Abstract
Abstract Viruses with genomes of positive strand (messenger-sense) RNA are found in animals, plants, and bacteria. Examples include poliovirus, foot and mouth disease virus, flockhouse virus (FHV), tobacco mosaic virus (TMV), cucumber mosaic virus (CMV), and the bacteriophages MS2 and 013 (1). However, they are most abundant in plants. About 77% (approximately 500) of known plant viruses have positive strand RNA genomes (2). Enzymes that replicate RNA templates (RNA-dependent RNA polymerase, RdRp) apparently do not have proof-reading activities and have error rates of 10-4 to 10-5 (3). This may well have limited the size of viral RNA genomes, the largest of which among plant viruses are the closteroviruses (about 20 kb) and among animal viruses the coronaviruses (about 32 kb). Because of the high error rate, RNA of a given virus may be a population in dynamic equilibrium, with viable mutants arising at a high rate on the one hand and being selected against on the other.
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