Chromatin dynamics during sea urchin embryogenesis: effects on the neural alpha tubulin PlTa2 gene expression

Expression of PlTa2 gene during sea urchin P. lividus development, is spatially confined to the neural territory and temporally activated from the blastula stage. To evaluate a possible involvement of chromatin modifications in regulation of PlTa2 gene expression we first searched for DNaseI hypersentive sites. We found four sites localized in the introns of the gene, when we used chromatin extracted from embryos at gastrula stage but not from morula stage. This result suggests a possible functional role of the introns in the activation of the expression of PlTa2 gene. Moreover, we used specific antibodies for RNA polymerase II and for different modified form of lysine 9, lysine 27 and lysine 4 of the H3 histone in quantitative Chromatin Immuno Precipitation experiments to emphasize the different state of chromatin during embryo development. Our analysis shows high H3K9 acetylation and H3K4 trimethylation degree in nucleosomes located at a2 promoter region in P. lividus embryos at gastrula stage, when the PlTa2 expression level is high. This observation agrees with conventional positive role assumed by these post-translational modifications in chromatin remodeling leading to increase promoter accessibility. Furthermore ChIP analysis shows also high H3K27 dimethylation degree during all development stage but ReChIP analysis shows no co-occupancy of this modification with RNA polymerase II in promoter region in embryos at gastrula stage. This observation is consistent with the hypothesis of a general repressive role of this modification in the non-neural territory of the embryo.