Core component of nucleosome. Nucleosomes wrap and compact DNA into chromatin, limiting DNA accessibility to the cellular machineries which require DNA as a template. Histones thereby play a central role in transcription regulation, DNA repair, DNA replication and chromosomal stability. DNA accessibility is regulated via a complex set of post-translational modifications of histones, also called histone code, and nucleosome remodeling. The nucleosome is a histone octamer containing two molecules each of H2A, H2B, H3 and H4 assembled in one H3-H4 heterotetramer and two H2A-H2B heterodimers. The octamer wraps approximately 147 bp of DNA. Localizes to both the large, transcriptionally active, somatic macronucleus (MAC) and the small, transcriptionally inert, germ line micronucleus (MIC). Monoubiquitination of Lys-124 gives a specific tag for epigenetic transcriptional repression. Acetylation occurs almost exclusively in the MAC. Belongs to the histone H2A family. To ensure consistency between histone entries, we follow the 'Brno' nomenclature for histone modifications, with positions referring to those used in the literature for the 'closest' model organism. Due to slight variations in histone sequences between organisms and to the presence of initiator methionine in UniProtKB/Swiss-Prot sequences, the actual positions of modified amino acids in the sequence generally differ. In this entry the following conventions are used: H2AK5ac = acetylated Lys-6; H2AK8ac = acetylated Lys-9; H2AK10ac = acetylated Lys-11; H2AK12ac = acetylated Lys-13; H2AK17ac = acetylated Lys-18; H2AS122ph = phosphorylated Ser-123; H2AK123ub1 = monoubiquitinated Lys-124; H2AS127ph = phosphorylated Ser-128.