RNA polymerase II is implicated in the RNA-templated RNA synthesis during 
replication of viroids and Hepatitis Delta Virus (HDV); however, neither the RNA 
template nor protein factor requirements for this process are well defined. We 
have developed an in vitro transcription system based on HeLa cell nuclear 
extract (NE), in which a segment of antigenomic RNA corresponding to the 
left-hand tip region of the HDV rod-like structure serves as a template for 
efficient and highly specific RNA synthesis. Accumulation of the unique RNA 
product is highly sensitive to alpha-amanitin in HeLa NE and only partially 
sensitive to this drug in NE from PMG cells that contain an allele of the 
alpha-amanitin-resistant subunit of pol II, strongly suggesting pol II 
involvement in this reaction. Detailed analysis of the RNA product revealed that 
it represents a chimeric molecule composed of a newly synthesized transcript 
covalently attached to the 5' half of the RNA template. Selection of the start 
site for transcription is remarkably specific and depends on the secondary 
structure of the RNA template, rather than on its primary sequence. Some 
features of this reaction resemble the RNA cleavage-extension process observed 
for pol II-arrested complexes in vitro. A possible involvement of the described 
reaction in HDV replication is discussed.