Sulphur responsiveness of the Chlamydomonas reinhardtii LHCBM9 promoter

A Rapid and Reversible Molecular "Switch" Regulating Protein Expression in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii, a prominent chassis in synthetic biology, faces limitations in regulating the expression of exogenous genes. A destabilization domain (DD)/Shield-1 system, originally derived from mammals, offers a ligand-dependent control of stability, making it a valuable tool. This system utilises the destabilization domain to induce rapid degradation of target protein unless stabilised by Shield-1, a synthetic ligand. Upon the addition of Shield-1,the degradation is halted, leading to the accumulation and stabilisation of the target protein. This system has demonstrated successful regulation of foreign protein expression in mammals, parasites, and plants. In this study, the DD/Shield-1 system was harnessed to regulate the expression of the paromomycin resistance gene and luciferase encoding gene in Chlamydomonas, revealing its capability for rapid, stable, and reversible protein expression regulation in microalgae, serving as a molecular switch. Furthermore, this regulation exhibits reagent dependency, enhancing its applicability in practical production. A strain with induced expression of the gene-editing protein, LbCas12a, was successfully constructed and then tested for gene editing. The findings not only enrich the toolkit for Chlamydomonas molecular studies but offer a promising technique for regulating the expression and validating the functionality of exogenous proteins in microalgae.

A Blue Light-Responsive Strong Synthetic Promoter Based on Rational Design in Chlamydomonas reinhardtii

Chlamydomonas reinhardtii (C. reinhardtii) is a single-cell green alga that can be easily genetically manipulated. With its favorable characteristics of rapid growth, low cost, non-toxicity, and the ability for post-translational protein modification, C. reinhardtii has emerged as an attractive option for the biosynthesis of various valuable products. To enhance the expression level of exogenous genes and overcome the silencing of foreign genes by C. reinhardtii, synthetic promoters such as the chimeric promoter AR have been constructed and evaluated. In this study, a synthetic promoter GA was constructed by hybridizing core fragments from the natural promoters of the acyl carrier protein gene (ACP2) and the glutamate dehydrogenase gene (GDH2). The GA promoter exhibited a significant increase (7 times) in expressing GUS, over the AR promoter as positive control. The GA promoter also displayed a strong responsiveness to blue light (BL), where the GUS expression was doubled compared to the white light (WL) condition. The ability of the GA promoter was further tested in the expression of another exogenous cadA gene, responsible for catalyzing the decarboxylation of lysine to produce cadaverine. The cadaverine yield driven by the GA promoter was increased by 1-2 times under WL and 2-3 times under BL as compared to the AR promoter. This study obtained, for the first time, a blue light-responsive GDH2 minimal fragment in C. reinhardtii, which delivered a doubling effect under BL when used alone or in hybrid. Together with the strong GA synthetic promoter, this study offered useful tools of synthetic biology to the algal biotechnology field.

Challenges and advances towards the rational design of microalgal synthetic promoters in Chlamydomonas reinhardtii

Microalgae hold enormous potential to provide a safe and sustainable source of high-value compounds, acting as carbon-fixing biofactories that could help to mitigate rapidly progressing climate change. Bioengineering microalgal strains will be key to optimizing and modifying their metabolic outputs, and to render them competitive with established industrial biotechnology hosts, such as bacteria or yeast. To achieve this, precise and tuneable control over transgene expression will be essential, which would require the development and rational design of synthetic promoters as a key strategy. Among green microalgae, Chlamydomonas reinhardtii represents the reference species for bioengineering and synthetic biology; however, the repertoire of functional synthetic promoters for this species, and for microalgae generally, is limited in comparison to other commercial chassis, emphasizing the need to expand the current microalgal gene expression toolbox. Here, we discuss state-of-the-art promoter analyses, and highlight areas of research required to advance synthetic promoter development in C. reinhardtii. In particular, we exemplify high-throughput studies performed in other model systems that could be applicable to microalgae, and propose novel approaches to interrogating algal promoters. We lastly outline the major limitations hindering microalgal promoter development, while providing novel suggestions and perspectives for how to overcome them.

