Persistence of ambigrammatic narnaviruses requires translation of the reverse open reading frame

Narnaviruses are RNA viruses detected in diverse fungi, plants, protists, arthropods and nematodes. Though initially described as simple single-gene non-segmented viruses encoding RNA-dependent RNA polymerase (RdRp), a subset of narnaviruses referred to as “ambigrammatic” harbor a unique genomic configuration consisting of overlapping open reading frames (ORFs) encoded on opposite strands. Phylogenetic analysis supports selection to maintain this unusual genome organization, but functional investigations are lacking. Here, we establish the mosquito-infecting Culex narnavirus 1 (CxNV1) as a model to investigate the functional role of overlapping ORFs in narnavirus replication. In CxNV1, a reverse ORF without homology to known proteins covers nearly the entire 3.2 kb segment encoding the RdRp. Additionally, two opposing and nearly completely overlapping novel ORFs are found on the second putative CxNV1 segment, the 0.8 kb “Robin” RNA. We developed a system to launch CxNV1 in a naïve mosqu...