Patent Application
20190256875
The present invention relates to a field of genetically modified fungal cells and converting galacturonic acid to meso-galactaric acid, more precisely to a method of producing meso-galactaric acid. The invention further relates to recombinant fungal cells having a specific combination of modifications including but not limited to expression of uronate dehydrogenase enzyme, reduced D-galacturonic acid reductase activity, and furthermore reduced meso-galactaric acid catabolism, as well as uses and methods related thereto.
meso-Galactaric acid, also known as mucic acid or galactaric acid, is an aldaric acid having terminal carboxylic acid groups. In nature, it occurs as a metabolite in the bacterial oxidative catabolic pathway for D-galacturonic acid which produces α-ketoglutarate, an intermediate of TCA cycle, as final product (Chang and Feingold, 1970; Dagley and Trudgill, 1965). In the pathway, an uronate dehydrogenase (UDH) oxidizes D-galacturonic acid resulting in formation of galactaro-1,4-lactone (
As a dicarboxylic acid, meso-galactaric acid is an attractive renewable alternative to be used as a platform chemical in polymers. It can be, for instance, chemically reduced to adipic acid or 2,5-furandicarboxylic acid (FDCA). Adipic acid has a huge market and it is widely used as a precursor in polymers such as nylon. FDCA is considered as promising renewable replacement for the fossil-based terephthalic acid that is used e.g. in PET plastic.
Production of meso-galactaric acid from D-galacturonic acid via chemical (Rautiainen et al., 2015) or biochemical (Benz et al., 2014; Mojzita et al., 2010) oxidation has been described in the literature. In addition, a combined process was recently reported including enzymatic hydrolysis of pectin, oxidation of resulting D-galacturonic acid to meso-galactaric acid by engineered E. coli and chemical reduction of meso-galactaric acid to adipic acid (H. Zhang et al., 2016). In the biochemical route, the filamentous fungus Trichoderma reesei was recently engineered for meso-galactaric acid production by disrupting the homologous D-galacturonic acid catabolism and introducing a heterologous UDH from the bacterial oxidative pathway (Mojzita et al., 2010; WO2010/072902 (A1)). The same strategy was also tested with Aspergillus niger that is, in contrast to T. reesei, efficient in pectin hydrolysis allowing a consolidated bioprocessing of pectin rich biomass (Kuivanen et al., 2014). However, expression of UDH in A. niger did not result in efficient meso-galactaric acid production.
Therefore, a need still exists for micro-organisms, specifically fungal cells, having ability to effectively produce meso-galactaric acid. The present invention improves the biochemical production of meso-galactaric acid in recombinant micro-organisms.
An object of the present invention is to provide methods and means for converting inexpensive pectin rich biomass such as sugar beet pulp or citrus processing waste, CPW, to galactarate and furthermore providing dicarboxylic acid, namely mucic acid i.e. meso-galactaric, for e.g. polymer industry.
More specifically an object of the present invention is to provide a method to solve the problems of time consuming, expensive and inefficient production of meso-galactaric acid. In the present invention the pectin rich biomass is converted in a single fermentative process to the desired product, thus taking into account both ecological compatibility and sustainable development issues.
The objects of the invention are achieved by a method and an arrangement, which are characterized by what is stated in the independent claims. The specific embodiments of the invention are disclosed in the dependent claims.
The present invention provides a method and genetically modified fungal cells for production of meso-Galactaric acid from D-galacturonic acid or galacturonate (i.e. any salt or ester of galacturonic acid) by utilizing very specific combinations of genetic modifications.
Surprisingly, the present invention is able to overcome the drawbacks of the prior art e.g. the problem that galactarates are catabolized in some microorganisms, specifically fungi. The present invention provides a way to stop the galactarate catabolism. Indeed, one benefit of this invention is that fungal cells may be utilized for meso-galactaric acid production. Fungi are especially efficient in producing pectinases for the hydrolysis of the pectin in pectin rich biomass. The pectinases hydrolyse the pectin to produce galacturonate, the substrate for the production of galactarate. The present invention enables production of galactarate from pectin rich biomass, by just applying some spores of the engineered fungus to e.g. citrus peels. The following autonomous fermentation process will convert it to the desired product galactarate.
Also, by the present invention it is possible to identify genes that are essential for galactarate catabolism. One or more said genes may be e.g. inhibited or knocked out. The resulting fungal strain is now able to catabolize galactarate. The resulting strain is converting galacturonate to galactarate but is not catabolizing galactarate.
In one aspect, the present invention relates to a method for producing meso-galactaric acid, said method comprising
contacting a fungal cell genetically modified to express uronate dehydrogenase enzyme, genetically modified to have reduced D-galacturonic acid reductase activity, and modified to reduce meso-galactaric acid catabolism, with a biomaterial comprising galacturonic acid, and
recovering the resulting meso-galactaric acid.
In another aspect, the present invention relates to a fungal cell that has been genetically modified to express uronate dehydrogenase enzyme, genetically modified to have reduced D-galacturonic acid reductase activity, and is capable of converting D-galacturonic acid to meso-galactaric acid, wherein said fungal cell has been further modified to reduce meso-galactaric acid catabolism.
Further, the present invention relates to a method for treating bio-material comprising galacturonic acid, which method comprises that a fungal cell genetically modified to express uronate dehydrogenase enzyme, genetically modified to have reduced D-galacturonic acid reductase activity, and modified to reduce meso-galactaric acid catabolism, is contacted with said biomaterial under suitable culture conditions.
