Splitting of RNA-dependent RNA polymerase is common in Narnaviridae: Identification of a type II divided RdRp from deep-sea fungal isolates

Until recently, it was accepted that RNA-dependent RNA polymerase (RdRp) is the only essential gene for non-retro RNA viruses and is encoded by a single open reading frame (ORF) in their genomes. However, divided RdRps that are coded by two ORFs were discovered in fungal RNA viruses in a few independent reports. This discovery showed higher plasticity of viral RdRp than was expected. Among these divided RdRps, the division site was common; specifically, the first part of the RdRp contains motifs F, A, and B, whereas the latter part possesses motifs C and D. These RdRps are designated as type I divided RdRp and have been limited to viruses in a specific clade of Narnaviridae. In this study, to further understand the plasticity of RdRp, we explored viruses from deep sea-derived fungal strains as an untapped resource with a focus on Aspergillus section Versicolores. Seven strains were found to be infected by a total of 13 viruses, and the viral RNA genomes were determined by fragmented and primer-ligated double-stranded RNA sequencing technology. Among them, six strains belong to Narnaviridae. One of the strains, Aspergillus tennesseensis narnavirus 1, which infects an Aspergillus tennesseensis, has a divided RdRp with a new division site (referred to as type II divided RdRp). A couple of sequences for possible type II divided RdRps were also detected in public metagenomic data sets. Our findings reveal that different types of divisions in RdRp are present in the virosphere, and two types of RdRp splitting occurred independently within Narnaviridae.

Automated summary

Recently discovered divided RdRps in fungal RNA viruses challenge the previous understanding that RdRp is encoded by a single ORF, demonstrating higher plasticity in viral RdRp. The identification of two types of RdRp splitting in specific clades of Narnaviridae suggests independent evolutionary events within the virosphere.