Biological Modules

Full Adder

With a logic diagram, a full adder can be illustrated with 5 logic gates (2 XOR, 2 AND and 1 OR) A and B represent the two operands and Cin and Cout the carries.

In our biological system, this diagram represents the input part.

A general inducer has been used to start the counting in order to differentiate the off state where there is no counting and the on state adding 0+0. TetR gene is put after a constitutive promoter and every input genes (Carry system genes, Blue and Red light receptor genes) are after a PtetR promoter that is repressed in presence of TetR. When we want the counting to be started, we add ATC (Anhydrotetracycline) that bind TetR and let the transcription of the input genes.
When there is a carry diffusing from the previous colony, the AHL will bind the LuxR protein produced by luxR gene. This can then activate the Plux promoter which induces the production of PhiC31 recombinase.
In absence of blue (480 nm) or red (660 nm) light, both light receptor systems will be activated and induce the production of respectively Bxb1 and tp901.1.

And this diagram combines the four logic gates.

The production of TP 901.1 and Bxb1 are the inputs (A and B) and PhiC31 is produced after the detection of a carry.
The OR gate is composed by the two LuxI genes (ouput of AND1 and AN2 gates). If there is one or the other AND gate activated, LuxI is transcripted and AHL is produced. The expression of the carry (AHL) needs to be strong for the diffusion, that implies a strong expression of LuxI gene. That is why it was placed under control of strong promoter. We choose then T7 promoter, known as very specific and strong promoter, for AND 1 and AND2 promoters.