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). Highly expressed in growing cells, but only at low levels in starved cells. Phosphorylated to form H3S10ph. H3S10ph promotes subsequent H3K14ac formation by GCN5. H3S10ph is only found in the mitotically dividing MIC, but not in the amitotically dividing MAC. H3S10ph is correlated with chromosome condensation during mitotic or meiotic micronuclear divisions (By similarity). Acetylation of histone H3 leads to transcriptional activation. H3K14ac formation by GCN5 is promoted by H3S10ph. H3K9acK14ac is the preferred acetylated form of newly synthesized H3. Acetylation occurs almost exclusively in the MAC (By similarity). Methylated to form H3K4me. H3K4me is only found in the transcriptionally active MAC. Methylated to form H3K9me in developing MACs during conjugation, when genome-wide DNA elimination occurs. At this stage, H3K9me specifically occurs on DNA sequences being eliminated (IES), probably targeted by small scan RNAs (scnRNAs) bound to IES, and is required for efficient IES elimination. H3K9me is required for the interaction with the chromodomains of PDD1 and PDD3 (By similarity). The full-length protein H3S (slow migrating) is converted to H3F (fast migrating) by proteolytic removal of the first 6 residues. H3F is unique to MIC, and processing seems to occur regularly each generation at a specific point in the cell cycle (By similarity). Belongs to the histone H3 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: H3K4ac = acetylated Lys-5; H3K4me1/2/3 = mono-, di- and trimethylated Lys-5; H3K9ac = acetylated Lys-10; H3K9me3 = trimethylated Lys-10; H3S10ph = phosphorylated Ser-11; H3K14ac = acetylated Lys-15; H3K18ac = acetylated Lys-19; H3K23ac = acetylated Lys-24; H3K27ac = acetylated Lys-28; H3K27me1/2/3 = mono-, di- and trimethylated Lys-28; H3K36ac = acetylated Lys-37; H3K36me1/3/3 = mono-, di- and trimethylated Lys-37; H3K56ac = acetylated Lys-57; H3K56me1 = monomethylated Lys-57; H3K79me1 = monomethylated Lys-80.