A general approach for receptor and antibody-targeted detection of native proteins utilizing split-luciferase reassembly

Chimeric Protein Switch Biosensors

Rapid detection of protein and small-molecule analytes is a valuable technique across multiple disciplines, but most in vitro testing of biological or environmental samples requires long, laborious processes and trained personnel in laboratory settings, leading to long wait times for results and high expenses. Fusion of recognition with reporter elements has been introduced to detection methods such as enzyme-linked immunoassays (ELISA), with enzyme-conjugated secondary antibodies removing one of the many incubation and wash steps. Chimeric protein switch biosensors go further and provide a platform for homogenous mix-and-read assays where long wash and incubation steps are eradicated from the process. Chimeric protein switch biosensors consist of an enzyme switch (the reporter) coupled to a recognition element, where binding of the analyte results in switching the activity of the reporter enzyme on or off. Several chimeric protein switch biosensors have successfully been developed for analytes ranging from small molecule drugs to large protein biomarkers. There are two main formats of chimeric protein switch biosensor developed, one-component and multi-component, and these formats exhibit unique advantages and disadvantages. Genetically fusing a recognition protein to the enzyme switch has many advantages in the production and performance of the biosensor. A range of immune and synthetic binding proteins have been developed as alternatives to antibodies, including antibody mimetics or antibody fragments. These are mainly small, easily manipulated proteins and can be genetically fused to a reporter for recombinant expression or manipulated to allow chemical fusion. Here, aspects of chimeric protein switch biosensors will be reviewed with a comparison of different classes of recognition elements and switching mechanisms.

A rapid nucleic acid detection platform based on phosphorothioate-DNA and sulfur binding domain

Nucleic acid detection plays a key role in diverse diagnosis and disease control. Currently available nucleic acid detection techniques are challenged by trade-offs among speed, simplicity, precision and cost. Here, we described a novel method, designated SENSOR (Sulfur DNA mediated nucleic acid sensing platform), for rapid nucleic acid detection. SENSOR was developed from phosphorothioate (PT)-DNA and sulfur binding domain (SBD) which specifically binds double-stranded PT-modified DNA. SENSOR utilizes PT-DNA oligo and SBD as targeting module, which is linked with split luciferase reporter to generate luminescence signal within 10 min. We tested detection on synthesized nucleic acid and COVID-19 pseudovirus, achieving attomolar sensitivity combined with an amplification procedure. Single nucleotide polymorphisms (SNP) could also be discriminated. Indicating SENSOR a new promising nucleic acid detection technique.

Recent progress in homogeneous immunosensors based on fluorescence or bioluminescence using antibody engineering

Homogeneous immunosensors integrate the advantages of both biosensors and immunoassays; they include speed, high sensitivity, and accuracy. They have been developed rapidly in the past few years and offer a cost-effective alternative technology with rapidity, sensitivity, and user-friendliness, which has been applied in a wide variety of applications. This review introduces the current directions of immunosensor development, focusing on fluorescent and bioluminescent immunosensors and highlighting the advantages, improvements, and key approaches to overcome the limitations of each.

Experimental and Analytical Framework for "Mix-and-Read" Assays Based on Split Luciferase

The use of immunodetection assays including the widely used enzyme-linked immunosorbent assay (ELISA) in applications such as point-of-care detection is often limited by the need for protein immobilization and multiple binding and washing steps. Here, we describe an experimental and analytical framework for the development of simple and modular "mix-and-read" enzymatic complementation assays based on split luciferase that enable sensitive detection and quantification of analytes in solution. In this assay, two engineered protein binders targeting nonoverlapping epitopes on the target analyte were each fused to nonactive fragments of luciferase to create biosensor probes. Binding proteins to two model targets, lysozyme and Sso6904, were isolated from a combinatorial library of Sso7d mutants using yeast surface display. In the presence of the analyte, probes were brought into close proximity, reconstituting enzymatic activity of luciferase and enabling detection of low picomolar concentrations of the analyte by chemiluminescence. Subsequently, we constructed an equilibrium binding model that relates binding affinities of the binding proteins for the target, assay parameters such as the concentrations of probes used, and assay performance (limit of detection and concentration range over which the target can be quantified). Overall, our experimental and analytical framework provides the foundation for the development of split luciferase assays for detection and quantification of various targets.

