Figure 6.

Initiation of PSTVd replication by circularized (−)PSTVd RNA. Viroid replication may involve as many as six separate steps: i.e., synthesis of multimeric (−)RNAs and (+)RNAs (steps 1 and 4), cleavage of these multimeric RNAs (steps 2 and 5), and circularization of the resulting linear (−)RNA and (+)RNA monomers (steps 3 and 6). During normal asymmetric replication (A), multimeric linear (−)PSTVd RNAs (boxed) serve as template for synthesis of (+)PSTVd progeny. In the symmetric rolling circle mechanism used by ASBVd and certain satellite RNAs (B), the corresponding multimeric (−)strand RNAs are cleaved and circularized before acting as template for (+)strand synthesis. Thus, the initial replicative steps after inoculation with precisely full length (−)PSTVd or (−)TPMVd RNA may resemble steps 3–6 of the symmetric mechanism.