Stabilization and translation of synthetic operon-derived mRNAs in chloroplasts by sequences representing PPR protein-binding sites

Posttranscriptional tuning of gene expression over a large dynamic range in synthetic tobacco chloroplast operons

Achieving optimally balanced gene expression within synthetic operons requires regulatory elements capable of providing a spectrum of expression levels. In this study, we investigate the expression of gfp reporter gene in tobacco chloroplasts, guided by variants of the plastid atpH 5' UTR, which harbors a binding site for PPR10, a protein that activates atpH at the posttranscriptional level. Our findings reveal that endogenous tobacco PPR10 confers distinct levels of reporter activation when coupled with the tobacco and maize atpH 5' UTRs in different design contexts. Notably, high GFP expression was not coupled to the stabilization of monocistronic gfp transcripts in dicistronic reporter lines, adding to the evidence that PPR10 activates translation via a mechanism that is independent of its stabilization of monocistronic transcripts. Furthermore, the incorporation of a tRNA upstream of the UTR nearly abolishes gfp mRNA (and GFP protein), presumably by promoting such rapid RNA cleavage and 5' exonucleolytic degradation that PPR10 had insufficient time to bind and protect gfp RNA, resulting in a substantial reduction in GFP accumulation. When combined with a mutant atpH 5' UTR, the tRNA leads to an exceptionally low level of transgene expression. Collectively, this approach allows for tuning of reporter gene expression across a wide range, spanning from a mere 0.02-25% of the total soluble cellular protein. These findings highlight the potential of employing cis-elements from heterologous species and expand the toolbox available for plastid synthetic biology applications requiring multigene expression at varying levels.

A heat-inducible expression system for external control of gene expression in plastids

Inducible expression systems can overcome the trade-off between high-level transgene expression and its pleiotropic effects on plant growth. In addition, they can facilitate the expression of biochemical pathways that produce toxic metabolites. Although a few inducible expression systems for the control of transgene expression in plastids have been developed, they all depend on chemical inducers and/or nuclear transgenes. Here we report a temperature-inducible expression system for plastids that is based on the bacteriophage λ leftward and rightward promoters (pL/pR) and the temperature-sensitive repressor cI857. We show that the expression of green fluorescent protein (GFP) in plastids can be efficiently repressed by cI857 under normal growth conditions, and becomes induced over time upon exposure to elevated temperatures in a light-dependent process. We further demonstrate that by introducing into plastids an expression system based on the bacteriophage T7 RNA polymerase, the temperature-dependent accumulation of GFP increased further and was ~24 times higher than expression driven by the pL/pR promoter alone, reaching ~0.48% of the total soluble protein. In conclusion, our heat-inducible expression system provides a new tool for the external control of plastid (trans) gene expression that is cost-effective and does not depend on chemical inducers.

Full-length transcriptome analysis of Ophioglossum vulgatum: effects of experimentally identified chloroplast gene clusters on expression and evolutionary patterns

Genes with similar or related functions in chloroplasts are often arranged in close proximity, forming clusters on chromosomes. These clusters are transcribed coordinated to facilitate the expression of genes with specific function. Our previous study revealed a significant negative correlation between the chloroplast gene expression level of the rare medicinal fern Ophioglossum vulgatum and its evolutionary rates as well as selection pressure. Therefore, in this study, we employed a combination of SMRT and Illumina sequencing technology to analyze the full-length transcriptome sequencing of O. vulgatum for the first time. In particular, we experimentally identified gene clusters based on transcriptome data and investigated the effects of chloroplast gene clustering on expression and evolutionary patterns. The results revealed that the total sequenced data volume of the full-length transcriptome of O. vulgatum amounted to 71,950,652,163 bp, and 110 chloroplast genes received transcript coverage. Nine different types of gene clusters were experimentally identified in their transcripts. The chloroplast cluster genes may cause a decrease in non-synonymous substitution rate and selection pressure, as well as a reduction in transversion rate, transition rate, and their ratio. While expression levels of chloroplast cluster genes in leaf, sporangium, and stem would be relatively elevated. The Mann-Whitney U test indicated statistically significant in the selection pressure, sporangia and leaves groups (P < 0.05). We have contributed novel full-length transcriptome data resources for ferns, presenting new evidence on the effects of chloroplast gene clustering on expression land evolutionary patterns, and offering new theoretical support for transgenic research through gene clustering.

