Patent Grant
10480003
This application also incorporates by reference the attached Tables 1-4 found in paper form.
A sequence listing is submitted concurrently with the specification and is part of the specification and is hereby incorporated in its entirety by reference herein. This application also incorporates by reference the sequence listing found in computer-readable form in a *.txt file entitled, “2013-020-03_SeqListing_ST25.txt”, created on Aug. 5, 2016.
Field of the Invention
The present invention relates to synthetic biology, especially using operons and synthetic constructs to produce a non-natural or engineered cyclic by-pass of the photorespiration pathway in organisms such as plants, cyanobacteria and microbes.
Related Art
Oxygenic photosynthesis is the primary source of nearly all biological energy. In this process, light is converted into chemical energy which is used to fix CO2 in the CB cycle through the enzyme RuBisCO. The carboxylase activity of RuBisCO results in the addition of one molecule of CO2 to one molecule of ribulose-1,5-bisphosphate to create two molecules of 3-phosphoglycerate, thus fixing inorganic CO2 into triose phosphates. However, the competing oxygenase activity of RuBisCO results in the loss of fixed carbon through a process termed photorespiration. One of the ‘holy grails’ of photosynthesis research has been to engineer RuBisCO to improve CO2 fixation and reduce photorespiration; however, these attempts have been met with limited success. It has been shown that biochemical constraints as well as abiotic factors are crucial considerations in addressing the protein engineering of RuBisCO (1,2). Given this complexity, a more promising approach may be to accept the inherent ‘flaws’ of RuBisCO and improve net photosynthetic rates through engineered photorespiratory bypasses.
The role of photorespiration is highly debated, as it consumes much more energy and cellular resources than its carboxylase counterpart reaction. The fixed O2 from RuBisCO results in the toxic intermediate, 2-phosphoglycolate, which continues through the photorespiratory pathway (C2 cycle). This pathway is costly, because the 2-phosophoglycolate must be metabolized in order to detoxify the cell through an elaborate pathway involving more than a dozen enzymes (CHECK). Furthermore, the glycine decarboxylase conversion of glycine to serine, in the C2 cycle, releases both an ammonia and a CO2 molecule, resulting in a net loss of carbon and nitrogen. Previous work has bypassed the C2 cycle by introducing the glycolate catabolic pathway from Escherichia coli into Arabidopsis thaliana chloroplasts resulting in improved growth rates (2). The pathway introduced by Kebeish et al circumvented the loss of nitrogen; however, the glyoxylate carboligase decarboxylates glyoxylate, losing one CO2 molecule and thus still resulting in a net loss in carbon. Although the irrelevance of photorespiration can be inferred from this work, genome-scale metabolic modeling of cyanobacteria has suggested that photorespiration is essential for optimal photosynthesis (3).
Photorespiration produces the toxic intermediate 2-phosphoglycolate, which is recycled through the photorespiratory C2 cycle (
Introduction of additional, synthetic CO2 fixation pathways provide an approach to increasing photosynthesis, which circumvents the complexities associated with manipulating the C2 cycle (7). Of the six known CO2 fixation cycles in nature, only the 3-hydroxypropionate (3OHP) bi-cycle is completely oxygen insensitive (8,9), a key consideration when engineering pathways into oxygenic photoautotrophs. The 3OHP bi-cycle from the thermophilic anoxygenic phototroph Chloroflexus aurantiacus offers an attractive starting point for engineering efforts (10), because all of the necessary enzymes have been characterized (9). In this bi-cyclic pathway, bicarbonate is fixed by biotin-dependent acetyl-CoA carboxylase and propionyl-CoA carboxylase. The primary CO2 fixation product resulting from the first cycle is glyoxylate, which is then fed into the second cycle, in which another bicarbonate is fixed and pyruvate is generated as the final product (9).
Independent of photorespiration, various synthetic carbon fixation pathways have been proposed as a potential way to increase net photosynthetic yield (4). Of the six known carbon fixation cycles that exist in nature, only the Calvin-Benson cycle and the 3-hydroxypropionate bicycle lack enzymes that are oxygen sensitive (5), a key factor to consider when engineering pathways into oxygenic photoautotrophs. Further studies have expanded upon natural carbon fixation pathways to predict novel carbon fixation pathways by mining enzyme databases and building cycles in silico (6).
Carbon and carbon dioxide (CO2) fixation in cyanobacteria proceeds via the reductive pentosephospate cycle (Calvin-Benson cycle). The key carboxylase of that CO2 fixation cycle is ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO). The oxygenase side reaction of RuBisCO results in the formation of a toxic compound (2-phosphoglycolate), which has to be removed or ideally recycled. In cyanobacteria and plants this is achieved in a series of reactions (photorespiration) involving the loss of CO2 and NH3, which both have to be re-assimilated at the cost of additional energy.
Cyanobacteria convert CO2 into biomass using solar energy. The rate limiting step in this process is the fixation of CO2 by enzyme RuBisCO. RuBisCO is a very inefficient catalyst, because it has relatively low affinity to its substrates and is not able to discriminate between CO2 and O2. The use of O2 instead of CO2 leads to photorespiration. Phosphoglycolate produced from the oxygenase side reaction of RuBisCO is toxic to cells, because it completely inhibits triosephosphate isomerase at micro molar levels. Therefore, 2-phosphoglycolate is recycled to 3-phosphoglycerate via a series of reactions (
Others have described different methods of increasing photosynthetic carbon fixation. For example, Kebeish, Rashad et al. describe a method for increasing photosynthetic carbon fixation in rice in WO2010012796 A1, hereby incorporated by reference. Kreuzaler, Fritz et al. describe a method for increasing photosynthetic carbon fixation using glycolate dehydrogenase multi-subunit fusion protein in WO2011095528 A1, also hereby incorporated by reference. However, there is a need for other compositions and methods that provide more energetically and metabolically desirable approaches.
