Design of an intracellular aptamer-based fluorescent biosensor to track burden in E. coli

Cell burden impacts the performance of engineered synthetic systems. For this reason, there is great interest toward the development of tools to track burden and improve biotechnology applications. Fluorogenic RNA aptamers are excellent candidates for live monitoring of burden because their production is expected to impose a negligible load on transcription resources. Here we characterise the performance of a library of aptamers when expressed from different promoters in E. coli. We find that aptamer relative performance is dependent on the promoter and the strain, and that, contrary to expectation, aptamer expression impacts host fitness. By selecting two of the aptamers with brighter output and lower impact, we then design an intracellular biosensor able to report on the activation of the burden response in engineered cells. The sensor developed here adds to the collection of tools available for burden mitigation and may support bioprocessing applications where improved host performance is sought.