Identification and Functional Analysis of the psaD Promoter of Chlorella vulgaris Using Heterologous Model Strains

Genome-wide analysis of Catalase gene family reveal insights into abiotic stress response mechanism in Brassica juncea and B. rapa

Environmental stresses affect the yield and productivity of Brassica crops. Catalases are important antioxidant enzymes involved in reducing excess hydrogen peroxide produced by environmental stresses. In the present study, nine and seven CAT family members in two oilseed Brassica species (B. juncea and B. rapa) were identified with complete characterization based on gene and protein structure. Phylogenetic classification categorized CAT proteins into three classes and differentiated the monocot and dicot-specific CAT proteins. Further, the gene and protein characterizations revealed a high degree of conservation across the CAT family members. Differences were observed in the CAT-HEME binding affinity in CAT1, CAT2, and CAT3 isozymes, which could suggest their differential enzyme activities in different conditions. Furthermore, protein-protein interaction with other antioxidant proteins suggested their coordinated role in ROS scavenging mechanisms. Notably, the differential gene expression of BjuCATs and BraCATs and CAT enzyme activities suggested their crucial roles in major abiotic stresses faced by Brassica species. Promoter analysis in BjuCATs and BraCATs suggested the presence of abiotic-stress responsive cis-regulatory elements. Gene regulatory network analysis suggested miRNA and TF mediated stress response in BjuCATs and BraCATs. CAT family screening and characterization in Brassica sp. has established a basic ground for further functional validation in abiotic and heavy-metal stresses which can help in developing stress tolerant crops.

Advantages of Heterotrophic Microalgae as a Host for Phytochemicals Production

Currently, most commercial recombinant technologies rely on host systems. However, each host has their own benefits and drawbacks, depending on the target products. Prokaryote host is lack of post-transcriptional and post-translational mechanisms, making them unsuitable for eukaryotic productions like phytochemicals. Even there are other eukaryote hosts (e.g., transgenic animals, mammalian cell, and transgenic plants), but those hosts have some limitations, such as low yield, high cost, time consuming, virus contamination, and so on. Thus, flexible platforms and efficient methods that can produced phytochemicals are required. The use of heterotrophic microalgae as a host system is interesting because it possibly overcome those obstacles. This paper presents a comprehensive review of heterotrophic microalgal expression host including advantages of heterotrophic microalgae as a host, genetic engineering of microalgae, genetic transformation of microalgae, microalgal engineering for phytochemicals production, challenges of microalgal hosts, key market trends, and future view. Finally, this review might be a directions of the alternative microalgae host for high-value phytochemicals production in the next few years.

Heterologous Gene Expression System Using the Cold-Inducible CnAFP Promoter in Chlamydomonas reinhardtii

To increase the availability of microalgae as producers of valuable compounds, it is necessary to develop novel systems for gene expression regulation. Among the diverse expression systems available in microalgae, none are designed to induce expression by low temperature. In this study, we explored a cold-inducible system using the antifreeze protein (AFP) promoter from a polar diatom, Chaetoceros neogracile. A vector containing the CnAFP promoter (pCnAFP) was generated to regulate nuclear gene expression, and reporter genes (Gaussia luciferase (GLuc) and mVenus fluorescent protein (mVenus)) were successfully expressed in the model microalga, Chlamydomonas reinhardtii. In particular, under the control of pCnAFP, the expression of these genes was increased at low temperature, unlike pAR1, a promoter that is widely used for gene expression in C. reinhardtii. Promoter truncation assays showed that cold inducibility was still present even when pCnAFP was shortened to 600 bp, indicating the presence of a low-temperature response element between -600 and -477 bp. Our results show the availability of new heterologous gene expression systems with cold-inducible promoters and the possibility to find novel low-temperature response factors in microalgae. Through further improvement, this cold-inducible promoter could be used to develop more efficient expression tools.

