Reporter proteins for in vivo fluorescence without oxygen

Fluorescent reporter proteins such as green fluorescent protein are valuable noninvasive molecular tools for in vivo real-time imaging of living specimens. However, their use is generally restricted to aerobic systems, as the formation of their chromophores strictly requires oxygen. Starting with blue-light photoreceptors from Bacillus subtilis and Pseudomonas putida that contain light-oxygen-voltage-sensing domains, we engineered flavin mononucleotide-based fluorescent proteins that can be used as fluorescent reporters in both aerobic and anaerobic biological systems.

Real-time determination of intracellular oxygen in bacteria using a genetically encoded FRET-based biosensor

Background

Molecular oxygen (O₂) is one of the key metabolites of all obligate and facultative aerobic pro- and eukaryotes. It plays a fundamental role in energy homeostasis whereas oxygen deprivation, in turn, broadly affects various physiological and pathophysiological processes. Therefore, real-time monitoring of cellular oxygen levels is basically a prerequisite for the analysis of hypoxia-induced processes in living cells and tissues.

Results

We developed a genetically encoded Förster resonance energy transfer (FRET)-based biosensor allowing the observation of changing molecular oxygen concentrations inside living cells. This biosensor named FluBO (fluorescent protein-based biosensor for oxygen) consists of the yellow fluorescent protein (YFP) that is sensitive towards oxygen depletion and the hypoxia-tolerant flavin-binding fluorescent protein (FbFP). Since O₂ is essential for the formation of the YFP chromophore, efficient FRET from the FbFP donor domain to the YFP acceptor domain only occurs in the presence but not in the absence of oxygen. The oxygen biosensor was used for continuous real-time monitoring of temporal changes of O₂ levels in the cytoplasm of Escherichia coli cells during batch cultivation.

Conclusions

FluBO represents a unique FRET-based oxygen biosensor which allows the non-invasive ratiometric readout of cellular oxygen. Thus, FluBO can serve as a novel and powerful probe for investigating the occurrence of hypoxia and its effects on a variety of (patho)physiological processes in living cells.

The photophysics of LOV-based fluorescent proteins – new tools for cell biology

In this study photophysical characteristics of LOV-based fluorescent proteins which are essential for analytic methods as well as imaging approaches have been comparatively analyzed in detail.

A Calcium-gated Lid and a Large β-Roll Sandwich Are Revealed by the Crystal Structure of Extracellular Lipase from Serratia marcescens

Lipase LipA from Serratia marcescens is a 613-amino acid enzyme belonging to family I.3 of lipolytic enzymes that has an important biotechnological application in the production of a chiral precursor for the coronary vasodilator diltiazem. Like other family I.3 lipases, LipA is secreted by Gram-negative bacteria via a type I secretion system and possesses 13 copies of a calcium binding tandem repeat motif, GGXGXDXUX (U, hydrophobic amino acids), in the C-terminal part of the polypeptide chain. The 1.8-A crystal structure of LipA reveals a close relation to eukaryotic lipases, whereas family I.1 and I.2 enzymes appear to be more distantly related. Interestingly, the structure shows for the N-terminal lipase domain a variation on the canonical alpha/beta hydrolase fold in an open conformation, where the putative lid helix is anchored by a Ca(2+) ion essential for activity. Another novel feature observed in this lipase structure is the presence of a helical hairpin additional to the putative lid helix that exposes a hydrophobic surface to the aqueous medium and might function as an additional lid. The tandem repeats form two separated parallel beta-roll domains that pack tightly against each other. Variations of the consensus sequence of the tandem repeats within the second beta-roll result in an asymmetric Ca(2+) binding on only one side of the roll. The analysis of the properties of the beta-roll domains suggests an intramolecular chaperone function.

