Screening of a highly soluble and oxygen-independent blue fluorescent protein from metagenome

A short guide on blue fluorescent proteins: limits and perspectives

The advent of the so-called colorful biology era is in line with the discovery of fluorescent proteins (FPs), which can be widely used to detect the intracellular locations of macromolecules or to determine the abundance of metabolites in organelles. The application of multiple FPs that emit different spectra and colors could be implemented to precisely evaluate cellular events. FPs were initially established with the emergence of the green fluorescent protein (GFP) from jellyfish. Red fluorescent proteins (RFPs) from marine anemones and several corals adopt fluorescent chromophores that are similar to GFP. Chromophores of GFP and GFP-like FPs are formed through the oxidative rearrangement of three chromophore-forming residues, thereby limiting their application to only oxidative environments. Alternatively, some proteins can be fluorescent upon their interaction with cellular prosthetic cofactors and, thus, work in aerobic and anaerobic conditions. The modification of an NADPH-dependent blue fluorescent protein (BFP) also expanded its application to the quantization of NADPH in the cellular environment. However, cofactor-dependent BFPs have an intrinsic weakness of poor photostability with a high fluorescent background. This review explores GFP-derived and NADPH-dependent BFPs with a focus on NADPH-dependent BFPs, which might be technically feasible in the near future upon coupling with two-photon fluorescence microscopy or nucleic acid-mimickers. KEY POINTS: • Oxidation-dependent GFP-like BFPs and redox-free NADPH-dependent BFPs • GFPs of weak photostability and intensity with a high fluorescent background • Real-time imaging using mBFP under two-photon fluorescence microscopy.

Genetically encoded ATP and NAD(P)H biosensors: potential tools in metabolic engineering

To date, many metabolic engineering tools and strategies have been developed, including tools for cofactor engineering, which is a common strategy for bioproduct synthesis. Cofactor engineering is used for the regulation of pyridine nucleotides, including NADH/NAD+ and NADPH/NADP+, and adenosine triphosphate/adenosine diphosphate (ATP/ADP), which is crucial for maintaining redox and energy balance. However, the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP cannot be monitored in real time using traditional methods. Recently, many biosensors for detecting, monitoring, and regulating the intracellular levels of NADH/NAD+, NADPH/NADP+, and ATP/ADP have been developed. Although cofactor biosensors have been mainly developed for use in mammalian cells, the potential application of cofactor biosensors in metabolic engineering in bacterial and yeast cells has received recent attention. Coupling cofactor biosensors with genetic circuits is a promising strategy in metabolic engineering for optimizing the production of biochemicals. In this review, we focus on the development of biosensors for NADH/NAD+, NADPH/NADP+, and ATP/ADP and the potential application of these biosensors in metabolic engineering. We also provide critical perspectives, identify current research challenges, and provide guidance for future research in this promising field.

An Evaluation of a New Quantitative Point-of Care Diagnostic to Measure Glucose-6-phosphate Dehydrogenase Activity

Malaria continues to be one of the most crucial infectious burdens in endemic areas worldwide, as well as for travelers visiting malaria transmission regions. It has been reported that 8-aminoquinolines are effective against the Plasmodium species, particularly primaquine, for anti-hypnozoite therapy in P. vivax malaria. However, primaquine causes acute hemolytic anemia in individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency. Therefore, G6PD deficiency testing should precede hypnozoite elimination with 8-aminoquinoline. Several point-of-care devices have been developed to detect G6PD deficiency. The aim of the present study was to evaluate the performance of a novel, quantitative G6PD diagnostics based on a metagenomic blue fluorescent protein (mBFP). We comparatively evaluated the sensitivity and specificity of the G6PD diagnostic modality with standard methods using 120 human whole blood samples. The G6PD deficiency was spectrophotometrically confirmed. The performance of the G6PD quantitative test kit was compared with that of a licensed control medical device, the G6PD strip. The G6PD quantitative test kit had a sensitivity of 95% (95% confidence interval (CI): 89.3–100%) and a specificity of 100% (95% CI: 94.3–100%). This study shows that the novel diagnostic G6PD quantitative test kit could be a cost-effective and time-efficient, and universally mandated screening tool for G6PD deficiency.

