Fig. 2.

Epigenetic pre-patterning of developmental gene expression by histone modifications in zebrafish embryos. a Prior to ZGA, developmentally important genes are marked by trimethylated H3K4, H3K9, and/or H3K27 in the promoter region. These genes are transcriptionally inactive during this developmental period. b At the time of ZGA, activated genes (right) lose any repressive mark they might have acquired prior to ZGA and become enriched in H3K36me3 and RNAPII downstream of the TSS, while H3K4me3 tends to extend towards the coding region. A number of genes marked by H3K4me3 (with or without any other modification) pre-ZGA also remain transcriptionally inactive at the time of ZGA, and are only activated later in development (left). These genes retain or acquire various combinations of repressive modifications, such as H3K27me3 alone (i), H3K27me3 and H3K9me3 (ii), H3K9me3 alone (iii), or lose all marks altogether, including H3K4me3 (iv)