Flavin-binding fluorescent proteins (FPs) are genetically encoded in vivo reporters, which are derived from microbial and plant LOV photoreceptors. In this study, we comparatively analyzed ROS formation and light-driven antimicrobial efficacy of eleven LOV-based FPs. In particular, we determined singlet oxygen (1O2) quantum yields and superoxide photosensitization activities via spectroscopic assays and performed cell toxicity experiments in E. coli. Besides miniSOG and SOPP, which have been engineered to generate 1O2, all of the other tested flavoproteins were able to produce singlet oxygen and/or hydrogen peroxide but exhibited remarkable differences in ROS selectivity and yield. Accordingly, most LOV-FPs are potent photosensitizers, which can be used for light-controlled killing of bacteria. Furthermore, the two variants Pp2FbFP and DsFbFP M49I, exhibiting preferential photosensitization of singlet oxygen or singlet oxygen and superoxide, respectively, were shown to be new tools for studying specific ROS-induced cell signaling processes. The tested LOV-FPs thus further expand the toolbox of optogenetic sensitizers usable for a broad spectrum of microbiological and biomedical applications.

Biotechnological production in bacteria enables access to numerous valuable chemical compounds. Nowadays, advanced molecular genetic toolsets, enzyme engineering as well as the combinatorial use of biocatalysts, pathways, and circuits even bring new-to-nature compounds within reach. However, the associated substrates and biosynthetic products often cause severe chemical stress to the bacterial hosts. Species of the Pseudomonas clade thus represent especially valuable chassis as they are endowed with multiple stress response mechanisms, which allow them to cope with a variety of harmful chemicals. A built-in cell envelope stress response enables fast adaptations that sustain membrane integrity under adverse conditions. Further, effective export machineries can prevent intracellular accumulation of diverse harmful compounds. Finally, toxic chemicals such as reactive aldehydes can be eliminated by oxidation and stress-induced damage can be recovered. Exploiting and engineering these features will be essential to support an effective production of natural compounds and new chemicals. In this article, we therefore discuss major resistance strategies of Pseudomonads along with approaches pursued for their targeted exploitation and engineering in a biotechnological context. We further highlight strategies for the identification of yet unknown tolerance-associated genes and their utilisation for engineering next-generation chassis and finally discuss effective measures for pathway fine-tuning to establish stable cell factories for the effective production of natural compounds and novel biochemicals.

Diseases caused by multi-drug resistant pathogens have become a global concern. Therefore, new approaches suitable for treating these bacteria are urgently needed. In this study, we analyzed genetically encoded photosensitizers (PS) related to the green fluorescent protein (GFP) or light-oxygen-voltage (LOV) photoreceptors for their exogenous applicability as light-triggered antimicrobial agents. Depending on their specific photophysical properties and photochemistry, these PSs can produce different toxic ROS (reactive oxygen species) such as O2•− and H2O2 via type-I, as well as 1O2 via type-II reaction in response to light. By using cell viability assays and microfluidics, we could demonstrate differences in the intracellular and extracellular phototoxicity of the applied PS. While intracellular expression and exogenous supply of GFP-related PSs resulted in a slow inactivation of E. coli and pathogenic Gram-negative and Gram-positive bacteria, illumination of LOV-based PSs such as the singlet oxygen photosensitizing protein SOPP3 resulted in a fast and homogeneous killing of these microbes. Furthermore, our data indicate that the ROS type and yield as well as the localization of the applied PS protein can strongly influence the antibacterial spectrum and efficacy. These findings open up new opportunities for photodynamic inactivation of pathogenic bacteria.

The expression of biosynthetic genes in bacterial hosts can enable access to high-value compounds, for which appropriate molecular genetic tools are essential. Therefore, we developed a toolbox of modular vectors, which facilitate chromosomal gene integration and expression in Pseudomonas putida KT2440. To this end, we designed an integrative sequence, allowing customisation regarding the modus of integration (random, at attTn7, or into the 16S rRNA gene), promoters, antibiotic resistance markers as well as fluorescent proteins and enzymes as transcription reporters. We thus established a toolbox of vectors carrying integrative sequences, designated as pYT series, of which we present 27 ready-to-use variants along with a set of strains equipped with unique ‘landing pads’ for directing a pYT interposon into one specific copy of the 16S rRNA gene. We used genes of the well-described violacein biosynthesis as reporter to showcase random Tn5-based chromosomal integration leading to constitutive expression and production of violacein and deoxyviolacein. Deoxyviolacein was likewise produced after gene integration into the 16S rRNA gene of rrn operons. Integration in the attTn7 site was used to characterise the suitability of different inducible promoters and successive strain development for the metabolically challenging production of mono-rhamnolipids. Finally, to establish arcyriaflavin A production in P. putida for the first time, we compared different integration and expression modes, revealing integration at attTn7 and expression with NagR/PnagAa to be most suitable. In summary, the new toolbox can be utilised for the rapid generation of various types of P. putida expression and production strains.