Still, a further aspect of the present invention relates to use of the fungal cell of the present invention for producing meso-galactaric acid from pectin.
Still, a further aspect of the present invention relates to a method of preparing a fungal cell, said method comprising
transforming a fungal cell with at least one polynucleotide encoding uronate dehydrogenase enzyme for enhanced expression of said polynucleotide, and
reducing D-galacturonic acid reductase activity and meso-galactaric acid catabolism.
Still further, one aspect of the present invention relates to a method for identifying a polynucleotide or polypeptide of the meso-galactaric acid catabolic pathway, wherein the method comprises
contacting a wild type fungal cell with meso-galactaric acid,
carrying out a transcriptional analysis on a sample obtained from the wild type fungal cell contacted with meso-galactaric acid, and
identifying overexpressed polynucleotides compared to a sample obtained from a wild type fungal cell not contacted with meso-galactaric acid.
By the present invention the catabolism of meso-galactaric was disrupted and a fungal strain capable of meso-galactaric production was generated.
The consolidated process from pectin-rich biomass for the production was also demonstrated.
In the following the invention will be described in greater detail by means of specific embodiments with reference to the attached drawings, in which
Galacturonic acid in pectin-rich residues can be converted to useful compounds exploiting microorganisms. In the present invention, the catabolism of meso-galactaric acid was disrupted in a fungal cell together with at least disrupted endogenous D-galacturonic acid catabolism and expression of uronate dehydrogenase enzyme.
Pectin is structurally the most complex family of polysaccharides in nature and it can be found in plant cell walls. Pectin consist of a α-1,4-linked D-galacturonic acid backbone with sugar side chains. Approximately 70% of pectin mass is made up of galacturonic acid. There are five different types of pectin classified based on their structure: homogalacturonan (HG), xylogalacturonan (XGA), apiogalacturonan (AP), rhamnogalacturonan I (RG-I) and rhamnogalacturonan II (RG-II). HG is the most abundant and the simplest pectin since it is a linear galacturonic acid chain, which is partially methylesterified and acetylated. XGA is a HG substituted with xylose and AP is a HG substituted with D-apiofuranose. RG-II is the most structurally complex pectin. (Mohnen, 2008) It consists of a HG backbone with side chains consisting of 12 different sugars in over 20 different linkages. The sugars are D-glucuronic acid, L-rhamnose, D-galactose, L-arabinose, L-fucose, D-apiose, L-aceric acid, 2-O-methyl L-fucose, 2-O-methyl D-xylose, L-galactose, 2-keto-3-deoxy-D-lyxo-heptulosaric acid and 2-keto-3-deoxy-D-manno-octulosonic acid. (O'Neill et aL, 2004) In contrast to other pectin types, the RG-I backbone consists of a α-1,4-linked D-galacturonic acid and α-1,2-linked L-rhamnose repeating disaccharide unit with side chains consisting of various amount of L-arabinose and D-galactose. (Mohnen, 2008)
The main constituent of pectin, D-galacturonic acid, can be catabolized by some microorganisms. For example, the filamentous fungus, e.g. A. niger, is naturally capable of hydrolyzing pectin and catabolizing D-galacturonic acid to pyruvate and glycerol. In this reductive pathway, D-galacturonate (salt of galacturonic acid) is first reduced to L-galactonate by D-galacturonate reductase and then L-galactonate dehydratase removes a water molecule generating 3-deoxy-L-threo-hex-2-ulosonate. Next 2-keto-3-deoxy-L-galactonate aldolase splits 3-deoxy-L-threo-hex-2-ulosonate to pyruvate and L-glyceraldehyde. Finally, L-glyceraldehyde is reduced to glycerol by glyceraldehyde reductase. (Richard and Hilditch, 2009)
Meso-galactaric acid can be produced by the oxidation of D-galacturonic acid—the main constituent in pectin. In the biochemical oxidation route, a bacterial enzyme—uronate dehydrogenase—may be heterologously expressed in fungal and bacterial hosts resulting in production of meso-galactaric acid from D-galacturonic acid (
A nucleotide sequence encoding uronate dehydrogenase enzyme (EC 1.1.1.203) suitable for the present invention can be isolated from any organism producing this enzyme comprising eukaryotes and including animals (and man), plants, fungi, yeasts or prokaryotes including bacteria. Specifically a nucleotide sequence encoding uronate dehydrogenase enzyme of the present invention is isolated from a microbial source, such as from bacteria or fungi, in particular from bacteria. According to a specific embodiment of the invention a nucleotide sequence encoding uronate dehydrogenase enzyme of the present invention is obtainable from genus Agrobacterium, more specifically from A. tumefaciens. According to a another embodiment of the invention a nucleotide sequence encoding the enzyme is obtainable from a commercial culture collection strain, e.g. C 58, ATCC, American Type Culture Collection. According to a specific embodiment of the invention a nucleotide sequence encoding uronate dehydrogenase enzyme is obtainable from genus Pseudomonas. According to a very specific embodiment of the invention a nucleotide sequence encoding the enzyme is available in the GenBank, e.g. as GenBank accession number EU377538. In a further embodiment of the invention the uronate dehydrogenase enzyme is a heterologous (e.g. bacterial) uronate dehydrogenase enzyme, e.g. originating from Pseudomonas or Agrobacterium genera. In a specific embodiment uronate dehydrogenase enzyme is D-galacturonate dehydrogenase enzyme.
