NOVEL STRUCTURES OF 2 VIRUS-LIKE RNA ELEMENTS FROM A DISEASED ISOLATEOF THE DUTCH ELM DISEASE FUNGUS, OPHIOSTOMA-NOVO-ULMI

The nucleotide sequences of 2 of the 10 mitochondrial double-stranded (ds) RNA segments in a diseased isolate, Log1/3-8d(2) (Ld), of Ophiost oma novo-ulmi, RNA-7 (1057 nucleotides) and RNA-10 (317-330 nucleotide s), have been determined. Both RNAs are A-U-rich. but in Southern and Northern blots, no hybridization with mitochondrial DNA or RNA could b e detected. Only very short open reading frames were found in both RNA s. As most of its sequence is unrelated to any of the other Ld dsRNAs, RNA-7 may be regarded as a satellite RNA. Northern blotting detected a full-length single-stranded (ss) form of RNA-7 in nucleic acid extra cts from Ld The 5'- and 8'-terminal 39 nucleotides of ssRNA-7 are impe rfect inverted complementary repeats of each other, which could ca use ssRNA-7 to form a panhandle structure. In addition, the 5'-terminal n ucleotides 1-28 and 3'-terminal nucleotides 1032-1057 of ssRNA-7 each contained inverted complementary sequences, allowing the possibility f or each terminus to form separate stem-loop structures. The combinatio n of these two structural features has not been found previously in an y dsRNA or ssRNA virus. RNA-10 was shown to have an unusual structure, consisting of a mosaic of sequences derived from regions of the 5'- a nd 3'-termini, or just the 5'-terminus, of RNA-7 RNA-10 has a high deg ree of inverted complementarity, with the potential to be folded into a very stable hairpin structure. A model for the formation of RNA-10 i s presented, involving replicase-driven strand switching between (-)-s trand and (+)-strand templates during RNA synthesis, followed by utili zation of the nascent strand as a primer and template to form a snap-b ack RNA. (C) Academic Press.