Real-time monitoring of NADPH levels in living mammalian cells using fluorescence-enhancing protein bound to NADPHs

Nicotinamide adenine nucleotide phosphate (NADPH) has been known to be involved in the multiple pathways of cell metabolism. However, conventional quantification assays for NADPH have required breaking down the cell membranes of around one million cells per assay, and monitoring NADPH flux in living cells has been limited by a few available tools. Here, we visualized NADPH levels in human cervical cancer cells HeLa using metagenome-derived blue fluorescent protein (mBFP), which specifically binds to NADPH and enhances the intrinsic fluorescence of NADPH up to 10-fold when imaged by two-photon microscopy to reduce photodamage. Adding an oxidizing agent such as diamide to HeLa cells that expressed mBFP led to an immediate decrease of intracellular NADPH depending on glucose availability in culture media. Furthermore, inhibiting glucose-6-phosphate dehydrogenase (G6PD) in the pentose phosphate pathway with dehydroandrosterone (DHEA) and knockdown of G6PD transcripts gradually decreased NADPH when diamide was added to living cells. These results demonstrate that introducing a bacterial mBFP gene into mammalian cells is a straightforward approach to monitoring intracellular NADPH flux in real time at the single-cell level. Moreover, this strategy can be expanded to tracking the spatio-temporal changes in NADPH even in single-cell organelles such as mitochondria and chloroplasts, which will allow us to more precisely assess the efficacy of biochemically or biophysically metabolic perturbations in animal and plant cells.

Structure-Guided Generation of a Redox-Independent Blue Fluorescent Protein from mBFP

Fluorescent proteins, such as the green fluorescent protein, are used for detection of cellular components and events. However, green fluorescent protein and its derivatives have limited usage under anaerobic conditions and require a long maturation time. On the other hand, the NADPH-dependent blue fluorescent protein (BFP) without oxidative modification of residues is instantly functional in both aerobic and anaerobic systems. BFP proteins belong to a short-chain dehydrogenase/reductase (SDR) protein family, and their fluorescent property changes with reaction time in the presence of a substrate. With the aim of developing a better fluorescent reporter independent of redox state, we elucidated the crystal structure of a tetrameric mBFP from soil metagenomes with and without NADPH. Apart from the previously known regions, structure-guided mutational studies have identified several residues that contribute to the fluorescence of mBFP, including two aromatic residues (F97 and Y157) near the nicotinamide moiety of the bound NADPH. A single histidine mutation at Y157 (Y157H) has conferred more stabilized, time-independent fluorescence even in the presence of substrates. Furthermore, we discovered another SDR protein that can also emit blue fluorescence. These results open a new possibility for the development of BFP as a stable cellular reporter for widespread use, independent of subcellular environments.

Rapid and sensitive detection of NADPH via mBFP-mediated enhancement of its fluorescence

The reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) functions as a reducing agent involved in many biosynthetic and antioxidant reactions in cells. Therefore, a lots of detection or assaying method of this cofactor are developed and used broadly in various research and application fields. These detection or assay tools, however, have often some problems, such as the low sensitivity, susceptibility to environmental interference and time-consuming pretreatment steps, remaining hurdle to successful quantification of NADPH or its derivatives accurately and immediately. Herein, we present a rapid (assay time < 30 s) and sensitive (detection limit < 2 pmol) detection method of NADPH using metagenome-derived blue fluorescent protein (mBFP), a protein capable of significantly enhancing NADPH fluorescence upon binding to this cofactor. Our method takes advantage of the high specificity of mBFP to NADPH and the immediate fluorescence enhancement upon the addition of mBFP to a solution of interest containing NADPH. We can apply this detection scheme to directly quantitative assessment of NADP(H)-dependent enzyme activities in-vitro, and further accessed to quantitative assay of other nicotine amide cofactors, such as NAD+ and NADH, by coupling assay using NAD(H) kinase. Thus, our method enabled us to quantitatively assess the activity of nicotinamide cofactor-associated enzymes in both bacterial and human cell lysates.