The present invention provides for constructs and systems and methods for producing and expressing in an organism a synthetic carbon fixation cycle that also acts as a photorespiratory bypass based on half of the 3-hydroxypropionate bicycle characterized from Chloroflexus aurantiacus (Chloroflexus) (7). Bicarbonate is fixed through Acetyl CoA Carboxylase, and glyoxylate is inputted as a photorespiratory byproduct, resulting in a net gain in carbon through the generation of pyruvate.
In order to examine the potential and consequences of introducing carbon fixation pathways into oxygenic photoautotrophs, a synthetic pathway based on the 3-hydroxypropionate bicycle was introduced into the model cyanobacterium, Synechococcus elongatus sp. PCC 7942. The heterologously expressed pathway acts as a photorespiratory bypass as well as an additional carbon fixation cycle orthogonal to the endogenous Calvin-Benson cycle. The examples herein demonstrate the function of all six introduced enzymes, which not only has implications on increasing net-photosynthetic productivity, but also key enzymes in the pathway are involved in high-value products that are of biotechnological interest, such as 3-hydroxypropionate.
In one embodiment, the present invention provides for a construct or an expression cassette comprising a heterologous polynucleotide encoding a cluster of enzymes, wherein the cluster comprising a set of genes necessary for the expression of a synthetic photorespiratory bypass pathway in a host cell.
The expression cassette can be used to provide a cell comprising in its genome at least one stably incorporated expression cassette, where the expression cassette comprising a heterologous nucleotide sequence or a fragment thereof operably linked to a promoter that drives expression in the cell and operably linked to a ribosomal binding site that controls expressions efficiency in the cell.
The present invention further describes methods for production of a photorespiratory bypass in plant, cyanobacterial, algae, and other host organisms.
In one embodiment, a CO2-fixing synthetic photorespiratory bypass based on the 3OHP bi-cycle (
Herein described are methods for enhancing metabolic activity in an organism. In one embodiment, a method comprising introducing into an organism at least one expression cassette operably linked to a promoter that drives expression in the organism, where the expression cassette comprising a cluster of photorespiratory bypass enzymes derived from a bacteria, wherein the cluster comprising a set of photorespiratory bypass genes necessary for the expression of a synthetic photorespiratory bypass pathway to provide the non-native organism enhanced metabolic activity.
In other embodiments, methods for increasing improving the efficiency of photosynthesis by introduction of an expression cassette comprising a cluster of photorespiratory bypass genes in a photosynthetic organism.
In another embodiment, methods for increasing photosynthetic carbon fixation in a photosynthetic organism or plant.
SEQ ID NOS:1-18 are nucleotide sequences used for cloning.
SEQ ID NO:19 is the enzyme sequence for propionyl-CoA synthase (PCS), Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. AAL47820.2.
SEQ ID NO:20 is the enzyme sequence for (MCR) malonyl-CoA reductase in Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. AAS20429.1.
SEQ ID NO:21 is the enzyme sequence for HpcH/HpaI aldolase (MCL) in Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. ABY33428.1
SEQ ID NO:22 is the enzyme sequence for MaoC domain protein dehydratase (MCH) in Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. ABY33427.1.
SEQ ID NO:23 the enzyme sequence for L-carnitine dehydratase/bile acid-inducible protein F (MCT) in Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. ABY33429.1.
SEQ ID NO:24 is the enzyme sequence for MEH in Chloroflexus aurantiacus J-10-fl, having GenBank Accession No. ABY33434.1.
SEQ ID NO:25 is the enzyme sequence for MCR homolog, NAD-dependent epimerase/dehydratase:Short-chain dehydrogenase/reductase SDR in Erythrobacter sp. NAP1, having GenBank Accession No. EAQ29650.1.
SEQ ID NO:26 is the enzyme sequence for PCS homolog, acetyl-coenzyme A synthetase in Erythrobacter sp. NAP1, having GenBank Accession No. EAQ29651.1.
SEQ ID NO:27 is the enzyme sequence for MCL homolog HpcH/HpaI aldolase in Candidatus Accumulibacter phosphatis clade IIA str. UW-1, having GenBank Accession No. ACV35795.1.
SEQ ID NO:28 is the enzyme sequence for MCH homolog, MaoC domain protein dehydratase (MCH) in Candidatus Accumulibacter phosphatis clade IIA str. UW-1, having GenBank Accession No. ACV35796.1.
SEQ ID NO:29 is the enzyme sequence for MCT homolog, acyl-CoA transferase/carnitine dehydratase-like protein in Candidatus Accumulibacter phosphatis clade IIA str. UW-1, having GenBank Accession No. ACV35794.1.
SEQ ID NO:30 is the enzyme sequence for MEH homolog in Candidatus Accumulibacter phosphatis clade IIA str. UW-1, having GenBank Accession No. ACV35791.1.
SEQ ID NO:31 is the DNA sequence of the PMS4570 construct.
SEQ ID NO:32 is the DNA sequence of the PMS4591 construct.
SEQ ID NO:33 is the DNA sequence of the PMS4749 construct.
SEQ ID NO:34 is the DNA sequence of the PCS construct.
SEQ ID NO:35 is the DNA sequence of the pAM1573PMS construct.
SEQ ID NO:36 is the DNA sequence of the pNS3 construct.
