Notes on Genus: Ilarvirus

icosahedra
icosahedra

Type member: Tobacco streak virus

Contents

General Description

This is one of 6 genera in the family Bromoviridae. The name is derived from the isometric labile ringspot characteristics of the viruses in this group. In common with other members of the family, it has a tripartite genome and an encapsidated subgenomic RNA from which the coat protein is expressed. It differs from the other genera in having isometric particles of three different sizes. On the basis of serology and host relations, the genus has been divided into 10 subgroups.

Morphology

Virions isometric (icosahedral) to quasi-isometric, not enveloped, of three different sizes, 26-35 nm in diameter.

Genome

Tripartite, linear, single-stranded, positive sense RNA, with segments of 3300-3400 (RNA1), 2700 (RNA2) and 2100-2200 (RNA3) nucleotides. RNA4, a subgenomic fragment of RNA3, 900 nucleotides long, is also encapsidated. Each virion contains either one molecule of RNA1, or one of RNA2, or one each of RNA3 and RNA4. The 5'-terminus of each RNA has a methylated nucleotide cap (m7G5'ppp5'Gp...). There are stable hairpin structures, flanked by AUGC boxes, at the 3'-termini of all RNAs. The 3' terminus has neither a poly(A) tract nor a tRNA-like structure.

Genus Genomic Organization

There are four (or perhaps five) open reading frames, resulting in the following proteins:
  1. Replication protein 1a (RNA1) containing methyltransferase and helicase domains, c. 120 kDa
  2. Polymerase 2a (RNA2), RNA-dependent RNA polymerase, c. 100 kDa
  3. 2b protein (RNA2), possible ORF in a different reading frame to 2a in some members
  4. Movement protein 3a (5' half of RNA3), 30-35 kDa, involved in cell-to-cell transport
  5. Coat protein 3b (3' half of RNA3 but expressed from the subgenomic RNA4), c. 25 kDa

Type Member Genomic Organization

The probably five open reading frames:
  1. Replication protein 1a (RNA1) containing methyltransferase and helicase domains, 123 kDa
  2. Polymerase 2a (RNA2), RNA-dependent RNA polymerase, 92 kDa
  3. 2b protein (RNA2), probable ORF in a different reading frame to 2a, 22 kDa
  4. Movement protein 3a (5' half of RNA3), 32 kDa, involved in cell-to-cell transport
  5. Coat protein 3b (3' half of RNA3 but expressed from the subgenomic RNA4), 26 kDa