An Alternative Platform for Protein Expression Using an Innate Whole Expression Module from Metagenomic DNA

Many integrated gene clusters beyond a single genetic element are commonly trapped as the result of promoter traps in (meta)genomic DNA libraries. Generally, a single element, which is mainly the promoter, is deduced from the resulting gene clusters and employed to construct a new expression vector. However, expression patterns of target proteins under the incorporated promoter are often inconsistent with those shown in clones harboring plasmids with gene clusters. These results suggest that the integrated set of gene clusters with diverse cis- and trans-acting elements is evolutionarily tuned as a complete set for gene expression, and is an expression module with all the components for the expression of a nested open reading frame (ORF). This possibility is further supported by truncation and/or serial deletion analysis of this module in which the expression of the nested ORF is highly fluctuated or reduced frequently, despite being supported by plentiful cis-acting elements in the spanning regions around the ORF such as the promoter, ribosome binding site (RBS), terminator, and 3'-/5'-UTRs for gene expression. Here, we examined whether an innate module with a naturally overexpressed gene could be considered as a scaffold for an expression system. For a proof-of-principle study, we mined a complete expression module with an innately overexpressed ORF in E. coli from a metagenomics DNA library, and incorporated it into a vector that had no regulatory element for expressing the insert. We obtained successful expression of several inserts such as MBP, GFPuv, β-glucosidase, and esterase using this simple construct without tuning and codon optimization of the target insert.

NADPH-related processes studied with a SoxR-based biosensor in Escherichia coli

NADPH plays a crucial role in cellular metabolism for biosynthesis and oxidative stress responses. We previously developed the genetically encoded NADPH biosensor pSenSox based on the transcriptional regulator SoxR of Escherichia coli, its target promoter P_soxS and eYFP as fluorescent reporter. Here, we used pSenSox to study the influence of various parameters on the sensor output in E. coliduring reductive biotransformation of methyl acetoacetate (MAA) to (R)-methyl 3-hydroxybutyrate (MHB) by the strictly NADPH-dependent alcohol dehydrogenase of Lactobacillus brevis (LbAdh). Redox-cycling drugs such as paraquat and menadione strongly activated the NADPH biosensor and mechanisms responsible for this effect are discussed. Absence of the RsxABCDGE complex and/or RseC caused an enhanced biosensor response, supporting a function as SoxR-reducing system. Absence of the membrane-bound transhydrogenase PntAB caused an increased biosensor response, whereas the lack of the soluble transhydrogenase SthA or of SthA and PntAB was associated with a strongly decreased response. These data support the opposing functions of PntAB in NADP⁺ reduction and of SthA in NADPH oxidation. In summary, the NADPH biosensor pSenSox proved to be a useful tool to study NADPH-related processes in E. coli.

Real Time Monitoring of NADPH Concentrations in Corynebacterium glutamicum and Escherichia coli via the Genetically Encoded Sensor mBFP

Analyses of intracellular NADPH concentrations are prerequisites for the design of microbial production strains and process optimization. mBFP was described as metagenomics derived, blue fluorescent protein showing NADPH-dependent fluorescence. Characterization of mBFP showed a high specificity for binding of NADPH (K_D 0.64 mM) and no binding of NADH, the protein exclusively amplified fluorescence of NADPH. mBFP catalyzed the NADPH-dependent reduction of benzaldehyde and further aldehydes, which fits to its classification as short chain dehydrogenase. For in vivo NADPH analyses a codon-optimized gene for mBFP was introduced into Corynebacterium glutamicum WT and the phosphoglucoisomerase-deficient strain C. glutamicum Δpgi, which accumulates high levels of NADPH. For determination of intracellular NADPH concentrations by mBFP a calibration method with permeabilized cells was developed. By this means an increase of intracellular NADPH concentrations within seconds after the addition of glucose to nutrient-starved cells of both C. glutamicum WT and C. glutamicum Δpgi was observed; as expected the internal NADPH concentration was significantly higher for C. glutamicum Δpgi (0.31 mM) when compared to C. glutamicum WT (0.19 mM). Addition of paraquat to E. coli cells carrying mBFP led as expected to an immediate decrease of intracellular NADPH concentrations, showing the versatile use of mBFP as intracellular sensor.