Engineering Protein Switches for Rapid Diagnostic Tests

Biological signaling pathways are underpinned by protein switches that sense and respond to molecular inputs. Inspired by nature, engineered protein switches have been designed to directly transduce analyte binding into a quantitative signal in a simple, wash-free, homogeneous assay format. As such, they offer great potential to underpin point-of-need diagnostics that are needed across broad sectors to improve access, costs, and speed compared to laboratory assays. Despite this, protein switch assays are not yet in routine diagnostic use, and a number of barriers to uptake must be overcome to realize this potential. Here, we review the opportunities and challenges in engineering protein switches for rapid diagnostic tests. We evaluate how their design, comprising a recognition element, reporter, and switching mechanism, relates to performance and identify areas for improvement to guide further optimization. Recent modular switches that enable new analytes to be targeted without redesign are crucial to ensure robust and efficient development processes. The importance of translational steps toward practical implementation, including integration into a user-friendly device and thorough assay validation, is also discussed.

Wash- and Amplification-Free Digital Immunoassay Based on Single-Particle Motion Analysis

Digital enzyme-linked immunosorbent assay (ELISA) is a powerful analytical method for highly sensitive protein biomarker detection. The current protocol of digital ELISA requires multiple washing steps and signal amplification using an enzyme, which could be the potential drawback in in vitro diagnosis. In this study, we propose a digital immunoassay method, which we call "Digital HoNon-ELISA" (digital homogeneous non-enzymatic immunosorbent assay) for highly sensitive detection without washing and signal amplification. Target antigen molecules react with antibody-coated magnetic nanoparticles, which are then magnetically pulled into femtoliter-sized reactors. The antigens on the particles are captured by antibodies anchored on the bottom surface of the reactor via molecular tethers. Magnetic force enhances the efficiency of particle encapsulation in the reactors. Subsequent physical compartmentalization of the particles enhances the binding efficiency of antigen-carrying particles to the antibodies. The tethered particles show characteristic Brownian motion within a limited space by the molecular tethering, which is distinct from free diffusion or nonspecific binding of antigen-free particles. The number of tethered particles directly correlates with the concentration of the target antigen. Digital HoNon-ELISA was used with a prostate-specific antigen to achieve a detection of 0.093 pg/mL, which is over 9.0-fold the sensitivity of commercialized highly sensitive ELISA (0.84 pg/mL) and comparable to digital ELISA (0.055 pg/mL). This digital immunoassay strategy has sensitivity similar to digital ELISA with simplicity similar to homogeneous assay. Such similarity allows for potential application in rapid and simple digital diagnostic tests without the need for washing and enzymatic amplification.

Quantitative and Single-step Enzyme Immunosensing Based on an Electrochemical Detection Coupled with Lateral-flow System

A single-step electrochemical immunochromatography has been developed: the device was based on two pieces of nitrocellulose membrane, a sample pad with anti-mouse IgG antibody labeled with glucose oxidase (GOx-labeled antibody), a conjugate pad with glucose, and a Pt working electrode. Either antibody or antigen was immobilized on the membrane. The addition of a solution containing mouse IgG, a model target, allows for the dissolution of GOx-labeled antibody in the sample pad to form an immunocomplex. The produced immunocomplex was automatically separated by capturing to the antibody immobilized on the membrane with the sandwich structure or by passing through the membrane modified with an antigen for the competitive reaction. The separated GOx label arrived at the conjugate pad with glucose to undergo the enzyme reaction. Hydrogen peroxide generated by this reaction was detected at the Pt electrode prepared on the second nitrocellulose membrane downstream from the conjugate pad. The results demonstrated that the designed immunochromatography can be applied to quantitative detection with a single-step procedure, because both the GOx-labeled antibody for revealing the immunoreactions and the substrate for the enzyme reaction were prepared in the device. Moreover, the initial concentration of the GOx-labeled antibody permitted control of the detectable concentration for mouse IgG.

