Figure 1.

Rolling circle mechanism proposed for the replication of viroids. Members of the families Avsunviroidae (A) and Pospiviroidae (B) follow symmetric (two rolling circles) and asymmetric (one rolling circle) alternative versions of the mechanism, respectively. Orange and blue colors refer to (+) (by convention the most abundant in vivo) and (−) RNA polarities, respectively, with cleavage sites indicated by arrowheads. The enzymes and ribozymes catalyzing the replication steps are reported: the nuclear DNA Pol II and the plastid DNA-dependent NEP are redirected to transcribe RNA instead of DNA templates; the DNA ligase 1 is redirected to circularize RNA inestead of DNA substrates.