Nonplant Endornaviruses

dsRNAs are found not only in plants, but also frequently in a variety of fungi; in fact, they have been recorded in all major fungal taxonomic groups, including various plant pathogens. The genomes of most mycoviruses consist of dsRNAs, and these fungal dsRNA viruses have so far been classified into five families: Reoviridae, Totiviridae, Partitiviridae, Chrysoviri-dae, and Hypoviridae. Members of only one of these families, the Hypoviridae, contain a single large dsRNA genome about 10-12 kbp in length, in contrast to members of the other families that contain one or multiple small dsRNA genomes about 2-6 kbp in length. However, many unclassified dsRNAs are still found in various fungi, some of which are larger than 10 kbp in length.

The AG3 isolate of the fungus Rhizoctonia solani is the major cause of rhizoctonia disease in potato, and contains two large dsRNAs, L1 (25 kbp) and L2 (23 kbp), in some strains of the fungus. The nucleotide sequence of the L2 dsRNA has been partially determined, and was found to be similar to the RdRp regions of the ORFs encoded by plant endornaviruses.

The violet root rot fungus, Helicobasidium mompa, occurs on various plants, and contains various-sized dsRNAs. A large dsRNA (L1 dsRNA) in the V670 strain of H. mompa has been identified as a hypovirulence factor, and has been sequenced completely. It encodes a single long ORF (5373 aa), in which conserved RdRp and Hel motifs are found (Figure 1). A BLAST search using the aa sequence of this ORF revealed significant sequence similarities to the dsRNAs of members of the genus Endornavirus. These results indicated that the large dsRNA in H. mompa belongs to the genus Endornavirus, and it was thus named helicobasidium mompa endornavirus 1-670 (HmEV1-670).

Plant pathogens of the genus Phytophthora have many of the same biological properties as fungi. However, on the basis of sequence similarities they have been classified together with diatoms and brown algae in a protist group known as the stramenopiles or chromista. The complete nucleotide sequence of the 13.9 kbp dsRNA isolated from a Phytophthora isolate from Douglas fir has also been published recently. The dsRNA encodes a single long ORF (4548 aa), whose genome organization is very similar to that of known endornaviruses (Figure 1). Phylogenetic analyses support the classification of this dsRNA (named Phytophthora endornavirus 1 (PEV1)) as a member of the genus Endornavirus.

Therefore, entire nucleotide sequences have been published for a total of five endornaviruses, three from plants (OsEV, OrEV, and VfEV), one from a fungus (HmEV1-670) and one from a protist (PEV1). Despite the diversity of their hosts, a comparison of the genome organizations of the five endornaviruses indicates that they share a unique structure (Figure 1): all five endornaviruses encode a single unusually long ORF, in which conserved motifs for RdRp and Hel are found in the same regions, and they contain a site-specific nick in the 5' region of the coding strand.

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