Artificial control of transgene expression in Chlamydomonas reinhardtii chloroplast using the lac regulation system from Escherichia coli

Antimicrobial peptides (AMPs) from microalgae as an alternative to conventional antibiotics in aquaculture

The excessive use of conventional antibiotics has resulted in significant aquatic pollution and a concerning surge in drug-resistant bacteria. Efforts have been consolidated to explore and develop environmentally friendly antimicrobial alternatives to mitigate the imminent threat posed by multi-resistant pathogens. Antimicrobial peptides (AMPs) have gained prominence due to their low propensity to induce bacterial resistance, attributed to their multiple mechanisms of action and synergistic effects. Microalgae, particularly cyanobacteria, have emerged as promising alternatives with antibiotic potential to address these challenges. The aim of this review is to present some AMPs extracted from microalgae, emphasizing their activity against common pathogens and elucidating their mechanisms of action, as well as their potential application in the aquaculture industry. Likewise, the biosynthesis, advantages and disadvantages of the use of AMPs are described. Currently, biotechnology tolls are used to enhance the action of these peptides, such as genetically modified microalgae and recombinant proteins. Cyanobacteria are also mentioned as major producers of peptides, among them, the genus Lyngbya is described as the most important producer of bioactive peptides with potential therapeutic use. The majority of cyanobacterial AMPs are of the cyclic type, meaning that they have cysteine and disulfide bridges, thanks to this, their greater antimicrobial activity and selectivity. Likewise, we found that large hydrophobic aromatic amino acid residues increase specificity, and improve antibacterial efficacy. However, based on the results of this review, it is possible to highlight that while microalgae show potential as a source of AMPs, further research in this field is necessary to achieve safe and competitive production. Therefore, the data presented here can aid in the selection of microalgal species, peptide structures, and target bacteria, with the goal of establishing biotechnological platforms for aquaculture applications.

Antimicrobial peptides derived from microalgae for combating antibiotic resistance: Current status and prospects

Microalgae are photosynthetic cell factories that produce a spectrum of bioactive compounds extensively used for various applications. Owing to the increase in antibiotic resistance among microbial pathogens, there is a significant thrust for identifying new treatment strategies, and antimicrobial peptides (AMPs) generation is one such method. These AMPs have multiple roles and are active against bacteria, fungi, and viruses. Such peptides synthesized in microalgae have a significant role in medical application, managing aquaculture-associated diseases, and the food industry. To increase their effectiveness and novel peptides, genetically modified microalgae are used as cell factories. With the advancement of new technologies like the CRISPR-Cas system, new avenues are opened for developing novel AMPs using microalgae. This review gives us insight into the various AMPs produced by microalgae and multiple technologies involved in creating such therapeutically essential molecules.

Towards green biomanufacturing of high-value recombinant proteins using promising cell factory: Chlamydomonas reinhardtii chloroplast

Microalgae are cosmopolitan organisms in nature with short life cycles, playing a tremendous role in reducing the pressure of industrial carbon emissions. Besides, microalgae have the unique advantages of being photoautotrophic and harboring both prokaryotic and eukaryotic expression systems, becoming a popular host for recombinant proteins. Currently, numerous advanced molecular tools related to microalgal transgenesis have been explored and established, especially for the model species Chlamydomonas reinhardtii (C. reinhardtii hereafter). The development of genetic tools and the emergence of new strategies further increase the feasibility of developing C. reinhardtii chloroplasts as green factories, and the strong genetic operability of C. reinhardtii endows it with enormous potential as a synthetic biology platform. At present, C. reinhardtii chloroplasts could successfully produce plenty of recombinant proteins, including antigens, antibodies, antimicrobial peptides, protein hormones and enzymes. However, additional techniques and toolkits for chloroplasts need to be developed to achieve efficient and markerless editing of plastid genomes. Mining novel genetic elements and selectable markers will be more intensively studied in the future, and more factors affecting protein expression are urged to be explored. This review focuses on the latest technological progress of selectable markers for Chlamydomonas chloroplast genetic engineering and the factors that affect the efficiency of chloroplast protein expression. Furthermore, urgent challenges and prospects for future development are pointed out.