The origin of a polynucleotide sequence encoding uronate dehydrogenase enzyme of the present invention is not restricted to any specific genus or species. A person skilled in the art can find or isolate a polynucleotide sequence encoding uronate dehydrogenase enzyme of the present invention from other genera of bacteria or fungi or from other organisms.
Polynucleotide sequences encoding uronate dehydrogenase enzyme in various organisms can be isolated and the amino acid sequences encoded by the nucleotide sequences can be compared with the amino acid sequence of the uronate dehydrogenase enzyme isolated and characterized e.g. in WO2010/072902 A1. Homologues of uronate dehydrogenase enzymes can be identified by any conventional methods known in the art, for example by carrying out a BLAST or FASTA search.
A person skilled in the art can also identify a conserved region in the nucleotide or amino acid sequence and clone a gene fragment using PCR techniques. After sequencing the fragment the complete gene can be obtained for example by using cDNA library in a vector as described by Richard et al. (2001). A nucleotide sequence encoding uronate dehydrogenase enzyme can be identified also by nucleic acid hybridization.
Standard molecular biology methods can be used in the cloning of the uronate dehydrogenase enzyme i.e. in the isolation and enzyme treatments of DNA, in E. coli transformations, the isolation of a fragment comprising the uronate dehydrogenase gene by amplification in a PCR reaction (Coen D M, 2001) and in the techniques for codon change. The basic methods used are described in the standard molecular biology handbooks, e.g. Sambrook et al. (1989) and Sambrook and Russell (2001). Insertion of the nucleotide sequence under a strong promoter in an expression vector, transfer of the vector into suitable host cells and cultivation of the host cells in conditions provoking production of said enzyme. Methods for protein production by recombinant technology in different host systems are well known in the art (Gellissen, 2005). Cloning and expression of the D-galacturonic acid dehydrogenase in a heterologous host has also been well described e.g. in publications Mojzita et al. (2010) and WO2010/072902 (A1). These methods are suitable also for the present invention.
As used herein “a recombinant fungal cell” refers to any fungal cell that has been genetically modified to contain different genetic material compared to the fungal cell before modification. “The recombinant fungal cell” of the invention also refers to a host cell, which comprises a vector or plasmid comprising at least a polynucleotide encoding uronate dehydrogenase.
A recombinant micro-organism that has been “genetically modified to express” or “genetically modified to overexpress” includes embodiments, where a polynucleotide encoding a polypeptide has been transformed into a cell in such a manner that the cell is capable of producing an active polypeptide. As used herein, “expression” or “overexpression” achieved by a genetic modification of a fungal cell refers to excessive expression of a polynucleotide by producing more products (e.g. polypeptide) than an unmodified fungal cell. One or more copies of a polynucleotide may be transformed to a cell for expression or overexpression. The term also encompasses embodiments, where a promoter region has been modified to allow or to increase the expression of a polynucleotide in a fungal cell.
Activity and/or lack of activity of an uronate dehydrogenase enzyme can be confirmed by using known assay methods. E.g. Chang and Feingold (1969) and references therein have described the enzyme activity of uronate dehydrogenase.
An engineered fungal cell of the present invention comprises a genetic or non-genetic modification reducing D-galacturonic acid reductase (EC.1.1.1.19) activity. As used herein “reduced D-galacturonic acid reductase activity” refers to the presence of less D-galacturonic acid reductase activity, if any, compared to a wild type D-galacturonic acid reductase polypeptide, or lower D-galacturonic acid reductase activity (if any) in a cell compared to a cell comprising wild type D-galacturonic acid reductase. Reduced D-galacturonic acid reductase activity may result e.g. from down regulation of the polynucleotide or polypeptide expression, deletion of at least part of the D-galacturonic acid reductase polynucleotide or polypeptide, and/or lowered activity of D-galacturonic acid reductase polypeptide. An example of non-genetic methods includes but is not limited to a use of inhibitors (i.e. molecules that bind to a polynucleotide, a polypeptide or enzyme thereby decreasing its activity) of D-galacturonic acid reductases.
The fungal cell of the invention may comprise one or several genetic D-galacturonic acid reductase modifications. A genetic modification lowering D-galacturonic acid reductase activity may refer to a deletion or substitution of one or more nucleic acids or any fragment of a polynucleotide sequence encoding D-galacturonic acid reductase or any insertion of one or more nucleic acids or any nucleic acid sequence fragment into said polynucleotide sequence. Reduced activity of D-galacturonic acid reductase may be tested for example by a growth experiment, wherein a genetically modified fungal strain is grown on a medium in the presence of 2% D-galacturonic acid as a sole carbon source e.g. described in WO2010/072902 A1. As an example a mutant strain having no D-galacturonic acid reductase activity is not able to grow under these conditions.
In some embodiments, the genetic modification down regulates the expression of D-galacturonic acid reductase polynucleotide or polypeptide. As used herein “down regulated expression” refers to decreased expression, including lack of expression, of the gene or polypeptide of interest compared to a wild type fungal cell without the genetic modification. Lack of expression or decreased expression can be proved for example by western, northern or southern blotting or quantitative PCR or any other suitable method known to a person skilled in the art. Genetic modification leading to down-regulation of a polynucleotide or polypeptide refers to a deletion of D-galacturonic acid reductase polynucleotide, or one or more nucleotides or a fragment thereof, or one or more nucleotides or a fragment of a regulatory sequence (i.e. a sequence that regulates the expression of a polynucleotide (e.g. promoter area) or polypeptide) decreasing the expression of a polynucleotide or polypeptide. Also, any nucleotide insertions or substitutions (one or more nucleotides including long nucleotide sequences) in a polynucleotide or in a regulatory sequence of this polynucleotide may have an effect of decreasing the expression of a polynucleotide or polypeptide and thus, may be utilized in the present invention. Also, epigenetic modifications such as DNA methylation are known to block expression of genes and can be utilized in the present invention. Furthermore, e.g. RNA interference is also known to down-regulate translation of polypeptides. In certain embodiments, the genetic modification includes temporary or permanent silencing of D-galacturonic acid reductase gene.
