Real-time Observation of the DNA Strand Exchange Reaction Mediated by Rad51
February 13th, 2019
Fluorescence resonance energy transfer-based real-time observation systems of the DNA strand exchange reaction mediated by Rad51 were developed. Using the protocols presented here, we are able to detect the formation of reaction intermediates and their conversion into products, while also analyzing the enzymatic kinetics of the reaction.
This method can help decipher the steps in DNA strand exchange and reveals the molecule roles of different proteins involved in combination during repair. With this technique we can monitor DNA strand exchange in real time without any disruption to the reaction and use the operating theater to determine the kinetics of each reaction step. With some minor adjustments, this technique can be applied to study the activities of purified recombination proteins derived from other species, such as mammals and plants.
Demonstrating the procedure will be Kentaro Ito, a Post Doc from my laboratory. To begin, prepare the reaction buffer A according to the text protocol. Next, add 36-nanomole 16FA-oligonucleotide to the buffer for the DNA pairing assay.
Incubate the mixture at 37 degrees Celsius for five minutes. Add RAD-51 protein at a final concentration of 1.5 micro moles to the mixture and incubate it at 37 degrees celsius for five minutes. Add SWI5-SFR1 protein at a final concentration of 0.15 micro moles to the mixture, and incubate it at 37 degrees Celsius for an additional five minutes.
Transfer 1.5 milliliters of the mixture in a 1.0 by 1.0 cm quartz cuvette containing a magnetic stirrer. Insert the cuvette into a spectrofluorometer and proceed to adjusting the temperature controller and the magnetic stirrer. Record the change in FAM fluorescence emission at 525 nanometers upon excitation at 493 nanometers at one second intervals for 100 seconds.
Finally, with a syringe, inject ROX labeled double-stranded DNA at a final concentration of 36 nano moles into the cuvette. Record the change in fluorescence emission at one second intervals for 30 minutes. To compare the fluorescence spectra between the substrates and the final products, insert each cuvette containing the 16 FA-oligonucleotides with or without RAD-51 into a spectrofluorometer and incubate at 37 degrees Celsius for five minutes.
Then, record the FAM fluorescence emission at 500 to 600 nano meters upon excitation at 493 nanometers. To test the effect of RAD-51 on the fluorescence spectra, add RAD-51 at a final concentration of 1.5 micro moles. And incubate the mixture at 37 degrees Celsius for five minutes.
Finally, record the fluorescence spectra from 500 to 600 nanometers upon excitation at 493 nanometers. The maximum FRET efficiency is obtained by measuring the maximum reduction in the fluorescence intensity when all single-stranded DNA substrate is converted into double-stranded DNA in the pairing assay. Or by measuring the maximum increase in the intensity when all double-stranded DNA substrate is converted into single-stranded DNA in the displacement assay.
In both assays, addition of RAD-51 protein did not affect fluorescence emission of FAM, or its quenching efficiency by rocks. Effects of the spontaneous reactions between substrate DNAs and the subsequent photo bleaching are small. As shown by the negligible changes in the emission of FAM without RAD-51, compared to the substantial changes seen with RAD-51.
Adding the SWI5-SFR1 complex strongly stimulates the pairing activity of RAD-51. The pairing reaction is simulated using a three step model, consisting of formation of the first three-strand intermediate, transition into the second intermediate, and release of the single-stranded DNA and formation of the hetero duplex. A comparison of the three step model with a two step model indicates that the three step model is a better fit for simulating DNA pairing, without or with SWI5-SFR1 complex.
The calculated equilibrium constant of each reaction step, with or without SWI5-SFR1, shows that the SWI5-SFR1 complex does not stimulate the formation of the first three-strand intermediate, but strongly stimulates the transition between the two three-strand intermediates, and the release of SS DNA. It is essential to verify that each batch of purified protein is free of nuclease and helicase contamination.
Overview
This article presents a method for real-time observation of DNA strand exchange reactions mediated by Rad51 using fluorescence resonance energy transfer. The technique allows for the detection of reaction intermediates and analysis of enzymatic kinetics without disrupting the reaction.
Key Study Components
Area of Science
- Biochemistry
- Molecular Biology
- Genetics
Background
- DNA strand exchange is a crucial process in DNA repair.
- Rad51 plays a significant role in homologous recombination.
- Understanding the kinetics of this reaction can provide insights into DNA repair mechanisms.
- Real-time monitoring techniques enhance the study of these complex biochemical processes.