Recent biotechnological developments in reshaping the microalgal genome: A signal for green recovery in biorefinery practices

The use of renewable energy sources as a substitute for nonrenewable fossil fuels is urgently required. Algae biorefinery platform provides an excellent alternate to overcome future energy problems. However, to let this viable biomass be competent with existing feedstocks, it is necessary to exploit genetic manipulation and improvement in upstream and downstream platforms for optimal bio-product recovery. Furthermore, the techno-economic strategies further maximize metabolites production for biofuel, biohydrogen, and other industrial applications. The experimental methodologies in algal photobioreactor promote high biomass production, enriched in lipid and starch content in limited environmental conditions. This review presents an optimization framework combining genetic manipulation methods to simulate microalgal growth dynamics, understand the complexity of algal biorefinery to scale up, and identify green strategies for techno-economic feasibility of algae for biomass conversion. Overall, the algal biorefinery opens up new possibilities for the valorization of algae biomass and the synthesis of various novel products.

Manipulation of the microalgal chloroplast by genetic engineering for biotechnological utilization as a green biofactory

The chloroplast is an essential organelle in microalgae for conducting photosynthesis, thus enabling the photoautotrophic growth of microalgae. In addition to photosynthesis, the chloroplast is capable of various biochemical processes for the synthesis of proteins, lipids, carbohydrates, and terpenoids. Due to these attractive characteristics, there has been increasing interest in the biotechnological utilization of microalgal chloroplast as a sustainable alternative to the conventional production platforms used in industrial biotechnology. Since the first demonstration of microalgal chloroplast transformation, significant development has occurred over recent decades in the manipulation of microalgal chloroplasts through genetic engineering. In the present review, we describe the advantages of the microalgal chloroplast as a production platform for various bioproducts, including recombinant proteins and high-value metabolites, features of chloroplast genetic systems, and the development of transformation methods, which represent important factors for gene expression in the chloroplast. Furthermore, we address the expression of various recombinant proteins in the microalgal chloroplast through genetic engineering, including reporters, biopharmaceutical proteins, and industrial enzymes. Finally, we present many efforts and achievements in the production of high-value metabolites in the microalgal chloroplast through metabolic engineering. Based on these efforts and advances, the microalgal chloroplast represents an economically viable and sustainable platform for biotechnological applications in the near future.

CITRIC: cold-inducible translational readthrough in the chloroplast of Chlamydomonas reinhardtii using a novel temperature-sensitive transfer RNA

Background

The chloroplast of eukaryotic microalgae such as Chlamydomonas reinhardtii is a potential platform for metabolic engineering and the production of recombinant proteins. In industrial biotechnology, inducible expression is often used so that the translation or function of the heterologous protein does not interfere with biomass accumulation during the growth stage. However, the existing systems used in bacterial or fungal platforms do not transfer well to the microalgal chloroplast. We sought to develop a simple inducible expression system for the microalgal chloroplast, exploiting an unused stop codon (TGA) in the plastid genome. We have previously shown that this codon can be translated as tryptophan when we introduce into the chloroplast genome a trnW_UCA gene encoding a plastidial transfer RNA with a modified anticodon sequence, UCA.

Results

A mutated version of our trnW_UCA gene was developed that encodes a temperature-sensitive variant of the tRNA. This allows transgenes that have been modified to contain one or more internal TGA codons to be translated differentially according to the culture temperature, with a gradient of recombinant protein accumulation from 35 °C (low/off) to 15 °C (high). We have named this the CITRIC system, an acronym for cold-inducible translational readthrough in chloroplasts. The exact induction behaviour can be tailored by altering the number of TGA codons within the transgene.

Conclusions

CITRIC adds to the suite of genetic engineering tools available for the microalgal chloroplast, allowing a greater degree of control over the timing of heterologous protein expression. It could also be used as a heat-repressible system for studying the function of essential native genes in the chloroplast. The genetic components of CITRIC are entirely plastid-based, so no engineering of the nuclear genome is required.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1033-5) contains supplementary material, which is available to authorized users.

