To do so, in the middle of the DNA strand, they place a short DNA snippet called a “terminator” that kicks RNA-P molecules off the DNA. The key is that the terminator kicks off RNA-P only when RNA-P is traveling in one direction; say from left to right, but not from right to left. The Stanford team then uses the integrators to cut out the terminator DNA snippet that’s in the middle of the longer DNA strand, turn it around, and then reinsert it. That means as the RNA-P travels down the DNA strand from left to right, it no longer recognizes the terminator. So it stays attached to the DNA and continues its transcription to RNA. Thus, the signal is turned on. In this case, if the RNA-Ps make it to the end point, they transcribe the gene for green fluorescent protein, lighting up the cell. Flip the terminator again and the RNA-P is kicked off, and the light turns off.
The Stanford team then showed that they could line up multiple transcriptors to carry out logical functions, creating standard logical circuits called AND gates, OR gates, XOR gates, and so on, which combine signals according to certain rules. (A computer’s processor is a vast assemblage of such gates.) They also showed that their novel biological circuit designs were adept at producing signals with large amplification and that they could be used to up the expression of a variety of genes, such as the production of fluorescent signals that made it simple to detect cells that were carrying out their programming.