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Defective interfering (DI) RNAs have been isolated from a broad spectrum of animal viruses and have recently been identified in plant virus infections. Because of their ubiquitous nature, DIs are thought to play an important role in virus replication and yields. DI RNAs have now been found in association with a natural isolate of turnip crinkle virus (TCV-B) and are generated de novo after inoculation of turnip with virus derived from cloned transcripts. DI RNA G, naturally found in the TCV-B isolate, is a mosaic molecule with 5' and 3' viral segments and a repeat of 36 nucleotides at the beginning of the 3' segment. The 5'-terminal 21 nucleotides of DI RNA G were not similar to genomic TCV sequences but did resemble sequences found at the 5' end of other small RNAs associated with TCV (satellite RNAs). DI RNA G interferes with the accumulation of TCV genomic RNA and, unlike other DI RNAs, intensifies the symptoms of its helper virus. Infection of turnip with virus derived from cloned transcripts of TCV-B resulted in de novo generation of a DI RNA, DUI RNA. D11 RNA differed from DI RNA G by containing exact 5' and 3' ends of TCV as well as an internal virus segment.Defective interfering (DI) RNAs have been found associated with a wide variety of animal viruses. Delineated by Huang and Baltimore (1), DI RNAs are defective versions of viral RNAs that have lost essential coding sequences required for independent replication, maturation, or packaging. DI RNAs alone are not infectious; to be infectious DI RNAs require a helper virus to restore the deleted functions (2, 3). DI RNAs often compete with the nondefective virus for limited replication components, resulting in a decrease in the accumulation of helper virus. This interference with viral replication results in protection against viral-induced cytopathic effects in cell culture and in some cases has been implicated in virus disease modulation in whole animals (4). DI RNAs of animal viruses have also proved to be valuable subjects of study for many other important virological phenomena including the identification of cis-acting sequences important in replication and in encapsidation, fundamental studies on RNA replication and recombination, and use as transient expression vectors in animal cells (3).Although DI RNAs are generally considered as ubiquitous components of animal virus infections, they have not been common in plant virus infections (5-9). The only DI RNAs associated with an RNA plant virus that have been characterized at the molecular level are the small symptomattenuating RNAs associated with the cherry strain of tomato bushy stunt virus (TBSV) (6). This 396-base RNA, which is packaged along with TBSV and requires the virus for infectivity, is a mosaic molecule derived from the 5' and 3' ends of the virus and internal viral sequences. Like many animal virus DI RNAs, TBSV DI RNA competes with the helper virus resulting in a reduced level of virus accumulation and a marked attenuation of viral symptoms in infected plants (6). Evidence support...

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