RNA polymerase II (Pol II) is a well-characterized DNA-dependent RNA polymerase, 
which has also been reported to have RNA-dependent RNA polymerase (RdRP) 
activity. Natural cellular RNA substrates of mammalian Pol II, however, have not 
been identified and the cellular function of the Pol II RdRP activity is 
unknown. We found that Pol II can use a non-coding RNA, B2 RNA, as both a 
substrate and a template for its RdRP activity. Pol II extends B2 RNA by 18 nt 
on its 3'-end in an internally templated reaction. The RNA product resulting 
from extension of B2 RNA by the Pol II RdRP can be removed from Pol II by a 
factor present in nuclear extracts. Treatment of cells with α-amanitin or 
actinomycin D revealed that extension of B2 RNA by Pol II destabilizes the RNA. 
Our studies provide compelling evidence that mammalian Pol II acts as an RdRP to 
control the stability of a cellular RNA by extending its 3'-end.