Picornavirales

Picornavirales is an order of viruses with vertebrate, invertebrate, protist and plant hosts.^[1] The name has a dual etymology.^[2] First, picorna- is an acronym for poliovirus, insensitivity to ether, coxsackievirus, orphan virus, rhinovirus, and ribonucleic acid.^[2] Secondly, pico-, meaning extremely small, combines with RNA to describe these very small RNA viruses.^[2] The order comprises viruses that historically are referred to as picorna-like viruses.

Picornavirales
Picornavirus replication cycle
Virus classification
(unranked):	Virus
Realm:	Riboviria
Kingdom:	Orthornavirae
Phylum:	Pisuviricota
Class:	Pisoniviricetes
Order:	Picornavirales

Characteristics

The families within this order share a number of common features:

• The virions are non-enveloped, icosahedral, and about 30 nanometers in diameter.
• The capsid has a "pseudo T=3" structure, and is composed of 60 protomers each made of three similar-sized but nonidentical beta barrels.
• The genome is made of one or a few single-stranded RNA(s) serving directly as mRNA, without overlapping open reading frames.
• The genome has a small protein, VPg, covalently attached to its 5' end, and usually a poly-adenylated 3' end.
• Each genome RNA is translated into polyprotein(s) yielding mature viral proteins through one or several virus-encoded proteinase(s).
• A hallmark of the Picornavirales is a conserved module of sequence domains, Hel-Pro-Pol, which is typical of (from the amino- to the carboxy-end of the polyprotein):
  • A helicase belonging to superfamily III
  • [the VPg is encoded between these two domains]
  • A chymotrypsin-like protease where the catalytic residue is typically a cysteine rather than a serine,
  • A polymerase belonging to superfamily I; this conserved module is a hallmark of the Picornavirales

The evolution of picorna-like viruses seems to have antedated the separation of eukaryotes into the extant crown groups.^[3]

Taxonomy

Phylogenetic tree of representative viruses from order Picornavirales

The following families are recognized:^[4]

• Caliciviridae
• Dicistroviridae
• Iflaviridae
• Marnaviridae
• Noraviridae
• Picornaviridae
• Polycipiviridae
• Secoviridae
• Solinviviridae