“Derailing the host machinery to achieve replication: how Viroid and Viroid-like RNAs successfully copy their genomes in hostile territory”

Circular infectious RNAs have been known for several decades. Their biology has been intriguing from the beginning partly due to the antithesis between their efficiency and tiny size. Amongst infectious circular RNAs viroids hold a special place not only because they were the first to be characterized as such but also because they have been extensively studied as a group. Viroids do not encode proteins and therefore have to rely for their biological cycle on the host factors. As a result, the identification and functional characterization of host factors enabling their biological cycle has been of prime importance to the community. With the advent of high throughput sequencing technologies, viroid-like infectious RNAs have been found in plants, fungi, and animals, including mammals, making understanding their biology even more interesting and important. In this review, we summarize what is known about the replication of these tiny yet very efficient infectious RNAs.