Identification of zygotically transcribed genes in zebrafish by 4-thio-UTP metabolic labeling
The first embryonic cell divisions rely on maternally stored mRNA and proteins. The zygotic genome is initially transcriptionally silenced and activated later in a process called zygotic genome activation (ZGA). ZGA in any species is still a poorly understood process; the timing of transcription onset is controversial and the identity of the first transcribed genes unclear. Zebrafish, Danio rerio, is a rapidly developing vertebrate model, which is accessible to experimentation and global studies before, during and after ZGA. Overall design: To accurately determine the onset of ZGA and to identify the first transcripts in zebrafish, we developed a metabolic labeling method, utilizing the ribonucleotide analog 4-thio-UTP, which allows efficient and specific affinity purification of newly transcribed RNA. Using deep sequencing, we characterized the onset of transcription in zebrafish embryos at 128-, 256-, and 512-cell stages. We identified 592 nuclear-encoded zygotically transcribed genes, comprising 670 transcript isoforms. Mitochondrial genomes were highly transcribed at all time points. Further, bioinformatic analysis revealed an enrichment of transcription factors and miRNAs among the newly transcribed genes, suggesting mechanistic roles for the early genes that are required to activate subsequent gene expression programs in development. Interestingly, analysis of gene-architecture revealed that zygotically transcribed genes are often intronless and short, reducing transcription and processing time of the transcript. The newly generated dataset enabled us to compare zygotically transcribed genes over a broad phylogenetic distance with fly and mouse early zygotic genes. This analysis revealed that short gene length is a common characteristic for early zygotically expressed genes. However, we detected a poor level of overlap for shared orthologs.