The linker histones constitute the major proteins bound to linker DNA, the DNA bridging nucleosome core particles. The composition of the linker histone fraction is tissue- and species-specific, as well as developmentally-regulated. As such, linker histones play a critical role in the higher order packaging of chromatin and thus, inevitably, in an impressive array of regulatory functions. Recently, we uncovered a mammalian oocyte-specific linker histone H1oo homologous to the cs-H1 of sea urchin and to the B4 of the frog in the course of a differential screening project. H1oo protein localized to the intact germinal vesicle (GV) of preovulatory oocytes, to the condensed chromosomes of ovulated oocytes arrested at metaphase (M) II, and to the first polar body. The pronuclei were both positive at the 1-cell stage. Nuclear staining, however, was reduced in 2-cell embryos and was no longer detectable at the 4-cell stage of embryonic development. These findings indicate oocyte-specific linker histones may play some roles in genome-wide reprogramming in vertebrate eggs.
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