The Viroids

More than seven years have passed since the first monograph on viroids was published. At that time, the existence of viroids as a novel type of pathogen far smaller than viruses had been amply demonstrated and some of their unusual molecular properties had been elucidated, but the entry of molecular biology into viroid research was still in its infancy. Since that time, our knowledge of the molecular properties of viroids has increased exponentially and viroids have become even more fasci- nating than was the case seven years ago. Today, aside from transfer RNA, viroids are probably the best known type of RNA-at least from a struc- tural standpoint. Much less is known of the mechanisms of viroid func- tion, such as the exact pathway and enzymology of viroid replication and the biochemistry of viroid pathogenesis. Recently, however, emphasis in viroid research has shifted from structural to functional themes and im- portant beginnings have been made in the elucidation of viroid struc- ture-function relationships. With the discovery of viroidlike RNAs within the capsids of certain plant viruses and the finding of surprising structural similarities between viroids and plant satellite RNAs, the conceptual gap between viroids and conventional viruses has significantly narrowed. Even beyond virology, connecting links with cellular RNAs have come to light and the long isolation of viroids land "viroidologists"J has come to an end.