Contrary to earlier beliefs, viruses are not the smallest causative agents of 
infectious diseases. Single-stranded RNAs as small as 246 nucleotides exist in 
certain higher plants and cause more than a dozen crop diseases. These RNAs have 
been termed viroids. Despite their extremely limited information content, 
viroids replicate autonomously in susceptible cells--that is, they do not 
require helper functions from simultaneously replicating conventional viruses. 
Viroids are covalently closed circular molecules with a characteristic rodlike 
secondary structure in which short helical regions are interrupted by internal 
and bulge loops. Viroids are not translated; they are replicated by a host 
enzyme (or enzymes) (probably RNA polymerase II) via oligomeric RNA 
intermediates by a rolling circle mechanism. Viroidlike satellite RNAs resemble 
viroids in size and molecular structure, but are found within the capsids of 
specific helper viruses on which they depend for their own replication. These 
RNAs are of great interest to molecular biology for at least two reasons: 1) 
they are the smallest and simplest replicating molecules known, and 2) they may 
represent living fossils of precellular evolution in a hypothetical RNA world.