Synthetic biology approaches for the production of plant metabolites in unicellular organisms

Synthetic biology is the repurposing of biological systems for novel objectives and applications. Through the co-ordinated and balanced expression of genes, both native and those introduced from other organisms, resources within an industrial chassis can be siphoned for the commercial production of high-value commodities. This developing interdisciplinary field has the potential to revolutionize natural product discovery from higher plants, by providing a diverse array of tools, technologies, and strategies for exploring the large chemically complex space of plant natural products using unicellular organisms. In this review, we emphasize the key features that influence the generation of biorefineries and highlight technologies and strategic solutions that can be used to overcome engineering pitfalls with rational design. Also presented is a succinct guide to assist the selection of unicellular chassis most suited for the engineering and subsequent production of the desired natural product, in order to meet the global demand for plant natural products in a safe and sustainable manner.

Outlook in the application of Chlamydomonas reinhardtii chloroplast as a platform for recombinant protein production

Microalgae, also called microphytes, are a vast group of microscopic photosynthetic organisms living in aquatic ecosystems. Microalgae have attracted the attention of biotechnology industry as a platform for extracting natural products with high commercial value. During last decades, microalgae have been also used as cost-effective and easily scalable platform for the production of recombinant proteins with medical and industrial applications. Most progress in this field has been made with Chlamydomonas reinhardtii as a model organism mainly because of its simple life cycle, well-established genetics and ease of cultivation. However, due to the scarcity of existing infrastructure for commercial production and processing together with relatively low product yields, no recombinant products from C. reinhardtii have gained approval for commercial production and most of them are still in research and development. In this review, we focus on the chloroplast of C. reinhardtii as an algal recombinant expression platform and compare its advantages and disadvantages to other currently used expression systems. We then discuss the strategies for engineering the chloroplast of C. reinhardtii to produce recombinant cells and present a comprehensive overview of works that have used this platform for the expression of high-value products.

Boosting riboswitch efficiency by RNA amplification

Riboswitches are RNA sensors that regulate gene expression in response to binding of small molecules. Although they conceptually represent simple on/off switches and, therefore, hold great promise for biotechnology and future synthetic biology applications, the induction of gene expression by natural riboswitches after ligand addition or removal is often only moderate and, consequently, the achievable expression levels are not very high. Here, we have designed an RNA amplification-based system that strongly improves the efficiency of riboswitches. We have successfully implemented the method in a biological system for which currently no efficient endogenous tools for inducible (trans)gene expression are available: the chloroplasts of higher plants. We further show that an HIV antigen whose constitutive expression from the chloroplast genome is deleterious to the plant can be inducibly expressed under the control of the RNA amplification-enhanced riboswitch (RAmpER) without causing a mutant phenotype, demonstrating the potential of the method for the production of proteins and metabolites that are toxic to the host cell.

Chlamydomonas as a model for biofuels and bio-products production

Developing renewable energy sources is critical to maintaining the economic growth of the planet while protecting the environment. First generation biofuels focused on food crops like corn and sugarcane for ethanol production, and soybean and palm for biodiesel production. Second generation biofuels based on cellulosic ethanol produced from terrestrial plants, has received extensive funding and recently pilot facilities have been commissioned, but to date output of fuels from these sources has fallen well short of what is needed. Recent research and pilot demonstrations have highlighted the potential of algae as one of the most promising sources of sustainable liquid transportation fuels. Algae have also been established as unique biofactories for industrial, therapeutic, and nutraceutical co-products. Chlamydomonas reinhardtii's long established role in the field of basic research in green algae has paved the way for understanding algal metabolism and developing genetic engineering protocols. These tools are now being utilized in C. reinhardtii and in other algal species for the development of strains to maximize biofuels and bio-products yields from the lab to the field.

Establishing Chlamydomonas reinhardtii as an industrial biotechnology host

Microalgae constitute a diverse group of eukaryotic unicellular organisms that are of interest for pure and applied research. Owing to their natural synthesis of value-added natural products microalgae are emerging as a source of sustainable chemical compounds, proteins and metabolites, including but not limited to those that could replace compounds currently made from fossil fuels. For the model microalga, Chlamydomonas reinhardtii, this has prompted a period of rapid development so that this organism is poised for exploitation as an industrial biotechnology platform. The question now is how best to achieve this? Highly advanced industrial biotechnology systems using bacteria and yeasts were established in a classical metabolic engineering manner over several decades. However, the advent of advanced molecular tools and the rise of synthetic biology provide an opportunity to expedite the development of C. reinhardtii as an industrial biotechnology platform, avoiding the process of incremental improvement. In this review we describe the current status of genetic manipulation of C. reinhardtii for metabolic engineering. We then introduce several concepts that underpin synthetic biology, and show how generic parts are identified and used in a standard manner to achieve predictable outputs. Based on this we suggest that the development of C. reinhardtii as an industrial biotechnology platform can be achieved more efficiently through adoption of a synthetic biology approach.