Role of GlnB and GlnK in ammonium control of both nitrogenase systems in the phototrophic bacterium Rhodobacter capsulatus

In most bacteria, nitrogen metabolism is tightly regulated and P(II) proteins play a pivotal role in the regulatory processes. Rhodobacter capsulatus possesses two genes (glnB and glnK) encoding P(II)-like proteins. The glnB gene forms part of a glnB-glnA operon and the glnK gene is located immediately upstream of amtB, encoding a (methyl-) ammonium transporter. Expression of glnK is activated by NtrC under nitrogen-limiting conditions. The synthesis and activity of the molybdenum and iron nitrogenases of R. capsulatus are regulated by ammonium on at least three levels, including the transcriptional activation of nifA1, nifA2 and anfA by NtrC, the regulation of NifA and AnfA activity by two different NtrC-independent mechanisms, and the post-translational control of the activity of both nitrogenases by reversible ADP-ribosylation of NifH and AnfH as well as by ADP-ribosylation independent switch-off. Mutational analysis revealed that both P(II)-like proteins are involved in the ammonium regulation of the two nitrogenase systems. A mutation in glnB results in the constitutive expression of nifA and anfA. In addition, the post-translational ammonium inhibition of NifA activity is completely abolished in a glnB-glnK double mutant. However, AnfA activity was still suppressed by ammonium in the glnB-glnK double mutant. Furthermore, the P(II)-like proteins are involved in ammonium control of nitrogenase activity via ADP-ribosylation and the switch-off response. Remarkably, in the glnB-glnK double mutant, all three levels of the ammonium regulation of the molybdenum (but not of the alternative) nitrogenase are completely circumvented, resulting in the synthesis of active molybdenum nitrogenase even in the presence of high concentrations of ammonium.

A microfluidic co-cultivation platform to investigate microbial interactions at defined microenvironments

Interspecies interactions inside microbial communities bear a tremendous diversity of complex chemical processes that are by far not understood. Even for simplified, often synthetic systems, the interactions between two microbes are barely revealed in detail. Here, we present a microfluidic co-cultivation platform for the analysis of growth and interactions inside microbial consortia with single-cell resolution. Our device allows the spatial separation of two different microbial organisms inside adjacent microchambers facilitating sufficient exchange of metabolites via connecting nanochannels. Inside the cultivation chambers cell growth can be observed with high spatio-temporal resolution by live-cell imaging. In contrast to conventional approaches, in which single-cell activity is typically fully masked by the average bulk behavior, the small dimensions of the microfluidic cultivation chambers enable accurate environmental control and observation of cellular interactions with full spatio-temporal resolution. Our method enables one to study phenomena in microbial interactions, such as gene transfer or metabolic cross-feeding. We chose two different microbial model systems to demonstrate the wide applicability of the technology. First, we investigated commensalistic interactions between an industrially relevant l-lysine-producing Corynebacterium glutamicum strain and an l-lysine auxotrophic variant of the same species. Spatially separated co-cultivation of both strains resulted in growth of the auxotrophic strain due to secreted l-lysine supplied by the producer strain. As a second example we investigated bacterial conjugation between Escherichia coli S17-1 and Pseudomonas putida KT2440 cells. We could show that direct cell contact is essential for the successful gene transfer via conjugation and was hindered when cells were spatially separated. The presented device lays the foundation for further studies on contactless and contact-based interactions of natural and synthetic microbial communities.

Efficient recombinant production of prodigiosin in Pseudomonas putida

Serratia marcescens and several other bacteria produce the red-colored pigment prodigiosin which possesses bioactivities as an antimicrobial, anticancer, and immunosuppressive agent. Therefore, there is a great interest to produce this natural compound. Efforts aiming at its biotechnological production have so far largely focused on the original producer and opportunistic human pathogen S. marcescens. Here, we demonstrate efficient prodigiosin production in the heterologous host Pseudomonas putida. Random chromosomal integration of the 21 kb prodigiosin biosynthesis gene cluster of S. marcescens in P. putida KT2440 was employed to construct constitutive prodigiosin production strains. Standard cultivation parameters were optimized such that titers of 94 mg/L culture were obtained upon growth of P. putida at 20°C using rich medium under high aeration conditions. Subsequently, a novel, fast and effective protocol for prodigiosin extraction and purification was established enabling the straightforward isolation of prodigiosin from P. putida growth medium. In summary, we describe here a highly efficient method for the heterologous biosynthetic production of prodigiosin which may serve as a basis to produce large amounts of this bioactive natural compound and may provide a platform for further in-depth studies of prodiginine biosynthesis.

