eLife assessment
This important study conducts genetic analyses utilizing zebrafish, mouse, and mouse embryonic stem cell models to elucidate the role of Rtf1, a component of the PAF1 complex, in early cardiac development. By combining marker gene expression analysis, single-cell transcriptomics, ChIP-seq, and chemical inhibition, the study provides convincing evidence that Rtf1-mediated RNAPII (Pol2) transcriptional pausing is required for early cardiac development and that attenuation of pause release by pharmacological inhibition of Cdk9, a component of the PTEF-b complex that regulates the transition between the pausing and elongation phases of transcription, can partially restore transcriptional pausing and cardiogenesis in zebrafish rtf1 mutants. The work will be of broad interest to developmental biologists.