Drawing a parallel between electromagnetic relay and biological relay
The structure of an electromagnetic relay provides a concise and explicit understanding of our design. As long as a certain voltage is added to both ends of the coil, a certain current will flow through the circuit, thus generating an electromagnetic effect. The armature will overcome the tension of the return spring and attach to the iron core under the electromagnetic force, driving the moving contact of the armature to attract to the static contact (normally open contact). When the power is down, the magnetism will disappear, and the armature will return to its original position due to the spring's force, so that the moving contact and the original static contact (normally closed contact) will be connected. Such processes enable the conduction and outage in the circuit.
Figure 1. The schematic diagram of an electromagnetic relay
When designing our biological relay, we achieve similar properties by imitating the components of an electromagnetic relay. The recombinase acts as an electromagnet which is responsible for the shifting of contacts. We connect the recombinase in the control circuit of the relay system. The output circuit is initially blocked by the terminator between two recombinase binding sites (connected to the normally closed contact). But when inducers are added (power supplied), the recombinase starts expressing (magnetized) and the protein molecules bind to site attB and site attP, leading to the reversing or cutting of the intervening gene and thus incapacitating the inhibition of the terminator (connected to the normally open contact)—the operational circuit reports. Besides the separation of two circuits, we can see that a small number of products from the controlling module can yield an amplified output through the reporting circuit. This characteristic resembles the ability of the relay system to control a high-voltage device by applying a low-voltage device.
Figure 2. Analogy of electrical relay and bio-relay.
Figure 3. The mechanism of the recombinase system.
Constructing Our Bio-Relay
Our biological relay aims to convert the positive corelative function of the promoter-output DNA system to a function of distinct digital signal, so that the output intensity should rise sharply in a specific response range. Hence, the system we develop must have a narrow hypersensitive section. Based on our model, we learn that the intensity of RBS will influence the response range of the system but will exert no impact on the accuracy of the hypersensitive section.
Figure 4. The ideal modification of the function.
In order to exclude the possible errors because of the combination of the recombinase and the self-contained binding sites inside the cell, we choose all the recombinases for our project from non-Escherichia coli phages. As we preconceive that the increasing intensity of input signals will gradually trigger all the recombinase sites along the concentration gradient and thus the results will contain various output values, we use a tetracycline-induced promoter Ptet in our reporting circuit instead of the common constitutive promoter. After the control circuit responds to the input and produces recombinase molecules, the reporting circuit is allowed to be fully reacted and to complete the transformation. Tetracycline is then added to induce the Ptet promoter so to begin the output expression. That is, the calculating process and the reading process are separated.
Demonstrate the relay system
Figure 5. The construction of biological ADC
We design an analog-digital converter on the basis of the recombinase system. Theoretically, by finetuning the sequence of the ribosome binding sites, we can get different sets of recombinases with successive responding ranges. Thus, as the input increases, the output signal activates the recombinases sequentially and reorganizes the corresponding DNA. Using florescence proteins to display the effect, we can obtain the graph as shown in Figure 6.
Figure 6
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