bilayer and interacting with the nucleocapsid, and the spike (S; 180–220 kDa) glycoprotein that makes up the petal-shaped peplomers. Some viruses, including human coronavirus OC43 (HCoV-OC43), contain a third glycoprotein (HE; 65 kDa) that causes hemagglutination and has acetylesterase activity.

Figure 41-1

Human coronavirus OC43. Note the characteristic large, widely spaced spikes that form a "corona" around the virion (297,000×). (Courtesy of FA Murphy and EL Palmer.)

The genome organizations of representative corona viruses are shown in Figure 41-2. The gene order for the proteins encoded by all coronaviruses is Pol-S-E-M-N-3'. Several open reading frames encoding nonstructural proteins and the HE protein differ in number and gene order among coronaviruses. The SARS virus contains a comparatively large number of interspersed genes for nonstructural proteins at the 3' end of the genome.

Figure 41-2

Genomic organization of coronaviruses. The SARS coronavirus (SARS-CoV) genome is about 29.7 kb. Boxes shaded in yellow represent open reading frames (ORFs) encoding structural proteins; ...

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