{"type": "root", "children": [{"type": "p", "children": [{"type": "t", "text": "\n A body of work has defined diverse roles for linker histone H1 variants and H1‐like proteins in chromatin organization, gene regulation, and cell differentiation. In the male germline, for example, the testis‐specific histone H1t is produced in mid‐pachytene spermatocytes and early spermatids, where it participates in chromatin remodeling events essential for proper meiotic progression and spermatogenesis. Its misregulation—as seen when factors such as BRDT fail to engage its promoter—leads to abnormal chromatin structure and consequent infertility."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "1"}]}, {"type": "t", "text": " In addition, a spermatid‐specific H1‐like protein, HILS1, is expressed in elongating and elongated spermatids where it appears to mediate nuclear condensation via mechanisms distinct from those of conventional linker histones."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "2"}]}, {"type": "t", "text": ""}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Structural analyses of the carboxyl‐terminal domains of somatic H1 variants such as H1(0) and H1t have revealed that these intrinsically disordered regions become highly structured upon binding DNA, thereby facilitating higher‐order chromatin folding."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "3"}]}, {"type": "t", "text": " In embryonic stem cells and in somatic cells, other H1 variants (including H1d, H1c, H1.2, and H1.3) are unequally distributed across the genome; for instance, these proteins are typically depleted from GC‐rich, gene‐dense regions and active promoters yet enriched at heterochromatic domains such as major satellites and lamina‐associated regions. Notably, H1.2 displays a particularly repressive pattern correlating with low gene expression."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "4"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Moreover, post‐translational modifications such as phosphorylation further modulate H1 variant functions. Detailed mass spectrometry studies have identified multiple phosphorylation states of H1t in rodent testes, suggesting that the extent of phosphorylation—not necessarily a strict sequential order—is critical for regulated chromatin decondensation during histone replacement by transition proteins and protamines."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "6"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n Beyond canonical H1 proteins, H1‐like factors such as HP1BP3 expand the functional repertoire of this protein family. HP1BP3, which shares structural features with H1 histones, has been implicated in the organization of nucleolar protein networks and in coupling chromatin to RNA processing events. Specifically, HP1BP3 not only contributes to global gene expression regulation and proper chromatin dynamics during development but also promotes co‐transcriptional processing of primary microRNAs through stabilization of Drosha on active miRNA genes."}, {"type": "fg", "children": [{"type": "fg_f", "ref": "7"}]}, {"type": "t", "text": "\n "}]}, {"type": "t", "text": "\n \n "}, {"type": "p", "children": [{"type": "t", "text": "\n In summary, the H1 family—from classical linker histones such as H1(0), H1c, H1d, H1.2, H1.3, and the testis‐specific H1t to H1‐like proteins like HILS1 and HP1BP3—plays multifaceted roles in mediating chromatin compaction, regulating transcription, and driving the dynamic chromatin rearrangements required for proper meiotic progression, stem cell function, and overall cellular homeostasis.\n "}]}, {"type": "rg", "children": [{"type": "r", "ref": 1, "children": [{"type": "t", "text": "Enyuan Shang, Helen D Nickerson, Duancheng Wen, et al. "}, {"type": "b", "children": [{"type": "t", "text": "The first bromodomain of Brdt, a testis-specific member of the BET sub-family of double-bromodomain-containing proteins, is essential for male germ cell differentiation."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Development (2007)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1242/dev.004481"}], "href": "https://doi.org/10.1242/dev.004481"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "17728347"}], "href": "https://pubmed.ncbi.nlm.nih.gov/17728347"}]}, {"type": "r", "ref": 2, "children": [{"type": "t", "text": "Wei Yan, Lang Ma, Kathleen H Burns, et al. "}, {"type": "b", "children": [{"type": "t", "text": "HILS1 is a spermatid-specific linker histone H1-like protein implicated in chromatin remodeling during mammalian spermiogenesis."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Proc Natl Acad Sci U S A (2003)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1073/pnas.1837812100"}], "href": "https://doi.org/10.1073/pnas.1837812100"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "12920187"}], "href": "https://pubmed.ncbi.nlm.nih.gov/12920187"}]}, {"type": "r", "ref": 3, "children": [{"type": "t", "text": "Alicia Roque, Ibon Iloro, Imma Ponte, et al. "}, {"type": "b", "children": [{"type": "t", "text": "DNA-induced secondary structure of the carboxyl-terminal domain of histone H1."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2005)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M505636200"}], "href": "https://doi.org/10.1074/jbc.M505636200"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "16006555"}], "href": "https://pubmed.ncbi.nlm.nih.gov/16006555"}]}, {"type": "r", "ref": 4, "children": [{"type": "t", "text": "Kaixiang Cao, Nathalie Lailler, Yunzhe Zhang, et al. "}, {"type": "b", "children": [{"type": "t", "text": "High-resolution mapping of h1 linker histone variants in embryonic stem cells."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "PLoS Genet (2013)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1371/journal.pgen.1003417"}], "href": "https://doi.org/10.1371/journal.pgen.1003417"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "23633960"}], "href": "https://pubmed.ncbi.nlm.nih.gov/23633960"}]}, {"type": "r", "ref": 5, "children": [{"type": "t", "text": "Lluís Millán-Ariño, Abul B M M K Islam, Andrea Izquierdo-Bouldstridge, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Mapping of six somatic linker histone H1 variants in human breast cancer cells uncovers specific features of H1.2."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nucleic Acids Res (2014)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1093/nar/gku079"}], "href": "https://doi.org/10.1093/nar/gku079"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "24476918"}], "href": "https://pubmed.ncbi.nlm.nih.gov/24476918"}]}, {"type": "r", "ref": 6, "children": [{"type": "t", "text": "Bettina Sarg, Sabine Chwatal, Heribert Talasz, et al. "}, {"type": "b", "children": [{"type": "t", "text": "Testis-specific linker histone H1t is multiply phosphorylated during spermatogenesis. Identification of phosphorylation sites."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "J Biol Chem (2009)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1074/jbc.M805925200"}], "href": "https://doi.org/10.1074/jbc.M805925200"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "19043117"}], "href": "https://pubmed.ncbi.nlm.nih.gov/19043117"}]}, {"type": "r", "ref": 7, "children": [{"type": "t", "text": "Haoming Liu, Chunyang Liang, Rahul K Kollipara, et al. "}, {"type": "b", "children": [{"type": "t", "text": "HP1BP3, a Chromatin Retention Factor for Co-transcriptional MicroRNA Processing."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Mol Cell (2016)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1016/j.molcel.2016.06.014"}], "href": "https://doi.org/10.1016/j.molcel.2016.06.014"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "27425409"}], "href": "https://pubmed.ncbi.nlm.nih.gov/27425409"}]}, {"type": "r", "ref": 8, "children": [{"type": "t", "text": "Benjamin P Garfinkel, Naomi Melamed-Book, Eli Anuka, et al. "}, {"type": "b", "children": [{"type": "t", "text": "HP1BP3 is a novel histone H1 related protein with essential roles in viability and growth."}]}, {"type": "t", "text": " "}, {"type": "i", "children": [{"type": "t", "text": "Nucleic Acids Res (2015)"}]}, {"type": "t", "text": " DOI: "}, {"type": "a", "children": [{"type": "t", "text": "10.1093/nar/gkv089"}], "href": "https://doi.org/10.1093/nar/gkv089"}, {"type": "t", "text": " PMID: "}, {"type": "a", "children": [{"type": "t", "text": "25662603"}], "href": "https://pubmed.ncbi.nlm.nih.gov/25662603"}]}]}]}