Characterization of linker histone H1FOO during bovine in vitro embryo development.

Linker histones H1 are involved in various mechanisms, such as chromatin organization and gene transcription. In different organisms, a unique subtype can be found in the oocyte, however its function remains unclear. To assess the potential involvement of this oocyte linker histone (H1FOO) in chromatin modulation, we have cloned and sequenced the bovine H1FOO cDNA and followed its mRNA profile by quantitative RT-PCR in the oocyte and throughout bovine early embryo development. The highest level of mRNA was found in the germinal vesicle (GV) oocyte and diminished constantly throughout embryo development. In the 16-cell embryo and blastocyst, respectively, the mRNA levels were 200 and 2,000 times lower than in the GV oocyte. A specific antibody raised against bovine H1FOO was used to establish protein distribution in the oocyte and preimplantation embryo by immunocytochemistry. In the GV and metaphase II (MII) oocyte, as well as in the 1-, 2- and 4-cell embryo, H1FOO was localized in the cytoplasm and nucleus. The protein was uniformly spread within the cytoplasm, while it was concentrated onto the chromatin in the nucleus. In the 8- to 16-cell embryo, H1FOO's presence diminished in the cytoplasm, although it was still strongly expressed in nucleus. In the morula and blastocyst stages, the protein was totally lacking. By its position on chromatin, H1FOO could not only be involved in chromatin conformation but could also participate in activation or repression of genes during oogenesis and embryo development before embryonic genome activation.