What is Dr. Zubair Khalid's research focus?

Dr. Zubair Khalid specializes in molecular virology, mRNA vaccine development, and computational biology, with a focus on avian pathogens like IBDV and Avian Reovirus.

Where is Dr. Zubair Khalid currently working?

Dr. Zubair Khalid is a Postdoctoral Research Associate at the University of Maryland (UMD), specifically within the Department of Animal and Avian Sciences.

Chromatin Remodelers: How Cells Control DNA Access

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-08-11

Every human cell contains roughly two meters of DNA, yet the nucleus that houses it measures only about six micrometers across. To fit, DNA is wrapped around protein spools called histones, forming a repeating unit known as the nucleosome. This packaging solves a spatial problem but creates another: how do the molecular machines that read, copy, and repair DNA reach their target sequences when the DNA is tightly wound around histone proteins? The answer lies with a class of protein complexes called chromatin remodelers—ATP-powered molecular engines that physically move, eject, or restructure nucleosomes to control access to the genetic code.

What Are Chromatin Remodelers?

Chromatin remodelers are multi-subunit protein complexes that use energy from ATP hydrolysis to alter the position or composition of nucleosomes along DNA. They are the gatekeepers of the genome, determining which regions of DNA are accessible to transcription factors, RNA polymerase, and DNA repair machinery. Without them, genes could not be turned on or off in response to developmental cues, environmental signals, or cellular stress.

The Chromatin Problem: DNA Packaging

To understand what remodelers do, you must first appreciate the scale of the packaging problem. The fundamental unit of chromatin is the nucleosome: 147 base pairs of DNA wrapped 1.65 times around an octamer of histone proteins—two copies each of H2A, H2B, H3, and H4. Nucleosomes are connected by short stretches of linker DNA (20–80 base pairs) and further compacted into higher-order structures. This arrangement, described in detail under Chromatin Structure, means that most DNA sequences are not freely accessible. A transcription factor searching for its binding site must contend with a landscape where the DNA is occluded by histone surfaces at regular intervals.

The problem is particularly acute at gene promoters and enhancers, where regulatory proteins need to bind specific DNA sequences. If a nucleosome sits directly over a transcription factor binding site, that site is effectively hidden. The cell therefore needs a mechanism to move or remove nucleosomes at precise locations and times. This is the job of chromatin remodelers.

Remodelers vs. Histone Modifiers

A common point of confusion is the distinction between chromatin remodelers and histone modifiers. Histone modifiers are enzymes that add or remove chemical groups—such as acetyl, methyl, or phosphate—to the tails of histone proteins. These modifications do not move nucleosomes; instead, they change the chemical properties of chromatin, often by recruiting other proteins or altering histone-DNA contacts. For example, histone acetyltransferases (HATs) add acetyl groups to lysine residues on histone tails, which neutralizes the positive charge and weakens histone-DNA interactions, making the DNA more accessible.

Chromatin remodelers, by contrast, are physical machines. They use the energy from ATP hydrolysis to break histone-DNA contacts and reposition or restructure nucleosomes. The two systems are deeply interconnected—histone modifications often recruit remodelers to specific genomic locations, and remodelers can facilitate the deposition or removal of histone variants—but they are mechanistically distinct. For a fuller discussion of the chemical side of chromatin regulation, see Chromatin Modification.

How Chromatin Remodelers Work

All chromatin remodelers share a core ATPase subunit that belongs to the Snf2 family of helicase-related proteins. This ATPase is the engine that powers the remodeling reaction. Despite differences among remodeler families, the basic mechanism is conserved.

The ATPase Engine

The ATPase domain of a remodeler binds to both the histone octamer and the DNA wrapped around it. When ATP binds and is hydrolyzed to ADP and inorganic phosphate, the ATPase undergoes a conformational change that pulls DNA relative to the histone surface. This creates a transient "bulge" of DNA that propagates around the nucleosome, effectively shifting the position of the DNA relative to the histone octamer.

