Immunolocalization of Picornavirus RNA in infected cells with antibodies to Tyr-pUp, the covalent linkage unit between VPg and RNA.

The genomic RNA of picornaviruses is attached to a small protein (VPg) via a covalent bond between a tyrosine and a 5'-terminal uridine phosphate. The same structure is present in potyvirus and calicivirus families. VPgs play a key role in initiation of viral replication by acting as primers for RNA synthesis.