DNA-dependent RNA polymerase II purified from healthy plant tissue is capable of 
synthesizing linear (-)-viroid RNA copies of full length from (+)-viroid RNA 
templates in vitro. Together with the specific alpha-amanitin sensitivity of 
viroid replication observed in vivo, these findings suggest that viroids 
replicate by an entirely novel mechanism in which infecting viroid RNA molecules 
are copied by the host enzyme which is normally responsible for the synthesis of 
nuclear precursors to messenger RNA.