A pulse-detecting genetic circuit

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Mara Inniss, Jeffrey C. Way, Pamela A. Silver

Harvard Medical School, United States

Generation and detection of pulses by genetic circuits is important for the control of natural biological systems. While synthetic biological systems have incorporated pulse-generating circuits, pulse-detecting circuits have not yet been built. Here, we use components of the lambda phage repressor system to build a pulse-detecting genetic circuit in E. coli. Lambda cI protein dimers bind cooperatively to activate transcription from the PRM promoter. Mutations in cI that abolish DNA binding will be dominant-negative (cIDN); mutant protein heterodimerizes with wild-type protein diminishing its ability to bind DNA. We constructed a circuit that consists of a gene that expresses both cIDN – tagged with a degradation tag – and cI in response to tetracycline. A reporter construct that expresses a reporter such as lacZ, YFP, or luciferase under control of the PRM promoter generates a detectable output. Initially, in the absence of inducer, the reporter will not be expressed. In the continued presence of inducer the reporter will not be expressed due to the presence of cIDN. However, once inducer is removed, cIDN is rapidly degraded, cI forms active homodimers, and activates transcription of the reporter. Thus, this circuit responds to a pulse of inducer rather than sustained induction. The pulse-detecting genetic circuit presented here represents a novel architecture that can produce a non-monotonic input for downstream genes. This will allow the construction of more complex genetic circuits with more complicated dynamics.