Alternative hosts for functional (meta)genome analysis

Microorganisms are ubiquitous on earth, often forming complex microbial communities in numerous different habitats. Most of these organisms cannot be readily cultivated in the laboratory using standard media and growth conditions. However, it is possible to gain access to the vast genetic, enzymatic, and metabolic diversity present in these microbial communities using cultivation-independent approaches such as sequence- or function-based metagenomics. Function-based analysis is dependent on heterologous expression of metagenomic libraries in a genetically amenable cloning and expression host. To date, Escherichia coli is used in most cases; however, this has the drawback that many genes from heterologous genomes and complex metagenomes are expressed in E. coli either at very low levels or not at all. This review emphasizes the importance of establishing alternative microbial expression systems consisting of different genera and species as well as customized strains and vectors optimized for heterologous expression of membrane proteins, multigene clusters encoding protein complexes or entire metabolic pathways. The use of alternative host-vector systems will complement current metagenomic screening efforts and expand the yield of novel biocatalysts, metabolic pathways, and useful metabolites to be identified from environmental samples.

A novel T7 RNA polymerase dependent expression system for high-level protein production in the phototrophic bacterium Rhodobacter capsulatus

The functional expression of heterologous genes using standard bacterial expression hosts such as Escherichia coli is often limited, e.g. by incorrect folding, assembly or targeting of recombinant proteins. Consequently, alternative bacterial expression systems have to be developed to provide novel strategies for protein synthesis exceeding the repertoire of the standard expression host E. coli. Here, we report on the construction of a novel expression system that combines the high processivity of T7 RNA polymerase with the unique physiological properties of the facultative photosynthetic bacterium Rhodobacter capsulatus. This system basically consists of a recombinant R. capsulatus T7 expression strain (R. capsulatus B10S-T7) harboring the respective polymerase gene under control of a fructose inducible promoter. In addition, a set of different broad-host-range vectors (pRho) was constructed allowing T7 RNA polymerase dependent and independent target gene expression in R. capsulatus and other Gram-negative bacteria. The expression efficiency of the novel system was studied in R. capsulatus and E. coli using the yellow fluorescent protein (YFP) as model protein. Expression levels were comparable in both expression hosts and yielded up to 80mg/l YFP in phototrophically grown R. capsulatus cultures. This result clearly indicates that the novel R. capsulatus-based expression system is well suited for the high-level expression of soluble proteins.

TREX: a universal tool for the transfer and expression of biosynthetic pathways in bacteria

Secondary metabolites represent a virtually inexhaustible source of natural molecules exhibiting a high potential as pharmaceuticals or chemical building blocks. To gain broad access to these compounds, sophisticated expression systems are needed that facilitate the transfer and expression of large chromosomal regions, whose genes encode complex metabolic pathways. Here, we report on the development of the novel system for the transfer and expression of biosynthetic pathways (TREX), which comprises all functional elements necessary for the delivery and concerted expression of clustered pathway genes in different bacteria. TREX employs (i) conjugation for DNA transfer, (ii) randomized transposition for its chromosomal insertion, and (iii) T7 RNA polymerase for unimpeded bidirectional gene expression. The applicability of the TREX system was demonstrated by establishing the biosynthetic pathways of two pigmented secondary metabolites, zeaxanthin and prodigiosin, in bacteria with different metabolic capacities. Thus, TREX represents a valuable tool for accessing natural products by allowing comparative expression studies with clustered genes.

Mutual exchange of kinetic properties by extended mutagenesis in two short LOV domain proteins from Pseudomonas putida

We previously characterized a LOV protein PpSB2-LOV, present in the common soil bacterium Pseudomonas putida, that exhibits a plant phototropin LOV-like photochemistry [Krauss, U., Losi, A., Gartner, W., Jaeger, K. E., and Eggert, T. (2005) Phys. Chem. Chem. Phys. 7, 2804-2811]. Now, we have identified a second LOV homologue, PpSB1-LOV, found in the same organism with approximately 66% identical amino acids. Both proteins consist of a conserved LOV core flanked by short N- and C-terminal extensions but lack a fused effector domain. Although both proteins are highly similar in sequence, they display drastically different dark recovery kinetics. At 20 degrees C, PpSB2-LOV reverts with an average time constant of 137 s from the photoequilibrium to the dark state, whereas PpSB1-LOV exhibits an average dark recovery time constant of 1.48 x 10(5) s. Irrespective of the significant differences in their dark recovery behavior, both proteins showed nearly identical kinetics for the photochemically induced adduct formation. In order to elucidate the structural and mechanistic basis of these extremely different dark recovery time constants, we performed a mutational analysis. Six amino acids in a distance of up to 6 A from the flavin chromophore, which differ between the two proteins, were identified and interchanged by site-directed mutagenesis. The amino acid substitution R66I located near the FMN phosphate in LOV domains was identified in PpSB1-LOV to accelerate the dark recovery by 2 orders of magnitude. Vice versa, the corresponding substitution I66R slowed down the dark recovery in PpSB2-LOV by a factor of 10. Interestingly, the interchange of the C-terminal extensions between the two proteins also had a pronounced effect on the dark recovery time constants, thus highlighting a coupling of these protein regions to the chromophore binding pocket.

