Loading as a design parameter for genetic circuits

A significant problem when building complex biomolecular circuits is due to context-dependence: the dynamics of a system are altered upon changes to its context, potentially degrading the system's performance. Here, we study retroactivity, a specific type of context-dependence, by analyzing the effects of loads on a transcription factor applied by the transcription factor's target sites. In particular, we study this loading effect on the model of an activator-repressor oscillator, a widely studied motif in synthetic and systems biology. Our analysis indicates that strong activation and weak repression are key for a stable limit cycle. Repression can be effectively weakened by adding load to the repressor, while activation can be effectively weakened by adding load to the activator. Therefore, loading the repressor can be employed as a design parameter to establish a stable limit cycle. In contrast, loading the activator is deleterious to the clock