The population-shift mechanism can be used for rational re-engineering of structure-switching biosensors to enable their allosteric inhibition and activation. As a proof-of-principle example of this, we have introduced distal allosteric sites into molecular beacons, which are optical sensors for the detection of specific nucleic acid sequences. The binding of inhibitors and activators to these sites enabled the rational modulation of the sensor's target affinity-and thus its useful dynamic range-over 3 orders of magnitude. The convenience with which this was done suggests that the population-shift mechanism may prove to be a useful method by which allosteric regulation can be introduced into biosensors, "smart" biomaterials, and other artificial biotechnologies.