A novel RNA mycovirus in a hypovirulent isolate of the plant pathogen Diaporthe ambigua

Hypovirulent isolates of the fruit tree fungal pathogen Diaporthe ambigua h ave previously been shown to harbour a double-stranded (ds)RNA genetic elem ent of about 4 kb. In this study, we established the complete cDNA sequence of this dsRNA, which represents a replicative form of a positive-strand RN A virus that we have named D. ambigua RNA virus (DaRV). The nucleotide sequ ence of the genome is 4113 bp and has a GC content of 53%. Two large ORFs a re present in the same reading frame. They are most probably translated by readthrough of a UAG stop codon in the central part of the genome. The long est possible translation product (p125) has a predicted molecular mass of a bout 125 kDa. A significant homology can be found to the non-structural pro teins of carmoviruses of the positive-strand RNA virus family Tombusviridae . These proteins also include the conserved RNA-dependent RNA polymerase (R DRP) domain. In contrast to the genome organization of these plant viruses, no ORF is present at the 3' end of the DaRV genome that encodes a coat pro tein. Therefore, it is proposed that DaRV is not encapsidated but that it o ccurs as RNA-RDRP complexes and/or that it might be associated with cell me mbranes. interestingly, six putative transmembrane helices are predicted in the N-terminal part of p56 (translation product of the first ORF, N-termin al part of p125), which might direct and anchor the viral complex to membra nes. DaRV is a mycovirus with a unique genome organization and has a distan t relationship to the plant virus family Tombusviridae.