Transcription termination by bacterial RNA polymerase (RNAP) occurs at sequences 
coding for a GC-rich RNA hairpin followed by a U-rich tract. We used 
single-molecule techniques to investigate the mechanism by which three 
representative terminators (his, t500, and tR2) destabilize the elongation 
complex (EC). For his and tR2 terminators, loads exerted to bias translocation 
did not affect termination efficiency (TE). However, the force-dependent 
kinetics of release and the force-dependent TE of a mutant imply a forward 
translocation mechanism for the t500 terminator. Tension on isolated U-tracts 
induced transcript release in a manner consistent with RNA:DNA hybrid shearing. 
We deduce that different mechanisms, involving hypertranslocation or shearing, 
operate at terminators with different U-tracts. Tension applied to RNA at 
terminators suggests that closure of the final 2-3 hairpin bases destabilizes 
the hybrid and that competing RNA structures modulate TE. We propose a 
quantitative, energetic model that predicts the behavior for these terminators 
and mutant variants.