270: Human Topoisomerase IIIα–RMI1–RMI2 (TRR) processively relaxes negatively supercoiled DNA measured by optical tweezers

Spakmana D et al., Proc. Natl. Acad. Sci. U.S.A. 2026.123:e2406949123 - Optical tweezers and fluorescence imaging show human Topoisomerase IIIα–RMI1–RMI2 (TRR) processively relaxes highly negatively supercoiled DNA faster than PICH loops. Key terms: topoisomerase IIIα, TRR complex, DNA supercoiling, optical tweezers, ultrafine anaphase bridges.

Study Highlights:
Using torsionally constrained end-closed λ-DNA in a multichannel flow cell, the authors combined Optical DNA Supercoiling (ODS), dual-trap optical tweezers, and fluorescence imaging to measure supercoiling density and visualize TRR binding in real time. They find TRR relaxes hyper-negatively supercoiled DNA in a highly processive manner, with single TRR complexes performing thousands (>3,000) of strand-passage reactions and exhibiting a force-dependent rate described by an Arrhenius relation (δ ≈ 1.1–1.4 nm in ensemble and single-molecule fits). Ensemble TRR activity was ~10-fold lower than E. coli TopoI under the same conditions, while single- complex rates indicate a single TRR can relax PICH-generated negative loops within the reported loop lifetime. These results support a mechanistic role for TRR in relaxing PICH-generated negative supercoils that could facilitate ultrafine anaphase bridge resolution.

Conclusion:
Human topoisomerase IIIα–RMI1–RMI2 (TRR) relaxes highly negatively supercoiled DNA in a force-dependent, highly processive manner and can act within the lifetime of PICH-generated negative loops, supporting a role in ultrafine anaphase bridge resolution.

QC:
This episode was checked against the original article PDF and publication metadata for the episode release published on 2026-01-25.

QC Scope:
- article metadata and core scientific claims from the narration
- excludes analogies, intro/outro, and music
- transcript coverage: Substantive auditing of the transcript's presentation of TRR mechanism, speed and processivity, binding preferences, dwell times, single-molecule assay setup, relation to PICH loop dynamics, and implications for ultrafine anaphase bridge resolution.
- transcript topics: TRR mechanistic relaxation of negatively supercoiled DNA; Single-molecule optical DNA supercoiling (ODS) assay with dual-trap optical tweezers; TRR processivity and burst dynamics; Force dependence and Arrhenius behavior (δ values); AT-rich sequence binding preferences of TRR; PICH loop dynamics and ultrafine bridge context

QC Summary:
- factual score: 10/10
- metadata score: 10/10
- supported core claims: 8
- claims flagged for review: 0
- metadata checks passed: 4
- metadata issues found: 0

Metadata Audited:
- article_doi
- article_title
- article_journal
- license

Factual Items Audited:
- TRR relaxes hyper-negatively supercoiled DNA in a highly processive manner
- A single TRR complex can catalyze thousands of strand passages (>3,000 Lk) without dissociating
- Ensemble relaxation rate ~35 Lk/s; TRR is ~10-fold slower than EcTopoI; δ ~ 1.1 nm
- Single TRRmCherry complexes exhibit force-dependent relaxation with δ ~ 1.4 nm
- TRR binds preferentially to AT-rich sequences and melted bubbles
- TRR remains bound to DNA for tens of minutes after relaxation

QC result: Pass.