Characteristic H3 N-tail dynamics in the nucleosome core particle, nucleosome, and chromatosome

High-Speed Atomic Force Microscopy Reveals the Nucleosome Sliding and DNA Unwrapping/Wrapping Dynamics of Tail-less Nucleosomes

Each nucleosome contains four types of histone proteins, each with a histone tail. These tails are essential for the epigenetic regulation of gene expression through post-translational modifications (PTMs). However, their influence on nucleosome dynamics at the single-molecule level remains undetermined. Here, we employed high-speed atomic force microscopy to visualize nucleosome dynamics in the absence of the N-terminal tail of each histone or all of the N-terminal tails. Loss of all tails stripped 6.7 base pairs of the nucleosome from the histone core, and the DNA entry-exit angle expanded by 18° from that of wild-type nucleosomes. Tail-less nucleosomes, particularly those without H2B and H3 tails, showed a 10-fold increase in dynamics, such as nucleosome sliding and DNA unwrapping/wrapping, within 0.3 s, emphasizing their role in histone-DNA interactions. Our findings illustrate that N-terminal histone tails stabilize the nucleosome structure, suggesting that histone tail PTMs modulate nucleosome dynamics.

Nucleosome conformation dictates the histone code

Histone post-translational modifications (PTMs) play a critical role in chromatin regulation. It has been proposed that these PTMs form localized 'codes' that are read by specialized regions (reader domains) in chromatin-associated proteins (CAPs) to regulate downstream function. Substantial effort has been made to define [CAP: histone PTM] specificities, and thus decipher the histone code and guide epigenetic therapies. However, this has largely been done using the reductive approach of isolated reader domains and histone peptides, which cannot account for any higher-order factors. Here, we show that the [BPTF PHD finger and bromodomain: histone PTM] interaction is dependent on nucleosome context. The tandem reader selectively associates with nucleosomal H3K4me3 and H3K14ac or H3K18ac, a combinatorial engagement that despite being in cis is not predicted by peptides. This in vitro specificity of the BPTF tandem reader for PTM-defined nucleosomes is recapitulated in a cellular context. We propose that regulatable histone tail accessibility and its impact on the binding potential of reader domains necessitates we refine the 'histone code' concept and interrogate it at the nucleosome level.

Contributions of histone tail clipping and acetylation in nucleosome transcription by RNA polymerase II

The N-terminal tails of histones protrude from the nucleosome core and are target sites for histone modifications, such as acetylation and methylation. Histone acetylation is considered to enhance transcription in chromatin. However, the contribution of the histone N-terminal tail to the nucleosome transcription by RNA polymerase II (RNAPII) has not been clarified. In the present study, we reconstituted nucleosomes lacking the N-terminal tail of each histone, H2A, H2B, H3 or H4, and performed RNAPII transcription assays. We found that the N-terminal tail of H3, but not H2A, H2B and H4, functions in RNAPII pausing at the SHL(-5) position of the nucleosome. Consistently, the RNAPII transcription assay also revealed that the nucleosome containing N-terminally acetylated H3 drastically alleviates RNAPII pausing at the SHL(-5) position. In addition, the H3 acetylated nucleosome produced increased amounts of the run-off transcript. These results provide important evidence that the H3 N-terminal tail plays a role in RNAPII pausing at the SHL(-5) position of the nucleosome, and its acetylation directly alleviates this nucleosome barrier.

Arginine anchor points govern H3 tail dynamics

Chromatin is dynamically reorganized spatially and temporally, and the post-translational modification of histones is a key component of this regulation. The basic subunit of chromatin is the nucleosome core particle, consisting of two copies each of the histones H2A, H2B, H3, and H4 around which ∼147 base pairs of DNA wrap. The intrinsically disordered histone termini, or tails, protrude from the core and are heavily post-translationally modified. Previous studies have shown that the histone tails exist in dynamic ensembles of DNA-bound states within the nucleosome. Histone tail interactions with DNA are involved in nucleosome conformation and chromatin organization. Charge-modulating histone post-translational modifications (PTMs) are poised to perturb the dynamic interactions between histone tails and DNA. Arginine side chains form favorable interactions with DNA and are sites of charge-modulating PTMs such as citrullination. Our current focus is on the H3 tail, the longest histone tail. Four arginine residues are relatively evenly spaced along the H3 tail sequence, suggesting multivalent interactions with DNA poised for regulation by PTMs. In this study, we use NMR nuclear spin relaxation experiments to investigate the contribution of arginine residues to H3 tail dynamics within the nucleosome core particle. By neutralizing arginine via mutation to glutamine, we begin to work towards a comprehensive understanding of the contribution of individual residues to H3 tail dynamics. We find that neutralization of arginine residues results in increased regional mobility of the H3 tails, with implications for understanding the direct effects of arginine citrullination. Altogether, these studies support a role for dynamics within the histone language and emphasize the importance of charge-modulating histone PTMs in regulating chromatin dynamics, starting at the level of the basic subunit of chromatin.

Dynamic structures of intrinsically disordered proteins related to the general transcription factor TFIIH, nucleosomes, and histone chaperones

Advances in structural analysis by cryogenic electron microscopy (cryo-EM) and X-ray crystallography have revealed the tertiary structures of various chromatin-related proteins, including transcription factors, RNA polymerases, nucleosomes, and histone chaperones; however, the dynamic structures of intrinsically disordered regions (IDRs) in these proteins remain elusive. Recent studies using nuclear magnetic resonance (NMR), together with molecular dynamics (MD) simulations, are beginning to reveal dynamic structures of the general transcription factor TFIIH complexed with target proteins including the general transcription factor TFIIE, the tumor suppressor p53, the cell cycle protein DP1, the DNA repair factors XPC and UVSSA, and three RNA polymerases, in addition to the dynamics of histone tails in nucleosomes and histone chaperones. In complexes of TFIIH, the PH domain of the p62 subunit binds to an acidic string formed by the IDR in TFIIE, p53, XPC, UVSSA, DP1, and the RPB6 subunit of three RNA polymerases by a common interaction mode, namely extended string-like binding of the IDR on the positively charged surface of the PH domain. In the nucleosome, the dynamic conformations of the N-tails of histones H2A and H2B are correlated, while the dynamic conformations of the N-tails of H3 and H4 form a histone tail network dependent on their modifications and linker DNA. The acidic IDRs of the histone chaperones of FACT and NAP1 play important roles in regulating the accessibility to histone proteins in the nucleosome.

Histone tail network and modulation in a nucleosome

The structural unit of eukaryotic chromatin is a nucleosome, comprising two histone H2A/H2B heterodimers and one histone (H3/H4)₂ tetramer, wrapped around by ∼146-bp core DNA and linker DNA. Flexible histone tails sticking out from the core undergo posttranslational modifications that are responsible for various epigenetic functions. Recently, the functional dynamics of histone tails and their modulation within the nucleosome and nucleosomal complexes have been investigated by integrating NMR, molecular dynamics simulations, and cryo-electron microscopy approaches. In particular, recent NMR studies have revealed correlations in the structures of histone N-terminal tails between H2A and H2B, as well as between H3 and H4 depending on linker DNA, suggesting that histone tail networks exist even within the nucleosome.