Enhanced brightness of bacterial luciferase by bioluminescence resonance energy transfer

Using the lux operon (luxCDABE) of bacterial bioluminescence system as an autonomous luminous reporter has been demonstrated in bacteria, plant and mammalian cells. However, applications of bacterial bioluminescence-based imaging have been limited because of its low brightness. Here, we engineered the bacterial luciferase (heterodimer of luxA and luxB) by fusion with Venus, a bright variant of yellow fluorescent protein, to induce bioluminescence resonance energy transfer (BRET). By using decanal as an externally added substrate, color change and ten-times enhancement of brightness was achieved in Escherichia coli when circularly permuted Venus was fused to the C-terminus of luxB. Expression of the Venus-fused luciferase in human embryonic kidney cell lines (HEK293T) or in Nicotiana benthamiana leaves together with the substrate biosynthesis-related genes (luxC, luxD and luxE) enhanced the autonomous bioluminescence. We believe the improved luciferase will forge the way towards the potential development of autobioluminescent reporter system allowing spatiotemporal imaging in live cells.

Use of Bacterial Luciferase as a Reporter Gene in Eukaryotic Systems

Reporter gene assays are powerful tools for monitoring dynamic molecular changes and for evaluating the responses that occur at the genetic elements within cells in response to exogenous molecules. In general, various protein systems can be used as reporter genes, including luciferases. Here, the present protocol introduces a unique reporter gene system for monitoring molecular events in cells using bacterial luciferase (lux), which can generate blue-green light suitable for gene reporter applications with the highest cost performance. The protocol also guides the assay conditions and necessary components for using of lux gene (lux) as a eukaryotic reporter system. The lux system can be applied to monitor variety of molecular events inside mammalian cellular systems.

Kinetics of the thermal inactivation and the refolding of bacterial luciferases in Bacillus subtilis and in Escherichia coli differ

Here we present a study of the thermal inactivation and the refolding of the proteins in Gram positive Bacillus subtilis. To enable use of bacterial luciferases as the models for protein thermal inactivation and refolding in B. subtilis cells, we developed a variety of bright luminescent B. subtilis strains which express luxAB genes encoding luciferases of differing thermolability. The kinetics of the thermal inactivation and the refolding of luciferases from Photorhabdus luminescens and Photobacterium leiognathi were compared in Gram negative and Gram positive bacteria. In B. subtilis cells, these luciferases are substantially more thermostable than in Escherichia coli. Thermal inactivation of the thermostable luciferase P. luminescens in B. subtilis at 48.5°С behaves as a first-order reaction. In E.coli, the first order rate constant (Kt) of the thermal inactivation of luciferase in E. coli exceeds that observed in B. subtilis cells 2.9 times. Incubation time dependence curves for the thermal inactivation of the thermolabile luciferase of P. leiognathi luciferase in the cells of E. coli and B. subtilis may be described by first and third order kinetics, respectively. Here we shown that the levels and the rates of refolding of thermally inactivated luciferases in B. subtilis cells are substantially lower that that observed in E. coli. In dnaK-negative strains of B. subtilis, both the rates of thermal inactivation and the efficiency of refolding are similar to that observed in wild-type strains. These experiments point that the role that DnaKJE plays in thermostability of luciferases may be limited to bacterial species resembling E. coli.

A bioluminescent arsenite biosensor designed for inline water analyzer

Whole-cell biosensors based on the reporter gene system can offer rapid detection of trace levels of organic or metallic compounds in water. They are well characterized in laboratory conditions, but their transfer into technological devices for the surveillance of water networks remains at a conceptual level. The development of a semi-autonomous inline water analyzer stumbles across the conservation of the bacterial biosensors over a period of time compatible with the autonomy requested by the end-user while maintaining a satisfactory sensitivity, specificity, and time response. We focused here on assessing the effect of lyophilization on two biosensors based on the reporter gene system and hosted in Escherichia coli. The reporter gene used here is the entire bacterial luciferase lux operon (luxCDABE) for an autonomous bioluminescence emission without the need to add any substrate. In the cell-survival biosensor that is used to determine the overall fitness of the bacteria when mixed with the water sample, lux expression is driven by a constitutive E. coli promoter P_rpoD. In the arsenite biosensor, the arsenite-inducible promoter P _ars involved in arsenite resistance in E. coli controls lux expression. Evaluation of the shelf life of these lyophilized biosensors kept at 4 °C over a year evidenced that about 40 % of the lyophilized cells can be revived in such storage conditions. The performances of the lyophilized biosensor after 7 months in storage are maintained, with a detection limit of 0.2 μM arsenite for a response in about an hour with good reproducibility. These results pave the way to the use in tandem of both biosensors (one for general toxicity and one for arsenite contamination) as consumables of an autonomous analyzer in the field.

