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Department of Genetics, Forestry and Agricultural Biotechnology Institute (FABI), University of Pretoria, Pretoria 0002, South Africa1
ARC-Fruit, Vine and Wine Research Institute, Private Bag X5013, Stellenbosch 7599, South Africa2
Author for correspondence: Oliver Preisig. Fax +27 12 420 39 60. e-mail Oliver.Preisig{at}fabi.up.ac.za
Hypovirulent isolates of the fruit tree fungal pathogen Diaporthe ambigua have previously been shown to harbour a double-stranded (ds)RNA genetic element 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 RNA virus that we have named D. ambigua RNA virus (DaRV). The nucleotide sequence of the genome is 4113 bp and has a GC content of 53%. Two large ORFs are 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 longest possible translation product (p125) has a predicted molecular mass of about 125 kDa. A significant homology can be found to the non-structural proteins of carmoviruses of the positive-strand RNA virus family Tombusviridae. These proteins also include the conserved RNA-dependent RNA polymerase (RDRP) 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 protein. Therefore, it is proposed that DaRV is not encapsidated but that it occurs as RNARDRP complexes and/or that it might be associated with cell membranes. Interestingly, six putative transmembrane helices are predicted in the N-terminal part of p56 (translation product of the first ORF, N-terminal part of p125), which might direct and anchor the viral complex to membranes. DaRV is a mycovirus with a unique genome organization and has a distant relationship to the plant virus family Tombusviridae.
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