Introduction
Global photosynthetic productivity is limited by the enzymatic assimilation of CO2 into organic carbon compounds. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO), the carboxylating enzyme of the Calvin-Benson (CB) cycle, poorly discriminates between CO2 and O2, leading to photorespiration and the loss of fixed carbon and nitrogen. With the advent of synthetic biology, it is now feasible to design, synthesize and introduce biochemical pathways in vivo. We engineered a synthetic photorespiratory bypass based on the 3-hydroxypropionate bi-cycle into the model cyanobacterium, Synechococcus elongatus sp. PCC 7942. The heterologously expressed cycle is designed to function as both a photorespiratory bypass and an additional CO2-fixing pathway, supplementing the CB cycle. We demonstrate the function of all six introduced enzymes and identify bottlenecks to be targeted in subsequent bioengineering. These results have implications for efforts to improve photosynthesis, and for the “green” production of high-value products of biotechnological interest.
Herein is further described a synthetic pathway for CO2 fixation using photorespiratory bypass based on the 3-hydroxypropionate bi-cycle in a heterologous host environment.
Definitions
An “expression vector” or “expression cassette” is a nucleic acid construct, generated recombinantly or synthetically, with a series of specified nucleic acid elements that permit transcription of a particular nucleic acid in a host cell. The expression vector can be part of a plasmid, virus, or nucleic acid fragment. Typically, the expression vector includes a nucleic acid to be transcribed operably linked to a promoter.
By “host cell” is meant a cell that contains an expression vector and supports the replication or expression of the expression vector. Host cells may be prokaryotic cells including but not limited to, cyanobacteria including but not limited to, Synechococcus elongatus, plants, or eukaryotic cells including but not limited to, algae, yeast, insect, amphibian, or mammalian cells such as CHO, HeLa and the like, e.g., cultured cells, explants, and cells in vivo.
The terms “polypeptide,” “peptide” and “protein” are used interchangeably herein to refer to a polymer of amino acid residues. The terms apply to amino acid polymers in which one or more amino acid residue is an artificial chemical mimetic of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers and non-naturally occurring amino acid polymer. Amino acid polymers may comprise entirely L-amino acids, entirely D-amino acids, or a mixture of L and D amino acids. The use of the term “peptide or peptidomimetic” in the current application merely emphasizes that peptides comprising naturally occurring amino acids as well as modified amino acids are contemplated.
Any “gene” is meant to refer to the polynucleotide sequence that encodes a protein, i.e., after transcription and translation of the gene a protein is expressed. As understood in the art, there are naturally occurring polymorphisms for many gene sequences. Genes that are naturally occurring allelic variations for the purposes of this invention are those genes encoded by the same genetic locus.
Any “bacterial microcompartment gene”, “microcompartment gene” as referred to herein is meant to include any polynucleotide that encodes a Pfam00936 domain or Pfam03319 domain protein or variants thereof. When referring to the bacterial compartments or microcompartments, it is meant to include any number of proteins, shell proteins or enzymes (e.g., dehydrogenases, aldolases, lyases, etc.) that comprise or are encapsulated in the compartment.
The terms “isolated,” “purified,” or “biologically pure” refer to material that is substantially or essentially free from components that normally accompany it as found in its native state. Purity and homogeneity are typically determined using analytical chemistry techniques such as polyacrylamide gel electrophoresis or high performance liquid chromatography. A protein that is the predominant species present in a preparation is substantially purified. The term “purified” denotes that a nucleic acid or protein gives rise to essentially one band in an electrophoretic gel.
The terms “identical” or percent “identity,” in the context of two or more polypeptide sequences (or two or more nucleic acids), refer to two or more sequences or subsequences that are the same or have a specified percentage of amino acid residues or nucleotides that are the same e.g., 60% identity, preferably 65%, 70%, 75%, 80%, 85%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% identity over a specified region (such as the first 100 amino acids of SEQ ID NOS:19-30), when compared and aligned for maximum correspondence over a comparison window, or designated region as measured using one of the following sequence comparison algorithms or by manual alignment and visual inspection. Such sequences are then said to be “substantially identical.” This definition also refers to the compliment of a test sequence.
For sequence comparison, typically one sequence acts as a reference sequence, to which test sequences are compared. When using a sequence comparison algorithm, test and reference sequences are entered into a computer, subsequence coordinates are designated, if necessary, and sequence algorithm program parameters are designated. Default program parameters can be used, or alternative parameters can be designated. The sequence comparison algorithm then calculates the percent sequence identities for the test sequences relative to the reference sequence, based on the program parameters. For sequence comparison of nucleic acids and proteins, the BLAST and BLAST 2.0 algorithms and the default parameters discussed below are typically used.
The terms “nucleic acid” and “polynucleotide” are used interchangeably herein to refer to deoxyribonucleotides or ribonucleotides and polymers thereof in either single- or double-stranded form. The term encompasses nucleic acids containing known nucleotide analogs or modified backbone residues or linkages, which are synthetic, naturally occurring, and non-naturally occurring, which have similar binding properties as the reference nucleic acid, and which are metabolized in a manner similar to the reference nucleotides. Examples of such analogs include, without limitation, phosphorothioates, phosphoramidates, methyl phosphonates, chiral-methyl phosphonates, 2-O-methyl ribonucleotides, polypeptide-nucleic acids (PNAs). Unless otherwise indicated, a particular nucleic acid sequence also encompasses “conservatively modified variants” thereof (e.g., degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues (Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem., 260:2605-2608 (1985); Rossolini et al., Mol. Cell. Probes, 8:91-98 (1994)). The term nucleic acid can be used interchangeably with gene, cDNA, mRNA, oligonucleotide, and polynucleotide.
