THE VIROID

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Until about twelve years ago, it was generally believed that all infectious diseases of plants and animals were caused either by microorganisms, such as bacteria and fungi, or by viruses. However, in the last decade, dis ease-causing agents that are smaller and less complex than viruses have come to light, and today about a dozen diseases of higher plants are known to be caused by the small, naked molecules of RNA called viroids. Further, it has been shown that the agent of scrapie, a neurolog ical disease of sheep and goats, has unusual properties that distinguish it from both viruses and viroids. The term prion has been proposed for this agent, which ap pears to be even smaller than a viroid and which, in contrast to the viroid, contains an essential protein. This article will summarize what is known of the physical, chemical, and biological properties of these subviral pathogens, as well as examine their possible origins and relationships with normal cellular constituents. Because the smallest known viruses capable of replicating themselves contain genomes amounting to a molecular weight of about 1 million, it has long ap peared reasonable to assume that this size represents the minimum of genetic information required by a virus for its various functions: coding for virus-specified products and subjugating the metabolism of the host cell. Indeed, viruses with smaller genomes are known to exist but are not capable of independent replication; instead, certain functions must be provided by a helper virus present in the same cell. In the absence of helper virus, no repli cation of these defective, or satellite, viruses takes place. Viroids, on the other hand, introduce a far smaller amount of genetic information into their host cells than do viruses, and yet their replication apparently does not require the assistance of helper viruses. The existence of the viroid was therefore unexpected, and the story of its discovery is a surprising one.