Prospects of chloroplast metabolic engineering for developing nutrient-dense food crops

Addressing nutritional deficiencies in food crops through biofortification is a sustainable approach to tackling malnutrition. Biofortification is continuously being attempted through conventional breeding as well as through various plant biotechnological interventions, ranging from molecular breeding to genetic engineering and genome editing for enriching crops with various health-promoting metabolites. Genetic engineering is used for the rational incorporation of desired nutritional traits in food crops and predominantly operates through nuclear and chloroplast genome engineering. In the recent past, chloroplast engineering has been deployed as a strategic tool to develop model plants with enhanced nutritional traits due to the various advantages it offers over nuclear genome engineering. However, this approach needs to be extended for the nutritional enhancement of major food crops. Further, this platform could be combined with strategies, such as synthetic biology, chloroplast editing, nanoparticle-mediated rapid chloroplast transformation, and horizontal gene transfer through grafting for targeting endogenous metabolic pathways for overproducing native nutraceuticals, production of biopharmaceuticals, and biosynthesis of designer nutritional compounds. This review focuses on exploring various features of chloroplast genome engineering for nutritional enhancement of food crops by enhancing the levels of existing metabolites, restoring the metabolites lost during crop domestication, and introducing novel metabolites and phytonutrients needed for a healthy daily diet.

Plant organellar RNA maturation

Plant organellar RNA metabolism is run by a multitude of nucleus-encoded RNA-binding proteins (RBPs) that control RNA stability, processing, and degradation. In chloroplasts and mitochondria, these post-transcriptional processes are vital for the production of a small number of essential components of the photosynthetic and respiratory machinery-and consequently for organellar biogenesis and plant survival. Many organellar RBPs have been functionally assigned to individual steps in RNA maturation, often specific to selected transcripts. While the catalog of factors identified is ever-growing, our knowledge of how they achieve their functions mechanistically is far from complete. This review summarizes the current knowledge of plant organellar RNA metabolism taking an RBP-centric approach and focusing on mechanistic aspects of RBP functions and the kinetics of the processes they are involved in.

Engineering α-carboxysomes into plant chloroplasts to support autotrophic photosynthesis

The growth in world population, climate change, and resource scarcity necessitate a sustainable increase in crop productivity. Photosynthesis in major crops is limited by the inefficiency of the key CO₂-fixing enzyme Rubisco, owing to its low carboxylation rate and poor ability to discriminate between CO₂ and O₂. In cyanobacteria and proteobacteria, carboxysomes function as the central CO₂-fixing organelles that elevate CO₂ levels around encapsulated Rubisco to enhance carboxylation. There is growing interest in engineering carboxysomes into crop chloroplasts as a potential route for improving photosynthesis and crop yields. Here, we generate morphologically correct carboxysomes in tobacco chloroplasts by transforming nine carboxysome genetic components derived from a proteobacterium. The chloroplast-expressed carboxysomes display a structural and functional integrity comparable to native carboxysomes and support autotrophic growth and photosynthesis of the transplastomic plants at elevated CO₂. Our study provides proof-of-concept for a route to engineering fully functional CO₂-fixing modules and entire CO₂-concentrating mechanisms into chloroplasts to improve crop photosynthesis and productivity.

A CRR2-Dependent sRNA Sequence Supports Papillomavirus Vaccine Expression in Tobacco Chloroplasts