A “label” or “detectable label” is a composition detectable by spectroscopic, photochemical, biochemical, immunochemical, or chemical means. For example, useful labels include radioisotopes (e.g., 3H, 35S, 32P, 51Cr, or 125I), fluorescent dyes, electron-dense reagents, enzymes (e.g., alkaline phosphatase, horseradish peroxidase, or others commonly used in an ELISA), biotin, digoxigenin, or haptens and proteins for which antisera or monoclonal antibodies are available (e.g., proteins can be made detectable, e.g., by incorporating a radiolabel into the protein, and used to detect antibodies specifically reactive with the protein).
Detailed Description
In one embodiment, a synthetic metabolic pathway is selected to be synthesized and/or engineered in a host cell. A polynucleotide encoding the enzymes in the metabolic pathway can be inserted into a host organism and if needed, expressed using an inducible expression system.
In some embodiments, naturally existing or synthetic bacterial microcompartment operons which express microcompartment or shell proteins may be included. Prior strategies to produce microcompartment shells in heterologous hosts have transformed the host system with the natural operon sequences of the original organism. However, in a natural organism, the required shell proteins may not be placed together on the chromosome, they may be intermixed with enzymes or other proteins, and the ordering and regulatory mechanisms may not be useful in a new host organism.
In one embodiment, polynucleotides encoding enzymatic proteins in the photorespiratory by-pass pathway, are cloned into an appropriate plasmid, inserted into an expression vector, and used to transform cells from any host organism. Suitable host organisms include, but are not limited to, bacteria such as E. coli, B. subtilis, S. cerevisiae, cyanobacteria such as S. elongatus, plants such as Nicotiana tabacum and Camelina sativa, algae, fungi, or other eukaryotic organisms.
In one embodiment, the polynucleotides are in an inducible expression system which maintains the expression of the inserted genes silent unless an inducer molecule (e.g., IPTG) is added to the medium containing the host cell. The expression vector or construct may be a vector for coexpression or in some embodiments, it may be a neutral site vector for insertion into a host genome such as Synechococcous elongatus. The construct may include either inducible transcription elements or may be constitutively expressed in the host organism
Bacterial colonies are allowed to grow after gene expression has begun, or if required, after induction of gene expression. Thus, in some embodiments, expression vectors comprising a promoter operably linked to a heterologous nucleotide sequence or a fragment thereof, that encodes a microcompartment RNA or proteins are further provided. The expression vectors of the invention find use in generating transformed plants, plant cells, microorganisms, algae, fungi, and other eukaryotic organisms as is known in the art and described herein. The expression vector will include 5′ and 3′ regulatory sequences operably linked to a polynucleotide of the invention. “Operably linked” is intended to mean a functional linkage between two or more elements. For example, an operable linkage between a polynucleotide of interest and a regulatory sequence (i.e., a promoter) is functional link that allows for expression of the polynucleotide of interest. Operably linked elements may be contiguous or non-contiguous. When used to refer to the joining of two protein coding regions, by operably linked is intended that the coding regions are in the same reading frame. The vector may additionally contain at least one additional gene to be co-transformed into the organism. Alternatively, the additional gene(s) can be provided on multiple expression vectors or cassettes. Such an expression vectors is provided with a plurality of restriction sites and/or recombination sites for insertion of the polynucleotide that encodes a microcompartment RNA or polypeptide to be under the transcriptional regulation of the regulatory regions. The expression vector may additionally contain selectable marker genes.
The expression vector will include in the 5′-3′ direction of transcription, a transcriptional initiation region (i.e., a promoter), a cluster of bacterial compartment genes each preceded by a translational initiation site (RBS) specific to the organism and type of shell protein and followed by a translation termination signal (stop codon), and, optionally, a transcriptional termination region functional in the host organism. The regulatory regions (i.e., promoters, transcriptional regulatory regions, ribosomal binding sites and translational termination regions) and/or any targeting sequences may be native/analogous to the host cell or to each other. Alternatively, the regulatory regions and/or the targeting regions may be heterologous to the host cell or to each other. As used herein, “heterologous” in reference to a sequence that originates from a foreign species, or, if from the same species, is modified from its native form in composition and/or genomic locus by deliberate human intervention. For example, a promoter operably linked to a heterologous polynucleotide is from a species different from the species from which the polynucleotide was derived, or, if from the same/analogous species, one or both are substantially modified from their original form and/or genomic locus, or the promoter is not the native promoter for the operably linked polynucleotide.
In some embodiments, the selected photorespiratory pathway genes are placed onto the construct using the strategies described herein and shown in
In various embodiments, the synthetic operon contains the gene constructs as shown in
S. elongatus
S. elongatus
S. elongatus
S. elongatus
S. elongatus
S. elongatus
S. elongatus
S. elongatus
E. coli
E. coli
In other embodiments, the photorespiratory bypass pathway genes or constructs can be incorporated into multiple expression vectors and/or under multiple promoter control.
Where appropriate, the polynucleotides may be optimized for increased expression in the transformed organism. For example, the polynucleotides can be synthesized using preferred codons for improved expression.
Additional sequence modifications are known to enhance gene expression in a cellular host. These include elimination of sequences encoding spurious polyadenylation signals, exon-intron splice site signals, transposon-like repeats, and other such well-characterized sequences that may be deleterious to gene expression. The G-C content of the sequence may be adjusted to levels average for a given cellular host, as calculated by reference to known genes expressed in the host cell. When possible, the sequence is modified to avoid predicted hairpin secondary mRNA structures.
