H3.3/H2A.Z hybrid nucleosomes localized to the TSS of active genes, at sites that have previously been characterized as nucleosome depleted regions (NDRs). Upon modulating the salt concentration used in the nucleosome isolation, it was discovered that H3.3/H2A.Z nucleosomes are unstable PD0325901 mouse in vivo, causing them to dissociate from the DNA during extraction, leaving behind a NDR. Although a crystal structure is not available for this double hybrid, in vitro characterization of the H3.3/H2A.Z

nucleosome’s stability by salt induced dissociation revealed only very small differences compared to the stability of the canonical nucleosome, resulting in a puzzling discrepancy between in vivo and in vitro results [ 20]. However, a recent investigation into a post-translational modification (PTM) found not on the histone tail, but at H3K122, in the center of the nucleosome core, suggests a plausible explanation that could neatly resolve this discrepancy [ 21••]. Acetylation at H3K122 disrupts the interaction between the histone core and DNA, destabilizing the nucleosome [ 22••]. Furthermore, it co-localizes with H3.3 and H2A.Z in vivo, leading to the compelling hypothesis that K122 acetylation on H3.3, which is absent RGFP966 in the in vitro studies, may be responsible

for the destabilized H3.3/H2A.Z nucleosome in vivo [ 21••]. An alternative attractive explanation for the instability of the H2A.Z/H3.3 hybrid nucleosome

may lie with a newly characterized H2A.Z splice variant, H2A.Z.2.2 [ 23]. Due to its unique docking domain, this particular histone physically destabilizes the octameric core of the nucleosome. While it is unknown whether H2A.Z.2.2 co-localizes with H3.3 in the cell, the decreased stability observed in H2A.Z/H3.3 hybrid nucleosomes could be attributed to the splice variants. An additional key example of nucleosome conformation variability has also been documented for native CENP-A nucleosomes in vivo, which exhibit a surprising bi-stability across the human cell cycle, concurrent with cell-cycle regulated acetylation on K124, in the center of the CENP-A octameric core [ 24 and 25]. Thus, it Guanylate cyclase 2C is feasible that other histone variants display modification-dependent conformational oscillations that impact their inheritance and function in vivo. While nucleosomes have been shown to associate with specific locations within the genome, such as the localization of H3.3 and H2A.Z to TSS, the mechanisms underlying nucleosome positioning in the cell are still being debated. Both experimental and theoretical research have uncovered subtle structural motifs embedded within the primary sequence of DNA as a key component driving preferential nucleosome formation, albeit at subsaturating levels of histones [26 and 27].