Defective interfering (DI) RNAs were generated de novo in each of 12 independent isolates of tomato bushy stunt virus (TBSV) upon serial passage at high multiplicities of infection (m.o.i.) in plants, but not in any of 4 additional isolates after 11 serial passages at low m.o.i. The DI RNAs were detected in RNA isolated from virus particles and in 2.3 M LiCl-soluble RNA fractions isolated from inoculated leaves. Symptom attenuation leading to persistent infections was closely correlated with the passage in which DIs first developed. Comparisons of nucleotide sequences of 10 cDNA clones from 2 DI RNA populations and with a previously characterized TBSV DI RNA revealed the same four regions of sequence from the TBSV genome were strictly conserved in each of the DI RNAs: the virus 5′ leader sequence of 168 bases; a region of approximately 200-250 bases from the viral polymerase gene; approximately 70 bases from the 3′ terminus of the viral pl9 and p22 genes; and approximately 130 bases from the 3′terminal noncoding region. Conservation of the sequence motif present in all of the DIs suggests that there might be a common mechanism of DI formation as well as selection pressure to maintain sequences essential for replication and encapsidation.
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