In addition to polynucleotide polymerization, DNA polymerases and bacterial RNA 
polymerase can also remove nucleotides from the growing end of nucleic acid 
chains. For DNA polymerases this activity is an important factor in establishing 
fidelity in DNA synthesis. This report describes a novel in vitro activity of 
RNA polymerase II whereby it cleaves an RNA chain contained within an active 
elongation complex. These elongation complexes are arrested at a previously 
identified, naturally occurring transcriptional pause site in a human gene. The 
new 3'-end revealed by this cleavage remains associated with an active 
elongation complex and is capable of being extended by RNA polymerase II. 
Nascent RNA cleavage is evident after removal of free nucleotides and is 
dependent upon a divalent metal cation and transcription elongation factor SII. 
This function of SII could be important in its function as an activator of 
transcription elongation. It is also possible that the transcript cleavage 
activity of RNA polymerase II represents a proofreading function of the enzyme.