The knowledge of the DNA sequence of D-galacturonic acid reductase can be used to inactivate the corresponding polynucleotide in a suitable fungal cell. The polynucleotide can be inactivated e.g. by preventing its expression or by mutation or deletion of the polynucleotide or part thereof. There are various techniques for inactivating a gene. These techniques make use of the nucleotide sequence of the gene or of the polynucleotide sequence in the proximity of the gene. In a specific embodiment the recombinant fungal cell has been genetically modified by deleting a polynucleotide encoding the D-galacturonic acid reductase or part thereof. For example gene knockout methods are suitable for disrupting a natural pathway for utilization of D-galacturonic acid, namely deleting the nucleotide sequence that encodes a polypeptide having D-galacturonic acid reductase activity, or any part thereof. These methods have been described for example in publications Mojzita et al. (2010) and WO2010/072902 A1, they are well-known to a person skilled in the art and they can be used for the present invention.
Examples of D-galacturonic acid reductase are not limited to gar1 or gar2 in H. jecorina and the gaaA in Aspergillus niger. Indeed, as used herein “D-galacturonic acid reductase” refers to any fungal polypeptide having D-galacturonic acid reductase acivity.
A “fragment” or “part” of a given sequence means any part of that sequence, for example one or several nucleotides or amino acids or a truncated form of the sequence.
As used herein, “a polynucleotide” refers to any polynucleotide, such as single or double-stranded DNA (genomic DNA or cDNA) or RNA, comprising a nucleic acid sequence encoding a specific polypeptide or a conservative sequence variant thereof.
Herein, the term “polypeptide” refer to polymers of amino acids of any length. As used herein “enzyme” refers to a polypeptide or group of polypeptides having activity as a catalyst. Enzymes accelerate, or catalyze, chemical reactions.
In connection with polynucleotides, the term “conservative sequence variant” refers to nucleotide sequence modifications, which do not significantly alter biological properties of the encoded polypeptide. Conservative nucleotide sequence variants include variants arising from the degeneration of the genetic code and from silent mutations. Nucleotide substitutions, deletions and additions are also contemplated.
Identity of any sequence or fragments thereof compared to the sequence of this disclosure refers to the identity of any sequence compared to the entire sequence of the present invention. As used herein, the %identity between the two sequences is a function of the number of identical positions shared by the sequences (i.e. % identity=# of identical positions/total # of positions×100), taking into account the number of gaps, and the length of each gap, which need to be introduced for optimal alignment of the two sequences. The comparison of sequences and determination of identity percentage between two sequences can be accomplished using mathematical algorithms available in the art. This applies to both amino acid and nucleic acid sequences.
Sequence identity may be determined for example by using BLAST (Basic Local Alignment Search Tools) or FASTA (FAST-All). In the searches, setting parameters “gap penalties” and “matrix” are typically selected as default.
Polypeptides or polynucleotides “from”, “derived from”, “originated from” or “obtained from” a particular organism encompass products isolated from said organism, as well as modifications thereof. A protein derived from a particular organism may be a recombinantly produced product, which is identical to, or a modification of the naturally occurring protein. The protein may also be modified e.g. by glycosylation, phosphorylation or other chemical modification. Products derived from the particular organism also encompass mutants and natural variants of the products, where one or more nucleic acid and/or amino acid is deleted, inserted and/or substituted.
By “homologous” is meant something originating from the same species as the host. By “heterologous” is meant something from another species as the host or from other genera as the host. If the host organism is a fungus, the heterologous nucleotide sequence may mean a nucleotide sequence from a prokaryote, such as from bacteria.
Fungal cells efficient in pectin hydrolysis are also capable of catabolizing the resulting meso-galactaric acid via unknown catabolic pathway. This catabolic pathway has not been described before. In this study, transcriptomics approach was used to identify genes involved in meso-galactaric acid catabolism in fungal cells. Catabolism of meso-galactaric acid was disrupted in a fungal cell by deleting a polynucleotide of said catabolic pathway (see Tables 1 and 2, Protein IDs are based on Joint Genome institute (JGI), JGI Aspergillus niger ATCC 1015 v4.0 genomic database). Altogether seven different genes of said catabolic pathway were deleted in the experiments.
Meso-galactaric acid catabolism may be reduced by genetic or non-genetic methods. An example of non-genetic methods includes but is not limited to a use of inhibitors (i.e. molecules that bind to a polynucleotide, a polypeptide or enzyme thereby decreasing its activity) of a polynucleotide, a polypeptide or polypeptides of the meso-galactaric acid pathway. Genetic modifications reducing D-galacturonic acid reductase activity described earlier in the present disclosure apply also to genetic modifications of polynucleotides of meso-galactaric acid catabolic pathway.
