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Ohlendorf R, Möglich A. Light-regulated gene expression in Bacteria: Fundamentals, advances, and perspectives. Front Bioeng Biotechnol 2022; 10:1029403. [PMID: 36312534 PMCID: PMC9614035 DOI: 10.3389/fbioe.2022.1029403] [Citation(s) in RCA: 8] [Impact Index Per Article: 4.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/27/2022] [Accepted: 09/29/2022] [Indexed: 11/13/2022] Open
Abstract
Numerous photoreceptors and genetic circuits emerged over the past two decades and now enable the light-dependent i.e., optogenetic, regulation of gene expression in bacteria. Prompted by light cues in the near-ultraviolet to near-infrared region of the electromagnetic spectrum, gene expression can be up- or downregulated stringently, reversibly, non-invasively, and with precision in space and time. Here, we survey the underlying principles, available options, and prominent examples of optogenetically regulated gene expression in bacteria. While transcription initiation and elongation remain most important for optogenetic intervention, other processes e.g., translation and downstream events, were also rendered light-dependent. The optogenetic control of bacterial expression predominantly employs but three fundamental strategies: light-sensitive two-component systems, oligomerization reactions, and second-messenger signaling. Certain optogenetic circuits moved beyond the proof-of-principle and stood the test of practice. They enable unprecedented applications in three major areas. First, light-dependent expression underpins novel concepts and strategies for enhanced yields in microbial production processes. Second, light-responsive bacteria can be optogenetically stimulated while residing within the bodies of animals, thus prompting the secretion of compounds that grant health benefits to the animal host. Third, optogenetics allows the generation of precisely structured, novel biomaterials. These applications jointly testify to the maturity of the optogenetic approach and serve as blueprints bound to inspire and template innovative use cases of light-regulated gene expression in bacteria. Researchers pursuing these lines can choose from an ever-growing, versatile, and efficient toolkit of optogenetic circuits.
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Affiliation(s)
- Robert Ohlendorf
- Department of Biological Engineering, Massachusetts Institute of Technology, Cambridge, MA, United States
| | - Andreas Möglich
- Department of Biochemistry, University of Bayreuth, Bayreuth, Germany
- Bayreuth Center for Biochemistry and Molecular Biology, Universität Bayreuth, Bayreuth, Germany
- North-Bavarian NMR Center, Universität Bayreuth, Bayreuth, Germany
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2
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Tang K, Beyer HM, Zurbriggen MD, Gärtner W. The Red Edge: Bilin-Binding Photoreceptors as Optogenetic Tools and Fluorescence Reporters. Chem Rev 2021; 121:14906-14956. [PMID: 34669383 PMCID: PMC8707292 DOI: 10.1021/acs.chemrev.1c00194] [Citation(s) in RCA: 16] [Impact Index Per Article: 5.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/08/2021] [Indexed: 12/15/2022]
Abstract
This review adds the bilin-binding phytochromes to the Chemical Reviews thematic issue "Optogenetics and Photopharmacology". The work is structured into two parts. We first outline the photochemistry of the covalently bound tetrapyrrole chromophore and summarize relevant spectroscopic, kinetic, biochemical, and physiological properties of the different families of phytochromes. Based on this knowledge, we then describe the engineering of phytochromes to further improve these chromoproteins as photoswitches and review their employment in an ever-growing number of different optogenetic applications. Most applications rely on the light-controlled complex formation between the plant photoreceptor PhyB and phytochrome-interacting factors (PIFs) or C-terminal light-regulated domains with enzymatic functions present in many bacterial and algal phytochromes. Phytochrome-based optogenetic tools are currently implemented in bacteria, yeast, plants, and animals to achieve light control of a wide range of biological activities. These cover the regulation of gene expression, protein transport into cell organelles, and the recruitment of phytochrome- or PIF-tagged proteins to membranes and other cellular compartments. This compilation illustrates the intrinsic advantages of phytochromes compared to other photoreceptor classes, e.g., their bidirectional dual-wavelength control enabling instant ON and OFF regulation. In particular, the long wavelength range of absorption and fluorescence within the "transparent window" makes phytochromes attractive for complex applications requiring deep tissue penetration or dual-wavelength control in combination with blue and UV light-sensing photoreceptors. In addition to the wide variability of applications employing natural and engineered phytochromes, we also discuss recent progress in the development of bilin-based fluorescent proteins.
