Mouse Oocytes and Early Embryos Express Multiple Histone H1 Subtypes

Oocytes and embryos of many species, including mammals, contain a unique linker (H1) histone, termed H1oo in mammals. It is uncertain, however, whether other H1 histones also contribute to the linker histone complement of these cells. Using immunofluorescence and radiolabeling, we have examined whether histone H10, which frequently accumulates in the chromatin of nondividing cells, and the somatic subtypes of H1 are present in mouse oocytes and early embryos. We report that oocytes and embryos contain mRNA encoding H10. A polymerase chain reactionbased test indicated that the poly(A) tail did not lengthen during meiotic maturation, although it did so beginning at the four-cell stage. Antibodies raised against histone H10 stained the nucleus of wild-type prophase-arrested oocytes but not of mice lacking the H10 gene. Following fertilization, H10 was detected in the nuclei of two-cell embryos and less strongly at the four-cell stage. No signal was detected in H10 -/- embryos. Radiolabeling revealed that species comigrating with the somatic H1 subtypes H1a and H1c were synthesized in maturing oocytes and in one- and twocell embryos. Beginning at the four-cell stage in both wild-type and H10 -/- embryos, species comigrating with subtypes H1b, H1d, and H1e were additionally synthesized. These results establish that histone H10 constitutes a portion of the linker histone complement in oocytes and early embryos and that changes in the pattern of somatic H1 synthesis occur during early embryonic development. Taken together with previous results, these findings suggest that multiple H1 subtypes are present on oocyte chromatin and that following fertilization changes in the histone H1 complement accompany the establishment of regulated embryonic gene expression.