Human hepatitis D virus and plant viroids: trans-kingdom similarities between small infectious circular RNAs

The limits of all scales, from the expanding universe to the subatomic particles in the physical world, and from dinosaurs to the minimal replicating entities in the biological world, spark a particular interest. The latter entities are viroids (often inducing plant diseases) and human Hepatits D (HDV), with genomes just composed of circular RNAs of 250-430 and 1700 nucleotides, respectively, adopting compact secondary structures. These RNAs either donot code for any protein or just for a small one in the HDV antigenomic strand, and therefore, depend for replicarion (via anRNA-RNA rolling-cycle mechanism including RNA elongation, cleavage and ligation) on host enzymes and, remarkably, on ribozymes "encoded" in the strand of either polarity of some viroids (family Avsunviroidae) and HDV. Their catalytic nature and small size have led to propose that they might be remants of the RNA world assumed to have existed on Earth before the advent of proteins and DNA. Even if identified in two separated kingdoms (Plantae and Animalia), research on viroids and HDV has benefited from a mutual cross-pollination.