MicroRNAs (miRNAs) play a major part in the post-transcriptional regulation of 
gene expression. Mammalian miRNA biogenesis begins with cotranscriptional 
cleavage of RNA polymerase II (Pol II) transcripts by the Microprocessor 
complex. Although most miRNAs are located within introns of protein-coding 
transcripts, a substantial minority of miRNAs originate from long noncoding 
(lnc) RNAs, for which transcript processing is largely uncharacterized. We show, 
by detailed characterization of liver-specific lnc-pri-miR-122 and genome-wide 
analysis in human cell lines, that most lncRNA transcripts containing miRNAs 
(lnc-pri-miRNAs) do not use the canonical cleavage-and-polyadenylation pathway 
but instead use Microprocessor cleavage to terminate transcription. 
Microprocessor inactivation leads to extensive transcriptional readthrough of 
lnc-pri-miRNA and transcriptional interference with downstream genes. 
Consequently we define a new RNase III-mediated, polyadenylation-independent 
mechanism of Pol II transcription termination in mammalian cells.