Seminar

Viroids: the simplest (and most ancient?) RNA replicons

Ricardo Flores, PhD

Despite their name, viroids are structurally, functionally and evolutionarily independent of viruses. Viroids are exclusively composed by a small circular RNA of 250-400 nucleotides (one order of magnitude lower than the smallest viral genomes), without protein-coding ability (all viruses encode in their genomes one or more proteins). Besides, some viroid RNAs display catalytic activity, in other words, contain ribozymes (of the hammerhead class) that mediate their replication. This last property, together with their structural simplicity, has led to regard viroids as molecular fossils of the «RNA World» presumed to have preceded our present-day world base on DNA and proteins. Therefore, viroids seem to fulfill the paradigm that the simplest is the oldest. In addition to their academic interest, viroids incite diseases in plants economically relevant and, consequently, they also have an applied side. Pathogens of this class have not been described so far in animals, although the RNA of human hepatitis delta virus shares remarkable parallelisms with viroids.