#chromatin remodeling

Transcription in Eukaryotes

FACT complex is involved during

FACT, facilitates chromatin transcription, is heterodimeric histone chaperone composed of SPT16 and SSRP1 in humans, Spt16-Pob3 plus Nhp6 in yeast, that remains associated with elongating Pol II during gene body transcription. Nucleosomes present strong barrier to polymerase progression, causing frequent pausing and backtracking requiring rescue. FACT binds both H2A-H2B dimers and H3-H4 tetramers simultaneously via acidic domains, destabilizing nucleosome without displacing octamer completely, allowing passage. Interaction with SPT5 DSIF complex and PAF1 complex tethers FACT to elongating polymerase ensuring rapid reassembly behind transcription machinery.

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 10: FACT complex in elongation; JBC, FACT facilitates transcription-dependent nucleosome alteration

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Regulation of chromatin structure

Which complex mediates sliding of nucleosomes along DNA?

ATP-dependent remodeling complexes reorganize chromatin by mobilizing nucleosomes without covalent modification. SWI/SNF family members contain Brg1 or Brm ATPase subunits that translocate DNA relative to octamer, peeling off contacts at entry site and propagating bulge around particle to shift position along duplex. Sliding exposes previously occluded transcription factor binding sites, facilitates activator binding and promoter clearance. Unlike ISWI which spaces nucleosomes regularly, SWI/SNF generates disordered arrays and ejects octamers at high density, acting as transcriptional co-activator at inducible genes requiring rapid access during development and stress response.

Ref: Clapier and Cairns, Annu Rev Biochem 2009: SWI/SNF Complex Mediates Nucleosome Sliding Mechanism

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Regulation of chromatin structure

Which enzyme removes acetyl groups from histones?

Histone acetylation neutralizes positive charge on lysine residues in flexible N-terminal tails, weakening interaction with negatively charged DNA and favoring open euchromatin. Histone deacetylases reverse this activation mark by hydrolyzing the N-acetyl amide bond, regenerating unmodified lysine. Loss of acetyl restores electrostatic attraction, tightening wrap around octamer and recruiting repressive complexes such as Sin3, NuRD and CoREST. Humans possess 18 HDAC enzymes divided into Zn2+-dependent classes I, II, IV and NAD+-dependent sirtuins, targeting both histone and non-histone substrates for transcriptional control.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 4: Chromatin Structure, Histone Acetylation and Deacetylation

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Regulation of chromatin structure

Which nucleosome remodeling complex uses ATP hydrolysis?

Repositioning nucleosomes for regulated transcription demands mechanical force breaking 14 histone-DNA contacts. ATP-dependent chromatin remodelers couple ATP hydrolysis to translocate DNA around octamer. SWI/SNF family member containing SWI2/SNF2 ATPase domain hydrolyzes ATP sliding and evicting nucleosomes at promoters and enhancers, generating nucleosome depleted regions permitting preinitiation complex assembly. HAT and HDAC add or remove acetyl groups modulating charge not movement via ATP. DNMT methylates cytosine without remodeling. SWI/SNF mutations frequent in human cancers demonstrating tumor suppressor role maintaining accessibility. Thus ATP remodeling assigned to SWI/SNF complex specifically.

Ref: Vignali et al., 2000 Science; Alberts et al., Molecular Biology of the Cell, Chapter 4: SWI/SNF Uses ATP Hydrolysis

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Eukaryotic chromatin and chromosome

SWI/SNF complex is involved in:

Chromatin structure generally impedes transcription factor binding, requiring active remodeling. ATP-dependent remodeling complexes utilize energy from ATP hydrolysis to slide, evict or restructure nucleosomes exposing DNA. SWI/SNF family, containing BRG1 or BRM ATPase, disrupts histone-DNA contacts creating nucleosome-depleted regions at enhancers and promoters, essential for inducible transcription activation and DNA repair. Mutations in SWI/SNF subunits drive rhabdoid tumors and other cancers. It cooperates with histone acetylation opening chromatin, contrasting with ISWI and CHD families spacing nucleosomes regularly. SWI/SNF governs accessibility rather than histone synthesis.

Ref: Clapier et al., Nat Rev Mol Cell Biol 2017; Alberts et al., Chapter 4: SWI/SNF Complex Chromatin Remodeling

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