The repressilator is a synthetic genetic regulatory network reported in a paper by Michael Elowitz and Stanislas Leibler. This network was designed from scratch to exhibit a stable oscillation which is reported via the expression of green fluorescent protein, and hence acts like an electrical oscillator system with fixed time periods. The network was implemented in Escherichia coli using standard molecular biology methods and observations were performed that verify that the engineered colonies do indeed exhibit the desired oscillatory behavior.
The repressilator consists of three genes connected in a feedback loop, such that each gene represses the next gene in the loop, and is repressed by the previous gene. In addition, green fluorescent protein is used as a reporter so that the behavior of the network can be observed using fluorescence microscopy.
The design of the repressilator was guided by two simple mathematical models, one continuous and deterministic and the other discrete and stochastic.
These models were analyzed to determine the values for the various rates which would yield a sustained oscillation. It was found that these oscillations were favoured by strong promoters coupled to efficient ribosome binding sites, tight transcriptional repression (low 'leakiness'), cooperative repression characteristics, and comparable protein and mRNA decay rates.
This analysis motivated two design features which were engineered into the genes:
Second, to reduce the disparity between the lifetimes of the repressor proteins and the mRNAs, a carboxy terminal tag based on the ssRA RNA sequence was added at the 3' end of each repressor gene. This tag is recognized by proteases which target the protein for degradation.
To study the behavior of the repressilator single cells were isolated and the resulting colonies were observed over time using fluorescence microscopy. Observations were limited by the fact that the colonies entered a stationary phase after about 10 hours and approximately 5 oscillations.
The repressilator is a milestone of synthetic biology which shows that genetic regulatory networks which perform a novel desired function can be designed and implemented. Further, this experiment gives new appreciation to the circadian clock found in many organisms, as they perform much more robustly than the repressilator.
- A Synthetic Oscillatory Network of Transcriptional Regulators; Michael Elowitz and Stanislas Leibler; Nature. 2000 Jan 20;403(6767):335-8.