Nidovirales is an order of viruses with animal
hosts. It includes the families Coronaviridae, Arterivirus, Roniviridae and
The order Nidovirales is named for the Latin nidus, meaning nest, as all
viruses in this order produce a 3' co-terminal nested set of subgenomic
mRNA's during infection.
This group consists of viruses which have positive-sense, single-stranded
RNA genomes. As positive-sense genomes, the viruses can use some host cell
proteins during replication and gene expression which occurs in the
cytoplasm of the host cell.
This group of viruses expresses structural proteins separately from the
nonstructural 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 or inactivating specific
proteins at the correct time in the virus life cycle, ensuring replication
occurs at the right time.
A large number of proteins have been identified on the genomes of
Nidovirales, but their 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''-phosphate phosphatase activities and cyclic nucleotide
Most, but not all, nidovirus subgenomic 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.
The virus which has the largest known nonsegmented RNA genome of 31.5kb, the
mouse hepatitis virus (MHV), is of the order nidovirales.
This order of viruses 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 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 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.
Nidovirales belongs to Group IV of the Baltimore classification system.
A virus belonging to this group—Cavally virus—has been isolated from
mosquitoes. It appears to be unrelated to the other member of this order and
probably is the first member of a new family.
Cavally virus and Nam Dinh virus have been identified as two serotypes of
the same species, Alphamesonivirus 1, and placed in a newly discovered
Gill-associated virus and yellow head virus, both isolated from prawns, are
members of this order. These latter two viruses have been classified into
the family Roniviridae genus Okavirus.