A major goal of synthetic biology is to design and build digital genetic circuits inside cells, effectively programming cellular functions. Achieved this could allow living cells to be engineered to perform decision-making tasks, similar to computers, but composed of genetic elements rather than electronic components. This technology could be applied to medical therapeutics, molecular detection, diagnostics, tissue engineering, bio-electronic interfaces, and many other science-fictionesque uses. However, genetic elements tend to be less predictable and more “leaky” than electronic components, and this has limited progress.
In a brand-new study from the University of Washington, Gander et al. overcame these obstacles by using CRISPR/Cas9 linked to the transcriptional repressor Mxi1 to make genetic circuits in yeast. The group designed and built a library of single-gene NOR gates (which give an output signal only when there are no input signals). Each gate consists of a gRNA-expressing promoter that can be fully silenced by either of two gRNA-dCas9-Mxi1 complexes. This system has three key features that make it ideal for engineering genetic circuits. First, NOR gates are what is known in logic and engineering fields as “functionally complete”, meaning that multiple NOR gates can be combined to create any other logical function. So, in principal, ANY digital circuit can be made by connecting NOR gates in the right arrangement. Second, the dCas9-Mxi1-based NOR gates have a very strong OFF state, making the output binary, so the system behaves like a true digital circuit. Third, the input and output signals are both gRNAs, so large numbers of gates can be “wired” together, like an electronic circuit. Gander et al. successfully constructed logic circuits with as many as seven gRNA layers.
Because this technology is so simple and easy to produce, it should be relatively easy to construct almost any arbitrary logic circuit. Complex arrangements of gRNA-dCas9-Mxi1 NOR gates could be placed in various cell types to carry out computational tasks inside cells. The idea of an engineered biological computer may not be very far-fetched.