Development of a highly sensitive in vitro system to detect and discriminate between vitamin D receptor agonists and antagonists based on split-luciferase technique

Split-luciferase techniques are widely used to detect protein-protein interaction and bioactive small molecules including some hormones and vitamins. Previously, we successfully expressed chimeric proteins of luciferase and the ligand binding domain (LBD) of the vitamin D receptor (VDR), LucC-LBD-LucN in COS-7 cells. The LucC-LBD-LucN biosensor was named split-luciferase vitamin D biosensor (SLDB). This biosensor can detect and discriminate between VDR agonists and antagonists in mammalian cells. In this study, we established an in vitro screening system for VDR ligands using the SLDB proteins expressed in Escherichia coli (E. coli) cells. Our in vitro screening system using cell lysate of recombinant E. coli cells could be completed within 30min, and its activity was unchanged after 10 freeze-thaw cycles. This highly sensitive and convenient system would be quite useful to screen VDR ligands with therapeutic potential for various bone-related diseases, age-related cognitive disorders, cancer, and immune disorders. In addition, our system might be applicable to diagnostic measurement of serum concentrations of 25-hydroxyvitamin D₃ and 1α,25-dihydroxyvitamin D₃.

A Modular Receptor Platform To Expand the Sensing Repertoire of Bacteria

Engineered bacteria promise to revolutionize diagnostics and therapeutics, yet many applications are precluded by the limited number of detectable signals. Here we present a general framework to engineer synthetic receptors enabling bacterial cells to respond to novel ligands. These receptors are activated via ligand-induced dimerization of a single-domain antibody fused to monomeric DNA-binding domains (split-DBDs). Using E. coli as a model system, we engineer both transmembrane and cytosolic receptors using a VHH for ligand detection and demonstrate the scalability of our platform by using the DBDs of two different transcriptional regulators. We provide a method to optimize receptor behavior by finely tuning protein expression levels and optimizing interdomain linker regions. Finally, we show that these receptors can be connected to downstream synthetic gene circuits for further signal processing. The general nature of the split-DBD principle and the versatility of antibody-based detection should support the deployment of these receptors into various hosts to detect ligands for which no receptor is found in nature.

A Tri-part Protein Complementation System Using Antibody-Small Peptide Fusions Enables Homogeneous Immunoassays

Protein-fragment complementation is a valuable tool for monitoring protein interactions. In complementation assays, the reporter fragments are directly fused to the interacting proteins, eliminating the possibility of monitoring native interactions. In principle, complementation could be achieved by placing the reporter fragments on antibodies which bind to the proteins of interest, enabling the monitoring of endogenous protein interactions or detection of a single protein in a homogeneous immunoassay. Previous reports have demonstrated proof-of-concept of this approach; however, current complementation systems have not met the practical requirements as suitable fusion partners for antibodies while providing the sensitivity needed for immunoassays. To surmount these challenges, we created a first-in-class, tri-part split luciferase consisting of two 11-residue peptides that are used as the antibody appendages. As an initial proof-of-concept, we used antibody-peptide fusions and found them to be capable of quantifying pg/mL concentrations of soluble or cell-bound HER2, proving this unique complementation system overcomes previous limitations and transforms this approach from merely possible to practical and useful. As shown herein, this dual-peptide system provides a rapid, simple, and sensitive "add-and-read" homogeneous immunoassay platform that can be broadly adapted as an alternative to traditional immunoassays, and in the future should enable complementation to be expanded to monitoring endogenous protein interactions.

Creation of Antigen-Dependent β-Lactamase Fusion Protein Tethered by Circularly Permuted Antibody Variable Domains

Antibody-based molecular switches that are able to recognize a range of exogenous antigens can be highly useful as a versatile biosensor. However, regulating the catalytic activity of enzymes by antibodies is still hard to achieve. Here, we describe a design method of unique antibody variable region Fv introduced with two circular permutations, called Clampbody. By tethering the Clampbody to a circularly permuted TEM-1 β-lactamase (BLA), we successfully constructed a genetically encoded molecular switch Cbody-cpBLA that shows antigen-dependent catalytic activity.

Combinatorial Pairwise Assembly Efficiently Generates High Affinity Binders and Enables a "Mix-and-Read" Detection Scheme

We show that a combinatorial library constructed by random pairwise assembly of low affinity binders can efficiently generate binders with increased affinity. Such a library based on the Sso7d scaffold, from a pool of low affinity binders subjected to random mutagenesis, contained putative high affinity clones for a model target (lysozyme) at higher frequency than a library of monovalent mutants generated by random mutagenesis alone. Increased binding affinity was due to intramolecular avidity generated by linking binders targeting nonoverlapping epitopes; individual binders of K_D ∼ 1.3 μM and 250 nM produced a bivalent binder with apparent K_D ∼ 2 nM. Furthermore, the bivalent protein retained thermal stability (T_M = 84.5 °C) and high recombinant expression yields in E. coli. Finally, when binders comprising the bivalent protein are fused to two of the three fragments of tripartite split-green fluorescent protein (GFP), target-dependent reconstitution of fluorescence occurs, thereby enabling a "mix-and-read" assay for target quantification.

