The prototype of a plant virus with double-stranded DNA is the cauliflower mosaic-virus (CaMV). Double-stranded plant viruses, too, have been considered in genetic engineering. It is tried to convert them into vectors in order to put foreign DNA into plant cells similar to the way the plasmid of Agrobacterium tumefaciens is used.
CaMV-DNA is normally not incorporated constantly into the host cell genome (R. SHEPHERD, R. J. WAKEMAN, 1971). CaMV is the collective name for a group of tightly related viral species usually transmitted by aphids. Each species has a narrow host specificity, overlapping with that of other species is rare. The virion is spherical with a diameter of about 50 nm. Most likely, the capsid consists of 420 identical subunits with molecular weights of 42,000. The DNA contains about 8,000 base pairs. In the case of one species, it has been sequenced and 8,024 base pairs were found. Neutron diffraction experiments showed that the DNA is sandwiched between the protein subunits. The core of the viral particle contains no proteins or nucleic acids. One of the three ring-shaped DNA molecules is broken up in three different places. Treatment of the viral DNA with SI-endonuclease, a DNAse breaking down single-stranded DNA, renders therefore three fragments of defined length. Further analyses showed that the ring is no closed, uniform structure, but that it results from the combining of three molecules kept together at their ends by complementary sequences.
Symptoms occur two to three weeks after infection and can be recognized by the mosaic-like lesions of infected leaves. The virus spreads systemically, its secondary symptoms are similar to those of the primary infection. Leaves that were infected during their development display deformed leaf blades.
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