Viroid processing: a model involving the central conserved region and hairpin I.

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Abstract
A model is proposed for the processing of oligomeric viroid replication intermediates into monomeric, circular progeny viroids. The model identifies a thermodynamically extremely stable base-paired configuration that partially or completely dimeric, as well as higher, viroid oligomers can assume and postulates that this structure, which involves structural features common to all viroids (the central conserved region and secondary hairpin I), is essential for precise cleavage and ligation. The model explains why recombinant plasmids containing tandem repeats of two or more viroid sequence equivalents are highly infectious when inoculated into viroid-susceptible plants, why certain plasmids containing partially duplicated viroid-specific inserts are less infectious, and why plasmids containing monomeric inserts are noninfectious or at best marginally infectious. The model also accounts for the fact that vector-derived sequences on either or both sides of the viroid sequence(s) of a restriction fragment are precisely excised and are lacking in progeny viroids.