The reaction cycle proceeds as follows:

Binding: The remodeler binds to a nucleosome, with its ATPase domain contacting the histone octamer and a segment of DNA near the entry site of the nucleosome.
ATP hydrolysis: ATP binds to the ATPase domain and is hydrolyzed, causing a conformational change that pulls ~1–3 base pairs of DNA into the nucleosome.
Bulge propagation: The DNA bulge travels around the histone octamer, breaking and reforming histone-DNA contacts as it moves.
Release: The remodeler releases the nucleosome, which now sits at a new position along the DNA.

This cycle repeats, allowing the remodeler to "walk" the nucleosome along the DNA. The directionality and processivity vary among remodeler families; some move nucleosomes toward the center of DNA fragments, while others move them toward the edges.

Nucleosome Dynamics

Remodelers do not only slide nucleosomes. They can also perform three other types of reactions:

Nucleosome ejection: Some remodelers can remove entire histone octamers from DNA, creating a stretch of naked DNA. This is particularly important at promoters, where transcription factors need access to multiple binding sites. The SWI/SNF family is especially adept at this reaction.
Histone dimer exchange: Remodelers can remove H2A-H2B dimers from the nucleosome, leaving a "hexasome" (a nucleosome lacking one H2A-H2B dimer). This can destabilize the nucleosome and facilitate transcription.
Histone variant exchange: Remodelers can replace canonical histones with variant forms. For example, the INO80 complex can exchange H2A for H2A.Z, a variant that destabilizes nucleosomes and is often found at promoters and enhancers. This process is discussed further under Nucleosome Chromatin.

The type of reaction a remodeler performs depends on its family, its associated subunits, and the context of the chromatin environment. For instance, the presence of specific histone modifications can bias a remodeler toward sliding versus ejection.

Major Families of Chromatin Remodelers

There are four major families of chromatin remodelers in eukaryotes: SWI/SNF, ISWI, CHD, and INO80. Each family is defined by the sequence of its ATPase domain and by the accessory subunits that confer functional specificity.

Family	ATPase	Key Functions	Distinguishing Features
SWI/SNF	BRG1 or BRM (in humans)	Nucleosome ejection, gene activation and repression	Contains bromodomains that bind acetylated histones
ISWI	SNF2H or SNF2L	Nucleosome assembly, spacing, and sliding	Contains SANT and SLIDE domains for DNA binding
CHD	CHD1–CHD9	Nucleosome sliding, links to histone methylation	Contains chromodomains that bind methylated histones
INO80	INO80, SRCAP, p400	DNA repair, transcription, histone variant exchange	Contains split ATPase domain; interacts with RuvB-like helicases

SWI/SNF: Activators and Repressors

The SWI/SNF family was the first chromatin remodeler discovered, identified through genetic screens in yeast for mutants that failed to switch mating type (SWI) and that grew poorly on sucrose (SNF). In humans, the two main ATPases are BRG1 (encoded by SMARCA4) and BRM (encoded by SMARCA2). SWI/SNF complexes are large (1–2 MDa) and contain 10–15 subunits, including actin and actin-related proteins.

SWI/SNF complexes are best known for their ability to eject nucleosomes, creating nucleosome-free regions at promoters and enhancers. This makes them potent activators of transcription. However, they can also repress genes by repositioning nucleosomes to occlude binding sites or by facilitating the formation of repressive chromatin structures. The bromodomains in SWI/SNF subunits bind to acetylated histone tails, targeting the complex to actively transcribed or poised regulatory regions.

Mutations in SWI/SNF subunits are found in approximately 20% of human cancers, making this family one of the most frequently mutated chromatin regulators in malignancy. This topic is explored further in the section on disease.

