|Group:||Group IV ((+)ssRNA)|
Viruses in this family are primarily soil-borne, some transmitted by fungal species of the order Chytridiales, others by no known vector. Virions may spread by water, root growth into infected soil, contact between plants, pollen, or seed, depending on the virus species. These viruses may be successfully transmitted by grafting or mechanical inoculation, and both the virion and the genetic material alone are infective.
Genome organization and replication
All Tombusviridae have a single linear genome, with the exception of Dianthoviruses, whose genome is in two parts. The genome is approximately 4.6-4.8kb in length, with a 5' cap, and it encodes 4-6 ORFs. The polymerase encodes an amber stop codon which is the site of a readthrough event within ORF1, producing two products necessary for replication. There is no helicase encoded by the virus.
Members of Tombusviridae replicate in the cytoplasm, by use of negative strand templates. The replication process leaves a surplus of positive sense (+)RNA strands, and it is thought that not only does the viral RNA act as a template for replication, but is also able to manipulate and regulate RNA synthesis.
The level of RNA synthesis has been shown to be affected by the cis-acting properties of certain elements on the RNA (such as RNA1 and 2), which include core promoter sequences which regulate the site of initiation for the complementary RNA strand synthesis. This mechanism is thought to be recognised by RNA-dependent RNA polymerase, found encoded within the genome.
Tombusviridae have been found to co-opt GAPDH, a host metabolic enzyme, for use in the replication center. GAPDH may bind to the (-)RNA strand and keep it in the replicase complex, allowing (+)RNA strands synthesized from it to be exported and accumulate in the host cell. Downregulation of GAPDH reduced viral RNA accumulation, and eliminated the surplus of (+)RNA copies.
Research has shown that infection of plants from tombusviruses contain defective interfering RNAs that are born directly from the viruses RNA genome, and no host genome. Viral DI RNAs with their small size and cis-acting elements are good templates both in vivo and in vitro on which to study RNA replication.
Sub-genomic RNA is used in the synthesis of some proteins; they are generated by premature termination of (-)strand synthesis. sgRNAs and sgRNA negative-sense templates are found in infected cells.
The family Tombusviridae includes the following genera:
- Genus Aureusvirus; type species: Pothos latent virus
- Genus Avenavirus; type species: Oat chlorotic stunt virus
- Genus Carmovirus; type species: Carnation mottle virus
- Genus Dianthovirus; type species: Carnation ringspot virus
- Genus Machlomovirus; type species: Maize chlorotic mottle virus
- Genus Necrovirus; type species: Tobacco necrosis virus A
- Genus Panicovirus; type species: Panicum mosaic virus
- Genus Tombusvirus; type species: Tomato bushy stunt virus
- Genus Umbravirus; type species: Carrot mottle virus
- Habili, N. and Symons, R. H. (1989). Evolutionary relationship between luteoviruses and other RNA plant viruses based on sequence motifs in their putative RNA polymerases and nucleic acid helicases. Nucleic Acids Research 17:23, 9543–55
- ICTVdB—The Universal Virus Database, version 3 00.074. Tombusviridae
- Wiley InterScience Encyclopedia of Life Sciences: Tombusviridae
- Lommel SA, Weston-Fina M, Xiong Z, Lomonossoff GP (September 1988). "The nucleotide sequence and gene organization of red clover necrotic mosaic virus RNA-2". Nucleic Acids Res. 16 (17): 8587–602. doi:10.1093/nar/16.17.8587. PMC 338578. PMID 3047682.
- Mizumoto H, Tatsuta M, Kaido M, Mise K, Okuno T (November 2003). "Cap-independent translational enhancement by the 3' untranslated region of red clover necrotic mosaic virus RNA1". J. Virol. 77 (22): 12113–21. doi:10.1128/JVI.77.22.12113-12121.2003. PMC 254280. PMID 14581548.
- Wang, R. and Nagy, P. (2008) Tomato bushy stunt virus Co-Opts the RNA-Binding Function of a Host Metabolic Enzyme for Viral Genomic RNA Synthesis. Cell Host & Microbe 3:3 178–187
- Castaño A, Ruiz L, Hernández C (2009) Insights into the translational regulation of biologically active open reading frames of Pelargonium line pattern virus. Virology 386(2):417–426