Page "Messenger RNA" Paragraph 26

Other mechanisms for circularization exist, particularly in virus mRNA.

Poliovirus mRNA uses a cloverleaf section towards its 5 ' end to bind PCBP2, which binds poly ( A )- binding protein, forming the familiar mRNA-protein-mRNA circle.

Barley Yellow Dwarf Virus, has binding between mRNA segments on its 5 ' end and 3 ' end ( called kissing stem loops ) circularizing the mRNA without any proteins involved.