The structural system in the small RNA viruses is the same for all the viruses examined so far, with the possible exceptions of the pea enation mosaic virus middle componenet, tobacco streak virus and alfalfa mosaic virus. While there is a greater variation in the structural arrangements of the different viruses, all appear to ppossess icosahedral symmetry or a structure based on the such a symmetry.

All the viruses contain single-stranded RNA. Most contain only one species of RNA with a mol. wt. of approxiamately 1.1 x 10 for the paheges and 2.6 x 10fot eh vertebrate viruses genomes in which two or three different species of RNA are required for inspection. The possession of a split genome may not be confined to the smal RNA viruses of plants in viewe of the recent observations with Nodamura virus referred to above. This virus is particularly interesting in this respect since it inffects both vertebrate and invertebrate hosts. Little is known about the RNAs of the other invertebrate viruses

The number of proteins in the viruses varies from 1 to 4. This great variation compared with, for example, the rhabdoviruses presumably can be tolerated because of the very effective packaging invol ved in icosahefral symmetry.

One of the most exciting prospects in the small RNA viruses is the understandin g of the way in which the replication of the prospects of the split genome viruses is organiozed. The determination of this requires a suitable technique which may be provided by the protoplast system (Takebe & Ots, 1969). A study of the replication of Nodamura virus in mice, bees or waxmoths, which it kills, and in mosquitoes, whick it does not kill, also provide an engaging comparison.

The small RNA viruses from different organisms each have characteristics properties. for example, most of the viruses from vertebrates have a single species of RNA of mol. wt. about 2.5 x10 and four capsid proteins whereas most of those from bacteria have an RNA of about 1 x 10 and two capsied proteins. These differences probably reflect the adaptations of the baic desig n of the vitrus to the various host systems.

the authors would like to thank Prigfessor J.B. Ba ncroft for the f migure he generously supplied and also t he Academic Press for permitting its use.


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