In one embodiment of the invention meso-galactaric acid catabolism has been decreased by deleting at least part of one or more polynucleotides encoding polypeptides participating in meso-galactaric acid catabolism or by decreasing expression or activity of one or more polypeptides participating in meso-galactaric acid catabolism. An engineered fungal strain combining the disrupted meso-galactaric acid catabolism, disrupted D-galacturonic acid catabolism and expression of a heterologous UDH produced meso-galactaric acid from D-galacturonic acid with an excellent yield. In addition, the strain was capable of consolidated bioprocess from pectin-rich biomass to meso-galactaric acid.
In one embodiment of the invention any polynucleotide of the meso-galactaric acid catabolic pathway, e.g. any one of those mentioned in Table 1 or 2, may be deleted according to the present invention. Alternatively, expression of the polynucleotides mentioned in Table 1 or 2 may be decreased or activity of one or more corresponding polypeptides may be reduced. Furthermore, anyone of the primers or probes mentioned in Table 1 or 2 may be used for identifying the polypeptide to be genetically modified.
As used herein, “catabolic pathway” refers to a pathway that breaks down molecules into smaller units that are either oxidized to release energy, or used in other reactions. Therefore, in this context a pathway is a series of chemical reactions occurring within a cell. The meso-galactaric acid catabolic pathway is a pathway breaking down meso-galactaric acids.
In some embodiments, the nucleic acid sequence to be genetically modified may comprise a polynucleotide sequence encoding a polypeptide having JGI ID number selected from the group consisting of 39114 (SEQ ID NO: 72), 1090836 (SEQ ID NO: 73), 1117792, 1141260, 1121140 (SEQ ID NO: 74), 1146483 and 1170646; or the nucleic acid sequence to be genetically modified may comprise a polynucleotide sequence having at least 80%, 85%, 90%, 95%, 96%, 97%, 98%, 99% or 100% nucleic acid sequence identity with a polynucleotide sequence encoding a polypeptide having JGI ID number selected from the group consisting of 39114, 1090836, 1117792, 1141260, 1121140, 1146483 and 1170646, and encoding a polypeptide of the meso-galactaric acid catabolic pathway.
The polypeptide 39114 has a predicted function of α-aminoadipate-semialdehyde dehydrogenase (EC 1.2.1.31) also known as α-aminoadipate reductase (AAR) (Napora-Wijata et al., 2014). The polypeptides 1090836 and 1121140 were predicted to be members of the protein families aldo/keto reductases and FAD-dependent oxidoreductases, respectively. The polypeptides 1117792, 1141260, 1146483 and 1170646 were predicted to be members of the protein families alcohol dehydrogenases, short-chain dehydrogenases/reductases, mandelate racemase/muconate lactonizing enzymes and D-isomer specific 2-hydroxyacid dehydrogenases respectively. Enzymatic activities of these proteins also remain unknown.
In a very specific embodiment the polypeptide (s) of the meso-galactaric acid catabolic pathway is (are) selected from the group consisting of AMP-dependent synthetase and ligase, α-aminoadipate-semialdehyde dehydrogenase, aldo/keto reductase, zinc-binding alcohol dehydrogenase, short-chain dehydrogenase/reductase, FAD-dependent oxidoreductase, mandelate racemase/muconate lactonizing enzyme and D-isomer specific 2-hydroxyacid dehydrogenase, and any combination thereof.
In some embodiments of the invention the slow transport of D-galacturonic acid into the cell might slow down the conversion of D-galacturonic acid to meso-galactaric acid. In this case the expression of genes coding for D-galacturonic acid transport molecules can facilitate the conversion of uronic acids. For example, if the host is yeast, such as S. cerevisiae, the introduction of a transporter to facilitate the transport of D-galacturonic acid into the fungal cell may be recommended. Expression of a transporter gene for D-galacturonic acid has been well described in publication WO2010/072902 (A1) and said methods and transporter genes are also suitable for the present invention.
In a specific embodiment in addition to any other modifications (e.g. UDH expression, deletion of at least part of a polynucleotide encoding D-galacturonic acid reductase and reduced meso-galactaric acid catabolism) the fungal cell has further been genetically modified by deleting at least part of a polynucleotide encoding 2-keto-3-deoxy-L-galactonate aldolase.
In one embodiment of the invention the recombinant fungal cell further comprises other genetic modifications than described above in the disclosure. “Other genetic modifications” include any genetic modifications e.g. addition of plasmids, insertions, substitutions, deletions or disruptions of one or more polynucleotides or parts thereof, e.g. one or more nucleotides. Also epigenetic modifications such as methylation are included in “other genetic modifications”. Furthermore, the cell may be genetically modified to produce or not to produce other compounds than meso-galactaric acid. Methods for any genetic modifications are generally well known and are described in various practical manuals describing laboratory molecular techniques.
The genetically modified fungal cells used in the invention are obtained by performing specific genetic modifications to a fungal cell. As used herein, a “recombinant fungal cell” refers to any fungal cell that has been genetically modified to contain different genetic material compared to the micro-organism before modification (e.g. comprise a deletion, substitution, disruption or insertion of one or more nucleic acids compared to the fungal cell before modification). The fungal cell of the invention is genetically modified to produce meso-galactaric acid. At least three different polynucleotides or any regulatory nucleotide sequence of a gene are modified according to the present invention.
“Fungi” “fungus” and “fungal” as used herein refer to any yeast and filamentous fungi i.e. moulds. A genetically modified fungal cell may also be referred to as a host cell. A fungal cell selected for the present invention is suitable for genetic manipulation and often can be cultured at cell densities useful for industrial production of a target product. A fungal cell selected may be maintained in a fermentation device. In one embodiment of the invention the fungal cell is a uracil auxotrophic cell. In another embodiment the fungal cell is naturally capable of degrading pectin. In a specific embodiment of the invention the fungal cell produces sufficiently pectinolytic enzymes to hydrolyse pectin and pectinolytic enzymes do not have to be supplemented.