Bacterial host and reporter gene optimization for genetically encoded whole cell biosensors

Whole-cell biosensors based on reporter genes allow detection of toxic metals in water with high selectivity and sensitivity under laboratory conditions; nevertheless, their transfer to a commercial inline water analyzer requires specific adaptation and optimization to field conditions as well as economical considerations. We focused here on both the influence of the bacterial host and the choice of the reporter gene by following the responses of global toxicity biosensors based on constitutive bacterial promoters as well as arsenite biosensors based on the arsenite-inducible P_ars promoter. We observed important variations of the bioluminescence emission levels in five different Escherichia coli strains harboring two different lux-based biosensors, suggesting that the best host strain has to be empirically selected for each new biosensor under construction. We also investigated the bioluminescence reporter gene system transferred into Deinococcus deserti, an environmental, desiccation- and radiation-tolerant bacterium that would reduce the manufacturing costs of bacterial biosensors for commercial water analyzers and open the field of biodetection in radioactive environments. We thus successfully obtained a cell survival biosensor and a metal biosensor able to detect a concentration as low as 100 nM of arsenite in D. deserti. We demonstrated that the arsenite biosensor resisted desiccation and remained functional after 7 days stored in air-dried D. deserti cells. We also report here the use of a new near-infrared (NIR) fluorescent reporter candidate, a bacteriophytochrome from the magnetotactic bacterium Magnetospirillum magneticum AMB-1, which showed a NIR fluorescent signal that remained optimal despite increasing sample turbidity, while in similar conditions, a drastic loss of the lux-based biosensors signal was observed.

Rapid in-vitro testing for chemotherapy sensitivity in leukaemia patients

Bioluminescent bacterial biosensors can be used in a rapid in vitro assay to predict sensitivity to commonly used chemotherapy drugs in acute myeloid leukemia (AML). The nucleoside analog cytarabine (ara-C) is the key agent for treating AML; however, up to 30 % of patients fail to respond to treatment. Screening of patient blood samples to determine drug response before commencement of treatment is needed. To achieve this aim, a self-bioluminescent reporter strain of Escherichia coli has been constructed and evaluated for use as an ara-C biosensor and an in vitro assay has been designed to predict ara-C response in clinical samples. Transposition mutagenesis was used to create a cytidine deaminase (cdd)-deficient mutant of E. coli MG1655 that responded to ara-C. The strain was transformed with the luxCDABE operon and used as a whole-cell biosensor for development an 8-h assay to determine ara-C uptake and phosphorylation by leukemic cells. Intracellular concentrations of 0.025 μmol/L phosphorylated ara-C were detected by significantly increased light output (P < 0.05) from the bacterial biosensor. Results using AML cell lines with known response to ara-C showed close correlation between the 8-h assay and a 3-day cytotoxicity test for ara-C cell killing. In retrospective tests with 24 clinical samples of bone marrow or peripheral blood, the biosensor-based assay predicted leukemic cell response to ara-C within 8 h. The biosensor-based assay may offer a predictor for evaluating the sensitivity of leukemic cells to ara-C before patients undergo chemotherapy and allow customized treatment of drug-sensitive patients with reduced ara-C dose levels. The 8-h assay monitors intracellular ara-CTP (cytosine arabinoside triphosphate) levels and, if fully validated, may be suitable for use in clinical settings.

Escherichia coli deletion mutants illuminate trade-offs between growth rate and flux through a foreign anabolic pathway

Metabolic engineers strive to improve the production yields of microbial fermentations, sometimes by mutating the genomes of production strains. Some mutations are detrimental to the health of the organism, so a quantitative and mechanistic understanding of the trade-offs could inform better designs. We employed the bacterial luciferase operon (luxABCDE), which uses ubiquitous energetic cofactors (NADPH, ATP, FMNH₂, acetyl-CoA) from the host cell, as a proxy for a novel anabolic pathway. The strains in the Escherichia coli Keio collection, each of which contains a single deletion of a non-essential gene, represent mutational choices that an engineer might make to optimize fermentation yields. The Keio strains and the parental BW25113 strain were transformed with a luxABCDE expression vector. Each transformant was propagated in defined M9 medium at 37°C for 48 hours; the cell density (optical density at 600 nanometers, OD₆₀₀) and luminescence were measured every 30 minutes. The trade-offs were visualized by plotting the maximum growth rate and luminescence/OD₆₀₀ of each transformant across a “production possibility frontier”. Our results show that some loss-of-function mutations enhance growth in vitro or light production, but that improvement in one trait generally comes at the expense of the other.

Comparison of 3 real-time, quantitative murine models of staphylococcal biofilm infection by using in vivo bioluminescent imaging

Biofilm formation represents a unique mechanism by which Staphylococcus aureus and other microorganisms avoid antimicrobial clearance and establish chronic infections. Treatment of these infections can be challenging, because the bacteria in the biofilm state are often resistant to therapies that are effective against planktonic bacteria of the same species. Effective animal models for the study of biofilm infections and novel therapeutics are needed. In addition, there is substantial interest in the use of noninvasive, in vivo data collection techniques to decrease the animal numbers required for the execution of infectious disease studies. To ad- dress these needs, we evaluated 3 murine models of implant-associated biofilm infection by using in vivo bioluminescent imaging techniques. The goal of these studies was to identify the model that was most amenable to development of sustained infections that could be imaged repeatedly in vivo by using bioluminescent technology. We found that the subcutaneous mesh and tibial intramedullary pin models both maintained consistent levels of bioluminescence for as long as 35 d after infection, with no implant loss experienced in either model. In contrast, a subcutaneous catheter model demonstrated significant incidence of incisional ab- scessation and implant loss by day 20 after infection. The correlation of bioluminescent measurements and bacterial enumeration was strongest with the subcutaneous mesh model. Among the 3 models we evaluated, the subcutaneous mesh model is the most appropriate animal model for prolonged study of biofilm infections by using bioluminescent imaging.

