Cauliflower mosaic virus
| Cauliflower mosaic virus | |
|---|---|
| Virus classification | |
| Group: | Group VII (dsDNA-RT) |
| Family: | Caulimoviridae |
| Genus: | Caulimovirus |
| Species | |
|
Cauliflower mosaic virus |
|
Cauliflower mosaic virus (CaMV) is the type member of the caulimoviruses, one of the six genera in the Caulimoviridae family, pararetroviruses that infect plants (Pringle, 1999). Pararetroviruses replicate through reverse transcription just like retroviruses, but the viral particles contain DNA instead of RNA (Rothnie et al., 1994).
Contents |
[edit] Structure
The CaMV particle is an icosahedron with a diameter of 52 nm built from 420 capsid protein (CP) subunits arranged with a triangulation T = 7, which surrounds a solvent-filled central cavity (Cheng et al., 1992). It contains a circular double-stranded DNA molecule of about 8.0 kB, interrupted by sitespecific discontinuities resulting from its replication by reverse transcription. After entering the host, the single stranded nicks in the viral DNA are repaired, forming a supercoiled molecule that binds to histones. This DNA is transcribed into a full length, terminally redundant 35S RNA and a subgenomic 19S RNA.
[edit] Genome
The promoter of the 35S RNA is a very strong constitutive promoter responsible for the transcription of the whole CaMV genome. It is well known for its use in plant transformation. It causes high levels of gene expression in dicot plants. However, it is less effective in monocots, especially in cereals. The differences in behavior are probably due to differences in quality and/or quantity of regulatory factors. The promoter was named CaMV 35S promoter ("35S promoter") because the coefficient of sedimentation of the viral transcript, whose expression is naturally driven by this promoter, is 35S. It is one of the most widely used, general-purpose constitutive promoters. It was discovered at the beginning of the 1980s, by Chua and collaborators at the Rockefeller University.
The 35S RNA is particularly complex, containing a highly structured 600 nucleotide long leader sequence with six to eight short open reading frames (ORFs) (Fütterer et al., 1988; Pooggin et al., 1998).
This leader is followed by seven tightly arranged longer ORFs that encode all the viral proteins (reviewed by Hohn and Fütterer, 1997). The mechanism of expression of these proteins is very special. The ORF VI protein (encoded by the 19S RNA) controls translation reinitiation of major open reading frames on the polycistronic 35S RNA, which is normally only happens on bacterial mRNA's. TAV function depends on its association with polysomes and eukaryotic initiation factor eIF3 (Park et al., 2001).
- ORF I - Movement protein
- ORF II - Insect transmission factor
- ORF III - the protein translated shows a non-specific DNA binding activity. it is a structural protein in virus particle.
- ORF IV - Capsid protein
- ORF V - Protease, reverse transcriptase and RNaseH
- ORF VI - Translational activator / Inclusion body protein
- ORF VII - Unknown (dispensable)
[edit] Replication
CaMV replicates by reverse transcription. Initially all the gaps present in the genome are sealed and the covalently closed DNA then associates with host histones to form a super coiled mini chromosome. Transcription of this mini chromosome produces 35s RNA which translates protein as well as forming dsDNA by the process of reverse transcription. New viral particles are produced which are targeted to inclusion bodies & are released outside.
The Cauliflower Mosaic Virus promoter (CaMV 35S) is used in most transgenic crops to activate foreign genes which have been artificially inserted into the host plant. It is inserted into transgenic plants in a form which is different to that found when it is present in its natural brassica plant hosts. This enables it to operate in a wide range of host-organism environments which would otherwise not be possible.
CaMV contains about 8 kb double-strand DNA genome and produces spherical particles. CaMV infections are systemic, and even its DNA is infectious when inoculated on abraded plant surfaces. The CaMV genome has 8 tightly packed genes, of which only two small genes, genes II and VII, are nonessential; as a result, only these two genes can be replaced/deleted without a loss of infectivity. In addition, modified CaMV genomes exceeding the natural genome size (8024 bp) by even a few hundred bp are not packaged into virions. These two factors seriously limit the size of DNA insert clonable in CaMV. Bacterial dhfr (dihydrofolate reductase) gene was inserted in the CaMV genome in place of gene II, and was successfully expressed in plants. Cauliflower mosaic virus (CaMV) is the type member of the caulimoviruses, one of the six genera in the Caulimoviridae family, pararetroviruses that infect plants (Pringle, 1999). Pararetroviruses replicate through reverse transcription just like retroviruses, but the viral particles contain DNA instead of RNA (Rothnie et al., 1994). Structure