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Affiliation(s)
- Kun Tang
- Institute
of Synthetic Biology, Heinrich-Heine-University
Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
| | - Hannes M. Beyer
- Institute
of Synthetic Biology, Heinrich-Heine-University
Düsseldorf, Universitätsstrasse 1, D-40225 Düsseldorf, Germany
| | - Matias D. Zurbriggen
- Institute
of Synthetic Biology and CEPLAS, Heinrich-Heine-University
Düsseldorf, Universitätsstrasse
1, D-40225 Düsseldorf, Germany
| | - Wolfgang Gärtner
- Retired: Max Planck Institute
for Chemical Energy Conversion. At present: Institute for Analytical Chemistry, University
Leipzig, Linnéstrasse
3, 04103 Leipzig, Germany
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3
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Lindner F, Diepold A. Optogenetics in bacteria - applications and opportunities. FEMS Microbiol Rev 2021; 46:6427354. [PMID: 34791201 PMCID: PMC8892541 DOI: 10.1093/femsre/fuab055] [Citation(s) in RCA: 13] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2021] [Accepted: 11/11/2021] [Indexed: 12/12/2022] Open
Abstract
Optogenetics holds the promise of controlling biological processes with superb temporal and spatial resolution at minimal perturbation. Although many of the light-reactive proteins used in optogenetic systems are derived from prokaryotes, applications were largely limited to eukaryotes for a long time. In recent years, however, an increasing number of microbiologists use optogenetics as a powerful new tool to study and control key aspects of bacterial biology in a fast and often reversible manner. After a brief discussion of optogenetic principles, this review provides an overview of the rapidly growing number of optogenetic applications in bacteria, with a particular focus on studies venturing beyond transcriptional control. To guide future experiments, we highlight helpful tools, provide considerations for successful application of optogenetics in bacterial systems, and identify particular opportunities and challenges that arise when applying these approaches in bacteria.
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Affiliation(s)
- Florian Lindner
- Max-Planck-Institute for Terrestrial Microbiology, Department of Ecophysiology, Karl-von-Frisch-Str. 10, 35043 Marburg, Germany
| | - Andreas Diepold
- Max-Planck-Institute for Terrestrial Microbiology, Department of Ecophysiology, Karl-von-Frisch-Str. 10, 35043 Marburg, Germany.,SYNMIKRO, LOEWE Center for Synthetic Microbiology, Marburg, Germany
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Ariyanti D, Ikebukuro K, Sode K. Artificial complementary chromatic acclimation gene expression system in Escherichia coli. Microb Cell Fact 2021; 20:128. [PMID: 34225717 PMCID: PMC8256508 DOI: 10.1186/s12934-021-01621-3] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/29/2021] [Accepted: 06/26/2021] [Indexed: 01/21/2023] Open
Abstract
BACKGROUND The development of multiple gene expression systems, especially those based on the physical signals, such as multiple color light irradiations, is challenging. Complementary chromatic acclimation (CCA), a photoreversible process that facilitates the control of cellular expression using light of different wavelengths in cyanobacteria, is one example. In this study, an artificial CCA systems, inspired by type III CCA light-regulated gene expression, was designed by employing a single photosensor system, the CcaS/CcaR green light gene expression system derived from Synechocystis sp. PCC6803, combined with G-box (the regulator recognized by activated CcaR), the cognate cpcG2 promoter, and the constitutively transcribed promoter, the PtrcΔLacO promoter. RESULTS One G-box was inserted upstream of the cpcG2 promoter and a reporter gene, the rfp gene (green light-induced gene expression), and the other G-box was inserted between the PtrcΔLacO promoter and a reporter gene, the bfp gene (red light-induced gene expression). The Escherichia coli transformants with plasmid-encoded genes were evaluated at the transcriptional and translational levels under red or green light illumination. Under green light illumination, the transcription and translation of the rfp gene were observed, whereas the expression of the bfp gene was repressed. Under red light illumination, the transcription and translation of the bfp gene were observed, whereas the expression of the rfp gene was repressed. During the red and green light exposure cycles at every 6 h, BFP expression increased under red light exposure while RFP expression was repressed, and RFP expression increased under green light exposure while BFP expression was repressed. CONCLUSION An artificial CCA system was developed to realize a multiple gene expression system, which was regulated by two colors, red and green lights, using a single photosensor system, the CcaS/CcaR system derived from Synechocystis sp. PCC6803, in E. coli. The artificial CCA system functioned repeatedly during red and green light exposure cycles. These results demonstrate the potential application of this CCA gene expression system for the production of multiple metabolites in a variety of microorganisms, such as cyanobacteria.
