Abstract
Coronaviruses have the largest known RNA genomes (∼30 kb), which are of positive sense. Together with toroviruses, they are classified in the family Coronaviridae, order Nidovirales. All coronaviruses have four common proteins, three in the envelope and one associated with the genome. Assembly of virus particles occurs at internal membranes. The genes for the structural proteins are at the 3′ end of the genome. Most of the genome (∼20 kb) is gene 1, which encodes 15–16 proteins associated with RNA replication and transcription. Translation of gene 1 involves ribosomal frameshifting. Transcription is by a discontinuous process which results in a 3′ co-terminal nested set of mRNAs, each of which has a common leader sequence transcribed from the 5′ terminus of the genome. Only the most 5′-proximal gene of each mRNA is translated. Recombination is a feature of coronavirus evolution. The outbreak of severe acute respiratory syndrome (SARS) has resulted in the discovery of more coronaviruses in humans, other mammals, and avian species, and the realization that the host range of coronaviruses is wider than previously acknowledged. Coronaviruses are associated with a wide range of diseases, including the respiratory and enteric systems, though not necessarily restricted to these, for example, some coronaviruses affect the central nervous system, kidneys, and gonads. The most widely used coronavirus vaccine (billions of doses annually) is against infectious bronchitis virus, which affects chickens.