| Nidovirales | |
|---|---|
 | |
| Life cycle of nidoviruses | |
Virus classification | |
| (unranked): | Virus |
| Realm: | Riboviria |
| Kingdom: | Orthornavirae |
| Phylum: | Pisuviricota |
| Class: | Pisoniviricetes |
| Order: | Nidovirales |
Nidovirales is an order ofenveloped,positive-strand RNA viruses which infectvertebrates andinvertebrates. Host organisms includemammals,birds,reptiles,amphibians,fish,arthropods,molluscs, andhelminths.[1] The order includes the familiesCoronaviridae,Arteriviridae,Roniviridae,Tobaniviridae, andMesoniviridae.[2]
Memberviruses have a viral envelope and apositive-sense, single-strandedRNA genome which iscapped andpolyadenylated.[3] Nidoviruses are named for the Latinnidus, meaning nest, as all viruses in this order produce a 3' co-terminal nested set ofsubgenomic mRNAs during infection.[4]
Nidoviruses have a viral envelope and a positive-sense, single-stranded RNA genome which is capped and polyadenylated.[3] The group expresses structural proteins separately from the nonstructural ones. The structural proteins are encoded at the 3' region of thegenome and are expressed from a set ofsubgenomic mRNAs.[citation needed]
Member viruses encode one mainproteinase and between one and three accessory proteinases which are mainly involved in expressing thereplicase gene. These proteinases are also responsible for activating or inactivating specific proteins at the correct time in the virus life cycle, ensuring replication occurs at the right time.[citation needed]
Nidoviruses can be distinguished from other RNA viruses by a constellation of seven conserved domains—5'-TM2-3CLpro-TM3-RdRp-Zm-HEL1-NendoU-3'—with the first three being encoded inORF1a and the remaining four inORF1b. TM2 and TM3 andtransmembrane domains; RdRp is theRNA-dependent RNA polymerase; Zm is a Zn-cluster binding domain fused with ahelicase (HEL1); 3CLpro is a3C-like protease; and NendoU is anuridylate-specificendonuclease. The 3CLpro has a catalyticHis-Cys dyad, and is related to theSARS coronavirus main proteinase (Mpro).[citation needed]
Most, but not all, nidovirus subgenomic RNAs contain a 5′ leader sequence derived from the 5′ end of the genomic RNA. Theframeshift that generatesORF1b frameshift occurs at aUUUAAAC heptanucleotide 'slippery' sequence located upstream of the ORF1a stop codon and a putative RNApseudoknot structure.[citation needed]
Many proteins have been identified on the genomes ofNidovirales, but their function has not yet been determined. Other enzymes that may be present in the genome includepapain-like proteases, ADP-ribose/poly(ADP-ribose)-binding orADP-ribose 1''-phosphate phosphatase activities andcyclic nucleotide phosphodiesterase.[citation needed]
The orderNidovirales can be divided into twoclades depending on the size of the genome: those with large genomes (26.3–31.7 kilobases) which included theCoronaviridae andRoniviridae (the large nidoviruses) and those with small genomes (the small nidoviruses)—a clade that includes the distantly relatedArteriviridae (12.7–15.7 kb).[citation needed]
The large nidoviruses encode both a 2'-O-methyltransferase and a 3'–5'exoribonuclease (ExoN)—the latter being very unusual for an RNA virus. They also encode a superfamily 1helicase, uridylate-specific endonuclease (an enzyme unique to nidoviruses) and several proteases.[citation needed]
Nidoviruses as a group have the largest RNA genomes of viruses. The largest reported genomes for RNA viruses are Crassostrea gigas nidovirus (CGNV)[6] and Pacific oyster nidoviirus 1 (PONV 1);[7] they are biseqmented, ~64kb in size, and infectPacific oysters. They are members of a widely distributed group of viruses that belong to the proposed family Megarnaviridae.[7] Previously, the largest reported genome was the 41.1kb, nonsegmented genome ofplanarian secretory cell nidovirus (PSCNV)[8] that infect aplanarianflatworm.[9]
The following suborders and families are recognized (-virineae denotessuborders and -viridae denotes families):[10]
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