Levels of histone H4 diacetylation decrease dramatically during sea urchin embryonic development and correlate with cell doubling rate.

Basic proteins in nuclei and nucleosomes at different stages of development in Arbacia punctulata sea urchins were analyzed directly by in situ protamine release of chromosomal proteins into Triton/acid/urea-polyacrylamide gels. The predominant protein band in the H4 region of 2-cell through 64-cell stage embryos migrates with the mobility expected for diacetylated histone H4 (i.e. H4aa), whereas after blastulation (approximately 300 cells) the predominant H4 species is the unmodified form, H4O. In early embryos this H4aa band is highly labeled in vivo with [3H]acetic acid. The ratio of H4aa:H4O is more than 20-fold greater at the rapidly dividing 2-cell stage than at pluteus stage. This is true for both newly synthesized H4 labeled with [3H]lysine and total H4 (stained). Enhanced acetylation is also found in nucleosomes. The relative amount of this acetylated H4 species correlates roughly with the rate of cell doubling during early embryogenesis, and decreases as the average nucleosomal repeat increases. The results are indicative of a dynamically changing chromatin structure through development, as well as an intimate role of diacetylated histone H4 in the maturation of newly replicated chromatin.