Photocaged compounds are applied for implementing precise, optochemical control of gene expression in bacteria. To broaden the scope of UV-light-responsive inducer molecules, six photocaged carbohydrates were synthesized and photochemically characterized, with the absorption exhibiting a red-shift. Their differing linkage through ether, carbonate, and carbamate bonds revealed that carbonate and carbamate bonds are convenient. Subsequently, those compounds were successfully applied in vivo for controlling gene expression in E. coli via blue light illumination. Furthermore, benzoate-based expression systems were subjected to light control by establishing a novel photocaged salicylic acid derivative. Besides its synthesis and in vitro characterization, we demonstrate the challenging choice of a suitable promoter system for light-controlled gene expression in E. coli. We illustrate various bottlenecks during both photocaged inducer synthesis and in vivo application and possibilities to overcome them. These findings pave the way towards novel caged inducer-dependent systems for wavelength-selective gene expression.This article is part of a Special Collection dedicated to the Biotrans 2021 conference. Please see our homepage for more articles in the collection.

No abstract

Genetically encoded photosensitizers are able to produce reactive oxygen species upon illumination and are exploited in a wide range of applications, especially in the medical field. In this work, we envisioned to further apply these genetically encoded photosensitizers for the light-dependent control of single enzymes in multi-step biocatalysis. One of the challenges in the application of several enzymes in a cascade is the unwanted cross-reactivity of these biocatalysts on reaction intermediates when all enzymes are simultaneously present in the reaction. As one strategy to address this issue, we investigated whether the introduction of genetically encoded photosensitizers as fusion tags would allow the selective inactivation of enzymes after successful transformation by simply turning on light. We tested five different photosensitizers as molecular biological fusion tags to inactivate the pyruvate decarboxylase variant E469G/W543H from Acetobacter pasteurianus. Dimeric photosensitizer tags, like the flavin-binding fluorescent proteins from Bacillus subtilis and Pseudomonas putida showed the tendency to form insoluble protein aggregates in combination with the tetrameric carboligase. Enzyme activity was, to some extent, retained in these aggregates, but the handling of the insoluble aggregates proved to be unfeasible. Monomeric photosensitizer tags appeared to be much more suitable when fused to the tetrameric enzyme. In the dark, the singlet oxygen photosensitizing protein (SOPP3)-tagged carboligase retained 79% of its activity as compared to the unfused enzyme. Upon blue light exposure, the SOPP3 tag showed the best specific inactivation and enabled complete inactivation of the carboligase within 30 min. SOPP3 is thus seen as a promising photosensitizer tag to be applied in future multi-step enzyme cascades to overcome the challenge of cross-reactivity.

Azulitox as a new cancer cell specific and phototoxic fusion polypeptide was generated and is composed of a photosensitizer domain and the cell-penetrating peptide P28. The photosensitizer domain (EcFbFP) was derived from a bacterial blue light receptor, which belongs to the family of light-oxygen-voltage-proteins and produces reactive oxygen species (ROS) upon excitation. P28 is derived from the cupredoxin protein azurin that is known to specifically penetrate cancer cells and bind to tumor suppressor protein p53. We show that the P28 domain specifically directs and translocates the fused photosensitizer into cancer cells. Under blue-light illumination, Azulitox significantly induced cytotoxicity. Compared to the extracellular application of EcFbFP, Azulitox caused death of about 90 % of cells as monitored by flow cytometry, which also directly correlated with the amount of ROS produced in the cells. Azulitox may pave new avenues towards targeted polypeptide-photosensitizer based photodynamic therapies with reduced systemic toxicity compared to conventional photosensitizers.