Introduction: Human papillomavirus (HPV) infection is the leading cause of cervical cancer, and vaccination with HPV L1 capsid proteins has been successful in controlling it. However, vaccination coverage is not universal, particularly in developing countries, where 80% of all cervical cancer cases occur. Cost-effective vaccination could be achieved by expressing the L1 protein in plants. Various efforts have been made to produce the L1 protein in plants, including attempts to express it in chloroplasts for high-yield performance. However, manipulating chloroplast gene expression requires complex and difficult-to-control expression elements. In recent years, a family of nuclear-encoded, chloroplast-targeted RNA-binding proteins, the pentatricopeptide repeat (PPR) proteins, were described as key regulators of chloroplast gene expression. For example, PPR proteins are used by plants to stabilize and translate chloroplast mRNAs. Objectives: To demonstrate that a PPR target site can be used to drive HPV L1 expression in chloroplasts. Methods: To test our hypothesis, we used biolistic chloroplast transformation to establish tobacco lines that express two variants of the HPV L1 protein under the control of the target site of the PPR protein CHLORORESPIRATORY REDUCTION2 (CRR2). The transgenes were inserted into a dicistronic operon driven by the plastid rRNA promoter. To determine the effectiveness of the PPR target site for the expression of the HPV L1 protein in the chloroplasts, we analyzed the accumulation of the transgenic mRNA and its processing, as well as the accumulation of the L1 protein in the transgenic lines. Results: We established homoplastomic lines carrying either the HPV18 L1 protein or an HPV16B Enterotoxin::L1 fusion protein. The latter line showed severe growth retardation and pigment loss, suggesting that the fusion protein is toxic to the chloroplasts. Despite the presence of dicistronic mRNAs, we observed very little accumulation of monocistronic transgenic mRNA and no significant increase in CRR2-associated small RNAs. Although both lines expressed the L1 protein, quantification using an external standard suggested that the amounts were low. Conclusions: Our results suggest that PPR binding sites can be used to drive vaccine expression in plant chloroplasts; however, the factors that modulate the effectiveness of target gene expression remain unclear. The identification of dozens of PPR binding sites through small RNA sequencing expands the set of expression elements available for high-value protein production in chloroplasts.

Transplastomic approaches for metabolic engineering

The plastid (chloroplast) genome of seed plants represents an attractive target of metabolic pathway engineering by genetic transformation. Although the plastid genome is relatively small, it can accommodate large amounts of foreign DNA that precisely integrates via homologous recombination, and is largely excluded from pollen transmission due to the maternal mode of plastid inheritance. Since the engineering of metabolic pathways often requires the expression of multiple transgenes, the possibility to conveniently stack transgenes in synthetic operons makes the transplastomic technology particularly appealing in the area of metabolic engineering. Absence of epigenetic gene silencing mechanisms from plastids and the possibility to achieve high transgene expression levels further add to the attractiveness of plastid genome transformation. This review focuses on engineering principles and available tools for the transplastomic expression of enzymes and pathways, and highlights selected recent applications in metabolic engineering.

Harnessing the Algal Chloroplast for Heterologous Protein Production

Photosynthetic microbes are gaining increasing attention as heterologous hosts for the light-driven, low-cost production of high-value recombinant proteins. Recent advances in the manipulation of unicellular algal genomes offer the opportunity to establish engineered strains as safe and viable alternatives to conventional heterotrophic expression systems, including for their use in the feed, food, and biopharmaceutical industries. Due to the relatively small size of their genomes, algal chloroplasts are excellent targets for synthetic biology approaches, and are convenient subcellular sites for the compartmentalized accumulation and storage of products. Different classes of recombinant proteins, including enzymes and peptides with therapeutical applications, have been successfully expressed in the plastid of the model organism Chlamydomonas reinhardtii, and of a few other species, highlighting the emerging potential of transplastomic algal biotechnology. In this review, we provide a unified view on the state-of-the-art tools that are available to introduce protein-encoding transgenes in microalgal plastids, and discuss the main (bio)technological bottlenecks that still need to be addressed to develop robust and sustainable green cell biofactories.

Riboswitch-mediated inducible expression of an astaxanthin biosynthetic operon in plastids

The high-value carotenoid astaxanthin (3,3'-dihydroxy-β,β-carotene-4,4'-dione) is one of the most potent antioxidants in nature. In addition to its large-scale use in fish farming, the pigment has applications as a food supplement and an active ingredient in cosmetics and in pharmaceuticals for the treatment of diseases linked to reactive oxygen species. The biochemical pathway for astaxanthin synthesis has been introduced into seed plants, which do not naturally synthesize this pigment, by nuclear and plastid engineering. The highest accumulation rates have been achieved in transplastomic plants, but massive production of astaxanthin has resulted in severe growth retardation. What limits astaxanthin accumulation levels and what causes the mutant phenotype is unknown. Here, we addressed these questions by making astaxanthin synthesis in tobacco (Nicotiana tabacum) plastids inducible by a synthetic riboswitch. We show that, already in the uninduced state, astaxanthin accumulates to similarly high levels as in transplastomic plants expressing the pathway constitutively. Importantly, the inducible plants displayed wild-type-like growth properties and riboswitch induction resulted in a further increase in astaxanthin accumulation. Our data suggest that the mutant phenotype associated with constitutive astaxanthin synthesis is due to massive metabolite turnover, and indicate that astaxanthin accumulation is limited by the sequestration capacity of the plastid.

