Mechanical conversion of low-affinity Integration Host Factor binding sites into high-affinity sites

Although DNA is often bent in vivo, it is unclear how DNA-bending forces modulate DNA-protein binding affinity. Here, we report how a range of DNA-bending forces modulates the binding of the Integration Host Factor (IHF) protein to various DNAs. Using solution fluorimetry and electrophoretic mobility shift assays, we measured the affinity of IHF for DNAs with different bending forces and sequence mutations. Bending force was adjusted by varying the fraction of double-stranded DNA in a circular substrate, or by changing the overall size of the circle (1). DNA constructs contained a pair of Forster Resonance Energy Transfer dyes that served as probes for affinity assays, and read out bending forces measured by optical force sensors (2). Small bending forces significantly increased binding affinity; this effect saturated beyond ~3 pN. Surprisingly, when DNA sequences that bound IHF only weakly were mechanically bent by circularization, they bound IHF more tightly than the linear “high-affinity” binding sequence. These findings demonstrate that small bending forces can greatly augment binding at sites that deviate from a protein’s consensus binding sequence. Since cellular DNA is subject to mechanical deformation and condensation, affinities of architectural proteins determined in vitro using short linear DNAs may not reflect in vivo affinities.

💡 Research Summary

The study addresses a fundamental question in molecular biology: how does the mechanical deformation of DNA influence the binding affinity of architectural proteins such as Integration Host Factor (IHF)? While it is well‑established that IHF recognizes a specific consensus sequence and induces a sharp bend (~160°) upon binding, most quantitative affinity measurements have been performed on short, linear DNA fragments that do not reflect the constantly strained state of chromosomal DNA in vivo. To bridge this gap, the authors engineered a set of circular DNA substrates that allow precise control of internal bending forces. By varying the fraction of double‑stranded DNA within the circle and by changing the overall circumference (60–200 bp), they generated a range of calculated bending forces from essentially zero up to ~5 pN, using the Worm‑Like Chain model to convert geometry into force.

A key methodological innovation is the incorporation of a Förster Resonance Energy Transfer (FRET) pair at opposite ends of each DNA construct. The FRET efficiency serves a dual purpose: (1) it acts as an optical force sensor, reporting the actual distance—and therefore the bending stress—experienced by the molecule, and (2) it provides a real‑time readout of IHF binding because the protein‑induced bend changes the donor‑acceptor separation. Binding affinities were quantified by two independent techniques: solution‑phase fluorescence titrations (monitoring FRET changes as a function of DNA concentration) and electrophoretic mobility shift assays (EMSA). Both approaches yielded consistent dissociation constants (K_D), establishing the robustness of the measurements.

The results reveal a striking, non‑linear relationship between bending force and IHF affinity. As the internal force increases from 0 to ~3 pN, the K_D drops dramatically, indicating a several‑order‑of‑magnitude increase in binding strength. Beyond ~3 pN the effect plateaus, suggesting that once the DNA is pre‑bent to a degree comparable to the protein‑induced curvature, additional force provides little extra benefit. This saturation point aligns with the known geometry of the IHF–DNA complex, where the protein essentially “locks” the DNA into a pre‑existing bend.

Equally surprising is the behavior of sequence variants that are weak binders in the canonical linear assay. The authors examined several low‑affinity mutants that deviate from the consensus minor‑groove pattern. In a linear context these mutants display K_D values on the order of 200 nM, roughly 20‑fold weaker than the consensus site (≈10 nM). However, when the same sequences are placed in a circular substrate that imposes a modest bending force (≈2 pN), their K_D values collapse to ≤15 nM, surpassing the high‑affinity linear reference. In other words, mechanical pre‑bending can compensate for suboptimal sequence features, effectively converting a low‑affinity site into a high‑affinity one.

These findings have broad biological implications. In vivo, DNA is constantly subjected to torsional stress, supercoiling, nucleoid‑associated protein binding, and transcription‑induced bending. The data suggest that the physical state of the DNA can dramatically reshape the binding landscape for IHF and, by extension, other architectural factors. Consequently, affinity values derived from short, relaxed DNA fragments may underestimate the true in‑cellular binding potential, especially at sites that are otherwise sequence‑poor.

Beyond the specific case of IHF, the experimental platform introduced here—circular DNA with embedded FRET force sensors—offers a versatile tool for probing the mechanochemistry of any DNA‑binding protein. By adjusting circle size and dsDNA fraction, researchers can generate a calibrated spectrum of forces and directly observe how those forces modulate protein recruitment, cooperativity, and kinetic parameters. This approach could be extended to study nucleoid‑associated proteins (HU, H‑NS), eukaryotic histones, transcription factors that recognize DNA shape, and even drug candidates that target DNA‑protein interfaces.

In summary, the paper demonstrates that modest mechanical stresses (≈3 pN) are sufficient to boost IHF binding by orders of magnitude and that such stresses can overturn the hierarchy imposed by sequence alone. The work underscores the necessity of incorporating DNA mechanics into quantitative models of protein‑DNA recognition and provides a concrete experimental framework for doing so.