Project Abstract: Developmental gene regulation is a multidimensional problem where signals converge to generate patterns of transcription at the proper location (spatial regulation) and correct time (temporal regulation). An equally important feature of this regulatory process involves generating the correct amount of transcription during development as many key regulatory genes function in dosage sensitive manners where too much or too little expression can lead to developmental defects or disease. At present, we have only a primitive understanding about how the transcriptional output of developmentally regulated genes is established. Experiments outlined in this proposal aim to directly address this problem by employing an in vivo imaging system in C. elegans larva where GFP molecules are directly tethered to nascent RNAs while they are being actively transcribed in the nucleus. This new platform enables us to directly quantify and model the real-time expression dynamics that ultimately dictate transcriptional output of individual genes throughout development. In Aim 1, we will use this system to dissect the mechanisms by which two transcription factors, BLMP-1 and LIN-42, modulate features of transcriptional bursting (including burst frequency, duration, and amplitude) to tune transcriptional levels of key microRNAs (including lin-4 and let-7) that dictate sequential patterns of cell fate specification. We will then probe the genomic and molecular mechanisms that BLMP-1 employs to prime future transcription by remodeling chromatin accessibility near target gene loci. In the final Aim, we will characterize the physical and functional interactions between LIN-42 and several conserved nuclear hormone receptors (NHR-23RORγ and NHR-85Rev- erbβ) that mediate temporal aspects of lin-4 and let-7 transcriptional activation. This avenue of research will reveal the principles by which chromatin remodeling and its impact on modulating the transcriptional output of cyclically expressed genes ensures the precision and robustness of cell fate specification during development.