Transcription elongation complex stability: the topological lock.

Transcription machinery from a variety of organisms shows striking mechanistic similarity. Both multi- and single subunit RNA polymerases have evolved an 8-10-base pair RNA-DNA hybrid as a part of a stably transcribing elongation complex. Through characterization of halted complexes that can readily carry out homopolymeric slippage synthesis, this study reveals that T7 RNA polymerase elongation complexes containing only a 4-base pair hybrid can nevertheless be more stable than those with the normal 8-base pair hybrid. We propose that a key feature of this stability is the topological threading of RNA through the complex and/or around the DNA template strand. The data are consistent with forward translocation as a mechanism to allow unthreading of the topological lock, as can occur during programmed termination of transcription.