Bioengineering of Microalgae: Recent Advances, Perspectives, and Regulatory Challenges for Industrial Application

Microalgae, due to their complex metabolic capacity, are being continuously explored for nutraceuticals, pharmaceuticals, and other industrially important bioactives. However, suboptimal yield and productivity of the bioactive of interest in local and robust wild-type strains are of perennial concerns for their industrial applications. To overcome such limitations, strain improvement through genetic engineering could play a decisive role. Though the advanced tools for genetic engineering have emerged at a greater pace, they still remain underused for microalgae as compared to other microorganisms. Pertaining to this, we reviewed the progress made so far in the development of molecular tools and techniques, and their deployment for microalgae strain improvement through genetic engineering. The recent availability of genome sequences and other omics datasets form diverse microalgae species have remarkable potential to guide strategic momentum in microalgae strain improvement program. This review focuses on the recent and significant improvements in the omics resources, mutant libraries, and high throughput screening methodologies helpful to augment research in the model and non-model microalgae. Authors have also summarized the case studies on genetically engineered microalgae and highlight the opportunities and challenges that are emerging from the current progress in the application of genome-editing to facilitate microalgal strain improvement. Toward the end, the regulatory and biosafety issues in the use of genetically engineered microalgae in commercial applications are described.

The establishment of new protein expression system using N starvation inducible promoters in Chlorella

Chlorella is a unicellular green microalga that has been used in fields such as bioenergy production and food supplementation. In this study, two promoters of N (nitrogen) deficiency-inducible Chlorella vulgaris N Deficiency Inducible (CvNDI) genes were isolated from Chlorella vulgaris UTEX 395. These promoters were used for the production of a recombinant protein, human granulocyte-colony stimulating factor (hG-CSF) in Chlorella vulgaris UTEX 395 and Chlorella sp. ArM0029B. To efficiently secrete the hG-CSF, the protein expression vectors incorporated novel signal peptides obtained from a secretomics analysis of Chlorella spp. After a stable transformation of those vectors with a codon-optimized hG-CSF sequence, hG-CSF polypeptides were successfully produced in the spent media of the transgenic Chlorella. To our knowledge, this is the first report of recombinant protein expression using endogenous gene components of Chlorella.

Site-Specific Gene Knock-Out and On-Site Heterologous Gene Overexpression in Chlamydomonas reinhardtii via a CRISPR-Cas9-Mediated Knock-in Method

Chlamydomonas reinhardtii is being transformed from a model organism to an industrial organism for the production of pigments, fatty acids, and pharmaceuticals. Genetic modification has been used to increase the economic value of C. reinhardtii. However, low gene-editing efficiency and position-effects hinder the genetic improvement of this microorganism. Recently, site-specific double-stranded DNA cleavage using CRISPR-Cas9 system has been applied to regulate a metabolic pathway in C. reinhardtii. In this study, we proved that site-specific gene expression can be induced by CRISPR-Cas9-mediated double-strand cleavage and non-homologous end joining (NHEJ) mechanism. The CRISPR-Cas9-mediated knock-in method was adopted to improve gene-editing efficiency and express the reporter gene on the intended site. Knock-in was performed using a combination of ribonucleoprotein (RNP) complex and DNA fragment (antibiotics resistance gene). Gene-editing efficiency was improved via optimization of a component of RNP complex. We found that when the gene CrFTSY was targeted, the efficiency of obtaining the desired mutant by the knock-in method combined with antibiotic resistance was nearly 37%; 2.5 times higher than the previous reports. Additionally, insertion of a long DNA fragment (3.2 and 6.4 kb) and site-specific gene expression were analyzed. We demonstrated the knock-out phenotype of CrFTSY and on-site inserted gene expression of luciferase and mVenus at the same time. This result showed that CRISPR-Cas9-mediated knock-in can be used to express the gene of interest avoiding position-effects in C. reinhardtii. This report could provide a new perspective to the use of gene-editing. Furthermore, the technical improvements in genetic modification may accelerate the commercialization of C. reinhardtii.