A VIRAL DSRNA ELEMENT OF THE CHESTNUT BLIGHT FUNGUS WITH A DISTINCT GENETIC ORGANIZATION

We have sequenced overlapping complementary DNA clones representing the viral double-stranded (ds) RNA from hypovirulent strain NB58 of the chestnut blight fungus Clyphonectria parasitica. Clyphonectria hypovirus 2-NB58 (CHV2- NB58) dsRNA contains 12,507 base pairs, excluding the poly(A) tail at the 3' end of the plus strand, and is organizationally similar to the largest dsRNA from the virus of strain EP713 (CHV1-713; equivalent to HAV; Shapira at al., (1991), EMBO J. 10, 731-739). CHV2-NB58 and CHV1-713 dsRNAs share approximately 60% nucleotide sequence identity. On the poly(A)-containing strand of CHV2-NB58, a 487-residue nontranslated region precedes two Open reading frames, designated ORF A (438 codons) and ORF a (3291 codons). The connecting pentanucleotide sequence UAAUG (1802-1806) terminates ORF A and initiates ORF a. In contrast to the 69-kDa ORF A product of CHV1-713, the 50-kDa CHV2-NB58 ORF A product did not undergo autoproteolysis under the conditions tested, nor were motifs associated with cysteine proteases present in the CHV2-NB58 ORF A sequence. CHV2-NB58 ORF a products appear to be homologous with CHV1-713 ORF B products, and the motifs involved in autoproteolysis of the N-terminal 48 kDa of CHV1-713 ORF a were identified in the CHV2-NB58 ORF a product. Motifs associated with RNA polymerase and helicase activities were highly conserved between CHV2-NB58 and CHV1-713 and were found at similar genomic positions in the C-terminal half of ORF a. (C) 1994 Academic Press, Inc.