Comparison of human optimized bacterial luciferase, firefly luciferase, and green fluorescent protein for continuous imaging of cell culture and animal models

Bioluminescent and fluorescent reporter systems have enabled the rapid and continued growth of the optical imaging field over the last two decades. Of particular interest has been noninvasive signal detection from mammalian tissues under both cell culture and whole animal settings. Here we report on the advantages and limitations of imaging using a recently introduced bacterial luciferase (lux) reporter system engineered for increased bioluminescent expression in the mammalian cellular environment. Comparison with the bioluminescent firefly luciferase (Luc) system and green fluorescent protein system under cell culture conditions demonstrated a reduced average radiance, but maintained a more constant level of bioluminescent output without the need for substrate addition or exogenous excitation to elicit the production of signal. Comparison with the Luc system following subcutaneous and intraperitoneal injection into nude mice hosts demonstrated the ability to obtain similar detection patterns with in vitro experiments at cell population sizes above 2.5 × 10(4) cells but at the cost of increasing overall image integration time.

Fluorescence-PCR assays and isolation of luminescent bacterial clones using an automated plate reader

The genes responsible for luminescence in various species of the marine microorganism Photobacterium, have been used for many years as a tool by researchers and instructors. In particular, the lux operon of Photobacterium fischeri has been used by many instructors to teach recombinant DNA techniques. Two methods using an automated plate reader and multiwell plates were applied to a set of previously-published exercises. In these exercises that involve transfer of lux genes to Escherichia coli to create a luminescent phenotype, this technology was used to screen for Lux(+) colonies. It was found to be more convenient and more sensitive than the previously used method; that is, assaying bacterial plates by direct observation. Eight students synthesized four genomic libraries and isolated six Lux(+) clones. The fluorescent-detection feature of the plate reader was used to verify amplification of target sequence in polymerase chain reaction (PCR) reactions. Lux(+) E. coli colony lysates were examined. An exonuclease-activated, fluorescent DNA probe generated a signal on hybridization to an amplified portion of the luxA gene in each lysate tested. This method is suggested as a means of demonstrating the concept of real-time PCR without the expense of the specialized device typically used for this technique.

Effect of pH, EDTA, and anions on heavy metal toxicity toward a bioluminescent cyanobacterial bioreporter

The bioavailability and therefore toxicity of a metal depends on the chemical species present in a particular environment. We evaluated the effect of a series of factors that could potentially modify metal speciation on the toxicity of Hg, Cu, Zn, and Cd toward a recombinant strain of the freshwater cyanobacterium Anabaena sp. PCC 7120 with cloned lux operon of luminescent terrestrial bacterium Photorhabdus luminescens. The strain, denoted as Anabaena CPB4337, showed a high constitutive luminescence with no need to add exogenous aldehyde. The tested factors were pH, EDTA (as organic ligand), and anions PO(4)(3-), CO(3)(2-), and Cl(-). Chemical modeling and correlation analyses were used to predict metal speciation and link it with toxicity. In general, metal toxicity significantly correlated to the predicted metal free-ion concentration, although Zn-EDTA complexes and certain Hg chloro-complexes could also exhibit some toxicity to cyanobacteria. An interesting feature of metal toxicity to strain Anabaena CPB4337 was that low amounts of PO(4)(3-) and CO(3)(2-) increased metal toxicity; this effect could not be related to significant changes in metal speciation and could be attributed to a modulating effect of these anions on metal/uptake toxicity. The combination of toxicity studies that take into account a range of factors that might modulate metal toxicity with chemical modeling to predict changes in metal speciation might be useful for interpreting complex toxicity data. Finally, this cyanobacterial bioreporter, due to its ecological relevance as a primary producer, could be used as a tool for toxicity assessment in freshwater environments.

[Quantitative detection of NADH by in vitro bacterial luciferase bioluminescent]

OBJECTIVE

The study aimed at establishing the bacterial luciferase: FMN-NADH oxidoreductase bioluminescent system in vitro and evaluating its potential for quantitative detection of NADH.

METHODS

By optimizing the amount of substrates, we set up the coupled luciferase: FMN-NADH oxidoreductase bioluminescent system in vitro, based on the crude extract from Photobacterium leiognathi YL.

RESULTS

The in vitro coupled bacterial luciferase: FMN-NADH oxidoreductase bioluminescent system was: 1 mL crude extract, 27 mmol/L Dodecane 100 microL, 10 mmol/L FMN-Na 0.5 microL and 0.14 mmol/L NADH 300 microL. Furthermore, we developed a method for quantitative detection of NADH according to the bioluminescence of NADH catalyzed of bacterial luciferase: FMN-NADH oxidoreductase system in vitro. A good linear relationship of NADH concentration was in a range of 1.0 x 10(-10) to 1.0 x 10(-8) mol/L.

