Rtf1-dependent transcriptional pausing regulates cardiogenesis.

During heart development, a well-characterized network of transcription factors initiates cardiac gene expression and defines the precise timing and location of cardiac progenitor specification. However, our understanding of the post-initiation transcriptional events that regulate cardiac gene expression is still incomplete. The PAF1C component Rtf1 is a transcription regulatory protein that modulates pausing and elongation of RNA Pol II, as well as cotranscriptional histone modifications. Here we report that Rtf1 is essential for cardiogenesis in fish and mammals, and that in the absence of Rtf1 activity, cardiac progenitors arrest in an immature state. We found that Rtf1's Plus3 domain, which confers interaction with the transcriptional pausing and elongation regulator Spt5, was necessary for cardiac progenitor formation. ChIP-seq analysis further revealed changes in the occupancy of RNA Pol II around the transcription start site (TSS) of cardiac genes in morphants reflecting a reduction in transcriptional pausing. Intriguingly, inhibition of pause release in 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 gene expression levels in a cardiac developmental context.