Structural basis for the slow dark recovery of a full-length LOV protein from Pseudomonas putida

Blue-light photoreceptors containing light–oxygen–voltage (LOV) domains regulate a myriad of different physiological responses in both eukaryotes and prokaryotes. Their light sensitivity is intricately linked to the photochemistry of the non-covalently bound flavin mononucleotide (FMN) chromophore that forms a covalent adduct with a conserved cysteine residue in the LOV domain upon illumination with blue light. All LOV domains undergo the same primary photochemistry leading to adduct formation; however, considerable variation is found in the lifetime of the adduct state that varies from seconds to several hours. The molecular mechanism underlying this variation among the structurally conserved LOV protein family is not well understood. Here, we describe the structural characterization of PpSB1-LOV, a very slow cycling full-length LOV protein from the Gram-negative bacterium Pseudomonas putida KT2440. Its crystal structure reveals a novel dimer interface that is mediated by N- and C-terminal auxiliary structural elements and a unique cluster of four arginine residues coordinating with the FMN-phosphate moiety. Site-directed mutagenesis of two arginines (R61 and R66) in PpSB1-LOV resulted in acceleration of the dark recovery reaction approximately by a factor of 280. The presented structural and biochemical data suggest a direct link between structural features and the slow dark recovery observed for PpSB1-LOV. The overall structural arrangement of PpSB1-LOV, together with a complementary phylogenetic analysis, highlights a common ancestry of bacterial LOV photoreceptors and Per-ARNT-Sim chemosensors.

Lights on and action! Controlling microbial gene expression by light

Light-mediated control of gene expression and thus of any protein function and metabolic process in living microbes is a rapidly developing field of research in the areas of functional genomics, systems biology, and biotechnology. The unique physical properties of the environmental factor light allow for an independent photocontrol of various microbial processes in a noninvasive and spatiotemporal fashion. This mini review describes recently developed strategies to generate photo-sensitive expression systems in bacteria and yeast. Naturally occurring and artificial photoswitches consisting of light-sensitive input domains derived from different photoreceptors and regulatory output domains are presented and individual properties of light-controlled expression systems are discussed.

Yeast two-hybrid studies on interaction of proteins involved in regulation of nitrogen fixation in the phototrophic bacterium Rhodobacter capsulatus

Rhodobacter capsulatus contains two PII-like proteins, GlnB and GlnK, which play central roles in controlling the synthesis and activity of nitrogenase in response to ammonium availability. Here we used the yeast two-hybrid system to probe interactions between these PII-like proteins and proteins known to be involved in regulating nitrogen fixation. Analysis of defined protein pairs demonstrated the following interactions: GlnB-NtrB, GlnB-NifA1, GlnB-NifA2, GlnB-DraT, GlnK-NifA1, GlnK-NifA2, and GlnK-DraT. These results corroborate earlier genetic data and in addition show that PII-dependent ammonium regulation of nitrogen fixation in R. capsulatus does not require additional proteins, like NifL in Klebsiella pneumoniae. In addition, we found interactions for the protein pairs GlnB-GlnB, GlnB-GlnK, NifA1-NifA1, NifA2-NifA2, and NifA1-NifA2, suggesting that fine tuning of the nitrogen fixation process in R. capsulatus may involve the formation of GlnB-GlnK heterotrimers as well as NifA1-NifA2 heterodimers. In order to identify new proteins that interact with GlnB and GlnK, we constructed an R. capsulatus genomic library for use in yeast two-hybrid studies. Screening of this library identified the ATP-dependent helicase PcrA as a new putative protein that interacts with GlnB and the Ras-like protein Era as a new protein that interacts with GlnK.