CONCLUSION

The bacterial luciferase: FMN-NADH oxidoreductase system used to measure NADH concentration was a good attempt to detect living bacterial cells in the fields of environment, food sanitation and other related.

Crystal structure of the bacterial luciferase/flavin complex provides insight into the function of the beta subunit

Bacterial luciferase from Vibrio harveyi is a heterodimer composed of a catalytic alpha subunit and a homologous but noncatalytic beta subunit. Despite decades of enzymological investigation, structural evidence defining the active center has been elusive. We report here the crystal structure of V. harveyi luciferase bound to flavin mononucleotide (FMN) at 2.3 A. The isoalloxazine ring is coordinated by an unusual cis-Ala-Ala peptide bond. The reactive sulfhydryl group of Cys106 projects toward position C-4a, the site of flavin oxygenation. This structure also provides the first data specifying the conformations of a mobile loop that is crystallographically disordered in both prior crystal structures [(1995) Biochemistry 34, 6581-6586; (1996) J. Biol. Chem. 271, 21956 21968]. This loop appears to be a boundary between solvent and the active center. Within this portion of the protein, a single contact was observed between Phe272 of the alpha subunit, not seen in the previous structures, and Tyr151 of the beta subunit. Substitutions at position 151 on the beta subunit caused reductions in activity and total quantum yield. Several of these mutants were found to have decreased affinity for reduced flavin mononucleotide (FMNH(2)). These findings partially address the long-standing question of how the beta subunit stabilizes the active conformation of the alpha subunit, thereby participating in the catalytic mechanism.

Rapid detection of tetracyclines and their 4-epimer derivatives from poultry meat with bioluminescent biosensor bacteria

Tetracycline (TC) specific luminescent bacterial biosensors were used in a rapid TC residue assay sensitized to meet the EU maximum residue limit (MRL) for TC residues in poultry muscle tissue (100 microg kg(-1)) by membrane-permeabilizing and chelating agents polymyxin B and EDTA. Sensitivities of 5 ng g(-1) for doxycycline, 7.5 ng g(-1) for chlortetracycline, and 25 ng g(-1) for tetracycline and oxytetracycline were reached. Except for doxycycline, the MRLs of these tetracyclines include their 4-epimer metabolites. In the biosensor assay, all four 4-epimers showed induction capacity and antimicrobial activity, and antimicrobial activity was also observed in the inhibition assay, although with lower efficiency than that of the corresponding parent compound in both assays. The biosensor assay is an inexpensive and rapid high-throughput screening method for the detection of 4-epimer TC residues along with their parent compounds.

[Construction of recombinate luminescence bacteria vector to evaluate the genetoxic of environment pollutant]

Recombinate luminescence bacteria have the important role in evaluating water toxicity. Two recombinate luminescence bacteria vectors PUCD-uvrA and PUCD-alkA were constructed to investigate the impaired mechanism of pollutant genetic toxicity. The genes of uvrA and alkA were amplified by PCR from E. coli W3110, sequenced after ligated with pGEM-T easy vector. The PCR products and PUCD615 vector were all digested with BamH I, EcoR I, then be linked and imported into JM109 with electrotransformation. Several clones were selected and identificated by PCR and sequencing. The results reviewed that the length of the uvrA and the alkA fragments were 237 bp, 326 bp. When they were sequenced and blasted in GenBank, the homology of sequences reached 99% indicated the amplified results correct. The results of sequencing ligated with PUCD615 reviewed that the fragments of uvrA and alkA had been inserted into the multiple clone site correctly, the insert direction and reading frame were also exactly. Optimizing the condition of ligation and transformation, the large fragment of PUCD615 and the short inserted sequences can been ligated successfully.

[Effect of salts on luminescence of natural and recombinant luminescent bacterial biosensors]

Effect of cations K+, Na+, Mg2+, and Ca2+ and anions SO4(2-), HCO3(-), and CO3(2-) on the luminescence intensity of the marine luminescent bacterium Photobacterium phorphoreum (Microbiosensor B-17 677f) and the recombinant strain Escherichia coli with cloned lux operon of P. leiognathi (Ekolyum-9). It is found that small concentrations of chlorides and sulfates of the cations studied had a concentration-dependent stimulatory effect on bacterial bioluminescence; as the concentration of agents increased, activation was succeeded by quenching. The strength of the inhibitory effect, which is characterized by EC50, decreased in the series Ca2+ > Na+ > Mg2+ > K+. Carbonates and hydrocarbonates had a pronounced inhibitory effect on the bioluminescence intensity, determined by an increase in pH. We showed that some types of highly mineralized water with a high hydrocarbonate content have a marked inhibitory effect on the luminescence intensity of microbial luminescent biosensors, mimicking the effect of chemical pollutants.