SpyTag/SpyCatcher Cyclization Enhances the Thermostability of Firefly Luciferase

SpyTag can spontaneously form a covalent isopeptide bond with its protein partner SpyCatcher. Firefly luciferase from Photinus pyralis was cyclized in vivo by fusing SpyCatcher at the N terminus and SpyTag at the C terminus. Circular LUC was more thermostable and alkali-tolerant than the wild type, without compromising the specific activity. Structural analysis indicated that the cyclized LUC increased the thermodynamic stability of the structure and remained more properly folded at high temperatures when compared with the wild type. We also prepared an N-terminally and C-terminally shortened form of the SpyCatcher protein and cyclization using this truncated form led to even more thermostability than the original form. Our findings suggest that cyclization with SpyTag and SpyCatcher is a promising and effective strategy to enhance thermostability of enzymes.

SpyTag/SpyCatcher Cyclization Enhances the Thermostability of Firefly Luciferase

SpyTag can spontaneously form a covalent isopeptide bond with its protein partner SpyCatcher. Firefly luciferase from Photinus pyralis was cyclized in vivo by fusing SpyCatcher at the N terminus and SpyTag at the C terminus. Circular LUC was more thermostable and alkali-tolerant than the wild type, without compromising the specific activity. Structural analysis indicated that the cyclized LUC increased the thermodynamic stability of the structure and remained more properly folded at high temperatures when compared with the wild type. We also prepared an N-terminally and C-terminally shortened form of the SpyCatcher protein and cyclization using this truncated form led to even more thermostability than the original form. Our findings suggest that cyclization with SpyTag and SpyCatcher is a promising and effective strategy to enhance thermostability of enzymes.

Steric-dependent label-free and washing-free enzyme amplified protein detection with dual-functional synthetic probes

Enzyme-catalyzed signal amplification with an antibody-enzyme conjugate is commonly employed in many bioanalytical methods to increase assay sensitivity. However, covalent labeling of the enzyme to the antibody, laborious operating procedures, and extensive washing steps are necessary for protein recognition and signal amplification. Herein, we describe a novel label-free and washing-free enzyme-amplified protein detection method by using dual-functional synthetic molecules to impose steric effects upon protein binding. In our approach, protein recognition and signal amplification are modulated by a simple dual-functional synthetic probe which consists of a protein ligand and an inhibitor. In the absence of the target protein, the inhibitor from the dual-functional probe would inhibit the enzyme activity. In contrast, binding of the target protein to the ligand perturbs this enzyme-inhibitor affinity due to the generation of steric effects caused by the close proximity between the target protein and the enzyme, thereby activating the enzyme to initiate signal amplification. With this strategy, the fluorescence signal can be amplified to as high as 70-fold. The generality and versatility of this strategy are demonstrated by the rapid, selective, and sensitive detection of four different proteins, avidin, O6-methylguanine DNA methyltransferase (MGMT), SNAP-tag, and lactoferrin, with four different probes.

Detection of protein cage assembly with bisarsenic fluorescent probes

We describe a method for the detection of specific protein-protein interactions in protein cages through the exploitation of designed binding sites for bisarsenic fluorescent probes. These sites are engineered to be protein-protein interface specific. We have adapted this method to ferritins; however, it could conceivably be applied to other protein cages. It is thought that this technique could be utilized in the thermodynamic and kinetic characterization of cage assembly mechanisms and in the high-throughput screening of protein cage libraries for the discovery of proteins with new assembly properties or of optimized conditions for assembly.