Purpose of Study
- To develop a system for observing DNA strand exchange in real time.
- To analyze the roles of different proteins in the reaction.
- To determine the kinetics of each step in the DNA strand exchange process.
Methods Used
- Preparation of reaction buffer and oligonucleotides for DNA pairing assays.
- Incubation of RAD-51 and SWI5-SFR1 proteins with DNA substrates.
- Use of a spectrofluorometer to monitor fluorescence changes.
- Analysis of reaction intermediates and products through fluorescence emission measurements.
Main Results
- The addition of SWI5-SFR1 significantly stimulates RAD-51's pairing activity.
- A three-step model better fits the DNA pairing simulation compared to a two-step model.
- The study provides quantitative data on the kinetics of DNA strand exchange.
- Fluorescence measurements indicated minimal effects from spontaneous reactions.
Conclusions
- The developed method allows for detailed kinetic analysis of DNA strand exchange.
- SWI5-SFR1 enhances the efficiency of RAD-51 in DNA pairing.
- This technique can be adapted for studying recombination proteins from various species.
Frequently Asked Questions
Executive Industry Relevance
Real-time observation of DNA strand exchange mediated by Rad51 provides a quantitative framework for evaluating recombination protein function, supporting target validation in DNA repair pathways. The FRET-based assay enables kinetic de-risking of mechanistic hypotheses by resolving transient intermediates, informing lead identification for modulators of homologous recombination. This approach enhances predictive confidence in early discovery by linking protein activity to functional DNA repair outcomes.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of Rad51-mediated strand exchange to validate therapeutic targets in DNA repair pathways.
- Operational Value: Resolves formation and maturation of three-strand DNA intermediates, clarifying mechanism of action for accessory proteins like Swi5-Sfr1.
- Predictive Value: Supports three-step kinetic modeling that improves confidence in target engagement and pathway modulation predictions.
Screening & Assay Development
- Scientific Value: Generates quantitative fluorescence readouts to measure strand exchange kinetics under varying protein conditions.
- Operational Value: Establishes a homogeneous, solution-based assay format compatible with spectrofluorometric detection for compound screening.
- Assay Readiness: Enables real-time monitoring without reaction disruption, supporting dose-response and time-course analyses for hit validation.
Translational & Preclinical Research
- Translational Value: Facilitates cross-species comparison of recombination protein activity, supporting extrapolation from model systems to human homologs.
- Mechanistic De-risking: Distinguishes effects of accessory proteins on specific reaction steps, reducing ambiguity in mechanism-based biomarker alignment.
- Preclinical Continuity: Provides a standardized method to assess functional conservation of Rad51 regulators across species, informing target selection.
Pipeline & Workflow Integration
The method fits within the discovery continuum from target validation through lead optimization, where kinetic profiling of DNA repair proteins informs go/no-go decisions prior to preclinical investment.
- Discovery Biology: Supports hypothesis testing by quantifying how Swi5-Sfr1 stimulates intermediate maturation and ssDNA release in Rad51-mediated strand exchange.
- Screening: Delivers reproducible, real-time FRET signals that enable comparison of inhibitor or activator effects on reaction kinetics.
- Analytics: Provides step-resolved rate constants and equilibrium constants that allow quantitative comparison of protein variants or conditions.
- Translational Research: Enables assessment of evolutionary conservation by applying the assay to recombination proteins from mammals and plants.
- Enterprise Reuse: Establishes a reusable platform for characterizing recombinase activity, reducing redundant assay development across projects.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence in target validation by resolving transient intermediates and defining rate-limiting steps.
- Operational Value: Delivers standardized, reproducible kinetic data through real-time fluorescence monitoring in a cuvette-based format.
- Strategic Value: Improves go/no-go decisions by quantifying how accessory proteins modulate specific steps of strand exchange, reducing mechanistic uncertainty.
- Portfolio Impact: Enables risk-adjusted prioritization of DNA repair targets based on quantitative kinetic profiles and evolutionary conservation data.
Implementation Considerations
- Requires expertise in protein purification, fluorescence spectroscopy, and kinetic data analysis.
- Dependent on access to a temperature-controlled spectrofluorometer with magnetic stirring and syringe injection capability.
- Necessitates standardized protein preparation to exclude nuclease and helicase contaminants that could confound reaction interpretation.
- Requires optimization of oligonucleotide labeling and concentration for each target species to ensure detectable FRET signals.
- Limited to in vitro systems; does not capture chromatin or nuclear context effects present in vivo.