Genetic tools for the investigation of Roseobacter clade bacteria

BACKGROUND

The Roseobacter clade represents one of the most abundant, metabolically versatile and ecologically important bacterial groups found in marine habitats. A detailed molecular investigation of the regulatory and metabolic networks of these organisms is currently limited for many strains by missing suitable genetic tools.

RESULTS

Conjugation and electroporation methods for the efficient and stable genetic transformation of selected Roseobacter clade bacteria including Dinoroseobacter shibae, Oceanibulbus indolifex, Phaeobacter gallaeciensis, Phaeobacter inhibens, Roseobacter denitrificans and Roseobacter litoralis were tested. For this purpose an antibiotic resistance screening was performed and suitable genetic markers were selected. Based on these transformation protocols stably maintained plasmids were identified. A plasmid encoded oxygen-independent fluorescent system was established using the flavin mononucleotide-based fluorescent protein FbFP. Finally, a chromosomal gene knockout strategy was successfully employed for the inactivation of the anaerobic metabolism regulatory gene dnr from D. shibae DFL12T.

CONCLUSION

A genetic toolbox for members of the Roseobacter clade was established. This provides a solid methodical basis for the detailed elucidation of gene regulatory and metabolic networks underlying the ecological success of this group of marine bacteria.

Singlet oxygen photosensitisation by the fluorescent protein Pp2FbFP L30M, a novel derivative of Pseudomonas putida flavin-binding Pp2FbFP

The flavin-binding protein Pp2FbFP L30M shows a high singlet oxygen quantum yield.

Novel biocatalysts for white biotechnology

White Biotechnology uses microorganisms and enzymes to manufacture a large variety of chemical products. Therefore, the demand for new and useful biocatalysts is steadily and rapidly increasing. We have developed methods for the isolation of new enzyme genes, constructed novel expression systems, and optimized existing enzymes for biotechnological applications by methods of directed evolution. Furthermore, we have isolated and characterized biocatalysts relevant for the preparation of enantiopure compounds.

Discovery of the first light-dependent protochlorophyllide oxidoreductase in anoxygenic phototrophic bacteria

In all photosynthetic organisms, chlorophylls function as light-absorbing photopigments allowing the efficient harvesting of light energy. Chlorophyll biosynthesis recurs in similar ways in anoxygenic phototrophic proteobacteria as well as oxygenic phototrophic cyanobacteria and plants. Here, the biocatalytic conversion of protochlorophyllide to chlorophyllide is catalysed by evolutionary and structurally distinct protochlorophyllide reductases (PORs) in anoxygenic and oxygenic phototrophs. It is commonly assumed that anoxygenic phototrophs only contain oxygen-sensitive dark-operative PORs (DPORs), which catalyse protochlorophyllide reduction independent of the presence of light. In contrast, oxygenic phototrophs additionally (or exclusively) possess oxygen-insensitive but light-dependent PORs (LPORs). Based on this observation it was suggested that light-dependent protochlorophyllide reduction first emerged as a consequence of increased atmospheric oxygen levels caused by oxygenic photosynthesis in cyanobacteria. Here, we provide experimental evidence for the presence of an LPOR in the anoxygenic phototrophic α-proteobacterium Dinoroseobacter shibae DFL12(T). In vitro and in vivo functional assays unequivocally prove light-dependent protochlorophyllide reduction by this enzyme and reveal that LPORs are not restricted to cyanobacteria and plants. Sequence-based phylogenetic analyses reconcile our findings with current hypotheses about the evolution of LPORs by suggesting that the light-dependent enzyme of D. shibae DFL12(T) might have been obtained from cyanobacteria by horizontal gene transfer.

