<i>Ambiviricota</i> , a novel ribovirian phylum for viruses with viroid-like properties
Jens H. Kuhn, Leticia Botella, Marcos de la Peña, Eeva J. Vainio, Mart Krupovìč, Benjamin D. Lee, Beatriz Navarro, Sead Sabanadzovic, Peter Simmonds, Massimo Turina
Abstract
ABSTRACT Fungi harbor a vast diversity of mobile genetic elements (MGEs). Recently, novel fungal MGEs, tentatively referred to as ‘ambiviruses,’ were described. ‘Ambiviruses’ have single-stranded RNA genomes of about 4–5 kb in length that contain at least two open reading frames (ORFs) in non-overlapping ambisense orientation. Both ORFs are conserved among all currently known ‘ambiviruses,’ and one of them encodes a distinct viral RNA-directed RNA polymerase (RdRP), the hallmark gene of ribovirian kingdom Orthornavirae . However, ‘ambivirus’ genomes are circular and predicted to replicate via a rolling-circle mechanism. Their genomes are also predicted to form rod-like structures and contain ribozymes in various combinations in both sense and antisense orientations—features reminiscent of viroids, virusoids, ribozyvirian kolmiovirids, and yet-unclassified MGEs (such as ‘epsilonviruses,’ ‘zetaviruses,’ and some ‘obelisks’). As a first step toward the formal classification of ‘ambiviruses,’ the International Committee on Taxonomy of Viruses (ICTV) recently approved the establishment of a novel ribovirian phylum, Ambiviricota , to accommodate an initial set of 20 members with well-annotated genome sequences.