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 alpha-amanitin. This is the first report of a well-defined product 
synthesized in vitro by an RNA-dependent RNA polymerase from healthy plants.