|Group:||Group IV ((+)ssRNA)|
This group consists of viruses which have positive-sense, single-stranded RNA genomes. It contains the virus which has the largest known nonsegmented RNA genome, the mouse hepatitis virus (MHV), which has a genome of 31.5kb. These genomes share the structure of eukaryotic mRNA, 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 expresses the 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, so ensure 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"-phosphatase activities and cyclic nucleotide phosphodiesterase.
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.
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.
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