Biosensors consist of two basic modules (see the figure): sensitive elements for recognizing and binding analytes, and transducer modules for transmitting and reporting signals.
Transcriptional biosensors (part a) are built by linking environment-responsive promoters to engineered gene circuits for programmed transcriptional changes. In the example shown, a transcriptional AND gate was designed to sense and report only the simultaneous presence of two environmental signals (for example, salicylate and arabinose) (Anderson et al., 2007). At one gate input, the researchers encoded an environment-responsive promoter (for example, PBAD) that activates transcription of a T7 RNA polymerase gene in response to a single environmental signal (for example, arabinose). The gene, however, carries internally encoded amber stop codons (red spikedcircles) that function to block translation of its transcript. Activation of the second gate input is the key to unlocking translation; specifically, translation can be induced when a second promoter (for example, Psal) activates transcription of the supD amber suppressor tRNA in response to a second unique signal (for example, salicylate). In other words, only when the two environmental signals are simultaneously present can the T7 RNA polymerase be faithfully expressed and used to activate an output T7 promoter. This is an example of how sophisticated specificity can be programmed into a transducer module by creatively linking the sensory information of multiple sensitive elements. Furthermore, the design is transcriptionally modular in that different sets of environment-responsive promoters can be interfaced to the AND gate.
Translational biosensors (part b) are typically built by linking RNA aptamer domains to RNA regulatory domains. The example shown is an OFF ‘antiswitch’. Here, the small molecule theophylline is recognized and bound by the aptamer stem of the RNA biosensor. This causes a conformational change in the molecule that liberates the antisense domain from its sequestering stem loop and allows it to inhibit translation of an output reporter (Bayer and Smolke, 2005).
From: A2, SYNTHETIC BIOLOGY: APPLICATIONS COME OF AGE
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