Plant Viruses with a Multipartite Genome

Abstract

If a viral genome consists of two or more nucleic acid molecules of different length, which are separately encapsidated, the resulting particles are likely to differ in sedimentation rate but will not necessarily differ in size and buoyant density. The different classes of particles, which are called the components of the virus, can be separated by physical means. This chapter summarizes the different modes of particle heterogeneity that have been described and the methods suited for the detection and separation. If the size of the capsid is adapted to the size of the genome parts, the particles can be separated by electrophoresis in sieving media provided that an adequate pore size can be found and by sedimentation in density gradients. Since the RNA:protein ratio, and thus the buoyant density, does not differ appreciably, the particles cannot be separated by banding in equilibrium gradients. Tubular (tobraviruses) and bacilliform viruses (alfalfa mosaic virus) as well as spherical viruses (tobacco streak virus, pea enation mosaic virus) belong to this class. The particles of como- and nepoviruses do not differ in size, and thus cannot be separated by electrophoresis in sieving media, but they sediment and band differently. Separation of the parts of the genome can only be accomplished after the extraction of the RNA's. It is conceivable that viruses exist that have genome parts of the same length. The nepovirus tomato ring-spot virus approaches this situation. In the animal DNA virus, adeno-associated virus the complementary strands are present in different particles. In preparations of bromo- and cucumoviruses, a similar RNA is found encapsidated together with the smallest part of the genome.