Development of a novel cellulase biosensor that detects crystalline cellulose hydrolysis using a transcriptional regulator

Translating New Synthetic Biology Advances for Biosensing Into the Earth and Environmental Sciences

The rapid diversification of synthetic biology tools holds promise in making some classically hard-to-solve environmental problems tractable. Here we review longstanding problems in the Earth and environmental sciences that could be addressed using engineered microbes as micron-scale sensors (biosensors). Biosensors can offer new perspectives on open questions, including understanding microbial behaviors in heterogeneous matrices like soils, sediments, and wastewater systems, tracking cryptic element cycling in the Earth system, and establishing the dynamics of microbe-microbe, microbe-plant, and microbe-material interactions. Before these new tools can reach their potential, however, a suite of biological parts and microbial chassis appropriate for environmental conditions must be developed by the synthetic biology community. This includes diversifying sensing modules to obtain information relevant to environmental questions, creating output signals that allow dynamic reporting from hard-to-image environmental materials, and tuning these sensors so that they reliably function long enough to be useful for environmental studies. Finally, ethical questions related to the use of synthetic biosensors in environmental applications are discussed.

Biosensor-Based Directed Evolution of Methanol Dehydrogenase from Lysinibacillus xylanilyticus

Methanol dehydrogenase (Mdh), is a crucial enzyme for utilizing methane and methanol as carbon and energy sources in methylotrophy and synthetic methylotrophy. Engineering of Mdh, especially NAD-dependent Mdh, has thus been actively investigated to enhance methanol conversion. However, its poor catalytic activity and low methanol affinity limit its wider application. In this study, we applied a transcriptional factor-based biosensor for the direct evolution of Mdh from Lysinibacillus xylanilyticus (Lxmdh), which has a relatively high turnover rate and low K_M value compared to other wild-type NAD-dependent Mdhs. A random mutant library of Lxmdh was constructed in Escherichia coli and was screened using formaldehyde-detectable biosensors by incubation with low methanol concentrations. Positive clones showing higher fluorescence were selected by fluorescence-activated cell sorting (FACS) system, and their catalytic activities toward methanol were evaluated. The successfully isolated mutants E396V, K318N, and K46E showed high activity, particularly at very low methanol concentrations. In kinetic analysis, mutant E396V, K318N, and K46E had superior methanol conversion efficiency, with 79-, 23-, and 3-fold improvements compared to the wild-type, respectively. These mutant enzymes could thus be useful for engineering synthetic methylotrophy and for enhancing methanol conversion to various useful products.

Genetically Encoded Biosensor-Based Screening for Directed Bacteriophage T4 Lysozyme Evolution

Lysozyme is widely used as a model protein in studies of structure–function relationships. Recently, lysozyme has gained attention for use in accelerating the degradation of secondary sludge, which mainly consists of bacteria. However, a high-throughput screening system for lysozyme engineering has not been reported. Here, we present a lysozyme screening system using a genetically encoded biosensor. We first cloned bacteriophage T4 lysozyme (T4L) into a plasmid under control of the araBAD promoter. The plasmid was expressed in Escherichia coli with no toxic effects on growth. Next, we observed that increased soluble T4L expression decreased the fluorescence produced by the genetic enzyme screening system. To investigate T4L evolution based on this finding, we generated a T4L random mutation library, which was screened using the genetic enzyme screening system. Finally, we identified two T4L variants showing 1.4-fold enhanced lytic activity compared to native T4L. To our knowledge, this is the first report describing the use of a genetically encoded biosensor to investigate bacteriophage T4L evolution. Our approach can be used to investigate the evolution of other lysozymes, which will expand the applications of lysozyme.

Engineering Prokaryotic Transcriptional Activator XylR as a Xylose-Inducible Biosensor for Transcription Activation in Yeast

Biosensors regulated by specific substrates are needed to develop genetic tools to meet the needs of engineering microbial cell factories. Here, a xylose-inducible biosensor (xylbiosensor), comprising the Escherichia coli activation factor XylR, fusion activation domain (AD) VPRH, and a hybrid promoter with operator xylO, was established in Yarrowia lipolytica. The addition of xylose to an engineered Y. lipolytica strain harboring the xylbiosensor could trigger significant transcriptional activation of target genes, such as mcherry and the xylose utilization gene. Furthermore, a novel promoter Pleu-Pxo-Ptef was developed to construct a bidirectional expression system. The xylbiosensor showed good portability in Saccharomyces cerevisiae, suggesting its potential value in other eukaryotic cells. This study is the first to construct a "turn-on" xylbiosensor induced by xylose addition based on a prokaryotic activator XylR and eukaryotic universal AD. The xylbiosensor exhibits potential in pathway engineering for xylose utilization and xylose-derived product biosynthesis in yeast.

