Chromatin dynamics during sea urchinembryogenesis: effects on the neural alphatubulin PlTa2 gene expression

Expression of PlTa2 gene during sea urchin P.lividus development, is spatially confined to theneural territory and temporally activated from theblastula stage. To evaluate a possible involvementof chromatin modifications in regulation of PlTa2gene expression we first searched for DNaseIhypersentive sites. We found four sites localized inthe introns of the gene, when we used chromatinextracted from embryos at gastrula stage but notfrom morula stage. This result suggests a possiblefunctional role of the introns in the activation of theexpression of PlTa2 gene. Moreover, we usedspecific antibodies for RNA polymerase II and fordifferent modified form of lysine 9, lysine 27 andlysine 4 of the H3 histone in quantitativeChromatin Immuno Precipitation experiments toemphasize the different state of chromatin duringembryo development. Our analysis shows highH3K9 acetylation and H3K4 trimethylation degreein nucleosomes located at a2 promoter region in P.lividus embryos at gastrula stage, when the PlTa2expression level is high. This observation agreeswith conventional positive role assumed by thesepost-translational modifications in chromatinremodeling leading to increase promoteraccessibility. Furthermore ChIP analysis showsalso high H3K27 dimethylation degree during alldevelopment stage but ReChIP analysis shows noco-occupancy of this modification with RNApolymerase II in promoter region in embryos atgastrula stage. This observation is consistent withthe hypothesis of a general repressive role of thismodification in the non-neural territory of theembryo.