The expression vector can also comprise a selectable marker gene for the selection of transformed cells. Selectable marker genes are utilized for the selection of transformed cells or tissues. Marker genes include genes encoding antibiotic resistance, such as those encoding neomycin phosphotransferase II (NEO) and hygromycin phosphotransferase (HPT), as well as genes conferring resistance to herbicidal compounds, such as glufosinate ammonium, bromoxynil, imidazolinones, and 2,4-dichlorophenoxyacetate (2,4-D). Additional selectable markers include phenotypic markers such as β-galactosidase and fluorescent proteins such as green fluorescent protein (GFP) (Su et al. (2004) Biotechnol Bioeng 85:610-9 and Fetter et al. (2004) Plant Cell 16-215-28), cyan florescent protein (CYP) (Bolte et al. (2004) J. Cell Science 117:943-54 and Kato et al. (2002) Plant Physiol 129:913-42), and yellow fluorescent protein (PhiYFP™ from Evrogen, see, Bolte et al. (2004) J. Cell Science 117:943-54). The above list of selectable marker genes is not meant to be limiting. Any selectable marker gene can be used in the present invention.
In another embodiment, it may be beneficial to express the gene from an inducible promoter, particularly from an inducible promoter. The gene product may also be co-expressed with a targeting polypeptide or fragment thereof, such that the polypeptide is in the C-terminal or N-terminal region of any other gene in the construct.
In various embodiments, the photorespiratory bypass is produced from another organism in a non-native bacterial host cell, such as E. coli, by construction of a synthetic operon as described herein. As described in the Examples, in one embodiment, the enzymes from Chloroflexus aurantiacus J-10-fl are used to create the photorespiratory bypass pathway is engineered to be produced in S. elongatus.
In one embodiment, an in-vitro transcription/translation system (e.g., Roche RTS 100 E. coli HY) can be used to produce cell-free expression products.
In some embodiments, the photorespiratory bypass may be expressed inside a microcompartment in the non-native host organism to provide the host organism enhanced enzymatic activity that is sequestered to encapsulate reactions that would otherwise be toxic to the cell but however, be non-toxic or have low toxicity levels to humans, animals and plants or other organisms that are not the target.
In some embodiments, the metabolic pathway is preferably incorporated into the genome of the host microorganism or eukaryote (e.g., plant, algae, yeast/fungi) to provide the new or enhanced metabolic activity described herein of enhanced carbon fixation.
Genes which encode the enzymes or proteins to carry out these enhanced reactions or activities and which will be encapsulated by the microcompartment may be targeted to the microcompartment by adding encapsulation tags specific for the microcompartment shell. Methods and compositions describing this in greater detail are described previously by some of the inventors in U.S. application Ser. No. 13/367,260 filed on Feb. 6, 2012, published as US-2002/02104590-A1 (“Design and Implementation of Novel and/or Enhanced Bacterial Microcompartments for Customizing Metabolism”) and hereby incorporated by reference in its entirety. Such encapsulation tags and the genes encoding the proteins to be encapsulated may be incorporated in the expression vector itself or by co-expression of such encapsulation tagged genes which are on a second vector added to the host cell.
We express a synthetic carbon fixation cycle that also acts as a photorespiratory bypass based on half of the 3-hydroxypropionate bicycle characterized from Chloroflexus aurantiacus (Chloroflexus) (7). Bicarbonate is fixed through Acetyl CoA Carboxylase, and glyoxylate is inputted as a photorespiratory byproduct, resulting in a net gain in carbon through the generation of pyruvate.
We designed a CO2-fixing synthetic photorespiratory bypass based on the 3OHP bi-cycle (
To implement the proposed cycle shown in
The intermediates involved in the last two steps needed to complete the pathway in S. elongatus, MCR and PCS, are toxic to cells. Accumulation of 3OHP, the product of MCR, can lead to organic acid toxicity (19); propionyl-CoA, the product of PCS inhibits both pyruvate dehydrogenase and citrate synthase (20). The potential toxicity in conjunction with the difficulty of successfully reconstituting multi-step metabolic pathways (21) presented major challenges. Moreover, both MCR and PCS are large multi-domain enzymes, potentially presenting difficulty in proper folding and expression. For these reasons, mcr and pcs were driven by the IPTG-inducible promoter, pTrc (
To relieve the bottleneck, a second copy of mct, driven by the IPTG-inducible pTrc promoter, was added upstream of mcr. We tested two strategies for introducing the additional mct gene: 1) adding a duplicate Chloroflexus mct to generate PMS4749 and 2) introducing a synthetic mct homolog (referred to as ApMCT) from the β-proteobacterium ‘Candidatus Accumulibacter phosphatis’ (22,23) resulting in PMS4591 (
Nevertheless, the double transformants encoding either a second mct gene from Chloroflexus or the Accumulibacter gene (PCS/PMS4749 or PCS/PMS4591, respectively) were generated (
In order to estimate if the resulting enzyme activities were high enough to allow the functioning of the synthetic photorespiratory bypass, we calculated the carbon assimilation rate of a S. elongatus wild-type culture using the equation dS/dt=(μ/Y)×X (25), which correlates the specific substrate consumption (dS) over time (dt) with the specific growth rate (μ). The established growth yield (Y) corresponds to a bacterial cell dry mass of 1 g formed per 0.5 g of carbon fixed (approx. 50% of bacterial cell dry mass is carbon). Although X usually refers to the concentration of living cells, in this case it is used to account for the amount of total protein per 1 g cell dry mass (in bacteria approx. 50% of cell dry mass is protein). We assumed a typical doubling time of 8 h for a wild-type culture under laboratory conditions with ambient CO2, which corresponds to a p of 0.087 h−1. This would require a net carbon assimilation rate of 121 nmol min−1 (mg protein)−1. Taking into account an estimated loss of up to 25% of the fixed carbon due to photorespiration (26) results in 80 nmol min−1 (mg protein)−1 for the oxygenase activity of RuBisCO and the production of glycolate. To efficiently reassimilate glycolate in the synthetic bypass the minimal specific activities of the involved enzymes need to be at least as high as the rate of glycolate generation. Based on that estimate all but one of the introduced enzymes were well above the required threshold (
This study is, to our knowledge, the first successful effort to express a synthetic CO2-fixing photorespiratory bypass in a photoautotrophic organism, the cyanobacterium S. elongatus PCC7942. Unlike previous studies, our pathway differs by directly avoiding the net loss of nitrogen and carbon in the photorespiratory C2 cycle, which actually results in a net gain in carbon fixation through the enzyme acetyl-CoA carboxylase (ACC).
