A unique Nodavirus with novel features: Mosinovirus expresses two subgenomic RNAs, a capsid gene of unknown origin, and a suppressor of the antiviral RNAi pathway.

Journal of virology

PubMedID: 25210176

Schuster S, Zirkel F, Kurth A, van Cleef KW, Drosten C, van Rij RP, Junglen S. A unique Nodavirus with novel features: Mosinovirus expresses two subgenomic RNAs, a capsid gene of unknown origin, and a suppressor of the antiviral RNAi pathway. J Virol. 2014;.
Insects are a reservoir for many known and novel viruses. We discovered an unknown virus, tentatively named Mosinovirus (MoNV), in mosquitoes from a tropical rainforest region in Côte d'Ivoire. The MoNV genome consists of two segments of positive-sense RNA of 2,972 nt (RNA 1) and 1,801 nt (RNA 2) in length. Its putative RNA-dependent RNA polymerase shares 43% amino acid identity with its closest relative Pariacoto virus (family Nodaviridae). Unexpectedly, for the putative capsid protein maximal pairwise identity of 16% was found to Lake Sinai virus 2, an unclassified virus with a non-segmented RNA genome. Moreover, MoNV virions are non-enveloped and about 50 nm in diameter, larger than any of the known nodaviruses. Mature MoNV virions contain capsid proteins of ~56 kDa, which do not seem to be cleaved from a longer precursor. Northern blot analyses revealed that MoNV expresses two subgenomic RNAs of 580 nt (RNA 3) and 292 nt (RNA 4). RNA 4 encodes a viral suppressor of RNAi that shares its mechanism with the B2 RNAi suppressor protein of other nodaviruses despite lacking recognizable similarity to these proteins. MoNV B2 binds long dsRNA and, accordingly, inhibits Dicer-2-mediated processing of dsRNA into siRNAs. Phylogenetic analyses indicate that MoNV is a novel member of the Nodaviridae family that acquired its capsid gene via reassortment from an unknown, distantly related virus beyond the family level.IMPORTANCE
The identification of novel viruses provides important information about virus evolution and diversity. Here, we describe an unknown unique nodavirus in mosquitoes, named Mosinovirus (MoNV). MoNV was classified as a nodavirus based on its genome organization and on phylogenetic analyses of the RNA-dependent RNA polymerase. Notably, its capsid gene was acquired from an unknown virus in distant relationship to nodaviruses. Another remarkable feature of MoNV was that, unlike other nodaviruses, it expresses two sgRNAs. One of the sgRNAs expresses a protein that counteracts antiviral defense of its mosquito host, whereas the function of the other sgRNA remains unknown. Our results show that complete genome segments can be exchanged beyond the species level and suggest that insects harbor a large repertoire of exceptional viruses.