TCV isolate, TCV-M, supports a family of satellite (sat-) RNAs. The virulent sat-RNA C intensifies TCV symptoms on turnip cultivar Just Right. In this thesis, I report that sat-RNA C (356 b) exacerbates symptoms on all hosts where TCV produces visible symptoms including cultivars of Brassica rapa and Arabidopsis thaliana. This finding has led to studies of TCV-resistance using A. thaliana, a small plant with a well characterized genome, as a host. After screening over 6,000 M2 A. thaliana plants from EMS treated Columbia cultivar seeds and 22 ecotypes of A. thaliana, ecotype Dijon was found to be resistant to TCV. A second isolate, TCV-B, supports an RNA species with a size similar to that of sat-RNA C. Northern hybridization and cDNA cloning and sequencing demonstrate that this RNA is actually a defective interfering (DI) RNA, denoted DI RNA G. Infection of turnip with virus derived from cloned transcripts of TCV-B resulted in de novo generation of a DI RNA, DI1 RNA. Unlike DI RNAs associated with other plant viruses (or animal viruses), TCV DI RNAs intensify TCV symptoms. To understand sequences required for DI RNA infectivity, a series of mutation have been generated in a full length cDNA copy of DI RNA G. Stepwise deletions at base 98 of DI RNA G, at which an Apa I linker had been inserted, has shown that DI RNA GA (DI RNA G with a 8-base insertion at base 98) harboring deletions of bases 74-98 or less are infectious. However, deletion of bases 73-98 or more abolishes RNA infectivity. DI RNA GA with a deletion of bases 107-124 is infectious. However, DI RNA GA harboring a deletion of bases 107-138 is not infectious. I have found that infectivity of RNA harboring 31- or 32-base deletions can be restored by inserting foreign sequences into deletion sites. This implies that at least 71 bases (including the 8-base insertion) in DI RNA GA near the 5\sp\prime end are not specifically required for RNA infectivity. By combining deletions of infectious clones to generate larger deletions (40 or 43 bases deleted), I demonstrate that the infectivity of the RNA is abolished. This result suggests that size of the DI RNA is important in maintaining RNA infectivity
