Notes on Genus: Bromovirus

icosahedra

Type member: Brome mosaic virus

Contents

Members of this Genus
General Description
Morphology
Electron Micrograph
Genome
Genus Genomic Organization
Genome Map
Type Member Genomic Organization
All Known Sequences
Curated Sequences

General Description

The genus Bromovirus (from the type member, brome mosaic virus) is one of 6 genera in the family Bromoviridae. 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 a single size, c. 26 nm in diameter and a smaller coat protein (about 20kDa).

Morphology

Virions icosahedral, not enveloped, 26 nm in diameter. 32 capsomeres per nucleocapsid.

Genome

Tripartite, linear, single-stranded, positive sense RNA, with segments of 3234 (RNA1), 2865 (RNA2) and 2144 (RNA3) nucleotides (in brome mosaic virus). RNA4, a subgenomic fragment of RNA3, 875 nucleotides long, is also encapsidated. Each virion contains either one molecule of RNA1 (B), or one of RNA2 (M), or one each of RNA3 and RNA4 (T). The 5'-terminus of each RNA has a methylated nucleotide cap (m7G5'ppp5'Gp...). There is a homologous region of about 200 nucleotides at the 3'-termini of all RNAs. The 3' terminus has no poly(A) tract but a tRNA-like structure which accepts tyrosine.

Genus Genomic Organization

There are four open reading frames (ORFs) resulting in the following proteins:
  1. Replication protein 1a (RNA1) containing methyltransferase and helicase domains, 109-110 kDa
  2. Polymerase 2a (RNA2), RNA-dependent RNA polymerase, 93-94 kDa
  3. Movement protein 3a (5' half of RNA3), 32-33 kDa, involved in cell-to-cell transport
  4. Coat protein 3b (3' half of RNA3 but expressed from the subgenomic RNA4), 20-21 kDa

Type Member Genomic Organization

The four open reading frames result in the following proteins:
  1. Replication protein 1a (RNA1) containing methyltransferase and helicase domains, 109 kDa
  2. Polymerase 2a (RNA2), RNA-dependent RNA polymerase, 94 kDa
  3. Movement protein 3a (5' half of RNA3), 32 kDa, involved in cell-to-cell transport
  4. Coat protein 3b (3' half of RNA3 but expressed from the subgenomic RNA4), 20 kDa