Viroids: The Noncoding Genomes

Viroids are independently replicating small circular RNAs which apparently do not code for proteins. They code, in a broad sense, for a conformation which is recognized and replicated by the host cell; two viroids also express ribozyme activities which probably mediate self-cleavage of the oligomeric replicative intermediates generated by a rolling circle mechanism. Viroids are classified into subgroups according to their sequence and to the presence and type of some conserved motifs. Viroid infections which induce symptoms, do so as a result of the direct interaction of the viroid itself, or a product of its replication, with a cellular target(s) of unknown nature.