Abstract
Human hepatitis delta virus (HDV) is a unique infectious agent whose genome is composed of a small circular RNA. Recent data, however, have reported the existence of highly divergent HDV-like circRNAs in the transcriptomes of diverse vertebrate and invertebrate species. The HDV-like genomes described in amniotes such as birds and reptiles encode self-cleaving RNA motifs or ribozymes similar to the ones present in the human HDV, whereas no catalytic RNA domains have been reported for the HDV-like genomes detected in metagenomic data from some amphibians, fish, and invertebrates. Herein, we describe the self-cleaving motifs of the HDV-like genomes reported in newts and fish, which belong to the characteristic class of HDV ribozymes. Surprisingly, HDV-like genomes from a toad and a termite show conserved type III hammerhead ribozymes, which belong to an unrelated class of catalytic RNAs characteristic of plant genomes and plant subviral circRNAs, such as some viral satellites and viroids. Sequence analyses revealed the presence of similar HDV-like hammerhead ribozymes encoded in two termite genomes, but also in the genomes of several dipteran species. In vitro transcriptions confirmed the cleaving activity for these motifs, with moderate rates of self-cleavage. These data indicate that all described HDV-like agents contain self-cleaving motifs from either the HDV or the hammerhead class. Autocatalytic ribozymes in HDV-like genomes could be regarded as interchangeable domains and may have arisen from cellular transcriptomes, although we still cannot rule out some other evolutionary explanations.