Real-Time Monitoring of Single Bacterium Lysis and Leakage Events by Chemiluminescence Microscopy

The small size of bacteria makes it difficult to study the biochemistry inside single cells. The amount of material inside is limited; therefore, an ultrasensitive method is required to interrogate single cells. Using a sensitive ICCD detector to record chemiluminescence (CL) from an optimized firefly luciferase-ATP bioluminescence reaction system, we report for the first time real-time imaging of lysis and leakage of single bacterium with 10-s temporal resolution. Movies are generated to visualize how the cell membrane was damaged by phage lysis, antibiotics attack, or dehydration, as well as the wall repair and cell recovery processes. The results show single-cell variations that are not obtainable from bulk measurements, confirming that CL microscopy of luciferase-expressing bacteria is a powerful tool for studying the fundamental biology of cells.

Enhanced bacterial protein expression during auto-induction obtained by alteration of lac repressor dosage and medium composition

The auto-induction method of protein expression in E. coli is based on diauxic growth resulting from dynamic function of lac operon regulatory elements (lacO and LacI) in mixtures of glucose, glycerol, and lactose. The results show that successful execution of auto-induction is strongly dependent on the plasmid promoter and repressor construction, on the oxygenation state of the culture, and on the composition of the auto-induction medium. Thus expression hosts expressing high levels of LacI during aerobic growth exhibit reduced ability to effectively complete the auto-induction process. Manipulation of the promoter to decrease the expression of LacI altered the preference for lactose consumption in a manner that led to increased protein expression and partially relieved the sensitivity of the auto-induction process to the oxygenation state of the culture. Factorial design methods were used to optimize the chemically defined growth medium used for expression of two model proteins, Photinus luciferase and enhanced green fluorescent protein, including variations for production of both unlabeled and selenomethionine-labeled samples. The optimization included studies of the expression from T7 and T7-lacI promoter plasmids and from T5 phage promoter plasmids expressing two levels of LacI. Upon the basis of the analysis of over 500 independent expression results, combinations of optimized expression media and expression plasmids that gave protein yields of greater than 1000 mug/mL of expression culture were identified.

A synthetic luxCDABE gene cluster optimized for expression in high-GC bacteria

The luxCDABE operon of the bioluminescent bacterium Photorhabdus luminescens has proven to be a superb transcriptional reporter. It encodes a luciferase (LuxA and LuxB) and the enzymes that produce its substrate (LuxC, LuxD and LuxE) so cells that express the cluster emit the 490-nm light spontaneously. The sequence of these genes is AT-rich (>69%) and for this and other reasons, they are not expressed efficiently in high-GC bacteria like Streptomyces coelicolor. We therefore constructed a synthetic luxCDABE operon encoding the P. luminescens Lux proteins optimized for expression in high-GC bacteria. We tested the genes using transcriptional fusions to S. coelicolor promoters having well-established expression profiles during this organism's life cycle. The hrdB gene encodes a housekeeping sigma factor; while ramC is important for the formation of the spore-forming cells called aerial hyphae and whiE is required for the production of a grey, spore-associated pigment that is deposited in the walls of developing spores. Using these fusions we demonstrated that our synthetic lux genes are functional in S. coelicolor and that they accurately report complex developmental gene expression patterns. We suggest that this lux operon and our procedure for generating synthetic high-GC genes will be widely useful for research on high-GC bacteria.

A novel lux operon in the cryptically bioluminescent fish pathogen Vibrio salmonicida is associated with virulence

The cold-water-fish pathogen Vibrio salmonicida expresses a functional bacterial luciferase but produces insufficient levels of its aliphatic-aldehyde substrate to be detectably luminous in culture. Our goals were to (i) better explain this cryptic bioluminescence phenotype through molecular characterization of the lux operon and (ii) test whether the bioluminescence gene cluster is associated with virulence. Cloning and sequencing of the V. salmonicida lux operon revealed that homologs of all of the genes required for luminescence are present: luxAB (luciferase) and luxCDE (aliphatic-aldehyde synthesis). The arrangement and sequence of these structural lux genes are conserved compared to those in related species of luminous bacteria. However, V. salmonicida strains have a novel arrangement and number of homologs of the luxR and luxI quorum-sensing regulatory genes. Reverse transcriptase PCR analysis suggests that this novel arrangement of quorum-sensing genes generates antisense transcripts that may be responsible for the reduced production of bioluminescence. In addition, infection with a strain in which the luxA gene was mutated resulted in a marked delay in mortality among Atlantic salmon relative to infection with the wild-type parent in single-strain challenge experiments. In mixed-strain competition between the luxA mutant and the wild type, the mutant was attenuated up to 50-fold. It remains unclear whether the attenuation results from a direct loss of luciferase or a polar disturbance elsewhere in the lux operon. Nevertheless, these findings document for the first time an association between a mutation in a structural lux gene and virulence, as well as provide a new molecular system to study Vibrio pathogenesis in a natural host.

Detection of rhythmic bioluminescence from luciferase reporters in cyanobacteria

The unicellular cyanobacterium Synechococcus elongatus PCC 7942 is the model organism for studying prokaryotic circadian rhythms. Although S. elongatus does not display an easily measurable overt circadian behavior, its gene expression is under circadian control; hence, a "behavior" is created by linking a cyanobacterial promoter to either the bacterial luxAB or firefly luc luciferase genes to create reporter fusions whose activity can be easily monitored by bioluminescence. Numerous vectors have been created in our lab for introducing luciferase reporter genes into the S. elongatus chromosome. A choice of methods and equipment to detect light production from the luciferase fusions provides a means for high-throughput, automated mutant screens as well as testing rhythms from two promoter fusions within the same cell culture.

