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).

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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 translated into a full length, terminally redundant 35S RNA and a subgenomic 19S RNA.

Genome

The promoter of the 35S RNA is very strong, and well known for its use in plant transformation. 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 leads to viral shedding. 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 
ORF IV  Capsid protein
ORF V   Protease, reverse transcriptase and RNaseH
ORF VI  Translational activator / Inclusion body protein 
ORF VII Unknown (dispensable)

Replication

CaMV replicates by reverse transcription. Initially all the gaps present in the genome gets sealed. The covalently closed DNA associates with host histones to form a super coiled mini chromosome. Transcription of the former produces 35s RNA which translates protein as well as forms dsDNA by the process of reverse transcription. New viral particles are produced which gets targeted to inclusion body & is released outside.

Process of Infection

CaMV produces a helper component ( product of coding region II & III) which attracts aphids thus leading to its transmission.

References

• Cheng, R.H., Olson, N.H., and Baker, T.S. (1992). Cauliflower mosaic virus: a 420 subunit (T=7), multilayer structure. Virology 186, 655-668.
• Fütterer, J., Gordon, K., Bonneville, JM., Sanfaçon, H., Pisan, B., Penswick, J.R., and Hohn, T. (1988). The leading sequence of caulimovirus large RNA can be folded into a large stem-loop structure. Nucl.Acids Res. 16, 8377-8390.
• Hohn, T. and Fütterer, J. (1997). The proteins and functions of plant pararetroviruses: knowns and unknowns. Crit.Rev.Plant Sci. 16, 133-161.
• Nassal, M. and Schaller, H. (1993). Hepatitis B virus replication. Trends Microbiol. 1, 221-228.
• Park HS, Himmelbach A, Browning KS, Hohn T, Ryabova LA (2001). A plant viral "reinitiation" factor interacts with the host translational machinery. Cell 106, 723-733.
• Pooggin, M.M., Hohn, T., and Futterer, J. (1998). Forced evolution reveals the importance of short open reading frame A and secondary structure in the cauliflower mosaic virus 35S RNA leader. J.Virol. 72, 4157-4169.
• Pringle, C.R. (1999). Virus nomenclature [editorial]. Arch. Virol. 144, 1463-1466.
• Rothnie, H.M., Chapdelaine, Y., and Hohn, T. (1994b). Pararetroviruses and retroviruses: a comparative review of viral structure and gene expression strategies. Adv.Virus Res. 44, 1-67.
• Seeger, C. and Mason, W.S. (2000). Hepatitis B virus biology. Microbiol.Mol.Biol.Rev. 64, 51-68.