bioRxiv2021
In anamniote embryos the major wave of zygotic genome activation (ZGA) starts during the mid-blastula transition. This major wave of ZGA is facilitated by several mechanisms, including dilution of repressive maternal factors and accumulation of activating transcription factors during the fast cell division cycles preceding the mid-blastula transition. However, a set of genes escape global genome repression and are activated substantially earlier, during what is called, the minor wave of genome activation. While the mechanisms underlying the major wave of genome activation have been studied extensively, the minor wave of genome activation is little understood. In zebrafish the earliest expressed RNA polymerase II (Pol II) transcribed genes are activated in a pair of large transcription bodies depleted of chromatin, abundant in elongating Pol II and nascent RNAs (Hadzhiev et al., 2019; Hilbert et al., 2021). This transcription body includes the miR-430 gene cluster required for maternal mRNA clearance. Here we explored the genomic, chromatin organisation and cis-regulatory mechanisms of the minor wave of genome activation occurring in the transcription body. By long read genome sequencing we identified a remarkable cluster of miR-430 genes with over 300 promoters and spanning 0.6 Mb, which represent the highest promoter density of the genome. We demonstrate that the miR-430 gene cluster is required for the formation of the transcription body and acts as a transcription organiser for minor wave activation of a set of zinc finger genes scattered on the same chromosome arm, which share promoter features with the miR-430 cluster. These promoter features are shared among minor wave genes overall and include the TATA-box and sharp transcription start site profile. Single copy miR-430 promoter transgene reporter experiments indicate the importance of promoter-autonomous mechanisms regulating escape from global repression of the early embryo. These results together suggest that formation of the transcription body in the early embryo is the result of high promoter density coupled to a minor wave-specific core promoter code for transcribing key minor wave ZGA genes, which are required for the overhaul of the transcriptome during early embryonic development.