Bioluminescence-based bioassays for rapid detection of nisin in food

We have developed a method for determining ultralow amounts of nisin in food samples that is based on luminescent biosensor bacteria. Modified bacterial luciferase operon luxABCDE was placed under control of the nisin-inducible nisA promoter in plasmid pNZ8048, and the construct was transformed into Lactococcus lactis strains NZ9800 and NZ9000. The nisRK genes of these strains allow them to sense nisin and relay the signal to initiate transcription from nisA promoter. The resulting luminescence can be directly measured from living bacteria without the addition of exogenous substrates. Induction leads to detectable luminescence within ten minutes. Lyophilization of the biosensor cells produced viable and inducible sensor elements that can be utilized as freshly cultivated cells for rapid detection of nisin. The linear dose-response relationship perceived in the assay facilitates quantification of nisin in samples. The sensitivity of the nisin bioassay was 0.1 pg/ml in pure solution and 3 pg/ml in milk, exceeding the performance of all previously reported methods.

Genetic labelling and application of the isoproturon-mineralizing Sphingomonas sp. strain SRS2 in soil and rhizosphere

AIMS

To construct a luxAB-labelled Sphingomonas sp. strain SRS2 maintaining the ability to mineralize the herbicide isoproturon and usable for monitoring the survival and distribution of strain SRS2 on plant roots in laboratory systems.

METHODS AND RESULTS

We inserted the mini-Tn5-luxAB marker into strain SRS2 using conjugational mating. In the transconjugant mutants luciferase was produced in varying levels. The mutants showed significant differences in their ability to degrade isoproturon. One luxAB-labelled mutant maintained the ability to mineralize isoproturon and was therefore selected for monitoring colonization of barley roots.

CONCLUSIONS

We successfully constructed a genetically labelled isoproturon-mineralizing-strain SRS2 and demonstrated its ability to survive in soil and its colonization of rhizosphere.

SIGNIFICANCE AND IMPACT OF THE STUDY

The construction of a luxAB-labelled strain SRS2 maintaining the degradative ability, provides a powerful tool for ecological studies serving as the basis for evaluating SRS2 as a bioremediation agent.

Construction and testing of a bacterial luciferase reporter gene system for in vivo measurement of nonsense suppression in Streptomyces

A reporter gene system, based on luciferase genes from Vibrio harvei, was constructed for measurement of translation nonsense suppression in Streptomyces. Using the site-directed mutagenesis the TCA codon in position 13 of the luxB gene was replaced by all of the three stop codons individually. By cloning of luxA and luxB genes under the control of strong constitutive Streptomyces promoter ermE* in plasmid pUWL201 we created Wluxl with the wild-type sequence and pWlux2, pWlux3 and pWlux4 plasmids containing TGA-, TAG- and TAA-stop codons, respectively. Streptomyces lividans TK 24 was transformed with the plasmids and the reporter system was tested by growth of the strain in the presence of streptomycin as a translation accuracy modulator. Streptomycin increased nonsense suppression on UAA nearly 10-fold and more than 20-fold on UAG. On the other hand, UGA, the most frequent stop signal in Streptomyces, the effect was negligible.

Bacterial luciferase activity and the intracellular redox pool in Escherichia coli

In this study, we analyzed the activity of a bacterial luciferase (LuxAB of Vibrio fischeri) expressed under the control of a consensus-type promoter, lacUV5, in Escherichia coli, and found that activity declines abruptly upon entry into the stationary growth phase. Since this decline was reproducibly observed in strains cultured in various growth media, we refer to this phenomenon as ADLA (Abrupt Decline of Luciferase Activity) and define the time point when activity begins to decline as T (0). Because the levels of luciferase proteins (LuxA and LuxB) remained constant before and after T (0), ADLA cannot be due to the repression of luciferase gene expression. Further analyses suggested that a decline in the supply of intracellular reducing power for luciferase was responsible for ADLA. We also found that ADLA was alleviated or did not occur in several mutants deficient in nucleoid proteins, suggesting that ADLA is a genetically controlled process involved in intracellular redox flow.

Transactivation of the epidermal growth factor receptor by cag+ Helicobacter pylori induces upregulation of the early growth response gene Egr-1 in gastric epithelial cells

BACKGROUND AND AIMS

Helicobacter pylori, in particular cytotoxin associated gene (cag)+ strains, have been shown to enhance gastric epithelial cell proliferation in vivo, an effect that likely contributes to gastric carcinogenesis. Early growth response gene 1 (Egr-1) is a crucial regulator of cell growth, differentiation, and survival, which is known to play a role in carcinogenesis and cancer progression. The aims of this study were to: (1) examine whether H pylori could upregulate Egr-1 in gastric epithelial cell lines; (2) determine whether there was a differential response to infection with different strains; (3) examine the role of the cag pathogenicity island in this process; and (4) elucidate the molecular mechanisms leading to Egr-1 upregulation.

