Viroids

Abstract

Viroids are the smallest known agents of infectious disease - small, highly structured, single-stranded, circular RNA molecules that lack detectable messenger RNA activity. Whereas viruses supply some or most of the genetic information required for their replication, viroids are regarded as obligate parasites of the cell's transcriptional machinery and infect only plants. Four of the nearly 30 species of viroids described to date contain hammerhead ribozymes, and phylogenetic analysis suggests that viroids may share a common origin with hepatitis delta virus and several other viroid-like satellite RNAs. Replication proceeds via a rolling-circle mechanism, and strand exchange can result in a variety of insertion/deletion events. The terminal domains of potato spindle tuber and related viroids, in particular, appear to have undergone repeated sequence exchange and/or rearrangement. Viroid populations often contain a complex mixture of sequence variants, and environmental stress (including transfer to different hosts) has been shown to result in a significant increase in sequence heterogeneity. The new field of synthetic biology offers exciting opportunities to determine the minimal size of a fully functional viroid genome. Much of the preliminary structural and functional information necessary is already available, but formidable obstacles still remain. © 2008 Springer Berlin Heidelberg.