Viroids, subviral pathogens of plants, are composed of a single-stranded 
circular RNA of 246-399 nucleotides. Within the 27 viroids sequenced, avocado 
sunblotch, peach latent mosaic and chrysanthemum chlorotic mottle viroids 
(ASBVd, PLMVd and CChMVd, respectively) can form hammerhead structures in both 
of their polarity strands. These ribozymes mediate self-cleavage of the 
oligomeric RNAs generated in the replication through a rolling circle mechanism, 
whose two other steps are catalyzed by an RNA polymerase and an RNA ligase. 
ASBVd, and presumably PLMVd and CChMVd, replicate and accumulate in the 
chloroplast, whereas typical viroids replicate and accumulate in the nucleus. 
PLMVd and CChMVd do not adopt a rod-like or quasi rod-like secondary structure 
as typical viroids do but have a highly branched conformation. A pathogenicity 
determinant has been mapped in a defined region of the CChMVd molecule.