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Affiliation(s)
- Dwi Ariyanti
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo, 184-8588, Japan
- Faculty of Biotechnology, Sumbawa University of Technology, Olat Maras, Moyo Hulu, Sumbawa, West Nusa Tenggara, 84371, Indonesia
| | - Kazunori Ikebukuro
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Naka-cho, Koganei, Tokyo, 184-8588, Japan.
| | - Koji Sode
- Joint Department of Biomedical Engineering, The University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, 27599, USA.
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Shono C, Ariyanti D, Abe K, Sakai Y, Sakamoto I, Tsukakoshi K, Sode K, Ikebukuro K. A Green Light-Regulated T7 RNA Polymerase Gene Expression System for Cyanobacteria. MARINE BIOTECHNOLOGY (NEW YORK, N.Y.) 2021; 23:31-38. [PMID: 32979137 DOI: 10.1007/s10126-020-09997-w] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 07/14/2020] [Accepted: 09/10/2020] [Indexed: 06/11/2023]
Abstract
In this study, we developed a green light-regulated T7 RNA polymerase expression system (T7 RNAP system), to provide a novel and versatile high-expression system for cyanobacteria without using any chemical inducer, realizing high expression levels comparable with previously reported for recombinant gene expression in cyanobacteria. The T7 RNAP system was constructed and introduced into Synechocystis sp. PCC6803. T7 RNAP was inserted downstream of the cpcG2 promoter, which is recognized and activated by the CcaS/CcaR two-component green-light-sensing system, to compose a vector plasmid, pKT-CS01, to achieve the induction of T7 RNAP expression only under green light illumination, with repression under red light illumination. The reporter gene, superfolder green fluorescent protein (sfGFP), was inserted downstream of the T7 promoter. Transcriptional analyses revealed that T7 RNAP was induced under green light but repressed under red light. Expression of the sfGFP protein derived from pKT-CS01 was observed under green light illumination and was approximately 10-fold higher than that in the control transformant, which expressed sfGFP directly under the cpcG2 promoter, which is directly regulated by CcaS/CcaR, under green light illumination. Comparison with the strong promoter expression systems Pcpc560 and PtrcΔlacO revealed that the expression of sfGFP by the T7 RNAP system was comparable with the levels obtained with strong promoters. These results demonstrated that the green light-regulated T7 RNAP gene expression system will be a versatile tool for future technological platform to regulate gene expression in cyanobacterial bioprocesses.
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Affiliation(s)
- Chika Shono
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
| | - Dwi Ariyanti
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
- Faculty of Biotechnology, Sumbawa University of Technology, Olat Maras, Moyo Hulu, Sumbawa, West Nusa Tenggara, 84371, Indonesia
| | - Koichi Abe
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
| | - Yuta Sakai
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
| | - Ippei Sakamoto
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
| | - Kaori Tsukakoshi
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan
| | - Koji Sode
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan.
- Joint Department of Biomedical Engineering, The University of North Carolina at Chapel Hill and North Carolina State University, Chapel Hill, NC, 27599, USA.
| | - Kazunori Ikebukuro
- Department of Biotechnology and Life Science, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan.
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Wiltbank LB, Kehoe DM. Diverse light responses of cyanobacteria mediated by phytochrome superfamily photoreceptors. Nat Rev Microbiol 2020; 17:37-50. [PMID: 30410070 DOI: 10.1038/s41579-018-0110-4] [Citation(s) in RCA: 67] [Impact Index Per Article: 16.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/25/2022]
Abstract
Cyanobacteria are an evolutionarily and ecologically important group of prokaryotes. They exist in diverse habitats, ranging from hot springs and deserts to glaciers and the open ocean. The range of environments that they inhabit can be attributed in part to their ability to sense and respond to changing environmental conditions. As photosynthetic organisms, one of the most crucial parameters for cyanobacteria to monitor is light. Cyanobacteria can sense various wavelengths of light and many possess a range of bilin-binding photoreceptors belonging to the phytochrome superfamily. Vital cellular processes including growth, phototaxis, cell aggregation and photosynthesis are tuned to environmental light conditions by these photoreceptors. In this Review, we examine the physiological responses that are controlled by members of this diverse family of photoreceptors and discuss the signal transduction pathways through which these photoreceptors operate. We highlight specific examples where the activities of multiple photoreceptors function together to fine-tune light responses. We also discuss the potential application of these photosensing systems in optogenetics and synthetic biology.
