Maturation of the mammalian zygote involves extensive remodelling of chromatin and epigenetic reprogramming of the new zygotic genome. This reprogramming requires changes in chromatin from a transcriptionally repressive to an active state and is essential for successful embryonic development. One important component of this reprogramming is a marked global increase in the levels of acetylation of histone 3 lysine 9 (H3K9ace) in both the male and female pronuclei. Environmental disturbances, such as IVF or culture in vitro, are known to perturb this reprogramming, and cause a marked increase in the level of H3K9ace in the zygote prior to syngamy.

Indirect immunolocalization of H3K9ace was used to examine the ontogeny and regulation of H3K9ace in the mouse zygote. H3K9ace levels in both parental genomes were very low immediately after fertilisation but these levels increased as the zygotic pronuclei matured and increased in size. When zygotes were cultured in vitro the rate of accumulation of H3K9ace and pronuclei size were greater than in zygotes matured within the reproductive tract. The mechanisms of acetylation were analysed by inhibition of histone acetylases (HATs) and histone deacetylases (HDACs). Inhibition of HATs by anacardic acid (100μM) (targets CBP/p300, PCAF/GCN5 and Tip60), but not KG501 (targets CBP/p300), caused a significant reduction in the accumulation of H3K9ace and also pronuclear size. Inhibition of HDACs with Trichostatin A (≥ 3nM) caused an increase in the level of H3K9ace in zygotes.

The results indicate both histone acetylases and deacetylases are active during zygotic maturation and that the level of acetylation results from an equilibrium between their activities. The increased acetylation caused by culture of zygotes in vitro indicates that culture causes a change in the equilibrium between HAT and HDAC activity.