The unique feature of our pathway is the additional carbon fixation, which must be accounted for in energy balance comparisons to other proposed photorespiratory bypasses. Therefore we have assumed the stoichiometrically correct values for the formation of two glycolate molecules per CO2 released in the C2 cycle (see Table 3). Thus, to reassimilate two glycolate molecules our cyclic bypass requires 6 ATP equivalents and 4 NAD(P)H, while fixing two additional molecules of bicarbonate, the form of inorganic carbon concentrated in the cytoplasm of cyanobacteria, and circumventing the loss of NH3. Note that if pyruvate, which derives from our bypass, is to be used for replenishing the CB cycle two more ATP equivalents are required per pyruvate molecule in gluconeogenesis by pyruvate phosphate dikinase, because it is AMP-forming. Nevertheless, the synthetic bypass compares favorably over the canonical photorespiratory C2 cycle of cyanobacteria in terms of energy demand: the combined function of the C2 cycle and CB cycle requires 11 ATP equivalents, 4 NAD(P)H, and 2 reduced ferredoxins to first refix the lost CO2 and NH3, as well as additionally fix two more CO2 molecules to arrive at the same level of net carbon fixation as the synthetic bypass (see
Whereas the vast majority of metabolic engineering efforts focus on introducing linear pathways for the anabolic production of molecules of interest, our approach introduces a self-sustaining metabolic cycle that fixes CO2 when glycolate/glyoxylate is available.
We demonstrate that concomitant expression and activity of all six enzymes necessary to reconstitute the synthetic bypass can be achieved. This required heterologous expression of ˜16 kbp of DNA and functional assembly of six multimeric enzymes ranging in molecular mass from 62-600 kDA.
However, an obvious physiological phenotype was not observed during growth experiments. The transformants exhibited only slight delay in growth when liquid cultures in air were inoculated from agar plates, but they reached the same doubling times and optical densities as the wild type.
Our results immediately suggest next steps toward improvement. For example, our initial design used enzymes derived from the thermophile Chloroflexus which are evolved to function at higher temperatures than the mesophilic growth conditions of plants and most cyanobacteria. This may underlie the low measured activity of heterologous PCS despite its strong overexpression in our transformants (
Likewise, an increase in ACCase activity may be required. Our present design relies on the native enzyme to fix bicarbonate. ACCase is required for fatty acid biosynthesis and endogenous levels of the enzyme may be insufficient to support optimal flux through the heterologously expressed cycle. However, overexpression of up to four separate subunits of the prokaryotic ACCases will significantly complicate DNA assembly and cloning strategies. Suitable alternatives may be eukaryotic ACCases, which have undergone gene fusion events creating one large single multi-functional gene (27).
In addition to the C2 cycle, cyanobacteria can make use of two other strategies, the decarboxylation and glycerate pathways (28,29) that consume glyoxylate; they potentially compete with the synthetic bypass for substrate. In contrast, plants contain only the C2 cycle, thus simplifying the fate of glyoxylate. With the localization of all six genes of our pathway to the chloroplast, only one additional enzyme, glycolate dehydrogenase, would be necessary to convert glyoxlate and bicarbonate to pyruvate. In fact, glycolate dehydrogenase has already been successfully targeted and expressed in chloroplasts of Arabidopsis (3).
Our results have implications beyond the optimization of photorespiration in plants and cyanobacteria. The successful introduction of half of the 3OHP bi-cycle into S. elongatus provides a platform in which to express the other half to attain the full bi-cyclic CO2 fixation pathway. Given that CO2 fixation limits the light-saturated rate of photosynthesis, the presence of two orthogonal CO2 fixation pathways is expected to significantly enhance the conversion of solar energy into biomass. Although appealing, introducing the whole 3OHP bi-cycle will result in substantial carbon flux towards pyruvate, which could be detrimental to organisms that have evolved carbon metabolism based on sugar phosphates.
On the other hand, pyruvate or intermediates in the synthetic bypass could be redirected for biotechnological applications, such as biofuels or replacements for chemical feedstocks that are currently petroleum-derived (19). For example, we have shown that 3OHP, a precursor for bioplastics, can be derived from malonyl-CoA by the heterologous expression of MCR in cyanobacteria. Developing cyanobacteria as production strains requires increasing their tolerance to higher concentrations of 3OHP; this has been accomplished in E. coli (19). Likewise the production of propionyl-CoA by the combined function of MCR and PCS in the synthetic bypass could be useful for the production of diverse polyhydroxyalkanoates like polyhydroxyvalerate, polyhydroxymethylvalerate or co-polymers.