ISWI: Nucleosome Assembly and Spacing

The ISWI (Imitation SWI) family is named after the Drosophila protein ISWI. In humans, the two ATPases are SNF2H and SNF2L. ISWI complexes are generally smaller than SWI/SNF complexes (300–500 kDa) and contain subunits such as ACF1, WSTF, and NURF.

ISWI complexes are masters of nucleosome spacing. They slide nucleosomes to create evenly spaced arrays, which is essential for chromatin assembly during DNA replication and for the formation of repressive chromatin. The SANT domain of ISWI binds to unmodified histone tails, and the SLIDE domain binds to linker DNA. This dual recognition allows ISWI to sense the length of linker DNA and position nucleosomes accordingly.

ISWI activity is generally associated with transcriptional repression, because evenly spaced nucleosomes can occlude regulatory elements. However, ISWI can also promote transcription by positioning nucleosomes at specific locations that facilitate the binding of certain transcription factors.

CHD: Linking to Methylation

The CHD (Chromodomain-Helicase-DNA binding) family is characterized by the presence of chromodomains, which are protein modules that bind to methylated lysine residues on histone tails. In humans, there are nine CHD genes, with CHD1–CHD5 being the most studied.

CHD1 binds to H3K4me3 (trimethylation of lysine 4 on histone H3), a mark associated with active gene promoters. CHD1 slides nucleosomes to maintain an open chromatin structure at these promoters. CHD3 and CHD4 are components of the NuRD (Nucleosome Remodeling and Deacetylase) complex, which couples nucleosome remodeling with histone deacetylation to repress transcription. This connection between remodelers and histone modifications is a recurring theme in chromatin biology.

INO80: DNA Repair and Transcription

The INO80 family is named after the yeast gene INO80, which was identified in a screen for mutants defective in inositol metabolism. In humans, the family includes INO80, SRCAP, and p400. These complexes are large (1–2 MDa) and contain RuvB-like helicases (RUVBL1 and RUVBL2) that are related to bacterial RuvB, a protein involved in Holliday junction migration during DNA recombination.

INO80 complexes have two major functions. First, they are involved in DNA double-strand break repair. When a double-strand break occurs, INO80 is recruited to the damage site and slides or evicts nucleosomes to allow access for repair factors. Second, INO80 complexes exchange histone H2A for H2A.Z at promoters and enhancers. H2A.Z-containing nucleosomes are less stable than canonical nucleosomes, which facilitates transcription factor binding.

Chromatin Remodelers in Gene Regulation

The ultimate function of chromatin remodelers is to control access to DNA, and this control is exercised most critically at gene regulatory elements: promoters, enhancers, and insulators.

Opening and Closing Chromatin

At gene promoters, the default state is often a positioned nucleosome that occludes the transcription start site. For a gene to be transcribed, this nucleosome must be moved or removed. SWI/SNF complexes are frequently recruited to promoters by sequence-specific transcription factors. For example, the tumor suppressor p53 recruits SWI/SNF to target gene promoters upon DNA damage, leading to nucleosome ejection and transcriptional activation.

The process of opening a promoter typically follows a sequence:

A transcription factor binds to its recognition sequence in linker DNA, adjacent to a nucleosome.
The transcription factor recruits a remodeler, such as SWI/SNF, through direct protein-protein interactions.
The remodeler slides or ejects the nucleosome, exposing the promoter.
Additional transcription factors and RNA polymerase bind to the exposed DNA.
Transcription initiates.

Closing chromatin is the reverse process. ISWI complexes can assemble and space nucleosomes over regulatory elements, making them inaccessible. This is a key mechanism of Gene Silencing, where genes are stably repressed. The balance between opening and closing is dynamic, allowing cells to respond rapidly to changing conditions.