Biocircuits in plants and eukaryotic algae

As one of synthetic biology's foundations, biocircuits are a strategy of genetic parts assembling to recognize a signal and to produce a desirable output to interfere with a biological function. In this review, we revisited the progress in the biocircuits technology basis and its mandatory elements, such as the characterization and assembly of functional parts. Furthermore, for a successful implementation, the transcriptional control systems are a relevant point, and the computational tools help to predict the best combinations among the biological parts planned to be used to achieve the desirable phenotype. However, many challenges are involved in delivering and stabilizing the synthetic structures. Some research experiences, such as the golden crops, biosensors, and artificial photosynthetic structures, can indicate the positive and limiting aspects of the practice. Finally, we envision that the modulatory structural feature and the possibility of finer gene regulation through biocircuits can contribute to the complex design of synthetic chromosomes aiming to develop plants and algae with new or improved functions.

A gene regulatory network for antenna size control in carbon dioxide-deprived Chlamydomonas reinhardtii cells

In green microalgae, prolonged exposure to inorganic carbon depletion requires long-term acclimation responses, involving modulated gene expression and the adjustment of photosynthetic activity to the prevailing supply of carbon dioxide. Here, we describe a microalgal regulatory cycle that adjusts the light-harvesting capacity at photosystem II (PSII) to the prevailing supply of carbon dioxide in Chlamydomonas (Chlamydomonas reinhardtii). It engages low carbon dioxide response factor (LCRF), a member of the squamosa promoter-binding protein (SBP) family of transcription factors, and the previously characterized cytosolic translation repressor nucleic acid-binding protein 1 (NAB1). LCRF combines a DNA-binding SBP domain with a conserved domain for protein-protein interaction. LCRF transcription is rapidly induced by carbon dioxide depletion. LCRF activates NAB1 transcription by specifically binding to tetranucleotide motifs present in its promoter. Accumulation of the NAB1 protein enhances translational repression of its prime target mRNA, encoding the PSII-associated major light-harvesting protein LHCBM6. The resulting truncation of the PSII antenna size helps maintaining a low excitation during carbon dioxide limitation. Analyses of low carbon dioxide acclimation in nuclear insertion mutants devoid of a functional LCRF gene confirm the essentiality of this novel transcription factor for the regulatory circuit.

Nuclear transformation of Chlamydomonas reinhardtii: A review

Chlamydomonas reinhardtii is a model organism with three sequenced genomes capable of genetic transformation. C. reinhardtii has the advantages of being low cost, non-toxic, and having a post-translational modification system that ensures the recombinant proteins have the same activity as natural proteins, thus making it a great platform for application in molecular biology and other fields. In this review, we summarize the existing methods for nuclear transformation of C. reinhardtii, genes for selection, examples of foreign protein expression, and factors affecting transformation efficiency, to provide insights into effective strategies for the nuclear transformation of C. reinhardtii.

Introducing Dunaliella LIP promoter containing light-inducible motifs improves transgenic expression in Chlamydomonas reinhardtii

Promoter of the light-inducible protein gene (LIP) of Dunaliella was recently isolated in our laboratory. The aim of this work is to find the light-inducible motif in the Dunaliella LIP promoter and verify its regulatory motif with a Gaussia luciferase reporter gene transformed in Chlamydomonas reinhardtii. 400 bp upstream to the translational start site of the Dunaliella LIP gene was gradually truncated and analyzed for the luciferase expression. Furthermore, this promoter comprising duplicated or triplicated light-responsive motifs was tested for its augmentation of light response. Two putative light-responsive motifs, GT-1 binding motif and sequences over-represented in light-repressed promoters (SORLIP) located in the 200 bp LIP promoter fragment were analyzed for their light responsibility. It is turned out that SORLIP was responsible for the light-inducible activity. With the copy number of SORLIP up to three showed stronger high light response compared with the native LIP promoter fragment. Therefore, we found a light-responsive DNA motif operating in Chlamydomonas and confirm a synthetic promoter including this motif displayed light inducibility in heterologously transformed green algae for the first time. This light-inducible expression system will be applied to various area of algal research including algal biotechnology.