A Developmental Requirement for HIRA-Dependent H3.3 Deposition Revealed at Gastrulation in Xenopus

Discovering how histone variants that mark distinct chromatin regions affect a developmental program is a major challenge in the epigenetics field. To assess the importance of the H3.3 histone variant and its dedicated histone chaperone HIRA, we used an established developmental model, Xenopus laevis. After the early rapid divisions exploiting a large maternal pool of both replicative H3.2 and replacement H3.3, H3.3 transcripts show a distinct peak of expression at gastrulation. Depletion of both H3.2 and H3.3 leads to an early gastrulation arrest. However, with only H3.3 depletion, defects occur at late gastrulation, impairing further development. Providing exogenous H3.3 mRNAs, but not replicative H3.2 mRNAs, rescues these defects. Notably, downregulation of the H3.3 histone chaperone HIRA similarly impairs late gastrulation, and we find a global defect in H3.3 incorporation into chromatin comparable to H3.3 depletion. We discuss how specific HIRA-dependent H3.3 deposition is required for chromatin dynamics during gastrulation. [Display omitted] ► H3.3 transcription peaks at gastrulation ► Lack of H3.3 expression impairs Xenopus development at late gastrulation ► The H3.3 chaperone HIRA is also required for late gastrulation ► HIRA-dependent H3.3 deposition is required during development Discovering how histone variants affect development is a major challenge in the epigenetics field. Szenker, Lacoste, and Almouzni show a unique requirement of the H3.3 histone variant at late gastrulation during Xenopus development. Depletion of either H3.3 or its chaperone HIRA resulted in similar phenotypes, with comparable losses of H3.3 incorporation into chromatin. These data support the view that a DNA synthesis-independent H3.3 deposition requiring HIRA is critical during early development.