CDE4 encodes a pentatricopeptide repeat protein involved in chloroplast RNA splicing and affects chloroplast development under low-temperature conditions in rice

Pentatricopeptide repeat (PPR) proteins play important roles in the post-transcriptional modification of organellar RNAs in plants. However, the function of most PPR proteins remains unknown. Here, we characterized the rice (Oryza sativa L.) chlorophyll deficient 4 (cde4) mutant which exhibits an albino phenotype during early leaf development, with decreased chlorophyll contents and abnormal chloroplasts at low-temperature (20°C). Positional cloning revealed that CDE4 encodes a P-type PPR protein localized in chloroplasts. In the cde4 mutant, plastid-encoded polymerase (PEP)-dependent transcript levels were significantly reduced, but transcript levels of nuclear-encoded genes were increased compared to wild-type plants at 20°C. CDE4 directly binds to the transcripts of the chloroplast genes rpl2, ndhA, and ndhB. Intron splicing of these transcripts was defective in the cde4 mutant at 20°C, but was normal at 32°C. Moreover, CDE4 interacts with the guanylate kinase VIRESCENT 2 (V2); overexpression of V2 enhanced CDE4 protein stability, thereby rescuing the cde4 phenotype at 20°C. Our results suggest that CDE4 participates in plastid RNA splicing and plays an important role in rice chloroplast development under low-temperature conditions.

Construction of DNA Tools for Hyperexpression in Marchantia Chloroplasts

Chloroplasts are attractive platforms for synthetic biology applications since they are capable of driving very high levels of transgene expression, if mRNA production and stability are properly regulated. However, plastid transformation is a slow process and currently limited to a few plant species. The liverwort Marchantia polymorpha is a simple model plant that allows rapid transformation studies; however, its potential for protein hyperexpression has not been fully exploited. This is partially due to the fact that chloroplast post-transcriptional regulation is poorly characterized in this plant. We have mapped patterns of transcription in Marchantia chloroplasts. Furthermore, we have obtained and compared sequences from 51 bryophyte species and identified putative sites for pentatricopeptide repeat protein binding that are thought to play important roles in mRNA stabilization. Candidate binding sites were tested for their ability to confer high levels of reporter gene expression in Marchantia chloroplasts, and levels of protein production and effects on growth were measured in homoplastic transformed plants. We have produced novel DNA tools for protein hyperexpression in this facile plant system that is a test-bed for chloroplast engineering.

Replicating minichromosomes as a new tool for plastid genome engineering

Plant molecular farming, that is, using plants as hosts for production of therapeutic proteins and high-value compounds, has gained substantial interest in recent years. Chloroplasts in particular are an attractive subcellular compartment for expression of foreign genes. Here, we present a new method for transgene introduction and expression in chloroplasts that, unlike classically used approaches, does not require transgene insertion into the chloroplast genome. Instead, the transgene is amplified as a physically independent entity termed a 'minichromosome'. Amplification occurs in the presence of a helper protein that initiates the replication process via recognition of specific sequences flanking the transgene, resulting in accumulation of extremely high levels of transgene DNA. Importantly, we demonstrate that such amplified transgenes serve as a template for foreign protein expression, are maintained stably during plant development and are maternally transmitted to the progeny. These findings indicate that the minichromosome-based approach is an attractive tool for transgene expression in chloroplasts and for organelle genome engineering.

Advancing organelle genome transformation and editing for crop improvement

Plant cells contain three organelles that harbor DNA: the nucleus, plastids, and mitochondria. Plastid transformation has emerged as an attractive platform for the generation of transgenic plants, also referred to as transplastomic plants. Plastid genomes have been genetically engineered to improve crop yield, nutritional quality, and resistance to abiotic and biotic stresses, as well as for recombinant protein production. Despite many promising proof-of-concept applications, transplastomic plants have not been commercialized to date. Sequence-specific nuclease technologies are widely used to precisely modify nuclear genomes, but these tools have not been applied to edit organelle genomes because the efficient homologous recombination system in plastids facilitates plastid genome editing. Unlike plastid transformation, successful genetic transformation of higher plant mitochondrial genome transformation was tested in several research group, but not successful to date. However, stepwise progress has been made in modifying mitochondrial genes and their transcripts, thus enabling the study of their functions. Here, we provide an overview of advances in organelle transformation and genome editing for crop improvement, and we discuss the bottlenecks and future development of these technologies.