Small molecule gated split-tyrosine phosphatases and orthogonal split-tyrosine kinases

Protein kinases phosphorylate client proteins, while protein phosphatases catalyze their dephosphorylation and thereby in concert exert reversible control over numerous signal transduction pathways. We have recently reported the design and validation of split-protein kinases that can be conditionally activated by an added small molecule chemical inducer of dimerization (CID), rapamycin. Herein, we provide the rational design and validation of three split-tyrosine phosphatases (PTPs) attached to FKBP and FRB, where catalytic activity can be modulated with rapamycin. We further demonstrate that the orthogonal CIDs, abscisic acid and gibberellic acid, can be used to impart control over the activity of split-tyrosine kinases (PTKs). Finally, we demonstrate that designed split-phosphatases and split-kinases can be activated by orthogonal CIDs in mammalian cells. In sum, we provide a methodology that allows for post-translational orthogonal small molecule control over the activity of user defined split-PTKs and split-PTPs. This methodology has the long-term potential for both interrogating and redesigning phosphorylation dependent signaling pathways.

Strategy of mutual compensation of green and red mutants of firefly luciferase identifies a mutation of the highly conservative residue E457 with a strong red shift of bioluminescence

Bioluminescence spectra of firefly luciferases demonstrate highly pH-sensitive spectra changing the color from green to red light when pH is lowered from alkaline to acidic. This reflects a change of ratio of the green and red emitters in the bimodal spectra of bioluminescence. We show that the mutations strongly stabilizing green (Y35N) or red (H433Y) emission compensate each other leading to the WT color of firefly luciferase. We further used this compensating ability of Y35N to search for strong red-shifting mutations in the C-domain of firefly luciferase by random mutagenesis. The discovered mutation E457K substantially increased the contribution of the red emitter and caused a 12 nm red shift of the green emitter as well. E457 is highly conservative not only in beetle luciferases but also in a whole ANL superfamily of adenylating enzymes and forms a conservative structural hydrogen bond with V471. Our results suggest that the removal of this hydrogen bond only mildly affects luciferase properties and that most of the effect of E457K is caused by the introduction of positive charge. E457 forms a salt bridge with R534 in most ANL enzymes including pH-insensitive luciferases which is absent in pH-sensitive firefly luciferases. The mutant A534R shows that this salt bridge is not important for pH-sensitivity but considerably improves in vivo thermostability. Although E457 is located far from the oxyluciferin-binding site, the properties of the mutant E457K suggest that it affects color by influencing the AMP binding.

Point mutations in firefly luciferase C-domain demonstrate its significance in green color of bioluminescence

Firefly luciferase is a two-domain enzyme that catalyzes the bioluminescent reaction of firefly luciferin oxidation. Color of the emitted light depends on the structure of the enzyme, yet the exact color-tuning mechanism remains unknown by now, and the role of the C-domain in it is rarely discussed, because a very few color-shifting mutations in the C-domain were described. Recently we reported a strong red-shifting mutation E457K in the C-domain; the bioluminescence spectra of this enzyme were independent of temperature or pH. In the present study we investigated the role of the residue E457 in the enzyme using the Luciola mingrelica luciferase with a thermostabilized N-domain as a parent enzyme for site-directed mutagenesis. We obtained a set of mutants and studied their catalytic properties, thermal stability and bioluminescence spectra. Experimental spectra were represented as a sum of two components (bioluminescence spectra of putative "red" and "green" emitters); λmax of these components were constant for all the mutants, but the ratio of these emitters was defined by temperature and mutations in the C-domain. We suggest that each emitter is stabilized by a specific conformation of the active site; thus, enzymes with two forms of the active site coexist in the reactive media. The rigid structure of the C-domain is crucial for maintaining the conformation corresponding to the "green" emitter. We presume that the emitters are the keto- and enol forms of oxyluciferin.

Approaches to engineer stability of beetle luciferases

Luciferase enzymes from fireflies and other beetles have many important applications in molecular biology, biotechnology, analytical chemistry and several other areas. Many novel beetle luciferases with promising properties have been reported in the recent years. However, actual and potential applications of wild-type beetle luciferases are often limited by insufficient stability or decrease in activity of the enzyme at the conditions of a particular assay. Various examples of genetic engineering of the enhanced beetle luciferases have been reported that successfully solve or alleviate many of these limitations. This mini-review summarizes the recent advances in development of mutant luciferases with improved stability and activity characteristics. It discusses the common limitations of wild-type luciferases in different applications and presents the efficient approaches that can be used to address these problems.

