Questions: Chromatin Remodeling Complexes and SWI/SNF Family

Question 1

A transcription factor binds a partially accessible promoter region but cannot recruit RNA Pol II or initiate transcription. When SWI/SNF is recruited to this locus and ATP is provided, transcription begins. What most likely happened?

• A. SWI/SNF added acetyl groups to histone tails, loosening their charge interactions with DNA
• B. SWI/SNF used ATP hydrolysis to slide or eject the nucleosome blocking the core promoter, exposing it for transcription factor binding
• C. SWI/SNF methylated the DNA promoter sequence, enabling RNA Pol II recognition
• D. SWI/SNF recruited additional transcription factors that bypassed the nucleosomal barrier

Answer: (B)

SWI/SNF complexes are ATP-dependent chromatin remodelers that physically disrupt histone-DNA contacts by translocating DNA across the histone surface, using ATP hydrolysis as the energy source. This can slide the nucleosome to a new position or eject it entirely, exposing previously occluded sequences. Histone acetylation is performed by histone acetyltransferases (HATs), a distinct class of enzyme — chromatin remodeling complexes physically move nucleosomes rather than chemically modifying histones, and they require ATP, not chemical modification, as their energy source.

Question 2

SWI/SNF complexes are present in every cell of an organism, yet they remodel chromatin at only specific gene loci rather than disrupting all nucleosomes globally. What explains this specificity?

• A. SWI/SNF only functions in euchromatin regions and is excluded from heterochromatin
• B. SWI/SNF must be recruited to specific loci by transcription factors that physically interact with its subunits
• C. Each cell type expresses a different ATPase subunit that recognizes specific DNA sequences
• D. SWI/SNF is activated only during S phase when chromatin is already partially disrupted

Answer: (B)

Gene-specificity arises from the recruitment model: transcription activators bind their recognition sequences (often in nucleosome-free regions or partially accessible sites) and then physically interact with SWI/SNF subunits, bringing the complex to nearby nucleosomes. Without this recruitment, SWI/SNF does not act. This is the mechanism by which signal-responsive gene activation works — a developmental signal activates a transcription factor, which then recruits SWI/SNF to the relevant promoter, enabling transcription. Only genes whose activators can recruit SWI/SNF will be remodeled.

Question 3

SWI/SNF complexes remodel chromatin by adding acetyl groups to histone tails, which reduces the positive charge on histones and weakens their interaction with DNA.

• T. True
• F. False

Answer: False

This describes histone acetyltransferases (HATs), not SWI/SNF. SWI/SNF complexes are ATP-dependent chromatin remodelers that physically move nucleosomes — sliding them along DNA, ejecting them, or exchanging histone variants — using the energy from ATP hydrolysis. Histone acetylation (by HATs) and chromatin remodeling (by SWI/SNF and related complexes) are distinct mechanisms that often work together but are fundamentally different in nature: one is a chemical modification to histones, the other is a mechanical repositioning of the entire nucleosome.

Question 4

ISWI chromatin remodeling complexes and SWI/SNF complexes both use ATP hydrolysis but can have opposing effects on gene expression in some contexts.

• T. True
• F. False

Answer: True

SWI/SNF typically disrupts or ejects nucleosomes at gene promoters, opening chromatin and facilitating transcription activation. ISWI complexes, by contrast, typically space nucleosomes at regular intervals and can contribute to chromatin compaction and transcriptional repression by maintaining tight, ordered packaging over gene bodies. Both consume ATP to reposition nucleosomes, but the outcomes can be opposed depending on context. This illustrates that chromatin remodeling is not inherently activating or repressing — the consequence depends on which complex acts, where, and in coordination with which other factors.

Question 5

Why does SWI/SNF remodel chromatin at specific genes rather than acting globally, and what determines which genes get remodeled?

Model answer: SWI/SNF lacks intrinsic DNA sequence specificity — it cannot recognize particular promoter sequences on its own. Instead, it depends entirely on being recruited by gene-specific transcription factors (activators) that bind their recognition sequences and then physically interact with SWI/SNF subunits. This recruitment model means that only genes whose regulatory sequences are occupied by the appropriate activators will attract SWI/SNF. The set of genes remodeled therefore depends on which transcription factors are active in a given cell type or developmental state — explaining how the same SWI/SNF complex can have different targets in different cell types and in response to different developmental or environmental signals.

This recruitment model has a key implication: signal-responsive gene activation often works by activating a transcription factor (e.g., via phosphorylation downstream of a signaling pathway), which then recruits SWI/SNF to the relevant gene promoters. Chromatin remodeling is thus downstream of signal transduction, connecting extracellular signals to changes in gene expression via physical remodeling of the nucleosomal landscape.