Intrinsic termination signals for multisubunit bacterial RNA polymerases (RNAPs) 
encode a GC-rich stem-loop structure followed by a polyuridine [poly(U)] tract, 
and it has been proposed that steric clash of the stem-loop with the exit pore 
of the RNAP imposes a shearing force on the RNA in the downstream RNA:DNA 
hybrid, resulting in misalignment of the active site. The structurally unrelated 
T7 RNAP terminates at a similar type of signal (TΦ), suggesting a common 
mechanism for termination. In the absence of a hairpin (passive conditions), T7 
RNAP slips efficiently in both homopolymeric A and U tracts, and we have found 
that replacement of the U tract in TΦ with a slippage-prone A tract still allows 
efficient termination. Under passive conditions, incorporation of a single G 
residue following a poly(U) tract (which is the situation during termination at 
TΦ) results in a "locked" complex that is unable to extend the transcript. Our 
results support a model in which transmission of the shearing force generated by 
steric clash of the hairpin with the exit pore is promoted by the presence of a 
slippery tracts downstream, resulting in alterations in the active site and the 
formation of a locked complex that represents an early step in the termination 
pathway.