ASSEMBLY OF SOMATIC HISTONE H1 ONTO CHROMATIN DURING BOVINE EARLY EMBRYOGENESIS

We have examined the distribution of somatic histone H1 in bovine oocy tes and preimplantation embryos, using an antibody that recognizes his tone H1 subtypes present in somatic cells. Immunoreactive H1 was not d etectable on the chromosomes of metaphase II of meiosis nor in the nuc lei of early cleavage-stage embryos. In most embryos, immunoreactive H 1 was assembled onto embryonic chromatin during the fourth to sixth ce ll cycle after fertilization. No immunoreactive somatic histone H1 was detected, however, when embryos were incubated in the presence of a-a manitin beginning early during the fourth cell cycle. These results in dicate that somatic subtypes of histone H1 are assembled onto embryoni c chromatin in a developmentally regulated manner that requires embryo nic transcription. Aphidicolin, an inhibitor of DNA replication, also inhibited the assembly of somatic histone H1 onto chromatin when prese nt at early stages of the 4th cell cycle. It is suggested that, becaus e the bulk of histone gene expression in proliferating cells occurs du ring DNA replication, expression of genes encoding immunoreactive H1 i s inhibited in embryos blocked before or soon after entering the S-pha se. These findings on the control of somatic histone H1 assembly onto chromatin in cattle show a remarkable similarity to those found in the mouse. Such evolutionary conservation suggests that the somatic histo ne H1 complement of chromatin may regulate critical aspects of chromat in activity during mammalian oogenesis or early embryogenesis. (C) 199 5 Wiley-Liss, Inc.