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

We have sequenced overlapping complementary DNA clones representing th e viral double-stranded (ds) RNA from hypovirulent strain NB58 of the chestnut blight fungus Clyphonectria parasitica. Clyphonectria hypovir us 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 organizationall y similar to the largest dsRNA from the virus of strain EP713 (CHV1-71 3; equivalent to HAV; Shapira at al., (1991), EMBO J. 10, 731-739). CH V2-NB58 and CHV1-713 dsRNAs share approximately 60% nucleotide sequenc e identity. On the poly(A)-containing strand of CHV2-NB58, a 487-resid ue nontranslated region precedes two Open reading frames, designated O RF A (438 codons) and ORF a (3291 codons). The connecting pentanucleot ide sequence UAAUG (1802-1806) terminates ORF A and initiates ORF a. I n contrast to the 69-kDa ORF A product of CHV1-713, the 50-kDa CHV2-NB 58 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 ho mologous with CHV1-713 ORF B products, and the motifs involved in auto proteolysis 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 C HV1-713 and were found at similar genomic positions in the C-terminal half of ORF a. (C) 1994 Academic Press, Inc.