In a specific embodiment the fungal cell is a mould or filamentous fungi. In another embodiment the fungal cell is selected from the genera Aspergillus, Hypocrea or Trichoderma. In another very specific embodiment the fungal cell is selected from the group consisting of Aspergillus niger, A. oryzae, A. terreus, A. nidulans, A. kawachii and A. fischeri and Trichoderma reesei. E.g. Aspergillus fungal cells are excellent organisms for the present invention because they produce efficiently pectinases that are hydrolysing pectin to the galacturonic acid which is the substrate of the process.
The genetically modified fungal strain of the present invention was the engineered to convert galacturonate to galactarate, but is not able to catabolize galactarate.
According to an embodiment of the invention, in the method of converting galacturonic acid to meso-galactaric acid, a fungal cell is contacted with a biomaterial comprising galacturonic acid or pectin in order to convert at least a portion of the galacturonic acid to meso-galactaric acid.
Thus, according to this embodiment, biomass comprising a sugar acid or a derivative thereof is fermented by a microorganism capable of converting D-galacturonic acid to meso-galactaric acid and optionally the desired compounds produced are recovered.
In one embodiment, the present invention is directed to a method, which comprises cultivating a genetically modified fungal cell under conditions allowing expression of the UDH.
In the method of the invention, a recombinant fungal cell is cultured in a growth and production medium that includes compounds for growth and energy. The medium used for producing meso-galactaric acid is any conventional medium, such as aqueous media, or solid media for culturing the fungal cell of the invention. For example, suitable media include but are not limited to the following: natural media composed of natural substrates, such as herbaceous or woody stems, seeds, leaves, corn meal, wheat germ, and oatmeal etc.; corn meal agar; potato dextrose agar; V-8 juice agar; dung agar; synthetic media; Czapek-Dox medium; glucose-asparagine and Neurospora crassa minimal medium; Yeast Extract Peptone (YP) media, Yeast Extract Peptone Dextrose (YPD) media, or any medium suitable for culturing filamentous fungi or yeast.
The carbon substrate used as a source for meso-galactaric acid production may be provided as pure substrates or from complex sources. Carbon substrate can be any carbon substrate, which can be directly or through one or more steps converted to D-galacturonic acid. In a specific embodiment of the invention, the carbon substrate is pectin or any source comprising pectin or rich in pectin (such as non-woody plant biomass e.g. fruit peels, citrus fruit and sugar beet). The medium may also contain alternative carbon sources, such as ethanol, glycerol, acetate, L-arabinose, D-galacturonate or amino acids.
In addition, the medium may consist of or contain complex, poorly defined elements, such as corn steep liquor or solids, or molasses. Sugars of the fermentation medium are also present. In case of oligomeric sugars, it may be necessary to add enzymes to the fermentation broth in order to digest these to the corresponding monomeric sugar. The medium will typically contain nutrients required by the particular micro-organism, including a source of nitrogen (such as amino acids, proteins, inorganic nitrogen sources such as ammonia or ammonium salts), and various vitamins and minerals. Also any other agents or compounds may be present in the medium according to the common general knowledge of the art.
Other fermentation conditions, such as temperature, cell density, selection of nutrients, and the like are not considered to be critical to the invention and are generally selected to provide an economical process. Such conditions are within the knowledge of a skilled person and can be selected depending on the fungal cell in question.
Temperatures during each of the growth phase or the production phase may range from above the freezing temperature of the medium to about 50° C., although the optimal temperature will depend somewhat on the particular micro-organism. In one embodiment a temperature is from about 15 to 37° C.
The pH of the process may or may not be controlled to remain at a constant pH, but may specifically be between about 3.0 and 8.0, depending on the production organism. Suitable buffering agents include, for example, calcium hydroxide, calcium carbonate, sodium hydroxide, potassium hydroxide, potassium carbonate, sodium carbonate, ammonium carbonate, ammonia, ammonium hydroxide and the like. In general, those buffering agents that have been used in conventional fermentation processes are also suitable here.
The fermentation is conducted aerobically or microaerobically. If desired, a specific oxygen uptake rate can be used as a process control. The process of the invention can be conducted continuously, batch-wise, or any combination thereof.
According to one embodiment of the present invention a fungal cell is cultured under suitable culture conditions for producing increased levels of meso-galactaric acid. The increase may be at least a 2, 3, 5, 10, 15, 20, 50 or 100 fold increase in meso-galactaric acid concentration compared to the unmodified strain or any strain with other modifications during cultivation. Alternatively, it may be at least a 2, 3, 5, 10, 15, 20, 50 or 100 fold increase in meso-galactaric acid yield per used carbon source in transformants compared to the unmodified strain or any strain with other modifications. It may also refer to at least a 2, 3, 5, 10, 15, 20, 50 or 100 fold increase in meso-galactaric acid production rate (g/l) compared to the unmodified strain or any strain with other modifications. In a very specific embodiment the meso-galactaric acid production reaches values 1 g/l, 2 g/l, 3 g/l, or 4 g/l. The increase of meso-galactaric acid production can be detected either intracellularly or in culture medium.
