Viroids are the simplest RNA-based pathogens known to date. They
consist of 246–401 single-stranded, circular, naked, and non-coding
RNAs. They infect higher plants, induce serious diseases – such as
Chrysanthemum chlorotic mottle, Avocado Sunblotch and Potato
Spindle Tuber – and therefore cause considerable economical losses
[1]. It has been shown recently that viroids can replicate in yeast
and thus in other organisms than plants [2]. Approximately 30 species
of viroids are currently known and classified into two families:
Pospiviroidae and Avsunviroidae. The latter are characterized by the
presence in their sequence of a hammerhead ribozyme (HHR) which
is a small catalytic RNA motif involved in their replication through a
symmetric rolling-circle mechanism (for recent reviews see Ding [3],
Owens & Hammond [4] and Flores et al. [5]). Actually, the presence of
hammerhead ribozymes is not restricted to the viroids and they are
largely distributed in the genomes of all kinds of organisms including
human