Notes on Genus: Polerovirus

icosahedra

Type member: Potato leafroll virus

General Description

The genus Polerovirus, one of three genera in the family Luteoviridae, is distinctive for its isometric particles of 25-30 nm diameter, monopartite genome and persistent, aphid transmission. It consists of viruses previously included in the genus Luteovirus but which have three (rather than two) ORFs upstream of the coat protein. These ORFs have some homology to members of the genus Sobemovirus. The name is from derived from the type member, potato leafroll virus.

Morphology

Virions are isometric (icosahedral), not enveloped, 25-30 nm in diameter, with 32 capsomeres per nucleocapsid. They contain 28 % nucleic acid.

Genome

Monopartite, linear, single stranded, positive sense RNA 5300-5700 nucleotides long. The 3' terminus has neither a poly (A) tract nor a tRNA-like structure. The 5' terminus has a genome-linked protein (VPg) (of about 17 kDa).

Genus Genomic Organization

There are a total of six potential ORFs. Of the three at the 5'-end, two show some sequence homology to members of the genus Sobemovirus. They may be expressed as one protein by translational frameshift. The three 3'-ORFs are expressed from a subgenomic RNA:

Coat protein
Coat protein readthrough involved in aphid transmission and possibly cell-to-cell movement
Movement protein overlapping the coat protein in a different reading frame

Type Member Genomic Organization

Six open reading frames have been identified :

0.	28 kDa	unknown function
1.	70 kDa	Replication protein
2.	36 kDa	Polymerase. Probably expressed as a larger protein entered earlier by frameshift from ORF1
3.	23 kDa	Coat protein, expressed from subgenomic RNA
4.	17 kDa	Movement protein, expressed from subgenomic RNA
5.	80 kDa	Coat protein readthrough expressed from subgenomic RNA, probably involved in aphid transmission

These ORFs are numbered to be consistent with other members of the family Luteoviridae.