Active Analyte Import Improves the Dynamic Range and Sensitivity of a Vitamin B₁₂ Biosensor

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.