METHODS AND RESULTS

We found that infection of AGS cells with cag+H pylori resulted in a rapid (1-2 hours) but transient increase in Egr-1 mRNA and protein levels whereas coculture with cag- isolates did not elicit this response. Furthermore, two independent cagE- isogenic mutants of H pylori also demonstrated impaired ability to upregulate Egr-1. Upregulation of Egr-1 protein was inhibited by the extracellular regulated kinase (ERK)1/2 inhibitor PD98059 and overexpression of dominant negative MEK1 downregulated Egr-1 luciferase reporter gene activity. Treatment of AGS cells with the epidermal growth factor receptor (EGFR) kinase inhibitors PD153035 and AG1478 resulted in a reduction in H pylori mediated Egr-1 upregulation, demonstrating that EGFR transactivation plays a role in this early cellular process.

CONCLUSIONS

Our findings show that cag+H pylori cause rapid induction of Egr-1 in gastric epithelial cells which may contribute to H pylori mediated pathogenesis.

Active Site Hydrophobicity Is Critical to the Bioluminescence Activity of Vibrio harveyi Luciferase

Vibrio harveyi luciferase is an alphabeta heterodimer containing a single active site, proposed earlier to be at a cleft in the alpha subunit. In this work, six conserved phenylalanine residues at this proposed active site were subjected to site-directed mutations to investigate their possible functional roles and to delineate the makeup of luciferase active site. After initial screening of Phe --> Ala mutants, alphaF46, alphaF49, alphaF114, and alphaF117 were chosen for additional mutations to Asp, Ser, and Tyr. Comparisons of the general kinetic properties of wild-type and mutated luciferases indicated that the hydrophobic nature of alphaF46, alphaF49, alphaF114, and alphaF117 was important to luciferase V(max) and V(max)/K(m), which were reduced by 3-5 orders of magnitude for the Phe --> Asp mutants. Both alphaF46 and alphaF117 also appeared to be involved in the binding of reduced flavin substrate. Additional studies on the stability and yield of the 4a-hydroperoxyflavin intermediate II and measurements of decanal substrate oxidation by alphaF46D, alphaF49D, alphaF114D, and alphaF117D revealed that their marked reductions in the overall quantum yield (phi( degrees )) were a consequence of diminished yields of luciferase intermediates and, with the exception of alphaF114D, emission quantum yield of the excited emitter due to the replacement of the hydrophobic Phe by the anionic Asp. The locations of these four critical Phe residues in relation to other essential and/or hydrophobic residues are depicted in a refined map of the active site. Functional implications of these residues are discussed.

Implication of a critical residue (Glu175) in structure and function of bacterial luciferase

Structural properties of a bacterial luciferase mutant, evolved by random mutagenesis, have been investigated. Bacterial luciferases (LuxAB) can be readily classed as slow or fast decay luciferases based on their rates of luminescence decay in a single turnover assay. By random mutagenesis, one of the mutants generated by a single mutation on LuxA at position 175 (E175G) resulted in the "slow decay" Xenorhabdus luminescens luciferase was converted into a luciferase with a significantly more rapid decay rate [Hosseinkhani, S., Szittner, R. and Meighen, E.A. (2005) Biochemical Journal 385, 575-580]. A single mutation (E175G), in a loop that connects alpha helix 5 and beta sheet 5 brought about changes in the kinetic and structural properties of the enzyme. Enhancement of tryptophan fluorescence was observed with a lower degree of fluorescence quenching by acrylamide upon mutation. Near- and far-UV circular dichroism spectra of the native and mutant forms suggested formation of an intermediate structure, further supported by 8-anilino-1-naphthalene-sulphonic acid (ANS) fluorescence which indicated lower exposure of hydrophobic residues as a result of mutation. Fluorescence quenching studies utilizing acrylamide indicated a more accessible fluor for the native form. Thus, the E175G point mutation appears to change the enzymatic decay rate by inducing a substantial tertiary structural change, without a large effect on secondary structural elements, as revealed by Fourier transform IR spectroscopy. Overall, the mutation caused structural changes that go beyond the simple change in orientation of Glu175.

Random mutagenesis of bacterial luciferase: critical role of Glu175 in the control of luminescence decay

Bacterial luciferases (LuxAB) can be readily classed as slow or fast decay luciferases based on their rates of luminescence decay in a single turnover assay. Luciferases from Vibrio harveyi and Xenorhabdus (Photorhabdus) luminescens have slow decay rates, and those from the Photobacterium genus, such as Photobacterium fisheri, P. phosphoreum and P. leiognathi, have rapid decay rates. By substitution of a 67-amino-acid stretch of P. phosphoreum LuxA in the central region of the LuxA subunit, the 'slow' X. luminescens luciferase was converted into a chimaeric luciferase with a significantly more rapid decay rate [Valkova, Szittner and Meighen (1999) Biochemistry 38, 13820-13828]. To understand better the role of specific residues in the classification of luciferases as slow and fast decay, we have conducted random mutagenesis on this region. One of the mutants generated by a single mutation on LuxA at position 175 [E175G (Glu175-->Gly)] resulted in the 'slow decay' X. luminescens luciferase being converted into a luciferase with a significantly more rapid decay rate. These results indicate the importance of Glu175 in LuxA as a critical residue for differentiating between 'slow' and 'fast' luciferases and show that this distinction is primarily due to differences in aldehyde affinity and in the decomposition of the luciferase-flavin-oxygen intermediate.

