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

Abstract

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 Importance of the KR-Rich Region of the Coat Protein of Ourmia melon virus for Host Specificity, Tissue Tropism, and Interference With Antiviral Defense

“…However, the mechanisms connecting silencing suppression and the phenomena involving tissue tropism, host range, pathogenicity, and RNA affinity need additional clarification. The ability of OuMV to infect a wide range of dicotyledonous plants (Rastgou et al 2009) and, in particular, Nicotiana spp. and Arabidopsis, may provide different avenues for basic and applied research on this phenomenon.…”

“…Sodium dodecyl sulfate polyacrylamide gel electrophoresis of total leaf extracts and subsequent Western blot were carried out as previously described (Rastgou et al 2009). Anti-CP antibodies (purified IgG from antiserum A255 from the IVV collection) were diluted 1:2,000 in blocking buffer.…”

“…In particular, OuMV RdRp shares distant similarity only with RdRp of narnaviruses, a group of viruses which mainly infects fungi; however, the MP is distantly related to MP of plant tombusviruses. The CP has limited but significant similarity with CP of plant and mammalian viruses, although its phylogenetic origin remains uncertain (Rastgou et al 2009). Another peculiarity of the OuMV CP is the unique morphology of the virions among plant viruses: bacilliform particles of three different lengths with pointed ends (Lisa et al 1988).…”

“…Three plus-strand RNA molecules form the OuMV genome and each segment encodes a single protein: RNA1 directs synthesis of an RNA-dependent RNA polymerase (RdRp), RNA2 encodes a movement protein (MP), and RNA3 specifies the 22-kDa coat protein (CP) (Rastgou et al 2009). Ourmiaviruses have affinities with other postive single stranded RNA multipartite viruses, such as the members of the family Bromoviridae, but their unique structural and molecular features have resulted in their classification in a novel viral family.…”

Molecular mechanisms regulating plant infection in the rice blast fungus Magnaporthe oryzae

“…The Narnaviruses lack the coat protein (CP) that forms the capside structure or the movement protein (MP) . Phylogenetically, the narnaviruses are the closest relatives of the Ourmiavirus plant genus (Rastgou, et al, 2009. Plant ourmiaviruses contain a tripartite ssRNA genome that codes for three proteins, the RdRp, the CP and the MP (Rastgou, et al, 2009).…”