Some satellite (sat-) and defective interfering (DI) RNAs associated with plant viruses intensify or ameliorate the symptoms of the virus. We recently demonstrated that the TCV coat protein (CP) is involved in symptom modulation by sat-RNA C. Two additional subviral RNAs have now been tested for effect of the CP on symptom modulation. DI RNA G, which normally intensifies the symptoms of TCV, is able to attenuate symptoms if the TCV CP is replaced with the CP of cardamine chlorotic fleck virus. DI RNA G had no effect on the symptoms of TCV with a single base alteration in the CP open reading frame, unlike sat-RNA C, which was able to ameliorate the symptoms of the mutant TCV. Using a hybrid sat-RNA constructed from sat-RNA C and TCV (which shares a similar 3'-end region with DI RNA G), the 3'-terminal 53 bases of sat-RNA C were found to be involved in symptom attenuation, which was directly correlated with the lack of detectable viral genomic RNA in whole plants. Sat-RNA D had no effect on the symptoms of mutant or wild-type TCV. The accumulation of TCV subviral RNAs in plants and protoplasts was also found to be strongly influenced by the presence or absence of the wild-type TCV CP.
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