Improving photosynthesis holds promise for increasing the sustainable production of food and biofuel crops to meet the challenges of global climate change and population growth, but introducing new pathways and cycles constitutes a daunting challenge. The synthetic photorespiratory bypass reported in this study provides both a precedent and a platform for future bioengineering efforts.
The vast majority of metabolic engineering efforts focus on introducing linear pathways to generate products of interest; however, our proposed pathway is specifically aimed at introducing a self-sustaining metabolic cycle that fixes CO2. Because of this inherent difference, optimization of expression levels adds a level of complexity. This is observed as increased expression of MCR and PCS with higher amounts of IPTG reduces growth (
In order to further characterize the effects of our pathway on growth, both the functional (pms4591) and non-functional (pms4570) pathways were introduced into a carboxysome mutant background (AK-0) which presumably produces more glyoxylate, as the carboxysome is involved decreasing RuBisCO oxygenase activity via the cyanobacterial CCM (15). In the mutant background, we demonstrate that transformants with the functional pathway display higher growth rates than transformants expressing the non-functional pathway (
One target for downstream enzyme that will need to be examined more closely in future work will be the role of acetyl-CoA carboxylase, as the cycle utilizes the endogenous copy to fix bicarbonate. As the cell is accustomed to solely using this enzyme for the producing malonyl-CoA, the primary building block for fatty acid biosynthesis, increased flux through this step may be necessary to optimize the heterologously expressed cycle. Furthermore, as demonstrated with the ApMCT homolog, mining genome databases for variants of the six enzymes which exhibits faster enzyme kinetics will also greatly improve the cycle. Finally, as we have now successfully introduced half of the 3-hydroxypropionate bicycle into Synechococcus, we now have a platform to express the other half and introduce the full bicycle, which would result in two truly separate and orthogonal carbon fixation pathways being expressed. Although appealing, given the concerns brought up by downstream metabolites produced from synthetic carbon fixation pathways, introducing the whole pathway may result in more pyruvate being generated, which could be detrimental to a cell that has not evolved to base its central carbon metabolism around pyruvate, rather than triose phosphates.
3-hydroxypropionate is an attractive chemical feedstock that may be used to replace chemicals that are currently petroleum-derived (11). One potential modification that can be made to our pathway is controlling the flux between MCR and PCS, where higher flux through MCR would result in an increase concentration of 3-hydroxypropionate in the cell, while still sustaining the orthogonal carbon fixation cycle. Generation of cyanobacterial strains that are more tolerant to higher concentrations of 3-hydroxypropionate, as has been done in E. coli (11), would facilitate this biotechnological use of generating bioplastics from a photoautotrophic source.
The conventional photorespiratory C2 cycle found in cyanobacteria in plants requires 10 ATP for the net fixation of two CO2 molecules; however, during this process there is a net loss of one CO2 and one ammonia molecule (refs). Comparatively, our engineered cycle requires six ATP for the net fixation of two bicarbonate molecules, with no net loss of carbon or nitrogen. Our results show that soluble expression and proper activity of all six genes necessary to reconstitute our pathway can be achieved; however, further studies are needed to optimize this pathway to yield higher growth rates in algae and plants. Concerning cyanobacteria, other than the C2 cycle, there are two other pathways, the decarboxylation and glycerate pathway, which use glyoxylate as a substrate, thus potentially competing with our pathway to use glyoxylate molecule in vivo. Plants only contain the C2 cycle, thus simplifying the fate of glyoxylate. With the localization of all six genes of our pathway to the chloroplast, only the addition of one enzyme, glycolate dehydrogenase, would be necessary to convert glyoxlate to pyruvate within the chloroplast, which has already been successfully targeted and expressed glycolate dehydrogenase to the chloroplast of Arabidopsis (2).
Materials and Methods:
Materials:
3-Hydroxypropionate was synthesized chemically from β-propiolactone. A solution (6 ml) of 5 M NaOH in water was stirred at room temperature and 1.25 ml β propiolactone was added drop-wise (0.025 mol). The solution was lyophilized and the dry powder was stored at room temperature.
Cloning, Strains, Growth Conditions:
All constructs were cloned using the BglBrick assembly format (11) in E. coli and subsequently cloned into various neutral site destination vectors, which allow for genomic integration into the S. elongatus genome by previously described transformation protocols (12,13). Plasmids, strains and primers that were generated and used are summarized in Tables 1 and 2.
S. elongatus strains were maintained in BG-11 medium under appropriate selection with constant light at 30 or 37° C.
Cloning, Heterologous Expression of Recombinant Enzymes in E. coli, and Purification—
The cloning, expression, and purification of the mesaconyl-C1-CoA hydratase (MCH) and mesaconyl-CoA C1:C4 CoA transferase (MCT) from C. aurantiacus was performed as previously described (9). Cloning, expression, and purification of the malyl-CoA lyase (MCL) from C. aurantiacus was described previously (14).
Heterologous Expression in E. coli, and Purification of ApMCT—
Competent E. coli BL21(DE3) cells were transformed with the plasmid pMct_Ap_JZ33, and 1 liter cultures were grown at 37° C. in LB medium with 100 μg ampicillin ml−1. At an OD600 nm of 0.6, the expression was induced with 1 mM IPTG. The cells were harvested after 4 h of growth and stored at −80° C. until use.
