ORGANIZATION AND EXPRESSION OF THE DOUBLE-STRANDED-RNA GENOME OF HELMINTHOSPORIUM-VICTORIAE 190S VIRUS, A TOTIVIRUS INFECTING A PLANT-PATHOGENIC FILAMENTOUS FUNGUS

The complete nucleotide sequence, 5178 bp, of the totivirus Helminthos porium victoriae 190S virus (Hv190SV) double-stranded RNA, was determi ned, Computer-assisted sequence analysis revealed the presence of two large overlapping ORFs; the 5'-proximal large ORF (ORF1) codes for the coat protein (CP) with a predicted molecular mass of 81 kDa, and the 3'-proximal ORF (ORF2), which is in the -1 frame relative to ORF1, cod es for an RNA-dependent RNA polymerase (RDRP). Unlike many other totiv iruses, the overlap region between ORF1 and ORF2 lacks known structura l information required for translational frameshifting. Using an antis erum to a C-terminal fragment of the RDRP, the product of ORF2 was ide ntified as a minor virion-associated polypeptide of estimated molecula r mass of 92 kDa. No CP-RDRP fusion protein with calculated molecular mass of 165 kDa was detected. The predicted start codon of the RDRP OR F (2605-AUG-2607) overlaps with the stop codon (2606-UGA-2608) of the CP ORF, suggesting RDRP is expressed by an internal initiation mechani sm, Hv190SV is associated with a debilitating disease of its phytopath ogenic fungal host, Knowledge of its genome organization and expressio n will be valuable for understanding its role in pathogenesis and for potential exploitation in the development of biocontrol measures.