Molecular characterization of a novel mycovirus in the cultivated mushroom, Lentinula edodes

BACKGROUND: In the 1970s, mycoviruses were identified that infected the edible mushroom Lentinula edodes (shiitake), but they were not regarded as causal agents for mushroom diseases. None of their genes has been sequenced. In this study, the dsRNA genome of a mycovirus recently found in a shiitake commercial strain was sequenced and its molecular structure was characterized. METHODS: A cDNA library was constructed from a dsRNA purified from the fruiting body of L. edodes. The virus was tentatively named L. edodes mycovirus HKB (LeV). Based on the deduced RNA-dependent RNA polymerase (RdRp) sequence, phylogenetic analysis of LeV was conducted. Because no virion particles associated with the dsRNA were observed by electron microscopic observation, atomic force microscopy (AFM) observation was chosen for achieving molecular imaging of the virus. RESULTS: The 11,282-bp genome of LeV was obtained. The genome encoded two open reading frames (ORFs). ORF1 coded for a hypothetical protein and ORF2 for a putative RdRp, respectively. In addition, a region coding for a NUDIX domain was present in ORF1. There was a 62-bp intergenic region between ORF1 and RdRp. Similarity with coat protein of mycoviruses was not found within the whole sequence. Based on phylogenetic analysis of the putative RdRp sequence, LeV grouped into a clade with dsRNA found in the basidiomycetes Phlebiopsis gigantea and Helicobasidium mompa. The clade was placed apart from the Totiviridae and Chrysoviridae families. As suggested from the genome sequence, AFM revealed that the structure of LeV was linear unencapsidated dsRNA. CONCLUSIONS: The results suggest that LeV represents a novel family of mycoviruses, found thus far only among the basidiomycetes.