PLANT VIRUSES

1. Gross Characteristics
   1. Highly varied in morphologies
   2. Often not very host specific
   3. No attachment mechanisms are known
   4. Only one gene needed - for capsomers - yet much larger
      (Because most plants possess RNA-dependent RNA polymerase)
   5. Multiple genomes - What's the evolutionary good of this?
   6. Both (+) and (-) RNA viruses are known
   7. Generally very stable and infectious in environment for a long time. Easily isolated from cigarettes and cigars, for example.
   8. Satellite viruses

2. EXAMPLES
   1. Tobacco Necrosis Virus, TNV
      1. Medium sized, ocosahedral (+)RNA
      2. (+)RNA = 1.4x10⁶ dal (= ______ cistrons)
      3. The RNA is infectious in purified form

   2. Tobacco Necrosis Satellite Virus, TNSV
      1. Very small icosahedral (+)RNA virion
      2. (+)RNA = 4 x 10⁵ dal (= ______ cistrons)
      3. Obligatory parasite of TNV infected cells

   3. Tobacco Mosaic Virus, TMV (one of the more studied plant viruses)
      1. Virion = 18 x 300 nm helical tube of 130 turns of 2130 capsomers
      2. First virus to be reconstituted
      3. (+)RNA = 2x10⁶ dal (= _____ cistrons)
      4. Purified RNA is infectious
      5. Method of normal assembly
         1. Aggregation of starter double disk
         2. Association of the double disk with the assembly initiation site on the RNA
            • about 800 bases away from the 3'-end
            • elsewhere on other tubular viruses
         3. Double disk accretes more capsomers
            • genomic RNA is sandwiched between the helically placed disk capsomers
            • remainder of RNA is threaded up through the middle of the tube
      6. Cotranslational disassembly
      7. A tRNA-like genome
         1. 3'-end wraps in a configuration very much like a tRNA
         2. 3'-end: A-C-C-C-Š (just like all tRNA's)
         

      8. Like eukaryotic mRNA, genomic RNA is "capped:" Š-G(A)-5'-5'-(7-methyl-G) (But many other plant viral RNA's are not capped.)

   4. Asters Yellows Virus - heat labile

3. Some Plant-Virus HURDLES
   1. The Problems of a polycistronic message in a monocistronic universe
   2. How to replicate the polycistronic message for more virionic RNA

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