Plant pathogenic RNAs and RNA catalysis.

The rolling circle replication of small circular plant pathogenic RNAs requires a processing step to convert multimeric intermediates to monomers which are then circularized. Eleven such RNAs are known so far, two are viroids, one is viroid-like and the remainder are satellite RNAs dependent on a helper virus for replication. The processing step is RNA-catalysed in all cases, at least in vitro. All plus forms of these RNAs self-cleave via the hammerhead structure whereas only eight of the minus RNAs self-cleave, five via the hammerhead structure and three via the hairpin structure. There are about 20 other viroids where the processing mechanism has yet to be determined but they are likely candidates for a new type of self-cleavage reaction which is predicted to be conserved in all these viroids. Hepatitis delta RNA is the only circular pathogenic RNA known to self-cleave in the animal kingdom. It is feasible that more single-stranded circular pathogenic RNAs are waiting to be discovered and these could be prospective for new types of self-cleavage reactions.