Defective interfering (DI) RNA molecules derived from the genomic L RNA segment of tomato spotted wilt virus (TSWV) were generated during sequential passage of the virus at high multiplicity. Characterization of DI RNAs from four distinct isolates by Northern blot analysis and sequence determination revealed that both the 5′ and 3′ genomic termini were retained in these molecules. Each DI RNA contained a single internal deletion of approximately 60% to 80% of the L RNA segment. All DI RNAs studied maintain an open reading frame (ORF) which suggests that these defective molecules should be translatable by ribosomes. Detection of only defective molecules with ORFs indicates either that association with ribosomes or translation is a prerequisite for the selection and maintenance of replicating DI RNAs, or that the truncated proteins produced play a role in their selection or replication. Analysis of the junction sites in the DI RNAs showed that short nucleotide sequences are repeated, one at the release and another at the reinitiation point on the L RNA. One of these is lost during the generation of the DI molecules. The presence of repeated sequences at the junction sites seems to be unique for tospovirus DI L RNAs; they have not been described for other DI systems of either positive- or negative-strand RNA viruses. A model for TSWV DI RNA generation is proposed in which the viral polymerase can ‘jump’ across the internal sequences from one secondary structure to another containing the repeated sequences, during the replication of the viral complementary L RNA segment.


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