Figure 2.

The rolling-circle mechanism in its (A) asymmetric and (B) symmetric variants is proposed for the replication of viroids of the families Pospiviroidae in the nucleus and Avsunviroidae in chloroplasts, respectively. In both cases, the positive and negative viroid RNA polarities are represented in orange and blue, respectively. Host proteins and viroid RNA motifs involved in the replicative cycle are indicated. Arrowheads indicate RNA cleavage sites. -P, -OH and >P, 5′-phosphate, 5′-hydroxyl and 2′,3′-phosphodiester groups, respectively; HF?, unknown host factor; HHR, hammerhead ribozyme; IMPa-4, importin alpha-4; NEP, nuclear-encoded chloroplastic DNA-dependent RNA polymerase; RPL5, ribosomal protein L5; TFIIIA-7ZF/-9ZF, transcription factor IIIA splicing variants with seven or nine zinc fingers, respectively; and Virp-1, bromodomain-containing protein 1.