Optical aptasensors for the analysis of the vascular endothelial growth factor (VEGF)

The vascular endothelial growth factor, VEGF, is an important biomarker for different diseases and clinical disorders. We present a series of optical aptasensor-based sensing platforms for VEGF that include the following: (i) A FRET-based sensor that involves the VEGF-induced separation of aptamer-functionalized quantum dots blocked by a quencher nucleic acid (detection limit 1 nM). (ii) A FRET-based sensor based on the VEGF-induced assembly of the aptamer subunits functionalized with QDs and a dye acceptor (Cy5), respectively (detection limit 12 nM). (iii) A chemiluminescence aptasensor based on VEGF-induced assembly of a hemin/G-quadruplex catalyst (detection limit 18 nM). (iv) A chemiluminescence aptasensor based on the VEGF-stimulated assembly of two aptamer subunits into the hemin/G-quadruplex catalyst (detection limit 2.6 nM). (v) A chemiluminescence resonance energy transfer (CRET) aptasensor based on the VEGF-induced assembly of a semiconductor QDs-hemin/G-quadruplex supramolecular structure (detection limit 875 pM). Furthermore, an amplified optical aptasensor system based on the Exonuclease III (Exo III) recycling of the VEGF analyte was developed. In this system, one aptamer subunit is modified at its 5' and 3' ends with QDs and a black hole quencher, respectively. The VEGF-induced self-assembly of the aptamer subunits result in the digestion of the quencher units and the autonomous recycling of the analyte, while triggering-on the luminescence of the QDs (detection limit 5 pM). The system was implemented to analyze VEGF in human sera samples.

Split-protein systems: beyond binary protein-protein interactions

It has been estimated that 650,000 protein-protein interactions exist in the human interactome (Stumpf et al., 2008), a subset of all possible macromolecular partnerships that dictate life. Thus there is a continued need for the development of sensitive and user-friendly methods for cataloguing biomacromolecules in complex environments and for detecting their interactions, modifications, and cellular location. Such methods also allow for establishing differences in the interactome between a normal and diseased cellular state and for quantifying the outcome of therapeutic intervention. A promising approach for deconvoluting the role of macromolecular partnerships is split-protein reassembly, also called protein fragment complementation. This approach relies on the appropriate fragmentation of protein reporters, such as the green fluorescent protein or firefly luciferase, which when attached to possible interacting partners can reassemble and regain function, thereby confirming the partnership. Split-protein methods have been effectively utilized for detecting protein-protein interactions in cell-free systems, Escherichia coli, yeast, mammalian cells, plants, and live animals. Herein, we present recent advances in engineering split-protein systems that allow for the rapid detection of ternary protein complexes, small molecule inhibitors, as well as a variety of macromolecules including nucleic acids, poly(ADP) ribose, and iron sulfur clusters. We also present advances that combine split-protein systems with chemical inducers of dimerization strategies that allow for regulating the activity of orthogonal split-proteases as well as aid in identifying enzyme inhibitors. Finally, we discuss autoinhibition strategies leading to turn-on sensors as well as future directions in split-protein methodology including possible therapeutic approaches.

Turn-on DNA damage sensors for the direct detection of 8-oxoguanine and photoproducts in native DNA

The integrity of the genetic information in all living organisms is constantly threatened by a variety of endogenous and environmental insults. To counter this risk, the DNA-damage response is employed for repairing lesions and maintaining genomic integrity. However, an aberrant DNA-damage response can potentially lead to genetic instability and mutagenesis, carcinogenesis, or cell death. To directly monitor DNA damage events in the context of native DNA, we have designed two new sensors utilizing genetically fragmented firefly luciferase (split luciferase). The sensors are comprised of a methyl-CpG binding domain (MBD) attached to one fragment of split luciferase for localizing the sensor to DNA (50-80% of the CpG dinucleotide sites in the genome are symmetrically methylated at cytosines), while a damage-recognition domain is attached to the complementary fragment of luciferase to probe adjacent nucleotides for lesions. Specifically, we utilized oxoguanine glycosylase 1 (OGG1) to detect 8-oxoguanine caused by exposure to reactive oxygen species and employed the damaged-DNA binding protein 2 (DDB2) for detection of pyrimidine dimer photoproducts induced by UVC light. These two sensors were optimized and validated using oligonucleotides, plasmids, and mammalian genomic DNA, as well as HeLa cells that were systematically exposed to a variety of environmental insults, demonstrating that this methodology utilizing MBD-directed DNA localization provides a simple, sensitive, and potentially general approach for the rapid profiling of specific chemical modifications associated with DNA damage and repair.