Pas de Trois: An Overview of Penta-, Tetra-, and Octo-Tricopeptide Repeat Proteins From Chlamydomonas reinhardtii and Their Role in Chloroplast Gene Expression

Penta-, Tetra-, and Octo-tricopeptide repeat (PPR, TPR, and OPR) proteins are nucleus-encoded proteins composed of tandem repeats of 35, 34, and 38-40 amino acids, respectively. They form helix-turn-helix structures that interact with mRNA or other proteins and participate in RNA stabilization, processing, maturation, and act as translation enhancers of chloroplast and mitochondrial mRNAs. These helical repeat proteins are unevenly present in plants and algae. While PPR proteins are more abundant in plants than in algae, OPR proteins are more abundant in algae. In Arabidopsis, maize, and rice there have been 450, 661, and 477 PPR proteins identified, respectively, which contrasts with only 14 PPR proteins identified in Chlamydomonas reinhardtii. Likewise, more than 120 OPR proteins members have been predicted from the nuclear genome of C. reinhardtii and only one has been identified in Arabidopsis thaliana. Due to their abundance in land plants, PPR proteins have been largely characterized making it possible to elucidate their RNA-binding code. This has even allowed researchers to generate engineered PPR proteins with defined affinity to a particular target, which has served as the basis to develop tools for gene expression in biotechnological applications. However, fine elucidation of the helical repeat proteins code in Chlamydomonas is a pending task. In this review, we summarize the current knowledge on the role PPR, TPR, and OPR proteins play in chloroplast gene expression in the green algae C. reinhardtii, pointing to relevant similarities and differences with their counterparts in plants. We also recapitulate on how these proteins have been engineered and shown to serve as mRNA regulatory factors for biotechnological applications in plants and how this could be used as a starting point for applications in algae.

Engineering Chloroplasts for High-Level Constitutive or Inducible Transgene Expression

Expression of transgenes from the plastid genome offers a number of attractions to biotechnologists, with the potential to attain very high protein accumulation levels arguably being the most attractive one. High-level transgene expression is of particular importance in resistance engineering (e.g., for expression of insecticidal proteins) and molecular farming (e.g., for expression of pharmaceutical proteins and industrial enzymes). Over the past decades, the production of many commercially valuable proteins in chloroplast-transgenic (transplastomic) plants has been attempted, including pharmaceutical proteins (e.g., subunit vaccines and protein antibiotics) and industrial enzymes. Although in some cases, spectacularly high foreign protein accumulation levels have been obtained, expression levels were disappointingly poor in other cases. In this review, I summarize our current knowledge about the factors influencing the efficiency of plastid transgene expression, and highlight possible optimization strategies to alleviate problems with poor expression levels. I also discuss available techniques for inducible expression of chloroplast transgenes.

Independent translation of ORFs in dicistronic operons, synthetic building blocks for polycistronic chloroplast gene expression

We designed a dicistronic plastid marker system that relies on the plastid's ability to translate polycistronic mRNAs. The identification of transplastomic clones is based on selection for antibiotic resistance encoded in the first open reading frame (ORF) and accumulation of the reporter gene product in tobacco chloroplasts encoded in the second ORF. The antibiotic resistance gene may encode spectinomycin or kanamycin resistance based on the expression of aadA or neo genes, respectively. The reporter gene used in the study is the green fluorescent protein (GFP). The mRNA level depends on the 5'-untranslated region of the first ORF. The protein output depends on the strengths of the ribosome binding, and is proportional with the level of translatable mRNA. Because the dicistronic mRNA is not processed, we could show that protein output from the second ORF is independent from the first ORF. High-level GFP accumulation from the second ORF facilitates identification of transplastomic events under ultraviolet light. Expression of multiple proteins from an unprocessed mRNA is an experimental design that enables predictable protein output from polycistronic mRNAs, expanding the toolkit of plant synthetic biology.