Marker-free genetic engineering of the chloroplast in the green microalga Chlamydomonas reinhardtii

The work applied a transgene expression method based on the replacement of an inactive rbcL gene as the selection marker in Chlamydomonas reinhardtii chloroplasts. The native rbcL gene in strain CC2653 has a point mutation that causes early translation termination, thus resulting in a photosynthesis mutant. Recovery of rbcL function for selection is offered along with the heterologous expression of the alcohol dehydrogenase ADH1 gene from Saccharomyces cerevisiae in the Chlamydomonas chloroplast. The CrCpADH1 gene was inserted via double homologous recombination in the psaB-rbcL chloroplast intergenic region of recipient strain CC2653, using the psaB and rbcL gene sequences for the double homologous recombination. This transformation conferred a functional rbcL gene and expression of the CrCpADH1 transgene in the recipient strain. This method alleviated the need to use antibiotics for selection, resulting in a negligible number of false positives during screening, and attaining a transformation efficiency greater than 90%. The approach also ensured segregation of chloroplast DNA copies, so as to achieve homoplasmy of the transformant chloroplast DNA, with a concomitant elimination of recipient strain Cp DNA. High levels of steady-state CrCpADH1 transcripts were detected in the homoplasmic transformants. However, CrCpADH1 protein levels were attenuated under continuous illumination growth conditions due to oxygen accumulation in the cells. Under conditions of low oxygen partial pressure, or anoxia, accumulation of CrCpADH1 protein in the cells and ethanol in the growth medium was observed. A metabolic pathway for ethanol production is proposed in Chlamydomonas, mediated by the chloroplast-localized CrCpADH1 transgenic enzyme.

Plastid non-coding RNAs: emerging candidates for gene regulation

Recent advances in transcriptomics and bioinformatics, specifically strand-specific RNA sequencing, have allowed high-throughput, comprehensive detection of low-abundance transcripts typical of the non-coding RNAs studied in bacteria and eukaryotes. Before this, few plastid non-coding RNAs (pncRNAs) had been identified, and even fewer had been investigated for any functional role in gene regulation. Relaxed plastid transcription initiation and termination result in full transcription of both chloroplast DNA strands. Following this, post-transcriptional processing produces a pool of metastable RNA species, including distinct pncRNAs. Here we review pncRNA biogenesis and possible functionality, and speculate that this RNA class may have an underappreciated role in plastid gene regulation.

Chlamydomonas reinhardtii as a viable platform for the production of recombinant proteins: current status and perspectives

Chlamydomonas reinhardtii has many advantages compared with traditional systems for the molecular farming of recombinant proteins. These include low production costs, rapid scalability at pilot level, absence of human pathogens and the ability to fold and assemble complex proteins accurately. Currently, the successful expression of several proteins with pharmaceutical relevance has been reported from the nuclear and the chloroplastic genome of this alga, demonstrating its usefulness for biotechnological applications. However, several factors affect the level of recombinant protein expression in Chlamydomonas such as enhancer elements, codon dependency, sensitivity to proteases and transformation-associated genotypic modification. The present review outlines a number of strategies to increase protein yields and summarizes recent achievements in algal protein production including biopharmaceuticals such as vaccines, antibodies, hormones and enzymes with implications on health-related approaches. The current status of bioreactor developments for algal culture and the challenges of scale-up and optimization processes are also discussed.