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Affiliation(s)
- Lisa B Wiltbank
- Department of Biology, Indiana University, Bloomington, IN, USA
| | - David M Kehoe
- Department of Biology, Indiana University, Bloomington, IN, USA.
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7
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Liu Z, Zhang J, Jin J, Geng Z, Qi Q, Liang Q. Programming Bacteria With Light-Sensors and Applications in Synthetic Biology. Front Microbiol 2018; 9:2692. [PMID: 30467500 PMCID: PMC6236058 DOI: 10.3389/fmicb.2018.02692] [Citation(s) in RCA: 52] [Impact Index Per Article: 8.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2018] [Accepted: 10/22/2018] [Indexed: 12/11/2022] Open
Abstract
Photo-receptors are widely present in both prokaryotic and eukaryotic cells, which serves as the foundation of tuning cell behaviors with light. While practices in eukaryotic cells have been relatively established, trials in bacterial cells have only been emerging in the past few years. A number of light sensors have been engineered in bacteria cells and most of them fall into the categories of two-component and one-component systems. Such a sensor toolbox has enabled practices in controlling synthetic circuits at the level of transcription and protein activity which is a major topic in synthetic biology, according to the central dogma. Additionally, engineered light sensors and practices of tuning synthetic circuits have served as a foundation for achieving light based real-time feedback control. Here, we review programming bacteria cells with light, introducing engineered light sensors in bacteria and their applications, including tuning synthetic circuits and achieving feedback controls over microbial cell culture.
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Affiliation(s)
- Zedao Liu
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
| | - Jizhong Zhang
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
| | - Jiao Jin
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
| | - Zilong Geng
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
| | - Qingsheng Qi
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
| | - Quanfeng Liang
- State Key Laboratory of Microbial Technology, Shandong University, Jinan, China
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Oliinyk OS, Chernov KG, Verkhusha VV. Bacterial Phytochromes, Cyanobacteriochromes and Allophycocyanins as a Source of Near-Infrared Fluorescent Probes. Int J Mol Sci 2017; 18:E1691. [PMID: 28771184 PMCID: PMC5578081 DOI: 10.3390/ijms18081691] [Citation(s) in RCA: 43] [Impact Index Per Article: 6.1] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/17/2017] [Revised: 07/28/2017] [Accepted: 07/28/2017] [Indexed: 12/21/2022] Open
Abstract
Bacterial photoreceptors absorb light energy and transform it into intracellular signals that regulate metabolism. Bacterial phytochrome photoreceptors (BphPs), some cyanobacteriochromes (CBCRs) and allophycocyanins (APCs) possess the near-infrared (NIR) absorbance spectra that make them promising molecular templates to design NIR fluorescent proteins (FPs) and biosensors for studies in mammalian cells and whole animals. Here, we review structures, photochemical properties and molecular functions of several families of bacterial photoreceptors. We next analyze molecular evolution approaches to develop NIR FPs and biosensors. We then discuss phenotypes of current BphP-based NIR FPs and compare them with FPs derived from CBCRs and APCs. Lastly, we overview imaging applications of NIR FPs in live cells and in vivo. Our review provides guidelines for selection of existing NIR FPs, as well as engineering approaches to develop NIR FPs from the novel natural templates such as CBCRs.
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Affiliation(s)
- Olena S Oliinyk
- Department of Biochemistry and Developmental Biology, Faculty of Medicine, University of Helsinki, 00290 Helsinki, Finland.
| | - Konstantin G Chernov
- Department of Biochemistry and Developmental Biology, Faculty of Medicine, University of Helsinki, 00290 Helsinki, Finland.
| | - Vladislav V Verkhusha
- Department of Biochemistry and Developmental Biology, Faculty of Medicine, University of Helsinki, 00290 Helsinki, Finland.
- Department of Anatomy and Structural Biology, and Gruss-Lipper Biophotonics Center, Albert Einstein College of Medicine, Bronx, NY 10461, USA.
