Questions: Synthetic Gene Circuits

Question 1

The toggle switch consists of two repressors, each repressing the other's promoter. Why does this mutual repression produce bistability rather than an intermediate state where both repressors are moderately expressed?

• A. Because the repressors are always expressed at maximum or zero — there is no intermediate expression level possible
• B. Because cooperative repression (Hill coefficient > 1) creates ultrasensitive switching: once one repressor gains even a slight advantage, it strongly suppresses the other, driving the system to one extreme or the other
• C. Because the two repressors always have different degradation rates
• D. Because bacterial cells cannot maintain two active promoters simultaneously

Answer: (B)

Mathematical analysis shows that bistability in the toggle switch requires cooperative repression (typically Hill coefficient >= 2). With cooperativity, the dose-response curve of each repressor is steep enough that a small asymmetry between the two repressors gets amplified into a large one. If repressor A is slightly higher than repressor B, the cooperative repression strongly suppresses B's promoter, reducing B's level further, which releases A's promoter even more — a positive feedback loop. The system settles at one of two stable states (A high / B low, or A low / B high). Without cooperativity (Hill = 1), the nullclines intersect only once and only a single intermediate steady state exists.

Question 2

The repressilator (A represses B, B represses C, C represses A) oscillates because it contains an odd number of repression steps in a feedback loop.

• T. True
• F. False

Answer: True

An odd number of repressions in a cycle creates net negative feedback: if A increases, it represses B, which de-represses C, which represses A — bringing A back down. This negative feedback, combined with the time delays inherent in transcription and translation, drives oscillations. An even number of repressions would create net positive feedback (mutual repression equivalent), producing bistability instead of oscillations. The repressilator's three-node ring is the minimal odd-numbered repression cycle, and its oscillatory behavior was predicted by ODE models before it was built, demonstrating the predictive power of dynamical systems theory in biology.

Question 3

Why are synthetic gene circuits important for systems biology beyond their potential applications in biotechnology?

Model answer: Synthetic circuits provide controlled experimental tests of systems biology theories. Natural gene regulatory networks are complex, with many interacting components and unknown parameters. By building a simple circuit from known parts (characterized promoters, repressors, reporters), researchers can test whether mathematical models correctly predict the circuit's behavior — oscillation period, switching threshold, noise properties. When predictions fail, the discrepancy reveals missing biology (context-dependent promoter behavior, metabolic burden, growth-rate feedback) that would be invisible in the complexity of natural networks. Synthetic circuits are the experimental physics of biology: controlled systems for testing quantitative theories.

The repressilator, for example, initially oscillated with much more variability than deterministic ODE models predicted, leading to important advances in understanding stochastic effects in gene circuits. The toggle switch revealed that host cell growth rate feeds back on circuit performance — a biological effect that pure circuit models missed. Each failure refined the theory.