TURNOVER OF HISTONE ACETYL GROUPS DURING SEA-URCHIN EARLY DEVELOPMENTIS NOT REQUIRED FOR HISTONE, HEAT-SHOCK AND ACTIN GENE-TRANSCRIPTION

Histone acetylation is an extremely complex, reversible and specific p rocess. In order to evaluate the importance of this modification for g ene expression during sea urchin development, acetyl group turnover of histone lysine residues was blocked by sodium butyrate. The continuou s presence of 15 Mm sodium butyrate in the incubation medium from the onset of development blocked gastrulation and resulted in chromatin co ntaining hyperacetylated histone molecules in amounts usually not foun d in nature. At the mesenchyme blastula stage, the expression of the e arly histone genes was shut off and the expression of the late genes w as switched on both in control and sodium butyrate-treated embryos. In vestigation of the early histone gene chromatin structure in butyrate- treated embryos revealed a random distribution of nucleosomes when the genes were transcriptionally active as compared to regular nucleosoma l packaging when genes were inactive. These changes in chromatin struc ture during development mimicked the chromatin structural transition o f the early histone genes in control embryos. In addition, the ability of heat shock genes to be induced at elevated temperature and repress ed at normal temperature was unaffected in butyrate treatment of embry os. Finally, the developmental profiles of the cytoskeletal CyIIIa act in gene expression in control and butyrate-treated embryos were very s imilar. The data presented suggest that turnover of histone acetyl gro ups and the overall level of histone acetylation are not determining f actors in the up and down regulation of a number of genes during early development of sea urchin.