Design of synthetic biological circuits often requires protein degradation to accomplish the reverse function in response to a negative signal. In Escherichia coli, protein degradation in the synthetic circuit is commonly achieved through the ssrA-tagged degradation system. In this work, we show that that the degradation kinetics for the native ssrA tag in each cell is in fact zeroth-order, rather than the commonly assumed first order. When measured in a population, the heterogeneity of the cells distorted the true kinetics and exhibited a first-order protein degradation kinetics as a population average. Furthermore we demonstrate that the zeroth-order kinetics significantly enhances the robustness of the gene-metabolic oscillator designed previously.

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