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).

The human Hepatitis B virus (HBV) is the type member of the hepadnaviruses, animal infecting pararetroviruses (Nassal and Schaller, 1993; Seeger and Mason, 2000). However, concerning sequence and gene structure, plant and animal pararetroviruses are more distantly related to each other than to true retroviruses.



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.

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. 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)