Exploring native genetic elements as plug-in tools for synthetic biology in the cyanobacterium Synechocystis sp. PCC 6803

Background

The unicellular cyanobacterium Synechocystis sp. PCC 6803 has been widely used as a photoautotrophic host for synthetic biology studies. However, as a green chassis to capture CO₂ for biotechnological applications, the genetic toolbox for Synechocystis 6803 is still a limited factor.

Results

We systematically characterized endogenous genetic elements of Synechocystis 6803, including promoters, ribosome binding sites, transcription terminators, and plasmids. Expression from twelve native promoters was compared by measuring fluorescence from the reporter protein EYFP in an identical setup, exhibiting an 8000-fold range of promoter activities. Moreover, we measured the strength of twenty native ribosome binding sites and eight native terminators, indicating their influence on the expression of the reporter genes. In addition, two shuttle vectors, pCA-UC118 and pCB-SC101, capable of replication in both Synechocystis 6803 and E. coli were constructed. Expression of reporter proteins were significantly enhanced in cells containing these new plasmids, thus providing superior gene expression platforms in this cyanobacterium.

Conclusions

The results of this study provide useful and well characterized native tools for bioengineering work in the model cyanobacterium Synechocystis 6803.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0897-8) contains supplementary material, which is available to authorized users.

Lighting Up Clostridium Difficile: Reporting Gene Expression Using Fluorescent Lov Domains

The uses of fluorescent reporters derived from green fluorescent protein have proved invaluable for the visualisation of biological processes in bacteria grown under aerobic conditions. However, their requirement for oxygen has limited their application in obligate anaerobes such as Clostridium difficile. Fluorescent proteins derived from Light, Oxygen or Voltage sensing (LOV) domains have been shown to bridge this limitation, but their utility as translational fusions to monitor protein expression and localisation in a strict anaerobic bacterium has not been reported. Here we demonstrate the utility of phiLOV in three species of Clostridium and its application as a marker of real-time protein translation and dynamics through genetic fusion with the cell division protein, FtsZ. Time lapse microscopy of dividing cells suggests that Z ring assembly arises through the extension of the FtsZ arc starting from one point on the circumference. Furthermore, through incorporation of phiLOV into the flagella subunit, FliC, we show the potential of bacterial LOV-based fusion proteins to be successfully exported to the extracellular environment.

Over-expression of recombinant proteins with N-terminal His-tag via subcellular uneven distribution in Escherichia coli

Specific tags with defined amino acid residues are widely used to purify or probe target proteins. Interestingly, the tagging system occasionally results in an increase of the recombinant protein expression in vivo. Here, we systematically examined this phenomenon using a poly-histidine (His)-tag fused to N- or C-terminal region of green, red, and blue fluorescent proteins by quantification and uneven distribution in cytoplasm of Escherichia coli. This effect was further supported by the distinct over-expression of several unrelated proteins, such as esterase, neopullulanase, and chloramphenicol acetyltransferase, tagging with the same tag. These results suggest that a poly-His-tag placed at N-terminal region can induce over-expression of recombinant protein via subcellular uneven distribution in vivo.

Rational design of a synthetic Entner-Doudoroff pathway for improved and controllable NADPH regeneration

NADPH is an essential cofactor for the biosynthesis of several high-value chemicals, including isoprenoids, fatty acid-based fuels, and biopolymers. Tunable control over all potentially rate-limiting steps, including the NADPH regeneration rate, is crucial to maximizing production titers. We have rationally engineered a synthetic version of the Entner-Doudoroff pathway from Zymomonas mobilis that increased the NADPH regeneration rate in Escherichia coli MG1655 by 25-fold. To do this, we combined systematic design rules, biophysical models, and computational optimization to design synthetic bacterial operons expressing the 5-enzyme pathway, while eliminating undesired genetic elements for maximum expression control. NADPH regeneration rates from genome-integrated pathways were estimated using a NADPH-binding fluorescent reporter and by the productivity of a NADPH-dependent terpenoid biosynthesis pathway. We designed and constructed improved pathway variants by employing the RBS Library Calculator to efficiently search the 5-dimensional enzyme expression space and by performing 40 cycles of MAGE for site-directed genome mutagenesis. 624 pathway variants were screened using a NADPH-dependent blue fluorescent protein, and 22 were further characterized to determine the relationship between enzyme expression levels and NADPH regeneration rates. The best variant exhibited 25-fold higher normalized mBFP levels when compared to wild-type strain. Combining the synthetic Entner-Doudoroff pathway with an optimized terpenoid pathway further increased the terpenoid titer by 97%.

