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Rapid molecular biosensing is an emerging application area for synthetic biology. Here, we engineer a portable biosensor for cyanuric acid (CYA), an analyte of interest for human and environmental health, using a LysR-type transcription regulator (LTTR) from Pseudomonas within the context of Escherichia coli gene expression machinery. To overcome cross-host portability challenges of LTTRs, we rationally engineered hybrid Pseudomonas-E. coli promoters by integrating DNA elements required for transcriptional activity and ligand-dependent regulation from both hosts, which enabled E. coli to function as whole-cell biosensor for CYA. To alleviate challenges of whole-cell biosensing, we adapted these promoter designs to function within a freeze-dried E. coli cell-free system to sense CYA. This portable, on-demand system robustly detects CYA within an hour from laboratory and real-world samples and works with both fluorescent and colorimetric reporters. This work elucidates general principles to facilitate the engineering of a wider array of LTTR-based environmental sensors.

NADH and NAD+ cofactors drive hundreds of biochemical reactions, and their ratio is a key metabolic marker of cellular state. Traditional assays to measure the NADH/NAD+ ratio is laborious, prone to inaccuracies, and not suitable for high-throughput screening. We report a genetically encoded ratiometric biosensor for NADH/NAD+ based on redox-responsive bacterial transcription factor Rex that overcomes these limitations. We engineered a Rex-regulated E. coli promoter with improved biosensor characteristics by tuning the affinity of Rex and the operator site. Since NADH is oxidized during aerobic respiration, we used the biosensor-reporter to investigate the effect of removing respiratory chain enzymes on NADH/NAD+ ratio during aerobiosis. We found that the NADH/NAD+ signal increased in five of the nine mutants by over 3-fold compared to wildtype, including an NADH dehydrogenase double mutant with 6-fold elevation. We also found that among several common carbon sources, E. coli grown on acetate exhibited higher NADH/NAD+ compared to E. coli grown on glucose. As a proof-of-concept for high-throughput redox screening, we were able to enrich high NADH mutants present at 1 in 10 000 among wildtype cells by biosensor-guided pooled screen. Thus, our Rex biosensor-reporter enables facile, noninvasive, high-throughput redox measurement to understand and engineer redox metabolism.

The study of allostery has a central place in biology because of the myriad roles of allosteric proteins in cellular function. As technologies for probing the spatiotemporal resolution of biomolecules have become increasingly sophisticated, so has our understanding of the diverse structural and molecular mechanisms of allosteric proteins. Studies have shown that the allosteric signal is transmitted a through a network of residue-residue interactions connecting distal sites on a protein. Linking structural and dynamical changes to the functional role of individual residues will give a more complete molecular view of allostery. In this work, we highlight new mutational technologies that enable a systems-level, quantitative description of allostery that dissect the role of individual residues through large-scale functional screens. A molecular model for predicting allosteric hot spots can be developed by applying statistical tools on the resulting large sequence-structure-function data sets. Design of allosteric proteins with new function is essential for engineering biological systems. Previous design efforts demonstrate that the allosteric network is a powerful functional constraint in the design of novel or enhanced allosteric proteins. We discuss how a priori knowledge of an allosteric network could improve rational design by facilitating better navigation of the design space. Understanding the molecular "rules" governing allostery would elucidate the molecular basis of dysfunction in disease-associated allosteric proteins, provide a means for designing tailored therapeutics, and enable the design of new sensors and enzymes for synthetic biology.