Nidovirales

Nidovirales is an order of enveloped, positive-sense, single-stranded RNA viruses that share a distinctive transcription strategy and large genomes. A hallmark is the nested set of subgenomic mRNAs produced by discontinuous transcription. Genomes are among the largest of RNA viruses, roughly 20–32 kilobases, with a 5' cap and a 3' poly(A) tail. Replication occurs in the cytoplasm on rearranged membranes, using a replication-transcription complex derived from ORF1a/1b polyproteins that are cleaved by viral proteases.

Taxonomy and hosts: The order includes four families: Coronaviridae, Arteriviridae, Mesoniviridae, and Tobaniviridae. Coronaviridae infects mammals and birds, including human pathogens such as SARS-CoV, MERS-CoV, and SARS-CoV-2; Arteriviridae includes arteriviruses like equine arteritis virus and porcine reproductive and respiratory syndrome virus; Mesoniviridae comprises insect-specific viruses; Tobaniviridae includes toroviruses and related viruses with various hosts.

Genomic organization and replication: The typical genome contains a large replicase gene (ORF1a/1b) followed by structural and accessory genes expressed from subgenomic RNAs. The spike glycoprotein in coronaviruses mediates receptor binding and entry; other nidoviruses use different attachment proteins. Coronaviruses uniquely encode a proofreading exonuclease, nsp14, contributing to replication fidelity and supporting their large genome size.

Pathogenicity and evolution: Nidoviruses include important human and animal pathogens and show capacity for cross-species transmission and recombination, factors in past outbreaks and the emergence of novel viruses.