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Wang F, Lv X, Xie W, Zhou P, Zhu Y, Yao Z, Yang C, Yang X, Ye L, Yu H. Combining Gal4p-mediated expression enhancement and directed evolution of isoprene synthase to improve isoprene production in Saccharomyces cerevisiae. Metab Eng 2017; 39:257-266. [DOI: 10.1016/j.ymben.2016.12.011] [Citation(s) in RCA: 47] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/29/2016] [Revised: 12/14/2016] [Accepted: 12/26/2016] [Indexed: 12/20/2022]
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10
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Löwe H, Hobmeier K, Moos M, Kremling A, Pflüger-Grau K. Photoautotrophic production of polyhydroxyalkanoates in a synthetic mixed culture of Synechococcus elongatus cscB and Pseudomonas putida cscAB. BIOTECHNOLOGY FOR BIOFUELS 2017; 10:190. [PMID: 28814973 PMCID: PMC5517840 DOI: 10.1186/s13068-017-0875-0] [Citation(s) in RCA: 55] [Impact Index Per Article: 7.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/05/2017] [Accepted: 07/11/2017] [Indexed: 05/20/2023]
Abstract
BACKGROUND One of the major challenges for the present and future generations is to find suitable substitutes for the fossil resources we rely on today. Cyanobacterial carbohydrates have been discussed as an emerging renewable feedstock in industrial biotechnology for the production of fuels and chemicals, showing promising production rates when compared to crop-based feedstock. However, intrinsic capacities of cyanobacteria to produce biotechnological compounds are limited and yields are low. RESULTS Here, we present an approach to circumvent these problems by employing a synthetic bacterial co-culture for the carbon-neutral production of polyhydroxyalkanoates (PHAs) from CO2. The co-culture consists of two bio-modules: Bio-module I, in which the cyanobacterial strain Synechococcus elongatus cscB fixes CO2, converts it to sucrose, and exports it into the culture supernatant; and bio-module II, where this sugar serves as C-source for Pseudomonas putida cscAB and is converted to PHAs that are accumulated in the cytoplasm. By applying a nitrogen-limited process, we achieved a maximal PHA production rate of 23.8 mg/(L day) and a maximal titer of 156 mg/L. We will discuss the present shortcomings of the process and show the potential for future improvement. CONCLUSIONS These results demonstrate the feasibility of mixed cultures of S. elongatus cscB and P. putida cscAB for PHA production, making room for the cornucopia of possible products that are described for P. putida. The construction of more efficient sucrose-utilizing P. putida phenotypes and the optimization of process conditions will increase yields and productivities and eventually close the gap in the contemporary process. In the long term, the co-culture may serve as a platform process, in which P. putida is used as a chassis for the implementation of synthetic metabolic pathways for biotechnological production of value-added products.
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Affiliation(s)
- Hannes Löwe
- Fachgebiet für Systembiotechnologie, Technische Universität München, Boltzmannstr 15, 85748 Garching, Germany
| | - Karina Hobmeier
- Fachgebiet für Systembiotechnologie, Technische Universität München, Boltzmannstr 15, 85748 Garching, Germany
| | - Manuel Moos
- Fachgebiet für Systembiotechnologie, Technische Universität München, Boltzmannstr 15, 85748 Garching, Germany
| | - Andreas Kremling
- Fachgebiet für Systembiotechnologie, Technische Universität München, Boltzmannstr 15, 85748 Garching, Germany
| | - Katharina Pflüger-Grau
- Fachgebiet für Systembiotechnologie, Technische Universität München, Boltzmannstr 15, 85748 Garching, Germany
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Construction of a Miniaturized Chromatic Acclimation Sensor from Cyanobacteria with Reversed Response to a Light Signal. Sci Rep 2016; 6:37595. [PMID: 27883080 PMCID: PMC5121610 DOI: 10.1038/srep37595] [Citation(s) in RCA: 21] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/26/2016] [Accepted: 10/27/2016] [Indexed: 12/26/2022] Open
Abstract
Cyanobacteria harbor unique photoreceptors, designated as cyanobacteriochromes (CBCRs). In this study, we attempted to engineer the chromatic acclimation sensor CcaS, a CBCR derived from the cyanobacterium Synechocystis sp. PCC 6803. The wild-type CcaS induces gene expression under green light illumination and represses it under red light illumination. We focused on the domain structure of CcaS, which consists of an N-terminal transmembrane helix; a GAF domain, which serves as the sensor domain; a linker region (L1); two PAS domains; a second linker region (L2); and a C-terminal histidine kinase (HK) domain. Truncated versions of the photoreceptor were constructed by removing the L1 linker region and the two PAS domains, and fusing the GAF and HK domains with a truncated linker region. Thus constructed “miniaturized CcaSs” were grouped into four distinct categories according to their responses toward green and red light illumination, with some showing improved gene regulation compared to the wild type. Remarkably, one of the miniaturized CcaSs induced gene expression under red light and repressed it under green light, a reversed response to the light signal compared to wild type CcaS. These characteristics of engineered photoreceptors were discussed by analyzing the CcaS structural model.
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