Design and fabrication of a label-free aptasensor for rapid and sensitive detection of endoglucanase

Endoglucanase (endocellulase, EC 3.2.1.4) is one of the most widely used enzymes in industry. Despite its importance, improved methods for the rapid, selective, quantitative assay of this enzyme have been slow to emerge. In this work, we have designed an aptasensor platform for ultrasensitive endoglucanase II detection based on DNA aptamer and reduced graphene oxide/Au nanoparticles (RGO/AuNPs). The surface morphology of RGO/AuNPs was characterized by various techniques such as transmission electron microscopy (TEM), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX) methods. The aptasensor characterization was monitored with electrochemical techniques. Using RGO/AuNPs as a nanomaterial can effectively increase the conductivity of biosensor electrode. Moreover, using an RGO/AuNPs/aptamer platform in the presence of endoglucanase, the sensor system is able to generate a signal, which significantly improves the selectivity of the aptasensor. The fabricated aptasensor exhibits high sensitivity and selectivity to endoglucanase II with a low limit of detection (LOD) ˂0.1 nM.

Genetically encoded biosensors for lignocellulose valorization

Modern society is hugely dependent on finite oil reserves for the supply of fuels and chemicals. Moving our dependence away from these unsustainable oil-based feedstocks to renewable ones is, therefore, a critical factor towards the development of a low carbon bioeconomy. Lignin derived from biomass feedstocks offers great potential as a renewable source of aromatic compounds if methods for its effective valorization can be developed. Synthetic biology and metabolic engineering offer the potential to synergistically enable the development of cell factories with novel biosynthetic routes to valuable chemicals from these sustainable sources. Pathway design and optimization is, however, a major bottleneck due to the lack of high-throughput methods capable of screening large libraries of genetic variants and the metabolic burden associated with bioproduction. Genetically encoded biosensors can provide a solution by transducing the target metabolite concentration into detectable signals to provide high-throughput phenotypic read-outs and allow dynamic pathway regulation. The development and application of biosensors in the discovery and engineering of efficient biocatalytic processes for the degradation, conversion, and valorization of lignin are paving the way towards a sustainable and economically viable biorefinery.

C1 Compound Biosensors: Design, Functional Study, and Applications

The microbial assimilation of one-carbon (C1) gases is a topic of interest, given that products developed using this pathway have the potential to act as promising substrates for the synthesis of valuable chemicals via enzymatic oxidation or C–C bonding. Despite extensive studies on C1 gas assimilation pathways, their key enzymes have yet to be subjected to high-throughput evolution studies on account of the lack of an efficient analytical tool for C1 metabolites. To address this challenging issue, we attempted to establish a fine-tuned single-cell–level biosensor system constituting a combination of transcription factors (TFs) and several C1-converting enzymes that convert target compounds to the ligand of a TF. This enzymatic conversion broadens the detection range of ligands by the genetic biosensor systems. In this study, we presented new genetic enzyme screening systems (GESSs) to detect formate, formaldehyde, and methanol from specific enzyme activities and pathways, named FA-GESS, Frm-GESS, and MeOH-GESS, respectively. All the biosensors displayed linear responses to their respective C1 molecules, namely, formate (1.0–250 mM), formaldehyde (1.0–50 μM), and methanol (5–400 mM), and they did so with high specificity. Consequently, the helper enzymes, including formaldehyde dehydrogenase and methanol dehydrogenase, were successfully combined to constitute new versatile combinations of the C1-biosensors.

Structural and functional analyses of the cellulase transcription regulator CelR

CelR is a transcriptional regulator that controls the expression of cellulases catalyzing cellulose hydrolysis. However, the structural mechanism of its regulation has remained unclear. Here, we report the first structure of CelR, in this case with cellobiose bound. CelR consists of a DNA-binding domain (DBD) and a regulatory domain (RD), and homodimerizes with each monomer bound to cellobiose. A hinge region (HR) in CelR connects the DBD with the RD, and Leu59 in the HR acts as a 'leucine lever' that transduces a transcriptional activation signal. Furthermore, an α4 helix mediates the ligand-binding signal for transcriptional activation. Tyr84 and Gln301 can potentially alter the ligand specificity of CelR. This study provides a pivotal step toward understanding transcription of the cellulases.