Cooperation with Histone Modifications

Chromatin remodelers do not work in isolation. They are intimately connected to the histone modification system. This cooperation operates in both directions:

Modifications recruit remodelers: Acetylation of histone tails recruits SWI/SNF via bromodomains. Methylation of H3K4 recruits CHD1 via chromodomains. Phosphorylation of histone H2AX (γH2AX) recruits INO80 to DNA damage sites.
Remodelers facilitate modifications: By moving nucleosomes, remodelers can expose histone tails to modifying enzymes. They can also evict nucleosomes containing modified histones, allowing their replacement with unmodified histones.

This two-way communication creates a regulatory circuit where chromatin state is continuously read, interpreted, and modified. The interplay is particularly important at enhancers, where the binding of pioneer transcription factors to nucleosomal DNA requires remodeler activity to open the chromatin and allow subsequent factor binding.

Chromatin Remodelers in Development and Disease

Given their central role in gene regulation, it is not surprising that chromatin remodelers are critical for development and that their dysfunction causes disease.

Developmental Roles

During development, cells must make stable decisions about which genes to express. Chromatin remodelers are essential for establishing and maintaining cell identity. For example:

Embryonic stem cells: The SWI/SNF ATPase BRG1 is required for the self-renewal of embryonic stem cells. BRG1 maintains the expression of pluripotency factors such as OCT4 and NANOG by keeping their promoters accessible.
Neural development: CHD5 is specifically expressed in the nervous system and is required for the maturation of neurons. Mice lacking CHD5 show defects in synaptic function and learning.
Hematopoiesis: The INO80 complex is required for the development of B cells and T cells. Deletion of INO80 in hematopoietic stem cells blocks their differentiation into mature immune cells.

The importance of remodelers in development is underscored by the fact that mutations in remodeler genes often cause developmental disorders. For example, mutations in CHD7 cause CHARGE syndrome, a condition characterized by heart defects, choanal atresia, retarded growth, genital hypoplasia, and ear abnormalities.

Cancer and Mutations

Chromatin remodelers are among the most frequently mutated genes in cancer. The SWI/SNF complex is particularly notable: mutations in its subunits are found in approximately 20% of all human tumors. Specific examples include:

***SMARCA4* (BRG1)**: Mutated in non-small cell lung cancer, medulloblastoma, and ovarian cancer. Loss of BRG1 leads to widespread changes in gene expression that promote uncontrolled cell proliferation.
***SMARCB1* (INI1/SNF5)**: Mutated in malignant rhabdoid tumors, an aggressive pediatric cancer. Loss of SMARCB1 is the defining genetic event in these tumors.
***ARID1A* (BAF250A)**: Mutated in ovarian clear cell carcinoma and gastric cancer. ARID1A is a subunit of the SWI/SNF complex that binds AT-rich DNA.

Why are remodeler mutations so oncogenic? The answer lies in their role as master regulators of gene expression. When a remodeler is lost, thousands of genes change expression simultaneously. This can activate oncogenes, silence tumor suppressors, and create a permissive environment for genomic instability. Notably, many remodeler mutations are "loss of function"—the protein is not produced or is inactive—suggesting that these complexes normally act as tumor suppressors.

The connection between remodelers and cancer has therapeutic implications. Because cancer cells with remodeler mutations are often dependent on residual remodeler activity, they may be vulnerable to inhibitors of the remaining complexes. This concept, called synthetic lethality, is being explored in clinical trials.

Methods to Study Chromatin Remodelers

Studying chromatin remodelers requires approaches that can detect changes in nucleosome position, chromatin accessibility, and protein-DNA interactions across the genome or in purified systems.

