<b>Biophysics Seminar</b><br>Delayed Switch-like Negative Feedback Ensures Orderly Mitotic Progression in Early Embryos<br><b>Speaker: Qiong Yang (Stanford University)</b></br>

Speaker: Qiong Yang (Stanford University)

Despite the variety and complexity of biological oscillators, the core design of many natural oscillators and some synthetic oscillators includes a common architecture of coupled positive and negative feedback. The positive feedback loops often function as a bistable trigger that prevents the negative feedback loops from reaching a stable steady-state. Here we combine theoretical modelling and experiments to determine constraints on detailed structures of the negative feedback affect the overall performance of the oscillator, using the clock-like early embryonic cycles of Xenopus laevis as a motivating example. We isolated the key response of the anaphase-promoting complex APC-Cdc20 to the cyclin dependent kinase complex Cdk1-cyclin B1 from both feedback loops, and by developing a real-time fluorescence-based assay for APC-Cdc20 activity in Xenopus laevis egg extracts we revealed that the steady response is significantly ultrasensitive. Theoretical analysis predicts that this high ultrasensitivity is indeed required for robust and precise oscillations. Further, we simultaneously compared this response together with three other mitotic events responding to their common regulator Cdk1-cyclin B1, and found that thresholds set by Cdk1 activity alone cannot determine correct mitotic timing. Instead, the time lag associated with activation of APC by Cdk1 may be the dominant factor, based on quantitative measurements of potential time delays within the negative feedback. Our results suggest that switch-like negative feedback with appropriate time delays may be a useful strategy for converting a positive-feedback-driven bistable switch into an autonomous, clock-like oscillator.