Nidovirales

Nidovirales
Virus classification
Group:	Group IV ((+)ssRNA)
Order:	Nidovirales
Families
Arteriviridae Coronaviridae Roniviridae

The Nidovirales are an order of viruses with animal hosts. It includes the families Coronaviridae,^[1][2] Arterivirus ^[3] and Roniviridae.

Virology

This group consists of viruses which have (+) sense single stranded RNA genomes. It contains the virus which has the largest known non segmented RNA genome; this is the mouse hepatitis virus (MHV) which has a genome of 31.5kb. These genomes share the structure of eukaryotic mRNA and so the viruses can use some host cell proteins during replication and gene expression which occurs in the cytoplasm of the host cell. Unlike many viruses they do not have any polymerase in the virus particle as the genome can be read directly as mRNA when it first enters the host cell.

This group of viruses express the structural proteins separately from the non-structural ones. The structural proteins are encoded at the 3’ region of the genome and are expressed from a set of subgenomic mRNAs. These viruses encode one main proteinase and between one and three accessory proteinases which are mainly involved in expressing the replicase gene. These proteinases are also responsible for activating/inactivating specific proteins at the correct time in the virus life cycle and so ensure replication occurs at the right time. There are still a large number of proteins which have been identified on the genomes of nidovirales but whose function has not yet been determined.

Other enzymes that may be present in the genome include papain-like proteases, ADP-ribose/poly(ADP-ribose)-binding and/or ADP-ribose 1"-phosphatase activities and cyclic nucleotide phosphodiesterase.

Most, but not all, nidovirus sub genomic RNAs contain a 5′ leader sequence derived from the 5′ end of the genomic RNA.

The frameshift that generates ORF1b frameshift occurs at a UUUAAAC heptanucleotide 'slippery' sequence located upstream of the ORF1a stop codon and a putative RNA pseudoknot structure.

Taxonomy

This order of viruses can be distinguished from other RNA viruses by a constellation of 7 conserved domains - 5'-TM2-3CLpro-TM3-RdRp-Zm-HEL1-NendoU-3' - with the first three being encoded in ORF1a and the remaining four in ORF1b. TM2 and TM3 and transmembrane domains; RdRp is the RNA polymerase; Zm is a Zn-cluster binding domain fused with a helicase (HEL1); 3CLpro is a 3C-like protease; and NendoU is an uridylate-specific endonuclease. The 3CLpro has a catalytic His-Cys dyad.

The Nidovirales can be divided into two clades depending on the size of the genome: those with large genomes (26.3-31.7 kilobases) which included the Coronaviridae and Roniviridae (the large nidoviruses) and those with small genomes (the small nidoviruses) - a clade that includes the distantly related Arteriviridae (12.7-15.7 kb). The large nidoviruses large nidoviruses encode both an 2'-O-methyltransferase and a 3'-5' exoribonuclease (ExoN) - the latter being very unusual for an RNA virus. They also encode a superfamily 1 helicase, uridylate-specific endonuclease (an enzyme unique to Nidoviruses) and several proteases.

A virus belonging to this group - Cavally virus - has been isolated from mosquitoes.^[4] It appears to be unrelated to the other member of this order and probably is the first member of a new family.

Another virus has been isolated from mosquitoes - Nam Dinh virus - that appears to belong a new family in this order.^[5]

Another virus - White bream virus - isolated from fish may also belong to this order.^[6]

Gill-associated virus and Yellow head virus, both isolated from prawns, are members of this order.^[7][8]

See also

• Animal viruses
• RNA viruses
• Coronavirus
• Arterivirus

References

1. ^ Enjuanes et al. (2008). "Coronavirus Replication and Interaction with Host". Animal Viruses: Molecular Biology. Caister Academic Press. ISBN 978-1-904455-22-6. http://www.horizonpress.com/avir. 
2. ^ Thiel V (editor). (2007). Coronaviruses: Molecular and Cellular Biology. Caister Academic Press. ISBN 978-1-904455-16-5 . http://www.horizonpress.com/cor. 
3. ^ Balasuriya and Snijder (2008). "Arteriviruses". Animal Viruses: Molecular Biology. Caister Academic Press. ISBN 978-1-904455-22-6. http://www.horizonpress.com/avir. 
4. ^ Zirkel F, Kurth A, Quan PL, Briese T, Ellerbrok H, Pauli G, Leendertz FH, Lipkin WI, Ziebuhr J, Drosten C, Junglen S (2011) An insect nidovirus emerging from a primary tropical rainforest. MBio. 2(3). pii: e00077-11. doi: 10.1128/mBio.00077-11
5. ^ Nga PT, Parquet Mdel C, Lauber C, Parida M, Nabeshima T, Yu F, Thuy NT, Inoue S, Ito T, Okamoto K, Ichinose A, Snijder EJ, Morita K, Gorbalenya AE (2011) Discovery of the first insect nidovirus, a missing evolutionary link in the emergence of the largest RNA virus genomes. PLoS Pathog 7(9):e1002215.
6. ^ Schütze H, Ulferts R, Schelle B, Bayer S, Granzow H, Hoffmann B, Mettenleiter TC, Ziebuhr J (2006) Characterization of White bream virus reveals a novel genetic cluster of nidoviruses. J Virol 80(23):11598-11609
7. ^ Cowley JA, Dimmock CM, Spann KM, Walker PJ (2000) Gill-associated virus of Penaeus monodon prawns: an invertebrate virus with ORF1a and ORF1b genes related to arteri- and coronaviruses. J Gen Virol 81(Pt 6):1473-1484
8. ^ Sittidilokratna N, Dangtip S, Cowley JA, Walker PJ (2008) RNA transcription analysis and completion of the genome sequence of yellow head nidovirus. Virus Res 136(1-2):157-165

• J Ziebuhr, EJ Snijder, AE Gorbalenya Virus-encoded proteinases and proteolytic processing in the Nidovirales Journal of General Virology, 2000

External links

• Nidovirales - ICTVdB - The Universal Virus Database, version 4.
• MeSH Nidovirales
• NIH/MeSH
• "The Nidoviruses" at Amazon.com