Figure 2.

Replication cycle of viroids. (A) Members of Pospiviroidae replicate in an asymmetric cycle. DNA-dependent RNA polymerase (polII) transcribes mature circular RNA into oligomeric (−) intermediates and these into oligomeric (+) intermediates. The (+) intermediates are enzymatically cleaved into monomers and ligated with DNA ligase 1 into mature circles. (B) Members of Avsunviroidae replicate in a symmetric cycle. Circular RNA is transcribed into oligomeric intermediates by nuclear-encoded polymerase (NEP). The intermediates are cleaved into monomeric units by the internal hammerhead ribozyme and ligated by tRNA ligase into circles. Molecules are shown here without structure, but the presence of thermodynamically stable as well as metastable structures (Figure 11) is critical for each step of the different replication cycles.