Enhanced chloroplast transgene expression in a nuclear mutant of Chlamydomonas

Chloroplast transformation in microalgae offers great promise for the production of proteins of pharmaceutical interest or for the development of novel biofuels. For many applications, high level expression of transgenes is desirable. We have transformed the chloroplast of Chlamydomonas reinhardtii with two genes, acrV and vapA, which encode antigens from the fish pathogen Aeromonas salmonicida. The promoters and 5' untranslated regions of four chloroplast genes were compared for their ability to drive expression of the bacterial genes. The highest levels of expression were obtained when they were placed under the control of the cis-acting elements from the psaA-exon1 gene. The expression of these chimeric genes was further increased when a nuclear mutation that affects a factor involved in psaA splicing was introduced in the genetic background of the chloroplast transformants. Accumulation of both the chimeric mRNAs and the recombinant proteins was dramatically increased, indicating that negative feedback loops limit the expression of chloroplast transgenes. Our results demonstrate the potential of manipulating anterograde signalling to alter negative regulatory feedback loops in the chloroplast and improve transgene expression.

Micro-algae come of age as a platform for recombinant protein production

A complete set of genetic tools is still being developed for the micro-alga Chlamydomonas reinhardtii. Yet even with this incomplete set, this photosynthetic single-celled plant has demonstrated significant promise as a platform for recombinant protein expression. In recent years, techniques have been developed that allow for robust expression of genes from both the nuclear and plastid genome. With these advances, many research groups have examined the pliability of this and other micro-algae as biological machines capable of producing recombinant peptides and proteins. This review describes recent successes in recombinant protein production in Chlamydomonas, including production of complex mammalian therapeutic proteins and monoclonal antibodies at levels sufficient for production at economic parity with existing production platforms. These advances have also shed light on the details of algal protein production at the molecular level, and provide insight into the next steps for optimizing micro-algae as a useful platform for the production of therapeutic and industrially relevant recombinant proteins.

Micro-algae come of age as a platform for recombinant protein production

A complete set of genetic tools is still being developed for the micro-alga Chlamydomonas reinhardtii. Yet even with this incomplete set, this photosynthetic single-celled plant has demonstrated significant promise as a platform for recombinant protein expression. In recent years, techniques have been developed that allow for robust expression of genes from both the nuclear and plastid genome. With these advances, many research groups have examined the pliability of this and other micro-algae as biological machines capable of producing recombinant peptides and proteins. This review describes recent successes in recombinant protein production in Chlamydomonas, including production of complex mammalian therapeutic proteins and monoclonal antibodies at levels sufficient for production at economic parity with existing production platforms. These advances have also shed light on the details of algal protein production at the molecular level, and provide insight into the next steps for optimizing micro-algae as a useful platform for the production of therapeutic and industrially relevant recombinant proteins.

Chloroplasts as expression platforms for plant-produced vaccines

Production of recombinant subunit vaccines from genes incorporated in the plastid genome is advantageous because of the attainable expression level due to high transgene copy number and the absence of gene silencing; biocontainment as a consequence of maternal inheritance of plastids and no transgene presence in the pollen; and expression of multiple transgenes in prokaryotic-like operons. We discuss the core technology of plastid transformation in Chlamydomonas reinhardtii, a unicellular alga, and Nicotiana tabacum (tobacco), a flowering plant species, and demonstrate the utility of the technology for the production of recombinant vaccine antigens.

Design and characterization of molecular tools for a Synthetic Biology approach towards developing cyanobacterial biotechnology

Cyanobacteria are suitable for sustainable, solar-powered biotechnological applications. Synthetic biology connects biology with computational design and an engineering perspective, but requires efficient tools and information about the function of biological parts and systems. To enable the development of cyanobacterial Synthetic Biology, several molecular tools were developed and characterized: (i) a broad-host-range BioBrick shuttle vector, pPMQAK1, was constructed and confirmed to replicate in Escherichia coli and three different cyanobacterial strains. (ii) The fluorescent proteins Cerulean, GFPmut3B and EYFP have been demonstrated to work as reporter proteins in cyanobacteria, in spite of the strong background of photosynthetic pigments. (iii) Several promoters, like P_rnpB and variants of P_rbcL, and a version of the promoter P_trc with two operators for enhanced repression, were developed and characterized in Synechocystis sp. strain PCC6803. (iv) It was shown that a system for targeted protein degradation, which is needed to enable dynamic expression studies, is working in Synechocystis sp. strain PCC6803. The pPMQAK1 shuttle vector allows the use of the growing numbers of BioBrick parts in many prokaryotes, and the other tools herein implemented facilitate the development of new parts and systems in cyanobacteria.