Temporal Regulation of Chromatin Organization During C. Elegans Embryogenesis

Abstract

In all eukaryotic cells, the genetic material is organized into a complex structure composed of DNA and protein: chromatin. Organization of chromatin structure in the nucleus influences all functions of the genome including gene expression and is dynamically regulated based on the needs of the cell. Chromatin re-organization is critical for avoiding the development and progression of pathological situations. In most animals, chromatin undergoes major restructuring as embryos develop. Accessible chromatin is a feature of naïve embryonic cells and is lost as cells become more specialized. As cells mature, the nucleus shrinks, chromatin compacts and silent heterochromatin domains are generated. Heterochromatin is typically associated with repressive post-translational modifications such as histone H3 lysine 9 (H3K9) methylation. While the machinery that organizes chromatin is known, the molecular cues that initiate chromatin re-organization in the early embryo are elusive. Using C. elegans as a model, I found that a conserved H3K9 methyltransferase is critical for the onset of heterochromatin formation. H3K9 methylation by MET-2/SETDB1 is temporally regulated by two conserved factors identified in this study. Nuclear accumulation of MET-2 and its binding partners are rate-limiting for heterochromatin formation. Polymerase II transcription, RNA interference pathways that are known to assemble heterochromatin, or mechanisms that rely on cell counting in the embryo do not dictate the timing of this transition. Instead, slowing the early embryonic cell cycles leads to precocious heterochromatin, suggesting that absolute time after fertilization is a key determinant. My studies delineate the temporal regulation of MET-2 and its binding partners, and their role in chromatin re-organization during embryogenesis. Orthologs of these worm proteins likely have similar roles in re-organizing chromatin in other systems and are implicated in human disease.