H1 linker histones (H1s) are key regulators of chromatin structure and function. The functions of different H1s during early embryogenesis, and mechanisms regulating their associations with chromatin are largely unknown. The developmental transitions of H1s during oocyte growth and maturation, fertilization and early embryogenesis, and in cloned embryos were examined. Oocyte-specific H1FOO, but not somatic H1s, associated with chromatin in oocytes (growing, GV-stage, and MII-arrested), pronuclei, and polar bodies. H1FOO associated with sperm or somatic cell chromatin within 5 min of intracytoplasmic sperm injection (ICSI) or somatic cell nuclear transfer (SCNT), and completely replaced somatic H1s by 60 min. The switching from somatic H1s to H1FOO following SCNT was developmentally regulated. H1FOO was replaced by somatic H1s during the late two- and four-cell stages. H1FOO association with chromatin can occur in the presence of a nuclear envelope and independently of pronucleus formation, is regulated by factors associated with the spindle, and is likely an active process. All SCNT constructs recapitulated the normal sequence of H1 transitions, indicating that this alone does not signify a high developmental potential. A paucity of all known H1s in two-cell embryos may contribute to precocious gene transcription in fertilized embryos, and the elaboration of somatic cell characteristics in cloned embryos.

Linker histones bind to the nucleosomes and linker DNA of chromatin fibers, causing changes in linker DNA structure and stabilization of higher order folded and oligomeric chromatin structures. Linker histones affect chromatin structure acting primarily through their ~100 residue C-terminal domain (CTD). We have previously shown that the ability of the linker histone H1° to alter chromatin structure was localized to two discontinuous 24-/25-residue CTD regions (Lu, X., and Hansen, J. C. (2004) J Biol Chem 279, 8701-8707). To determine the biochemical basis for these results, we have characterized chromatin model systems assembled with endogenous mouse somatic H1 isoforms, or recombinant H1° CTD mutants in which the primary sequence has been scrambled, the amino acid composition mutated, or the location of various CTD regions swapped. Our results indicate that specific amino acid composition plays a fundamental role in molecular recognition and function by the H1 CTD. Additionally, these experiments support a new molecular model for CTD function, and provide a biochemical basis for the redundancy observed in H1 isoform knockout experiments in vivo.Linker histones (e.g., H1, H5) are chromatin architectural proteins found in all eukaryotes (1, 2). They are abundant, with a stoichiometry of ~0.8 total linker histones per nucleosome in most tissues (3 and references therein). Linker histones are modularly structured proteins that have an ~35 residue unstructured N-terminal domain (NTD) 1 , a central globular winged helix domain, and an ~100 residue unstructured C-terminal domain (CTD) (4). Linker histones bind to chromatin fibers through interaction of the globular domain with nucleosomal sites(s) (1,2,5), and the CTD with linker DNA (6,7). Higher eukaryotes have at least six somatic linker histone isoforms, which differ primarily in their CTD primary sequences (1,8). The H1 isoform CTDs do, however, share a very similar and characteristic amino acid composition (9). At the molecular level, little is known about the actions of the isoforms. Linker histones are multifunctional, with roles in chromatin condensation (1,2,10,11), nucleosome spacing (12, 13), specific gene expression (13) and references therein), DNA methylation (13) and other nuclear processes. In addition to chromatin, linker histones bind to many nuclear proteins, e.g., † This work was supported by NIH grant GM45916 to JCH. *To whom correspondence should be addressed: Department of Biochemistry and Molecular Biology, Campus Delivery 1870, Colorado State University, Fort Collins, CO, 80523-1870. Tel.: 970-491-5440; Fax: 970-491-0494; E-mail: jeffrey.c The relationships between linker histones, nucleosomal arrays, and chromatin fiber structure are well documented (1,2,10). In vitro, nucleosomal arrays are in salt-dependent equilibrium between unfolded, folded, and oligomeric conformational states (10,11). Binding of linker histones to nucleosomal arrays affects chromatin structure in at least three distinct ways: the linker DNA between nucleosome...

The last 35 years has seen a substantial amount of information collected about the somatic H1 subtypes, yet much of this work has been overshadowed by research into highly divergent isoforms of H1, such as H5. Reports from several laboratories in the past few years have begun to call into question some of the traditional views regarding the general function of linker histones and their heterogeneity. Hence, the impression in some circles is that less is known about these ubiquitous nuclear proteins as compared with the core histones. The goal of the following review is to acquaint the reader with the ubiquitous somatic Hls by categorizing them and their characteristics into several classes. The reasons for our current state of misunderstanding is put into a historical context along with recent controversies centering on the role of H1 in the nucleus. Finally, we propose a model that may explain the functional role of H1 heterogeneity in chromatin compaction.