The blue fluorescent protein from Vibrio vulnificus CKM-1 is a useful reporter for plant research

BACKGROUND

The mBFP is an improved variant of NADPH-dependent blue fluorescent protein that was originally identified from the non-bioluminescent pathogenic bacteria Vibrio vulnificus CKM-1. To explore the application of mBFP in plants, the mBFP gene expression was driven by one of the three promoters, namely, leaf-specific (RbcS), hypoxia-inducible (Adh) or auxin-inducible (DR5) promoters, in different plant tissues such as leaves, roots and flowers under diverse treatments. In addition, the expressed mBFP protein was targeted to five subcellular compartments such as cytosol, endoplasmic reticulum, apoplast, chloroplast and mitochondria, respectively, in plant cells.

RESULTS

When the mBFP was transiently expressed in the tobacco leaves and floral tissues of moth orchid, the cytosol and apoplast exhibited brighter blue fluorescence than other compartments. The recombinant mBFP-mS₁C fusion protein exhibited enhanced fluorescence intensity that was correlated with more abundant RNA transcripts (1.8 fold) as compared with a control. In the root tips of horizontally grown transgenic Arabidopsis, mBFP could be induced as a reporter under hypoxia condition. Furthermore, the mBFP was localized to the expected subcellular compartments, except that dual targeting was found when the mBFP was fused with the mitochondria-targeting signal peptide. Additionally, the brightness of mBFP blue fluorescence was correlated with NADPH concentration.

CONCLUSION

The NADPH-dependent blue fluorescent protein could serve as a useful reporter in plants under aerobic or hypoxic condition. However, to avoid masking the mitochondrial targeting signal, fusing mBFP as a fusion tag in the C-terminal will be better when the mBFP is applied in mitochondria trafficking study. Furthermore, mBFP might have the potential to be further adopted as a NADPH biosensor in plant cells. Future codon optimization of mBFP for plants could significantly enhance its brightness and expand its potential applications.

Advanced in vivo applications of blue light photoreceptors as alternative fluorescent proteins

The ultimate ambition in cell biology, microbiology and biomedicine is to unravel complex physiological and pathophysiological processes within living organisms. To conquer this challenge, fluorescent proteins (FPs) are used as versatile in vivo reporters and biosensors to study gene regulation as well as the synthesis, localization and function of proteins in living cells. The most widely used FPs are the green fluorescent protein (GFP) and its derivatives and relatives. Their use as in vivo reporter proteins, however, is sometimes restricted by different environmental and cellular factors. Consequently, a whole range of alternative, cofactor-dependent reporter proteins have been developed recently. In this perspective, we summarize the advantages and limitations of the novel class of cyan-green fluorescent flavoproteins in comparison to members of the GFP family and discuss some correlated consequences for the use of FPs as in vivo reporters.

Bioprospecting in the genomic age

The genomic revolution promises great advances in the search for useful biocatalysts. Function-based metagenomic approaches have identified several enzymes with properties that make them useful candidates for a variety of bioprocesses. As DNA sequencing costs continue to decline, the volume of genomic data, along with their corresponding predicted protein sequences, will continue to increase dramatically, necessitating new approaches to leverage this information for gene-based bioprospecting efforts. Additionally, as new functions are discovered and correlated with this sequence information, the knowledge of the often complex relationship between a protein's sequence and function will improve. This in turn will lead to better gene-based bioprospecting approaches and facilitate the tailoring of desired properties through protein engineering projects. In this chapter, we discuss a number of recent advances in bioprospecting within the context of the genomic age.