Caulimoviridae

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General Overview

The Caulimoviridae are a family of viruses, including the following genera:

• Genus Badnavirus; type species: Commelina yellow mottle virus
• Genus Caulimovirus; type species: Cauliflower mosaic virus
• Genus Tungrovirus; type species: Rice tungro bacilliform virus
• Genus Soymovirus; type species: Soybean chlorotic mottle virus
• Genus Cavemovirus; type species: Cassava vein mosaic virus
• Genus Petuvirus; type species: Petunia vein clearing virus

Viruses belonging to the Caulimoviridae family are termed DNA reverse-transcribing viruses (or pararetroviruses) i.e. viruses that contain a reverse transcription stage in their replication cycle. This family contains all plant viruses that consist of a double-stranded DNA genome.

Virus Particle Structure

All viruses of this family are unenveloped. Virus particles contain nucleocapsids one of two forms; either bacilliform or isometric. The type of nucleocapsid incorporated into the virus structure determines the size of the virus. Bacilliform nucleocapsid viruses are approximately 35-50nm diameter and can be 900nm in length. Isometric nucleocapsid viruses are on average 45-50nm in diameter and show icosahdral symmetry.

Genome Structure and Replication

The genome of viruses from this family contain monopartite, double-stranded DNA in either an open circular or linear structure. The size of the genome is usually between 6000-8000 base pairs. Depending on the virus, DNA can contain either one open reading frame (ORF) as observed by the pertuvirus or up to eight ORFs such as in the soyamovirus. Proteins found to be encoded in this virus family genome include reverse transcriptase, proteases, nucleocapsids and transactivators - there are other proteins essential for replication that have yet to be assigned a specific function.

Replication takes place in both the cytoplasm and nucleus of host cells. Firstly, the viral genome enters the cytoplasm - when this occurs the viral DNA forms supercoiled mini-chromosome structures upon entering the host nucleus. The viral DNA is transcribed into polyadenylated RNA which is terminally redundant (due to transcription occurring twice for some parts of the DNA). Newly transcribed RNA enters the cytoplasm once more where it has two roles. It can either be used as a template for viral protein sysnthesis or it can undergo reverse transcription by viral encoded reverse transcriptase to make dsDNA. This DNA can then reenter the nucleus for amplification.

As replication requires the use of RNA intermediate, viruses from the Caulimoviridae family are not true dsDNA viruses - instead they are termed DNA reverse-transcribing viruses. As this property is also found in retroviruses, these families have been compared. However, there are several important differences between retroviruses and viruses from the caulimoviridae family. Unlike retroviruses, they do not require the integration of viral genome into the host's in order to replicate and for this reason their genome does not encode the enzymatic protein integrase.

References

• Notes on family; Caulimoviridae. http://www.dpvweb.net/notes/showfamily.php?family=Caulimoviridae
• Livia Stavolone, Antonio Ragozzino,and Thomas Hohn. Characterization of Cestrum yellow leaf curling virus: a new member of the family Caulimoviridae. Journal of General Virology 84 (2003), 3459-3464. http://vir.sgmjournals.org/cgi/content/full/84/12/3459
• 14th Swiss Plant Molecular and Cell Biology Conference, March 2005. http://www.unifr.ch/plantbio/skmb/abstracts.pdf