Rtf1-dependent transcriptional pausing regulates cardiogenesis
During heart development, an evolutionarily conserved network of cardiac transcription factors collaborate to define the precise timing and location of cardiac progenitor specification. Accumulating evidence suggests that cardiac progenitor specification is subject to transcriptional control beyond the level of transcription initiation. The PAF1C component Rtf1 is a multifunctional transcription regulatory protein that modulates pausing and elongation of RNA Pol II, as well as histone epigenetic modifications. By transient knockdown and CRISPR mutagenesis, we found that Rtf1 is essential for cardiogenesis and that without Rtf1 activity, cardiac progenitors arrest in an immature state. This role in early cardiogenesis was evolutionarily conserved between fish and mammals. We also found that Rtf1's Plus3 domain, which confers interaction with the pausing/elongation factor Spt5, was required for Rtf1's ability to support cardiac progenitor formation, while other regions of the protein were dispensable. We examined the occupancy of RNA Pol II at cardiac genes in rtf1 morphants using ChIP-seq and found that Pol II signals at the TSS of genes was reduced, suggesting a reduction in transcriptional pausing. Intriguingly, pharmacological or morpholino antisense reduction of pause release in rtf1 morphants and mutants restored the formation of cardiac cells and improved Pol II occupancy at the TSS of key cardiac genes. Our findings highlight the crucial role that transcriptional pausing plays in promoting normal levels of gene expression in a cardiac developmental context.