Genome-Wide Approaches

Several high-throughput methods allow researchers to map remodeler activity across the genome:

MNase-seq: Micrococcal nuclease (MNase) digests linker DNA, leaving only nucleosome-protected DNA. Sequencing this DNA reveals the positions of nucleosomes genome-wide. By comparing nucleosome positions in wild-type versus remodeler-mutant cells, researchers can identify regions where the remodeler is required for proper nucleosome positioning.
ATAC-seq: Assay for Transposase-Accessible Chromatin uses the Tn5 transposase to insert sequencing adapters into accessible (open) chromatin. Regions of open chromatin are enriched for remodeler binding sites and regulatory elements. ATAC-seq requires only 500–50,000 cells, making it suitable for rare cell populations.
ChIP-seq: Chromatin Immunoprecipitation followed by sequencing uses antibodies to enrich for DNA bound by a specific protein. For remodelers, ChIP-seq can identify the genomic locations where a remodeler is bound. For histone modifications, it can map the marks that recruit remodelers.

A typical ChIP-seq experiment involves crosslinking cells with 1% formaldehyde for 10 minutes at room temperature, quenching with 125 mM glycine, sonicating chromatin to fragments of 200–600 base pairs, immunoprecipitating with an antibody, reversing crosslinks, and sequencing the enriched DNA.

In Vitro Assays

Biochemical assays using purified components allow researchers to dissect the mechanism of remodeler action:

Nucleosome sliding assay: A nucleosome is assembled on a DNA fragment with a positioned nucleosome. The remodeler and ATP are added, and the reaction is incubated at 30°C for 30–60 minutes. The products are analyzed by native polyacrylamide gel electrophoresis, where nucleosomes at different positions migrate differently.
ATPase assay: The rate of ATP hydrolysis is measured by monitoring the conversion of ATP to ADP. This can be done using radiolabeled ATP or by coupling ATP hydrolysis to NADH oxidation, which is measured spectrophotometrically at 340 nm.
Restriction enzyme accessibility assay: A nucleosome is positioned over a restriction enzyme site. If the remodeler moves the nucleosome, the site becomes accessible, and the DNA is cut. The extent of cutting is quantified by gel electrophoresis.

These assays have revealed key parameters of remodeler function, such as the observation that SWI/SNF can move nucleosomes in 10-base-pair steps and that ISWI moves nucleosomes toward the center of DNA fragments.

Common Pitfalls and Misconceptions

Several misconceptions about chromatin remodelers are common among students encountering the topic for the first time.

Remodelers vs. Modifiers

The most frequent error is conflating chromatin remodelers with histone modifiers. Remodelers move nucleosomes using ATP; modifiers add or remove chemical groups to histones. A useful mnemonic: remodelers are "movers," modifiers are "markers." While they cooperate, they are distinct classes of enzymes with different mechanisms and often different functions. A histone acetyltransferase does not slide nucleosomes, and a remodeler does not acetylate histones.

Not All Remodeling Is Activation

A second misconception is that chromatin remodeling always leads to gene activation. While SWI/SNF is often associated with opening chromatin and activating transcription, ISWI and CHD complexes frequently repress genes by creating evenly spaced nucleosome arrays that occlude regulatory elements. The NuRD complex, which contains CHD3/CHD4, couples remodeling with deacetylation to actively repress transcription. Whether a remodeler activates or represses depends on the context, the specific complex, and the genomic location.

Oversimplifying Nucleosome Dynamics

A third error is thinking of nucleosomes as static obstacles that are either present or absent. In reality, nucleosomes are dynamic structures that can exist in multiple states: canonical nucleosomes, hexasomes (missing one H2A-H2B dimer), nucleosomes containing histone variants, and partially unwrapped states. Remodelers can interconvert these states, and the functional consequences depend on the specific state. A nucleosome containing H2A.Z is not the same as a canonical nucleosome, even though both are "nucleosomes."

Summary and Key Takeaways

Chromatin remodelers are essential regulators of DNA access. They use ATP hydrolysis to move, eject, or restructure nucleosomes, thereby controlling which regions of the genome are accessible to the transcription, replication, and repair machinery. The four major families—SWI/SNF, ISWI, CHD, and INO80—have distinct functions and are recruited to specific genomic locations by transcription factors and histone modifications. Their importance is underscored by the fact that mutations in remodeler genes cause developmental disorders and are among the most common genetic alterations in cancer.

