Chromatin Immunoprecipitation in the Cnidarian Model System Exaiptasia diaphana

Histone post-translational modifications (PTMs) and other epigenetic modifications regulate the chromatin accessibility of genes to the transcriptional machinery, thus affecting an organism's capacity to respond to environmental stimuli. Chromatin immunoprecipitation coupled with high-throughput sequencing (ChIP-seq) has been widely utilized to identify and map protein-DNA interactions in the fields of epigenetics and gene regulation. However, the field of cnidarian epigenetics is hampered by a lack of applicable protocols, partly due to the unique features of model organisms such as the symbiotic sea anemone Exaiptasia diaphana, whose high water content and mucus amounts obstruct molecular methods. Here, a specialized ChIP procedure is presented, which facilitates the investigation of protein-DNA interactions in E. diaphana gene regulation. The cross-linking and chromatin extraction steps were optimized for efficient immunoprecipitation and then validated by performing ChIP using an antibody against the histone mark H3K4me3. Subsequently, the specificity and effectiveness of the ChIP assay were confirmed by measuring the relative occupancy of H3K4me3 around several constitutively activated gene loci using quantitative PCR and by next-generation sequencing for genome-wide scale analysis. This optimized ChIP protocol for the symbiotic sea anemone E. diaphana facilitates the investigation of the protein-DNA interactions involved in organismal responses to environmental changes that affect symbiotic cnidarians, such as corals.