Evolutionary regain of lost network function*

Natural or synthetic genetic network modules can lose their function over long-term evolution if the function is costly. How populations can evolve to restore such broken function is poorly understood. To test the reversibility of evolutionary breakdown, we use a synthetic gene circuit (PF) integrated into yeast cells. In previous evolution experiments, mutations in a gene eliminated the fitness costs of PF activation, corrupting gene circuit function. Since PF activation also provides drug resistance, we grew such corrupted mutants in both drug and inducer, imposing selection to regain drug resistance and possibly PF function. We observe various adaptation scenarios with or without repairing lost gene circuit function. The data suggest interactions between intracellular gene network dynamics and evolutionary dynamics, with possible consequences for understanding the evolution of drug resistance and developing future synthetic biology applications.