Notes on Genus: Tymovirus

icosahedra

Type member: Turnip yellow mosaic virus

General Description

The genus Tymovirus (turnip yellow mosaic virus group) is one of three genera in the family Tymoviridae, all of which have a similar 30 nm icosahedral particles and which are related in phylogenetic analysis. Members differ from those of the other genera in having a single coat protein and by their sap and beetle transmission. Members of the genus Marafivirus have two coat proteins and are transmitted only by leafhoppers, while those of the genus Maculavirus have larger genomes, are phloem limited and have no known vectors.

Morphology

Virions isometric (icosahedral; T=3), not enveloped, 30 nm in diameter. 32 capsomeres per nucleocapsid (12 pentamers and 20 hexamers). Incomplete virus particles (empty capsids) are also present.

Virions often contain polyamines (several hundred molecules of spermine and spermidine).

Genome

Monopartite, linear, single stranded, positive sense RNA c. 6300 nucleotides long. The 3'-terminus has a tRNA-like structure accepting valine (in the type member). The 5' terminus has a methylated nucleotide cap.

Genus Genomic Organization

The RNA encodes three proteins, which in the well-characterised members are:

1.	69-85 kDa	Movement protein (cell-to-cell movement), overlapping with ORF2
2.	205-210 kDa	Replication protein
3.	c. 20 kDa	Coat protein, expressed from a 3'-coterminal subgenomic mRNA

Type Member Genomic Organization

The RNA encodes three proteins:

1.	69 kDa	Movement protein
2.	206 kDa	Replication protein with nucleotide binding and RNA polymerase functions. It may be autocatalytically proteolysed in vitro into products of 150 and 70 kDa
3.	20 kDa	Coat protein, expressed from a 3'-coterminal subgenomic mRNA of c. 800 nucleotides