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Segments of DNA can perform basic computing functions (NAND), and code the answer by emitting green flourescent protein (GFP) (image via Nature)


What happens when the world runs out of silicon? Biological circuits could replace minerals as storehouses of computing power. So far, it’s been much easier to store lots of information using DNA and programming that allows translation between DNA and binary code. What if cells could be programmed to activate certain genes? With the ongoing developments of analogue and digital programming in microbes, that may well happen soon thanks to an effort by MIT.


That takes some tinkering, though, because silicon circuits are much easier to design and many more transistors can be packed onto a chunk of silicon than within a single cell. But we’re working on it! So far, researchers at MIT have developed biological circuitry which allows a cell to convert analogue signals into digital ones, with a range of responses. A cell could detect the concentration of acid in the stomach, for example, which triggers different responses based on the intensity of the stimulus. Essentially, the living circuit is composed of a threshold module, which detects a range of analogue signals, and subsequently controls the expression of a recombinase gene, which is turned on or off by inverting it.


Expression of the gene regulates the response of the cell to the stimulus. A bacterium could be designed to detect a range of acid concentration, and respond within a certain preprogrammed range using this circuit design.


What kind of stimuli would a bacterial circuit be used for? Current investigations underway are to detect the levels of inflammation in the body, levels of glucose in the blood, and treat diseases of the gut with specially designed probiotics. Microbes are already used to produce medically important substances, such as penicillin and morphine. The gene circuitry under development would allow a microbe to produce insulin only in the presence of high blood glucose, for example, which could potentially change medicine in a powerful way.


Unlike silicon computer chips, however, microbes reproduce and exchange genes with each other. Engineering microbes to respond to environmental challenges could unleash a host of unforeseen consequences.


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