The current paradigm for tuning synthetic biological systems is through reengineering system components. Biological systems designed with the inherent ability to be tuned by external stimuli will be more versatile. We engineered E. coli cells to behave as an externally-tunable band-pass filter for enzyme activity and small molecules. A single strain of E. coli was constructed in which growth required enzyme activity in a very narrow range. This window of enzyme activity required for growth could be positioned within a four order of magnitude range simply by the addition of compounds to the growth medium. Inclusion in the genetic network of an enzyme-substrate pair that functions as an attenuator is a generalizable strategy that enables this tunability. The genetic circuit enabled a two-tiered genetic selection for engineered allosteric enzymes, which have potential applications in sensing in vitro and in live cells. The application of this strategy to other biological systems will increase their utility for biotechnological applications and their usefulness as a tool for gaining insight into nature's underlying design principles.