New Prodigiosin Derivatives Obtained by Mutasynthesis in Pseudomonas putida

The deeply red-colored natural compound prodigiosin is a representative of the prodiginine alkaloid family, which possesses bioactivities as antimicrobial, antitumor, and antimalarial agents. Various bacteria including the opportunistic human pathogen Serratia marcescens and different members of the Streptomycetaceae and Pseudoalteromonadaceae produce prodiginines. In addition, these microbes generally accumulate many structurally related alkaloids making efficient prodiginine synthesis and purification difficult and expensive. Furthermore, it is known that structurally different natural prodiginine variants display differential bioactivities. In the herein described mutasynthesis approach, 13 different derivatives of prodigiosin were obtained utilizing the GRAS (generally recognized as safe) classified strain Pseudomonas putida KT2440. Genetic engineering of the prodigiosin pathway together with incorporation of synthetic intermediates thus resulted in the formation of a so far unprecedented structural diversity of new prodiginine derivatives in P. putida. Furthermore, the formed products allow reliable conclusions regarding the substrate specificity of PigC, the final condensing enzyme in the prodigiosin biosynthesis pathway of S. marcescens. The biological activity of prodigiosin toward modulation of autophagy was preserved in prodiginine derivatives. One prodiginine derivative displayed more potent autophagy inhibitory activity than the parent compound or the synthetic clinical candidate obatoclax.

Light-responsive control of bacterial gene expression: precise triggering of thelacpromoter activity using photocaged IPTG

An optogenetic tool was established allowing for precise, gradual and homogeneous light-triggering oflac-based gene expression in a non-invasive fashion.

Natural biocide cocktails: Combinatorial antibiotic effects of prodigiosin and biosurfactants

Bacterial secondary metabolites are naturally produced to prevail amongst competitors in a shared habitat and thus represent a valuable source for antibiotic discovery. The transformation of newly discovered antibiotic compounds into effective drugs often requires additional surfactant components for drug formulation. Nature may also provide blueprints in this respect: A cocktail of two compounds consisting of the antibacterial red pigment prodigiosin and the biosurfactant serrawettin W1 is naturally produced by the bacterium Serratia marcescens, which occurs in highly competitive habitats including soil. We show here a combinatorial antibacterial effect of these compounds, but also of prodigiosin mixed with other (bio)surfactants, against the soil-dwelling bacterium Corynebacterium glutamicum taken as a model target bacterium. Prodigiosin exerted a combinatorial inhibitory effect with all tested surfactants in a disk diffusion assay which was especially pronounced in combination with N-myristoyltyrosine. Minimal inhibitory and bactericidal concentrations (MIC and MBC) of the individual compounds were 2.56 μg/mL prodigiosin and 32 μg/mL N-myristoyltyrosine, and the MIC of prodigiosin was decreased by 3 orders of magnitude to 0.005 μg/mL in the presence of 16 μg/mL N-myristoyltyrosine, indicative of synergistic interaction. Investigation of bacterial survival revealed similar combinatorial effects; moreover, antagonistic effects were observed at higher compound concentrations. Finally, the investigation of microcolony formation under combined application of concentrations just below the MBC revealed heterogeneity of responses with cell death or delayed growth. In summary, this study describes the combinatorial antibacterial effects of microbial biomolecules, which may have ecological relevance by inhibiting cohabiting species, but shall furthermore inspire drug development in the combat of infectious disease.

Flavin mononucleotide-based fluorescent proteins function in mammalian cells without oxygen requirement

Usage of the enhanced green fluorescent protein (eGFP) in living mammalian cells is limited to aerobic conditions due to requirement of oxygen during chromophore formation. Since many diseases or disease models are associated with acute or chronic hypoxia, eGFP-labeling of structures of interest in experimental studies might be unreliable leading to biased results. Thus, a chromophore yielding a stable fluorescence under hypoxic conditions is desirable. The fluorescence of flavin mononucleotide (FMN)-based fluorescent proteins (FbFPs) does not require molecular oxygen. Recently, the advantages of FbFPs for several bacterial strains and yeasts were described, specifically, their usage as a real time fluorescence marker in bacterial expression studies and their ability of chromophore formation under anaerobic conditions. Our objective was to verify if FbFPs also function in mammalian cells in order to potentially broaden the repertoire of chromophores with ones that can reliably be used in mammalian studies under hypoxic conditions. In the present study, we demonstrate for the first time, that FbFPs can be expressed in different mammalian cells, among them murine neural stem cells during proliferative and differentiated stages. Fluorescence intensities were comparable to eGFP. In contrast to eGFP, the FbFP fluorescence did not decrease when cells were exposed to defined hypoxic conditions neither in proliferating nor in differentiated cells. Thus, FbFPs can be regarded as an alternative to eGFP in studies that target cellular structures which are exposed to hypoxic conditions.