Complete genome sequence of the first endornavirus from the ascocarp of the ectomycorrhizal fungus Tuber aestivum Vittad.

Abstract

The genus Endornavirus was recently approved by the International Committee on Taxonomy of Viruses (ICTV) [1, 10] to be separated from the family Partitiviridae, and endornaviruses have only been described in plants and fungi [4, 15]. They share common properties such as symptom-free infection of their host, occurrence in lowcopy number in host cells, lack of true virions [7], and an efficient vertical transmission in their hosts [16]. Endornaviruses have unique plasmid-like properties that differ markedly from those of other conventionally encapsidated viruses [5, 12, 16]. The genomes of endornaviruses consist of linear double-stranded RNA (dsRNA) with a characteristic, single open reading frame (ORF) of up to 18 kb in length, often preceded by a site-specific nick at the 5’ end [5, 7, 8, 12, 18]. The predicted protein from this ORF harbours in its N-terminal part characteristic amino acid (aa) sequence motifs for viral RNA helicases (VHel) or viral methyltransferases (VMet), while aa sequence motifs for UDP-glycosyltransferases (UGT) and RNA-dependent RNA polymerases (RdRp) are located at close proximity to the 30 end [7]. The remaining polyprotein is unrelated to any known functional proteins in the database. Currently, the INSDC databases contain complete sequences of seven endornaviruses. Three are plant-associated viruses infecting cultivated and wild rice (Oryza sativa endornavirus [OsEV] and Oryza rufipogon endornavirus [OrEV]) [5, 11], respectively), as well as broad bean (Vicia faba endornavirus [VFV]) [13]. In addition, four fungal isolates are known. One isolate each was reported from the mycelia of the violet root rot fungus (Helicobasidium mompa endornavirus 1-670 [HmEV1-670]) [12] and from an informally designated plant-pathogenic oomycete Phytophtora spp. (Phytophtora endornavirus 1 [PEV1]) [6], while two isolates originate from the pathogenic ascomycete Gremmeniella abietina (Gremmeniella abietina type B RNA virus) [15]. Here, we report the complete nucleotide sequence of an isolate of a putatively novel species in the genus Endornavirus from an ectomycorrhizal fungus, isolated from the hypogeous ascomata of a black truffle (Tuber aestivum Vittad.). Accordingly, we propose the name Tuber aestivum endornavirus (TaEV). The TaEV genome structure is highly similar to that of other Ascomycetes-infecting endornaviruses, such as Gremmeniella abietina type B RNA virus [15], containing characteristic motifs for the DEAD-like helicase (DEXDc), VMet and RdRp.