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

Abstract

A chromatin-based transcriptionally repressive state develops during the two-cell stage in preimplantation mouse embryos. Correlated with the initial formation of this state is the expression of somatic histone H1, which could confer repression by promoting the formation of a transcriptionally repressive chromatin structure. To ascertain if the expression of histone H1 could play such a primary role in initiating the formation of this transcriptionally repressive state, the endogenous pool of somatic histone H1 in the two-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 complex, which is an accepted marker for genome activation, was then assessed during the two-cell stage. No significant inhibition was noted following the injection of 25 pg of histone H1. A transient inhibition was observed following 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 the chromatin-based transcriptionally repressive state that accompanies genome activation.