Efforts to program biology often draw inspiration from and comparisons to electronic systems. For example, genetic circuits have been designed that function as oscillators, switches, band-detect filters and logic gates. An important useful feature of such systems is their ability to be tuned in predictable ways by modification of system components (e.g. modification a repressor's affinity for its activator). However, this approach to tuning amounts to building a new device for each desired behavior and is unlike how more complex electronic systems are designed to function. For example, electronic band-pass filters can be built such that any desired band-width and cut-off frequencies can be set through adjusting knobs from outside the device. The ability to create analogous externally-tuneable biological systems would greatly expand their versatility. We demonstrate that appropriate placement of an enzyme-substrate pair functioning as an attenuator in the cellular network enables external tuning. We engineered E. coli cells to behave as a band-pass filter for enzyme activity and small molecules in which the band's position and band-width can be tuned externally. This band-pass filter paradigm transformed E. coli into a simple model of developmental differentiation, enabled non-intuitive patterning of cells in response to chemical gradients and facilitated the engineering of allosteric proteins. The application of this strategy to other biological systems will increase their utility for biotechnological applications and their usefulness as a tool for understanding biology.

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