Rapid H1 linker histone transitions following fertilization or somatic cell nuclear transfer: evidence for a uniform developmental program in mice.

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.