Background: H2A.Z is a histone variant that replaces H2A at strategic locations within the genome. Results: Expression of non-acetylatable H2A.Z blocks myogenic differentiation. Conclusion: H2A.Z acetylation is important for MyoD expression in myoblasts, which in turn is required for myogenesis.Significance: These results demonstrate that expression of a mutant H2A.Z can lead to dysregulation and inhibition of cellular differentiation programs.

Histones are the protein components of chromatin and are important for its organization and compaction. Although core histones are exclusively expressed during S phase of the cell cycle, there exist variants of canonical histones that are expressed throughout the cell cycle. These histone variants are often deposited at defined regions of the genome and they play important roles in a variety of cellular processes, such as transcription regulation, heterochromatin formation and DNA repair. In this chapter, we will focus on several histone variants that have been linked to transcription regulation, and highlight their physical and functional features that facilitate their activities in this context.

We have previously shown that the Ser15-phosphorylated p53 phosphoform, p53(Ser15), can localize at sites of ionizing radiation-induced DNA damage. In this study, we hypothesized that the non-specific DNA binding domain (NSDBD) of the p53 carboxy-terminus (C-terminus) mediates chromatin anchoring at sites of DNA damage to interact with two key mediators of the DNA damage response (DDR): ATM and 53BP1. Exogenous YFP-p53 fusion constructs expressing C-terminus deletion mutants of p53 were transfected into p53-null H1299 cells and tracked by microscopy and biochemistry to determine relative chromatin-binding pre- and postirradiation. We observed that exogenous YFP-p53(WT) and YFP-p53(Δ367-393) associated with ATM(Ser1981) and 53BP1 in the nuclear, chromatin-bound fractions after DNA damage. Of interest, YFP-p53(Δ1-299) fusion proteins, which lack transcriptional trans-activation and the Ser15-residue, bound to ATM(Ser1981) but not to 53BP1. In support of these data, we used subnuclear UV-microbeam and immunoprecipitation analyses of irradiated normal human fibroblasts (HDFs) that confirmed an interaction between endogenous p53 and ATM or 53BP1. Based on these observations, we propose a model whereby a pre-existing pool of p53 responds immediately to radiation-induced DNA damage using the C-terminus to spatially facilitate protein-protein interactions and the DDR at sites of DNA damage.

The eukaryotic genome is highly complex and compartmentalized into distinct physical and functional domains. Although the majority of genomic DNA is wrapped around histone proteins in the same manner to form repeating units of nucleosomes, the use of distinct histone variants and the myriad of posttranslational modifications (PTMs) on different histones and amino acid residues create great diversity among these nucleosomes. In this review, we summarize some of the most recent findings in the histone variant and PTM fields and update the readers on our current understanding of various histone-related pathways. Furthermore, we discuss how homotypic or heterotypic deposition of histone variants as well as symmetric or asymmetric histone PTMs within the nucleosome context can further expand the diversity and functionality of chromatin. Altogether, this review highlights the complexity of chromatin composition and organization and their regulatory functions within the eukaryotic genome.