A glimpse into the expanded genome content of Vibrio cholerae through identification of genes present in environmental strains

Vibrio cholerae has multiple survival strategies which are reflected both in its broad distribution in many aquatic environments and its high genotypic diversity. To obtain additional information regarding the content of the V. cholerae genome, suppression subtractive hybridization (SSH) was used to prepare libraries of DNA sequences from two southern California coastal isolates which are divergent or absent in the clinical strain V. cholerae O1 El Tor N16961. More than 1,400 subtracted clones were sequenced. This revealed the presence of novel sequences encoding functions related to cell surface structures, transport, metabolism, signal transduction, luminescence, mobile elements, stress resistance, and virulence. Flanking sequence information was determined for loci of interest, and the distribution of these sequences was assessed for a collection of V. cholerae strains obtained from southern California and Mexican environments. This led to the surprising observation that sequences related to the toxin genes toxA, cnf1, and exoY are widespread and more common in these strains than those of the cholera toxin genes which are a hallmark of the pandemic strains of V. cholerae. Gene transfer among these strains could be facilitated by a 4.9-kbp plasmid discovered in one isolate, which possesses similarity to plasmids from other environmental vibrios. By investigating some of the nucleotide sequence basis for V. cholerae genotypic diversity, DNA fragments have been uncovered which could promote survival in coastal environments. Furthermore, a set of genes has been described which could be involved in as yet undiscovered interactions between V. cholerae and eukaryotic organisms.

Sensitivity of dark mutants of various strains of luminescent bacteria to reactive oxygen species

Recent studies indicated that bioluminescence of the marine bacterium Vibrio harveyi may both stimulate DNA repair and contribute to detoxification of deleterious oxygen derivatives. Therefore, it was also proposed that these reactions can be considered biological roles of bacterial luminescence and might act as evolutionary drives in development of luminous systems. However, experimental evidence for the physiological role of luciferase in protection of cells against oxidative stress has been demonstrated only in one bacterial species, raising the question whether this is a specific or a more general phenomenon. Here we demonstrate that in the presence of various oxidants (hydrogen peroxide, cumene hydroperoxide, t-butyl hydroperoxide and ferrous ions) growth of dark mutants of different strains of Vibrio fischeri and Photobacterium leiognathi is impaired relative to wild-type bacteria, though to various extents. Deleterious effects of oxidants on the mutants could be reduced (with different efficiency) by addition of antioxidants, A-TEMPO or 4OH-TEMPO. These results support the hypotheses that (1) activities of bacterial luciferases may detoxify deleterious oxygen derivatives, and (2) significantly different efficiencies of this reaction are characteristic for various luciferases.

[Effect of glucose on luciferase expression in Photobacterium leiognathi]

Photobacterium leiognathi cultured in marine broth emits a luminescence that is temporarily enhanced and then extinguished by glucose. Glucose reduces the luciferase level and the expression of lux ABG mRNA in P. leiognathi. The amount of ATP in P. leiognathi is temporarily increased and then declines to the normal level. These results indicate that the extinguishing by glucose in P. leiognathi is induced by the interruption of the translation of luciferase.

Detection of ViableMycobacterium aviumSubsp.ParatuberculosisUsing Luciferase Reporter Systems

Plasmid- and phage-based firefly luciferase reporter constructs were evaluated as rapid detection systems for viable Mycobacterium avium subsp. paratuberculosis (MAP). A MAP strain bearing a luciferase-encoding plasmid was detectable at 100 cells/mL in skim milk and 1000 cells/mL in whole milk. Three luciferase-encoding mycobacteriophage were evaluated for detection of wild-type MAP. The best of these, phAE85, allowed detection of >1000 cells/mL within 24-48 h. Membrane filtration did not improve the sensitivity of detection for either plasmid or phage reporters. Luciferase reporters show promise for rapid detection of viable MAP.

Detection of alkanes, alcohols, and aldehydes using bioluminescence

We report a novel method for the rapid, sensitive, and quantitative detection of alkanes, alcohols, and aldehydes that relies on the reaction of bacterial luciferase with an aldehyde, resulting in the emission of light. Primary alcohols with corresponding aldehydes that are within the substrate range of the particular luciferase are detected after conversion to the aldehyde by an alcohol dehydrogenase. In addition, alkanes themselves may be detected by conversion to primary alcohols by an alkane hydroxylase, followed by conversion to the aldehyde by alcohol dehydrogenase. We developed a rapid bioluminescent method by genetically engineering the genes encoding bacterial luciferase, alcohol dehydrogenase, and alkane hydroxylase into a plasmid for simultaneous expression in an E. coli host cell line. Alkanes, alcohols, or aldehydes were detected within seconds, with sensitivity in the micromolar range, by measuring the resulting light emission with a microplate reader. We demonstrate the application of this method for the detection of alkanes, alcohols, and aldehydes and for the detection of alkane hydroxylase and alcohol dehydrogenase activity in vivo. This method is amenable to the high-throughput screening needs required for the identification of novel catalysts.