After culturing fungal cells the meso-galactaric acid can be recovered by disrupting the cells and/or directly from the culture medium without disrupting the cells. In one embodiment of the invention the method of producing meso-galactaric acid further comprises an optional step d) of isolating and purifying the meso-galactaric acid from the medium. Meso-galactaric acid may be isolated and purified from the medium by using any conventional methods known in the art such as ion exchange, two phase extraction, molecular distillation, melt crystallization, hexane extraction, CO2 extraction or distillation.
The meso-galactaric acid obtained by the method of the present invention can be used for preparing e.g. polymers. As an example, meso-galactaric acid may be chemically reduced to adipic acid and/or 2,5-furandicarboxylic acid (FDCA), precursors of polymers.
The present invention also relates to a method for identifying a polynucleotide or polypeptide of the meso-galactaric acid catabolic pathway, wherein the method comprises contacting a wild type fungal cell with meso-galactaric acid, carrying out a transcriptional analysis on a sample obtained from the wild type fungal cell contacted with meso-galactaric acid, and identifying overexpressed polynucleotides compared to a sample obtained from a wild type fungal cell not contacted with meso-galactaric acid. As used herein “wild type” refers to a fungal cell, which has not been genetically modified.
A transcriptional analysis may be carried out by any methods known to a person skilled in the art. Gene expression profiling is the measurement of the activity (the expression) of thousands of genes at once, to create a global picture of cellular function. These profiles can, for example, distinguish between cells that are actively dividing, or show how the cells react to a particular treatment. Many experiments of this sort measure an entire genome simultaneously, that is, every gene present in a particular cell. However, an entire genome does not need to be studied simultaneously.
A transcriptional analysis methods include but are not limited to hybridization-based microarrays, utilization of Expressed Sequence Tag libraries or chemical tag-based methods (e.g., serial analysis of gene expression), DNA microarray technology and RNA Sequencing.
DNA microarray technology measures the relative activity of previously identified target genes. Sequence based techniques, like serial analysis of gene expression (SAGE, SuperSAGE) are also used for gene expression profiling. SuperSAGE is especially accurate and can measure any active gene, not just a predefined set. The advent of next-generation sequencing has made sequence based expression analysis an increasingly popular, “digital” alternative to microarrays called RNA-Seq. RNA-seq (RNA sequencing, or also called whole transcriptome shotgun sequencing) uses next-generation sequencing to reveal the presence and quantity of RNA in a biological sample at a given moment in time. RNA-Seq is used to analyze the continually changing cellular transcriptome, specifically e.g. alternative gene spliced transcripts, post-transcriptional modifications, gene fusion, mutations/SNPs and changes in gene expression, and different populations of RNA to include mRNA, total RNA, small RNA, such as miRNA, tRNA, and ribosomal profiling.
In a specific embodiment of the invention total RNA was extracted from the fungal cell contacted with meso-galactaric acid for RNA sequencing, and optionally also from a fungal cell not contacted with meso-galactaric acid, for transcriptional analysis e.g. RNA sequencing.
In a specific embodiment the overexpressed polynucleotides were further analysed with enzyme prediction for potential carbohydrate metabolism. Such prediction programs are well known to a person skilled in the art and commercially available.
In one embodiment of the invention the method for identifying a polynucleotide or polypeptide of the meso-galactaric acid catabolic pathway further comprises: deleting a polynucleotide, which was identified as overexpressed or further analyzed as a potential polynucleotide or polypeptide of the meso-galactaric acid catabolic pathway according to said method, or part thereof from a fungal cell, and optionally identifying whether the fungal cell is capable of producing meso-galactaric acids. Methods suitable for decreasing meso-galactaric acid catabolism or deleting polynucleotides have been described above in the disclosure.
In a specific embodiment the method for identifying a polynucleotide or polypeptide of the meso-galactaric acid catabolic pathway further comprises genetic modification of a fungal cell capable of converting D-galacturonic acid to meso-galactaric acid by expressing uronate dehydrogenase enzyme, by reducing (e.g. by deleting at least part of a polynucleotide encoding D-galacturonic acid reductase) D-galacturonic acid reductase activity, and by reducing meso-galactaric acid catabolism. A cell to be modified may be the cell identified to have overexpressed polynucleotides or polypeptides, or any other cell (e.g. from different genera or species). Genetic modifications suitable for said fungal cell have been described above in the disclosure. In a more specific embodiment a putative gene was deleted using CRISPR/Cas9 technology, optionally together with in vitro synthesized single chimeric guide RNA (sgRNA), and as a result meso-galactaric acid catabolism was disrupted in the fungal cell. In a very specific embodiment of the invention any primer or primers of Table 1 may be utilized in the present invention.
Indeed, polynucleotide targeting (i.e. gene targeting) may be utilized in any genetic modifications of the present invention for deleting or replacing nucleotide sequences. Polynucleotide targeting uses homologous recombination to target desired changes to a specific endogenous polypeptide. Polynucleotide targeting may be carried out by any methods or techniques well known in the art. Methods for genetic targeting are described in various practical manuals describing laboratory molecular techniques. A person skilled in the art knows when and how to employ these methods. The success of polynucleotide targeting can be enhanced with the use of engineered nucleases. The nucleases create specific double-stranded break at desired locations in the genome, and harness the cell's endogenous mechanisms to repair the induced break by natural processes of homologous recombination and nonhomologous end-joining. Nucleases suitable for the present invention include but are not limited to, zinc finger nucleases, engineered homing endonucleases, transcription activator-like effector nucleases (TALENs), the CRISPR/Cas system and engineered meganucleases.
