Histone H2B is one of the 5 main histone proteins involved in the structure of chromatin in eukaryotic cells. Featuring a main globular domain and long N-terminal and C-terminal tails, H2B is involved with the structure of the nucleosomes. Structure Histone H2B is a lightweight structural protein made of 126 amino acids. Many of these amino acids have a positive charge at cellular pH, which allows them to interact with the negatively charged phosphate groups in DNA. Along with a central globular domain, histone H2B has two flexible histone tails that extend outwards – one at the N-terminal end and one at C-terminal end. These are highly involved in condensing chromatin from the beads-on-a-string conformation to a 30-nm fiber. Similar to other histone proteins, histone H2B has a distinct histone fold that is optimized for histone-histone as well as histone-DNA interactions. Two copies of histone H2B come together with two copies each of histone H2A, histone H3, and histone H4 to form the octamer core of the nucleosome to give structure to DNA. To facilitate this formation, histone H2B first binds to histone H2A to form a heterodimer. Two of these heterodimers then bind together with a heterotetramer made of histone H3 and histone H4, giving the nucleosome its characteristic disk shape. DNA is then wrapped around the entire nucleosome in groups of approximately 160 base pairs of DNA. The wrapping continues until all chromatin has been packaged with the nucleosomes. Function Histone H2B is a structural protein that helps organize eukaryotic DNA. It plays an important role in the biology of the nucleus where it is involved in the packaging and maintaining of chromosomes, regulation of transcription, and replication and repair of DNA. Histone H2B helps regulate chromatin structure and function through post-translational modifications and specialized histone variants. Acetylation and ubiquitination are examples of two post-translational modifications that affect the function of histone H2B in particular ways. Hyperacetylation of histone tails helps DNA-binding proteins access chromatin by weakening histone-DNA and nucleosome-nucleosome interactions. Furthermore, acetylation of a specific lysine residue binds to bromine-containing domains of certain transcription and chromatin regulatory proteins. This docking facilitates the recruitment of these proteins to the correct region of the chromosome. Ubiquitinated histone H2B is often found in regions of active transcription. Through the facilitation of chromatin remodeling, it stimulates transcriptional elongation and sets the stage for further modifications that regulate multiple elements of transcription.