Natural biological systems have evolved a diverse array of switches to realize their strategies for environmental response and development. Emerging applications of synthetic biology have begun to exploit such switches to achieve increasingly sophisticated designed behaviors. However, not all switch architectures allow facile design of the switching and memory properties. Furthermore, not all designs are built from components for which large families of variants exist, a requirement for building many orthogonal switch variants. Therefore, there is a critical need from genetic engineers for scalable strategies that yield custom bistable switches. Here, we use a sigma factor and its cognate anti-sigma factor to experimentally verify that ultrasensitivity from sequestration combined with positive feedback is sufficient to build a bistable switch. We show that sequestration allows us to predictably tune the switching boundaries, and we can easily tune our switch to function as a set-reset latch that can be toggled between two states by a pulse of inducer input.
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