E. coli cells containing recombinant N-terminal His10-tagged ApMCT were suspended in a two-fold volume of 50 mM Tris/HCl pH 7.5, 250 mM NaCl (buffer A). Cells were lysed by sonication (W-220F, Branson Ultrasonics) and the lysate was centrifuged for 40 min (40,000×g) at 4° C. A 1 ml HisTrap HP column (GE Healthcare) was equilibrated with buffer A. The cell extracts (40,000×g supernatants) were applied to the column at a flow rate of 1 ml min−1. The column was washed with buffer A containing 100 mM imidazole to remove nonspecifically bound proteins. Recombinant His-tagged ApMCT was eluted with 500 mM imidazole in buffer A.
Cell Extracts.
Cells were harvested during exponential phase by centrifugation at 6000×g. The cell pellets were resuspended in a 2 fold volume of 200 mM MOPS/KOH buffer (pH 7.5). The cell suspensions were sonicated and the cell lysates were centrifuged at 20,000×g and 4° C. for 30 min. The supernatants were either used directly for enzyme assays or stored at −80° C.
High Performance Liquid Chromatography (HPLC).
A Waters 2695e system (Waters, Milford, Mass.) with a photo diode array detector (Waters 2998) was used. Reaction products and standard compounds were detected by UV absorbance at 260 nm. A reversed phase C18 column (SymmetryShield 4.6×250, Waters) was equilibrated at a flow rate of 0.6 ml min−1 with 4% acetonitrile in 40 mM K2HPO4/HCOOH buffer (pH 4.2). A gradient of 26 min from 4 to 16% acetonitrile was applied. CoA-thioesters and free CoA were identified by retention times and UV spectra.
Enzyme Assays.
Malonyl-CoA reductase was measured using a spectrophotometric assay described previously (Hügler 2002), which was modified. The malonyl-CoA dependent oxidation of NADPH was montitored at 30° C. at a wavelength of 365 nm (ε365=3,400 M−1 cm−1) (Dawson 1986). The assay mixture (400 μl) contained 200 mM MOPS/KOH buffer (pH 7.5), 5 mM MgCl2, 0.4 mM NADPH, 1 mM malonyl-CoA, and cell extract. The reaction was started by addition of malonyl-CoA. Notably, two NADPH molecules are oxidized per one malonyl-CoA molecule that is reduced to 3-hydroxypropionate.
Propionyl-CoA synthase activity was either monitored spectrophotometrically or in an HPLC based assay. (i) The photometric assay described by Alber and Fuchs (Alber 2002) was slightly modified. The reaction mixture (400 μl) contained 200 mM MOPS/KOH buffer (pH 7.5), 0.4 mM NADPH, 5 mM 3-hydroxypropionate, 100 mM KCl, 2 mM ATP, 0.5 mM CoA, and cell extract. The reaction was started by addition of 3-hydroxypropionate and carried out at 30° C. (ii) The same reaction mixture was used for the HPLC based assay only with 1 mM of NADPH instead. Samples of 100 μl were withdrawn after different time points and stopped by addition of 10 μl formic acid. The samples were kept on ice before precipitated protein was removed by centrifugation at 16,000×g. The supernatants were subjected to HPLC analysis to confirm propionyl-CoA formation.
The concerted function of the malyl-CoA/β-methylmalyl-CoA/citramalyl-CoA lyase, mesaconyl-C1-CoA hydratase, mesaconyl-CoA C1:C4 CoA transferase, and mesaconyl-C4-CoA hydratase was demonstrated in an HPLC based assay. The reaction mixture (400 μl) contained 200 mM MOPS/KOH buffer (pH 7.5), 5 mM MgCl2, 0.5 mM propionyl-CoA, 5 mM glyoxylate, and cell extract. The reaction started by addition of glyoxylate was carried out at 30° C. Samples of 100 μl volume were withdrawn after different time points and treated as described above and subjected to HPLC analysis to confirm the formation of acetyl-CoA or other CoA-thioester intermediates.
MCT activity was measured in an HPLC-based assay.
With the growing attention on global warming and an emphasis on green technologies, the potential for improving photoautotrophic growth by genetic engineering of synthetic carbon fixations may provide a solution. This study sets a precedent and platform for future engineering efforts. Table 3 attached shows an energy balance comparison of photorespiratory pathways to achieve the same level carbon gain as the presently described 3OHP bypass.
The above examples are provided to illustrate the invention but not to limit its scope. Other variants of the invention will be readily apparent to one of ordinary skill in the art and are encompassed by the appended claims. All accessions, publications, databases, and patents cited herein are hereby incorporated by reference for all purposes.
Accumulibacter phosphatis clade IIA str. UW-1]
This application is continuation and non-provisional application of and claims priority to International Patent Application No. PCT/US2015/014929, filed on Feb. 6, 2015, which claims priority to U.S. Provisional Patent Application No. 61/936,788, filed on Feb. 6, 2014, both of which are hereby incorporated by reference in their entirety.
This invention was made with government support under Contract No. DE-AC02-05CH11231 awarded by the U.S. Department of Energy, under Grant No. MCB0851054 awarded by the National Science Foundation. The government has certain rights in the invention.
| Number | Name | Date | Kind |
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| 8349587 | Fischer et al. | Jan 2013 | B2 |
| 20120210459 | Kerfeld et al. | Aug 2012 | A1 |
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| WO 2011095528 | Aug 2011 | WO |
| WO 2013130934 | Feb 2013 | WO |
| WO 2013130394 | Sep 2013 | WO |
| WO-2013130394 | Sep 2013 | WO |
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| Number | Date | Country | |
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| 20170088850 A1 | Mar 2017 | US |
| Number | Date | Country | |
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| 61936788 | Feb 2014 | US |
| Number | Date | Country | |
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| Parent | PCT/US2015/014929 | Feb 2015 | US |
| Child | 15230332 | US |
