Proper packaging of eukaryotic genomic DNA into chromatin is important for proper DNA compaction and gene expression regulation. The nucleosome core particle comprises a histone octamer wrapped DNA, and can be modulated by the incoporation of distinct histone variants. Concerning H3, the replicative histone variants H3.1 and H3.2 contribute to the histone supply needed during DNA replication, while the histone variant H3.3 is incorporated throughout the cell cycle. Interestingly, the literature highlights a link between H3.3 and transcription. Moreover, the incorporation of H3.3 depends on a specific assembly pathway that involves the histone chaperone HIRA. The aim of my research project was to determine whether H3.3 and its incorporation via HIRA had specific roles. The context of embryonic development was an ideal situation giving that it requires the fine control of genes expression. The use of the vertebrate Xenopus laevis, which unlike mammals has only one replicative H3 variant: H3.2, allowed me to assess the specific function of H3.2 and H3.3 during development. I showed that despite their similarity these two histone variants are not interchangeable. Remarkably, downregulation of H3.3 expression in embryos or interference with its assembly pathway via HIRA leads to major developmental defects at gastrulation. This phenotype is accompanied by expression defects of mesodermal genes, including the marker Xbra. Moreover, these embryos show overall defects in their chromatin organization. Taken together, these data highlight the importance of H3.3 incorporation into chromatin during a key developmental transition, gastrulation
