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Molecular characterization of the plant virus genus Ourmiavirus and evidence of inter-kingdom reassortment of viral genome segments as its possible route of origin

¹ Plant Protection Department, Faculty of Horticultural Science & Plant Protection, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran
² Department of Plant Protection, College of Agriculture, Urmia University, Urmia, Iran
³ Plant Virology Research Center, Shiraz University, Shiraz, Iran
⁴ Istituto di Virologia Vegetale, CNR, Strada delle Cacce 73, 10135 Torino, Italy
⁵ National Center for Biotechnology Information, National Library of Medicine, National Institutes of Health, Bethesda, MD 20894, USA

Correspondence
Massimo Turina
m.turina{at}ivv.cnr.it

Ourmia melon virus (OuMV), Epirus cherry virus (EpCV) and Cassava virus C (CsVC) are three species placed in the genus Ourmiavirus. We cloned and sequenced their RNA genomes. The sizes of the three genomic RNAs of OuMV, the type member of the genus, were 2814, 1064 and 974 nt and each had one open reading frame. RNA1 potentially encoded a 97.5 kDa protein carrying the GDD motif typical of RNA-dependent RNA polymerases (RdRps). The putative RdRps of ourmiaviruses are distantly related to known viral RdRps, with the closest similarity and phylogenetic affinity observed with fungal viruses of the genus Narnaviridae. RNA2 encoded a 31.6 kDa protein which, expressed in bacteria as a His-tag fusion protein and in plants through agroinfiltration, reacted specifically with antibodies made against tubular structures found in the cytoplasm. The ORF2 product is significantly similar to movement proteins of the genus Tombusviridae, and phylogenetic analysis supported this evolutionary relationship. The product of OuMV ORF3 is a 23.8 kDa protein. This protein was also expressed in bacteria and plants, and reacted specifically with antisera against the OuMV coat protein. The sequence of the ORF3 protein showed limited but significant similarity to capsid proteins of several plant and animal viruses, although phylogenetic analysis failed to reveal its most likely origin. Taken together, these results indicate that ourmiaviruses comprise a unique group of plant viruses that might have evolved by reassortment of genomic segments of RNA viruses infecting hosts belonging to different eukaryotic kingdoms, in particular, fungi and plants.

The GenBank/EMBL/DDBJ accession numbers of the sequences reported in this paper are: EU770623 [GenBank] , EU770624 [GenBank] and EU770625 [GenBank] for RNA1, RNA2 and RNA3 of OuMV; EU770620 [GenBank] , EU770621 [GenBank] and EU770622 [GenBank] for RNA1, RNA2 and RNA3 of EpCV; and FJ157981 [GenBank] , FJ157982 [GenBank] and FJ157983 [GenBank] for RNA1, RNA2 and RNA3 of CsVC.

A supplementary figure, three tables and three sequence alignments (in FASTA format) are available with the online version of this paper.