RNA-dependent RNA polymerase from healthy tomato plant tissue accepts potato spindle tuber viroid (PSTV) RNA as a template for the in vitro synthesis of full-length RNA copies of the PSTV genome. Viroid transcription requires the presence of Mn2+ and/or Mg2+ ions and is not inhibited by concentrations of 10−5 M α-amanitin. This is the first report of a well-defined product synthesized in vitro by an RNA-dependent RNA polymerase from healthy plants.

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