Phenotypic, Transcriptomic, and Metabolomic Signatures of Root-Specifically Overexpressed OsCKX2 in Rice

Cytokinins are crucial signaling molecules that regulate plant growth and development. OsCKX2 irreversibly degrades nucleobase cytokinins by encoding cytokinin oxidase/dehydrogenase to control grain production in rice. In this study, OsCKX2 was specifically overexpressed in roots using RCc3 promoter to investigate the effects of root-source cytokinins on the growth of rice. OsCKX2 overexpressed (OE) rice showed retarded growth with lower cytokinin levels and biomass production. Shoot-specific transcriptome analysis between OsCKX2 OE rice and wild type (WT) revealed differentially expressed genes (DEGs) associated with cell division, cell wall structure, phytohormone signaling, and assimilation and catabolism. Metabolome analysis indicated that a majority of differential primary metabolites, such as amino acids and organic acids, increased, while lipids decreased in OsCKX2 OE rice. Integration of transcriptomic and metabolomic data showed that several DEGs and differential metabolites were related to glycolysis and tricarboxylic acid cycle (TCA). To conclude, reduced cytokinin levels via root-specific overexpression of OsCKX2 resulted in developmental defects, which confirmed the importance of root-source cytokinins in plant growth and morphogenesis.

GhYGL1d, a pentatricopeptide repeat protein, is required for chloroplast development in cotton

BACKGROUND

The pentatricopeptide repeat (PPR) gene family, which contains multiple 35-amino acid repeats, constitutes one of the largest gene families in plants. PPR proteins function in organelles to target specific transcripts and are involved in plant development and growth. However, the function of PPR proteins in cotton is still unknown.

RESULTS

In this study, we characterized a PPR gene YELLOW-GREEN LEAF (GhYGL1d) that is required for cotton plastid development. The GhYGL1d gene has a DYW domain in C-terminal and is highly express in leaves, localized to the chloroplast fractions. GhYGL1d share high amino acid-sequence homology with AtECB2. In atecb2 mutant, overexpression of GhYGL1d rescued the seedling lethal phenotype and restored the editing of accD and ndhF transcripts. Silencing of GhYGL1d led to the reduction of chlorophyll and phenotypically yellow-green leaves in cotton. Compared with wild type, GhYGL1d-silenced cotton showed significant deformations of thylakoid structures. Furthermore, the transcription levels of plastid-encoded polymerase (PEP) and nuclear-encoded polymerase (NEP) dependent genes were decreased in GhYGL1d-silenced cotton.

CONCLUSIONS

Our data indicate that GhYGL1d not only contributes to the editing of accD and ndhF genes, but also affects the expression of NEP- and PEP-dependent genes to regulate the development of thylakoids, and therefore regulates leaf variegation in cotton.

Engineered PPR proteins as inducible switches to activate the expression of chloroplast transgenes

The engineering of plant genomes presents exciting opportunities to modify agronomic traits and to produce high-value products in plants. Expression of foreign proteins from transgenes in the chloroplast genome offers advantages that include the capacity for prodigious protein output, the lack of transgene silencing and the ability to express multicomponent pathways from polycistronic mRNA. However, there remains a need for robust methods to regulate plastid transgene expression. We designed orthogonal activators that boost the expression of chloroplast transgenes harbouring cognate cis-elements. Our system exploits the programmable RNA sequence specificity of pentatricopeptide repeat proteins and their native functions as activators of chloroplast gene expression. When expressed from nuclear transgenes, the engineered proteins stimulate the expression of plastid transgenes by up to ~40-fold, with maximal protein abundance approaching that of Rubisco. This strategy provides a means to regulate and optimize the expression of foreign genes in chloroplasts and to avoid deleterious effects of their products on plant growth.

Prediction and large-scale analysis of primary operons in plastids reveals unique genetic features in the evolution of chloroplasts

While bacterial operons have been thoroughly studied, few analyses of chloroplast operons exist, limiting the ability to study fundamental elements of these structures and utilize them for synthetic biology. Here, we describe the creation of a plastome-specific operon database (link provided below) achieved by combining experimental tools and predictive modeling. Using a Reverse-Transcription-PCR based method and published data, we determined the transcription-state of 213 gene pairs from four plastomes of evolutionary distinct organisms. By analyzing sequence-based features computed for our dataset, we were able to highlight fundamental characteristics differentiating between operon pairs and non-operon pairs. These include an interesting tendency toward maintaining similar messenger RNA-folding profiles in operon gene pairs, a feature that failed to yield any informative separation in cyanobacteria, suggesting that it catches unique traits of operon gene expression, which have evolved post-endosymbiosis. Subsequently, we used this feature set to train a random-forest classifier for operon prediction. As our results demonstrate the ability of our predictor to obtain accurate (84%) and robust predictions on unlabeled datasets, we proceeded to building operon maps for 2018 sequenced plastids. Our database may now present new opportunities for promoting metabolic engineering and synthetic biology in chloroplasts.

