Single amino acid change in the helicase domain of the putative RNA replicase of turnip crinkle virus alters symptom intensification by virulent satellites.

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The virulent satellite [satellite C (sat C)] of turnip crinkle virus (TCV) is a small pathogenic RNA that intensifies symptoms in TCV-infected turnip plants (Brassica campestris). The virulence of sat C is determined by properties of the satellite itself and is influenced by the helper virus. Symptoms produced in infections with sat C differ in severity depending on the helper virus. The TCV-JI helper virus produces more severe symptoms than the TCV-B helper virus when inoculated with sat C. To find determinants in the TCV helper virus genome that affect satellite virulence, the TCV-JI genome was cloned and the sequence compared to the TCV-B genome. The genomes were found to differ by only five base changes, and only one of the base changes, at nucleotide position 1025, produced an amino acid change, an aspartic acid----glycine in the putative viral replicase. A chimeric TCV genome (TCV-B/JI) containing four of the five base changes (including the base change at position 1025) and a mutant TCV-B genome (TCV-B1025G) containing a single base substitution at position 1025 converted the TCV-B genome into a form that produces severe symptoms with sat C. The base change a position 1025 is located in the helicase of the putative viral replicase, and symptom intensification appears to result from differences in the rate of replication of the satellite supported by the two helper viruses.