Molecular characterization of a dsRNA mycovirus, Fusarium graminearum virus-DK21, which is phylogenetically related to hypoviruses but has a genome organization and gene expression strategy resembling those of plant potex-like viruses.

Fusarium graminearum causes a serious scab disease of small grains in Korea. The nucleotide sequence of the genomic RNA of a double-stranded RNA (dsRNA) virus, Fusarium graminearum virus-DK21 (FgV-DK21), from F. graminearum strain DK21, which is associated with hypovirulence in F. graminearum, was determined and compared to the genome sequences of other mycoviruses, including Cryponectria hypoviruses. The FgV-DK21 dsRNA consists of 6,621 [corrected] nucleotides, excluding the 3'-terminal poly(A) tail. The viral genome has 53- and 46-nucleotide 5' and 3' untranslated regions (UTRs), respectively, and four [corrected] putative open reading frames. A phylogenetic analysis of the deduced amino acid sequence of ORF1, which encodes a putative RNA-dependent RNA polymerase, and those of other mycoviruses revealed that this organism forms a distinct virus clade with other hypoviruses, and is more distantly related to other mycoviruses (3.8 to 24.0% identity). However, pairwise sequence comparisons of the nucleotide and deduced amino acid sequences of ORFs 2 through 4 [corrected] revealed no close relationships to other protein sequences currently available in GenBank. Analyses of RNA accumulation by Northern blot and primer extension indicated that these putative gene products are expressed from at least two different subgenomic RNAs (sgRNAs), in contrast to the cases in other hypoviruses. This study suggests the existence of a new, as yet unassigned, genus of mycoviruses that exhibits a potex-like genome organization and sgRNA accumulation.