The RNA editing factor DUA1 is crucial to chloroplast development at low temperature in rice

Low temperature stress hinders plant growth and chloroplast development and can limit the geographic range of cultivars. In rice, japonica cultivars have greater chilling tolerance than indica cultivars, but the molecular mechanism underlying chilling tolerance is unclear. Here, we report an RNA-binding protein, DUA1, cloned from the indica cultivar Dular, which exhibits a deficiency in chloroplast development at an early stage of development under low-temperature conditions. DUA1 shares high sequence homology with the pentatricopeptide repeat family and functions in plastid RNA editing under low-temperature conditions. Our data suggest that DUA1 can bind to the plastid-encoded rps8-182 transcript and disruption of DUA1 activity impairs editing. The RNA editing cofactor WSP1, a partner of DUA1, also participates in chloroplast development at low temperature. Western blot analysis indicates that WSP1 enhances DUA1 stability under low temperatures. DUA1 sequence analyses of rice core germplasm revealed that three major haplotypes of DUA1 and one haplotype showed substantial differences in chlorophyll content under low-temperature conditions. Variation at DUA1 may play an important role in the adaptation of rice to different growing regions.

Intercistronic expression elements (IEE) from the chloroplast of Chlamydomonas reinhardtii can be used for the expression of foreign genes in synthetic operons

Two intercistronic regions were identified as functional intercistronic expression elements (IEE) for the simultaneous expression of aphA-6 and gfp in a synthetic operon in the chloroplast of C. reinhardtii. Chlamydomonas reinhardtii, a biflagellate photosynthetic microalga, has been widely used in basic and applied science. Already three decades ago, Chlamydomonas had its chloroplast genome transformed and to this day constitutes the only alga routinely used in transplastomic technology. Despite the fact that over a 100 foreign genes have been expressed from the chloroplast genome, little has been done to address the challenge of expressing multiple genes in the form of operons, a development that is needed and crucial to push forward metabolic engineering and synthetic biology in this organism. Here, we studied five intercistronic regions and investigated if they can be used as intercistronic expression elements (IEE) in synthetic operons to drive the expression of foreign genes in the chloroplast of C. reinhardtii. The intercistronic regions were those from the psbB-psbT, psbN-psbH, psaC-petL, petL-trnN and tscA-chlN chloroplast operons, and the foreign genes were the aminoglycoside 3'-phosphotransferase (aphA-6), which confers resistance to kanamycin, and the green fluorescent protein gene (gfp). While all the intercistronic regions yielded lines that were resistant to kanamycin, only two (obtained with intercistronic regions from psbN-psbH and tscA-chlN) were identified as functional IEEs, yielding lines in which the second cistron (gfp) was translated and generated GFP. The IEEs we have identified could be useful for the stacking of genes for metabolic engineering or synthetic biology circuits in the chloroplast of C. reinhardtii.

Carboxysome encapsulation of the CO2-fixing enzyme Rubisco in tobacco chloroplasts

A long-term strategy to enhance global crop photosynthesis and yield involves the introduction of cyanobacterial CO2-concentrating mechanisms (CCMs) into plant chloroplasts. Cyanobacterial CCMs enable relatively rapid CO2 fixation by elevating intracellular inorganic carbon as bicarbonate, then concentrating it as CO2 around the enzyme Rubisco in specialized protein micro-compartments called carboxysomes. To date, chloroplastic expression of carboxysomes has been elusive, requiring coordinated expression of almost a dozen proteins. Here we successfully produce simplified carboxysomes, isometric with those of the source organism Cyanobium, within tobacco chloroplasts. We replace the endogenous Rubisco large subunit gene with cyanobacterial Form-1A Rubisco large and small subunit genes, along with genes for two key α-carboxysome structural proteins. This minimal gene set produces carboxysomes, which encapsulate the introduced Rubisco and enable autotrophic growth at elevated CO2. This result demonstrates the formation of α-carboxysomes from a reduced gene set, informing the step-wise construction of fully functional α-carboxysomes in chloroplasts.