Initiation of a chromatin-based transcriptionally repressive state in the preimplantation mouse embryo: Lack of a primary role for expression of somatic histone H1

A chromatin-based transcriptionally repressive state develops during the tw o-cell stage in preimplantation mouse embryos. Correlated with the initial formation of this state is the expression of somatic histone H1, which coul d confer repression by promoting the formation of a transcriptionally repre ssive chromatin structure. To ascertain if the expression of histone H1 cou ld play such a primary role in initiating the formation of this transcripti onally repressive state, the endogenous pool of somatic histone H1 in the t wo-cell embryo was greatly expanded by injection of 25 or 100 pg of histone H1 at the one-cell stage. The expression of the transcription-requiring co mplex, which is an accepted marker for genome activation, was then assessed during the two-cell stage. No significant inhibition was noted following t he injection of 25 pg of histone H1. A transient inhibition was observed fo llowing injection of 100 pg, but this was likely due to a delay in cleavage to the two-cell stage. We conclude that it is unlikely that the expression of somatic histone H1 is a major factor in the initial establishment of th e chromatin-based transcriptionally repressive state that accompanies genom e activation. Mol. Reprod. Dev. 55:241-248, 2000. (C) 2000 Wiley-Liss, Inc.