Cell-free systems provide a versatile platform for the development of low-cost, easy-to-use sensors for diverse analytes. However, sensor affinity dictates response sensitivity, and improving binding affinity can be challenging. Here, we describe efforts to address this problem while developing a biosensor for vitamin B12, a critical micronutrient. We first use a B12-responsive transcription factor to enable B12-dependent output in a cell-free reaction, but the resulting sensor responds to B12 far above clinically relevant concentrations. Surprisingly, when expressed in cells, the same sensor mediates a much more sensitive response to B12. The sensitivity difference is partly due to regulated import that accumulates cytoplasmic B12. Overexpression of importers further improves sensitivity, demonstrating an inherent advantage of whole-cell sensors. The resulting cells can respond to B12 in serum, can be lyophilized, and are functional in a minimal-equipment environment, showing the potential utility of whole-cell sensors as sensitive, field-deployable diagnostics.
Keywords: cell-free sensors; circuit tuning; diagnostics; whole-cell sensors.