Cucumber mosaic virus Y satellite RNA (Y-satRNA) induces distinctive yellow mosaic symptoms on tobacco, whereas S19 satellite RNA (S19-satRNA) causes an attenuated green mosaic on tobacco, although they show considerable sequence identity. Biological assays of infectious chimeric satellite RNA molecules synthesized from cDNA clones of Y-satRNA and S19-satRNA using common restriction sites showed that the determinant for the induction of yellow mosaic symptoms lies in the XI-I fragment, in which 14 nucleotide differences are found between the two satellite RNAs. To define more precisely the yellow mosaic determinant(s) in this fragment, several site-directed mutants of Y-satRNA were created. The replacement of AUU, at nucleotides 191 to 193 in Y-satRNA, with GC, which mimicks the S19-satRNA sequence at the corresponding site, abolished the ability of Y-satRNA to elicit a yellow mosaic. Conversely, a mutant RNA molecule derived from S19-satRNA in which GC at nucleotides 192 and 193 was changed to AUU induced the yellow mosaic symptoms. Thus, the phenotypes of two satellite RNAs on tobacco can be altered reciprocally by changing the sequences in this limited region.


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