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#histone tails

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Nucleosome Packing

Histone tails emerge from nucleosome core through:

High resolution nucleosome structure at 2.8 angstrom shows histone tails exit core not through gaps at sides but threading between DNA gyres through channels formed where minor grooves face octamer. At specific superhelical locations DNA grooves widen permitting basic tails to protrude without disrupting tight histone-DNA wrapping mediated by arginine anchors. Tail emergence through minor groove allows enzymes like GCN5 acetyltransferase and kinases access for modification while maintaining core stability. Major groove exit would clash with phosphate backbone, linker DNA path distinct from tail path. Thus tails emerge via minor groove regions.

Ref: Luger and Richmond 1998 Crystal; Alberts et al., Molecular Biology of the Cell, Chapter 4: Histone Tail Emergence Minor Groove

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Nucleosome Packing

Histone tails mainly function in:

Histone N-terminal tails protrude beyond DNA gyres mediating essential internucleosomal interactions crucial for folding beyond beads-on-string. Unmodified tails, particularly H4 residues 16 to 24 highly basic, bind acidic patch formed by H2A glutamates 56,61,64 and H2B helices on adjacent nucleosome, drawing nucleosomes together during 30 nanometer fiber formation as shown by Dorigo tetranucleosome arrays. Acetylation at H4 K16 abolishes affinity, decompacting fiber. Tail removal prevents fiber condensation in vitro. Roles include recruiting modifying and remodeling enzymes, but major structural function involves chromatin compaction not replication initiation or termination.

Ref: Dorigo et al., Science 2004; Lodish et al., Chapter 8: Histone Tails Function in Chromatin Compaction

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Eukaryotic chromatin and chromosome

Histone tails are site of:

Histone octamer contains flexible unstructured N-terminal tails and protruding C-termini extending beyond DNA gyres. These lysine, arginine and serine rich segments undergo extensive covalent post-translational modifications including acetylation, methylation, phosphorylation, ubiquitination and sumoylation catalyzed by writer enzymes and erased by erasers. Modifications alter electrostatic charge and create binding sites for effector proteins containing bromodomains, chromodomains and PHD fingers, forming combinatorial histone code that governs accessibility, transcription activation, repression, replication timing and DNA repair without altering fundamental histone DNA wrapping structure.

Ref: Strahl and Allis 2000 Histone Code Hypothesis; Alberts et al., Chapter 4: Histone Tails Site of Modifications

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