Frequently Asked Questions

What are chromatin remodelers?

Chromatin remodelers are multi-subunit protein complexes that use energy from ATP hydrolysis to alter the structure of chromatin. They physically move, eject, or restructure nucleosomes—the basic units of DNA packaging—to control access to the underlying DNA. They are distinct from histone modifiers, which add or remove chemical groups to histone proteins.

What is the function of chromatin remodelers?

The primary function of chromatin remodelers is to regulate DNA accessibility. By sliding nucleosomes along DNA, ejecting them entirely, or exchanging histone variants, remodelers expose or occlude specific DNA sequences. This controls whether transcription factors, RNA polymerase, and DNA repair proteins can access their target sites. Remodelers are therefore essential for gene activation, gene repression, DNA replication, and DNA repair.

How do chromatin remodelers work?

Chromatin remodelers work through a conserved ATP-dependent mechanism. The ATPase domain binds to both the histone octamer and the DNA wrapped around it. ATP hydrolysis causes a conformational change that pulls DNA relative to the histones, creating a DNA bulge that propagates around the nucleosome. This shifts the nucleosome position. The cycle repeats, allowing the remodeler to processively move nucleosomes. Some remodelers can also eject nucleosomes or exchange histone dimers.

What are the main families of chromatin remodelers?

The four main families are SWI/SNF, ISWI, CHD, and INO80. SWI/SNF complexes (ATPases BRG1/BRM) eject nucleosomes and activate or repress genes. ISWI complexes (SNF2H/SNF2L) assemble and space nucleosomes, often repressing transcription. CHD complexes (CHD1–CHD9) slide nucleosomes and link to histone methylation. INO80 complexes (INO80, SRCAP, p400) are involved in DNA repair and histone variant exchange.

Are chromatin remodelers the same as histone modifiers?

No. Chromatin remodelers are ATP-dependent machines that physically move nucleosomes. Histone modifiers are enzymes that add or remove chemical groups—acetyl, methyl, phosphate, ubiquitin—to histone tails. The two systems cooperate: histone modifications can recruit remodelers to specific locations, and remodelers can expose histone tails to modifying enzymes. But they are mechanistically distinct.

Why are chromatin remodelers important in cancer?

Mutations in chromatin remodeler genes are found in approximately 20% of human cancers, with SWI/SNF subunits being among the most frequently mutated. Loss of remodeler function leads to widespread changes in gene expression that can activate oncogenes, silence tumor suppressors, and promote genomic instability. Because cancer cells with remodeler mutations may depend on residual remodeler activity, they are potential targets for synthetic lethal therapies.

How do scientists study chromatin remodelers?

Scientists study remodelers using genome-wide methods such as MNase-seq (to map nucleosome positions), ATAC-seq (to map accessible chromatin), and ChIP-seq (to map remodeler binding and histone modifications). They also use biochemical assays with purified components, including nucleosome sliding assays, ATPase activity assays, and restriction enzyme accessibility assays, to dissect the molecular mechanism.

Key Takeaways

Chromatin remodelers are ATP-powered protein complexes that move, eject, or restructure nucleosomes to control DNA access.
They are distinct from histone modifiers, which chemically modify histone tails without moving nucleosomes.
The four major families—SWI/SNF, ISWI, CHD, and INO80—have specialized functions in gene activation, repression, DNA repair, and histone variant exchange.
Remodelers are recruited to specific genomic locations by transcription factors and histone modifications, integrating physical chromatin dynamics with the epigenetic code.
Mutations in remodeler genes cause developmental disorders and are among the most common genetic alterations in cancer.
Remodeling does not always mean activation; ISWI and CHD complexes frequently repress transcription by creating repressive nucleosome arrays.
Advanced genomic and biochemical methods allow researchers to map remodeler activity genome-wide and dissect their molecular mechanisms in vitro.