Transcription Elongation Complex Stability: The Topological Lock

Xiaoqing Liu & Craig T. Martin, J. Biol. Chem. 284, 36262-36270, 2009.

An elongation complex stabilized by only 4 base (homo) pairs can synthesize long RNA by repeated slippage events

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.

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