catalytic core domain of RNA-dependent RNA polymerase (RdRp) in the order Nidovirales of positive-sense single-stranded RNA [(+)ssRNA] viruses
This family contains the catalytic core domain of RdRP of Nidovirales, an order of enveloped, (+)ssRNA viruses which infect vertebrates and invertebrates. Host organisms include mammals, birds, reptiles, amphibians, fish, arthropods, mollusks, and helminths. The order Nidovirales currently comprises 88 formally recognized virus species of (+)ssRNA viruses which are classified into nine virus families: Abyssoviridae, Arteriviridae, Coronaviridae, Euroniviridae, Medioniviridae, Mesoniviridae, Mononiviridae, Roniviridae, and Tobaniviridae. Based on the genome size, the members of the order Nidovirales can be divided into two groups, large and small nidoviruses. The genomes of the large nidoviruses are well over 25 kb in length with size differences in the 5 kb range. Planarian secretory cell nidovirus (PSCNV), only member of the Mononiviridae family, has the largest known non-segmented RNA genome of 41.1 kb; its host is the planarian flatworm. The RdRp domain displays a right hand with three functional subdomains, called fingers, palm, and thumb. All RdRps contain conserved polymerase motifs (A-G), located in the palm (A-E motifs) and finger (F-G) subdomains. All these motifs have been implicated in RdRp fidelity such as processes of correct incorporation and reorganization of nucleotides.
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