In the present invention polynucleotide targeting may be used to insert polynucleotides into or delete polynucleotides from target polynucleotides. Polynucleotide targeting can be permanent or conditional. Polynucleotide targeting requires the creation of a specific vector for each target polynucleotide of interest. The term “vector” refers to a nucleic acid compound and/or composition that transduces a cell, thereby causing the cell to express polynucleotides and/or polypeptides other than those native to the cell, or in a manner not native to the cell.
It will be obvious to a person skilled in the art that, as the technology advances, the inventive concept can be implemented in various ways. The invention and its embodiments are not limited to the examples described above but may vary within the scope of the claims.
The Aspergillus niger strain ATCC 1015 (CBS 113.46) was used as a wild type. The A. niger ΔpyrG strain (deleted orotidine-5′-phosphate decarboxylase) and the platform strain for meso-galactaric acid production ΔgaaA-udh (deleted D-galacturonate reductase and introduced uronate dehydrogenase) were described previously (Mojzita et al., 2010; WO2010/072902 (A1)). All the plasmids were produced in Escherichia coli TOP10 cells. The Saccharomyces cerevisiae strain ATCC 90845 was used in the homologous recombination for the construction of deletion cassettes.
Luria Broth culture medium supplemented with 100 μm ml−1 of ampicillin and cultural conditions of 37° C. and 250 rpm were used for E. coli cultures. YP-medium (10 g yeast extract 1−1, and 20 g peptone 1−1) supplemented with 20 g D-glucose 1−1 was used for yeast pre-cultures. After the transformations in yeast, SCD-URA (uracil deficient synthetic complete media supplemented with 20 g D-glucose 1−1) plates were used for uracil auxotrophic selection. All the yeast cultivations were carried out at 30° C. and the liquid cultivations at 250 rpm. A. niger spores were generated on potato-dextrose plates and ˜108 spores were inoculated to 50 ml of YP medium (10 g yeast extract 1−1, 20 g peptone 1−1) containing 30 g gelatin 1−1 for pre-cultivations. Mycelia were pre-grown in 250-ml Erlenmeyer flasks by incubating overnight at 28° C., 200 rpm and harvested by vacuum filtration, rinsed with sterile water and weighted. In A. niger transformations, A. nidulans defined minimal medium (Barratt et al., 1965) plates supplemented with 1.2 M D-sorbitol and 20 g agar 1−1 and, in the case of CRISPR/Cas9 transformations, 400 μm/ml hygromycin were used. The minimal medium used in the phenotypic characterization in liquid cultivations contained 10 g meso-galactaric acid 1−1 with or without 0.5 g D-xylose 1−1 and the pH was adjusted to 7.0. These cultivations were inoculated with 10 g 1−1 (wet) of pre-grown mycelia. Alternatively, YP-medium supplemented with 10 g meso-galactaric acid 1−1 was used. Similar minimal or YP-medium supplemented with 20 g D-galacturonic acid 1−1 pH 5 was used in the cultivations for meso-galactaric acid production and were inoculated with 10 gl−1 (wet) of pre-grown mycelia. For the consolidated process, the minimal medium was supplemented with 40 g 1−1 of orange processing waste as described earlier (Kuivanen et al., 2015).
A. niger wild type strain was cultivated in the minimal medium supplemented with meso-galactaric acid. Samples of 2 ml were collected and the mycelium was harvested by vacuum filtration. The filtered mycelium was frozen with liquid nitrogen and stored at −80° C. Total RNA was extracted using the RNeasy Plant Mini Kit (Qiagen). RNA library preparation and sequencing was carried out by GATC (Constance, Germany) using the InView™ Transcriptome Explore package. The raw data was processed as described earlier (Kuivanen et al. 2016).
For the deletion of the genes identified in the RNA sequencing, deletion cassettes containing homologous 5′ and 3′ flanks (˜1.5 kb) for targeted integration and the selectable marker pyrG (A. niger) were constructed. The 5′ and 3′ flanks were amplified by PCR (KAPA HiFi DNA polymerase, Kapa Biosystems) with the primers described in table 1. The amplified flanks and pyrG were joined using yeast homologous recombination, produced in E. coli and the resulting cassettes were linearized with Notl (Thermo). The linearized cassettes (10 μm) were transformed to A. niger ΔpyrG strain with or without the pFC-332 plasmid expressing Cas9 (1 jig) (Nodvig et al., 2015) and two suitable in vitro synthesized sgRNAs (10 jig) (GeneArt™ Precision Synthesis Kit) as described in table 1. In the generation of meso-galactaric acid producing strain, the genes with the ID 39114, 1090836 and 1121140 were deleted from the strain ΔgaaA-udh. All the A. niger transformations were carried out using the protoplast transformation method. Correct integration of the transformed cassette into the genome was confirmed with colony PCR using Phire direct PCR kit (Thermo Scientific) and the primers listed in table 1.
Samples were removed from liquid cultivations at intervals and mycelium was separated from the supernatant by filtration. The concentration of mesogalactaric acid and D-galacturonic acid was determined by HPLC using a Fast Acid Analysis Column (100 mm×7.8 mm, BioRad Laboratories, Hercules, Calif.) linked to an Aminex HPX-87H organic acid analysis column (300 mm×7.8 mm, BioRad Laboratories) with 5.0 mM H2SO4 as eluent and a flow rate of 0.5 ml min−1. The column was maintained at 55° C. Peaks were detected using a Waters 2489 UV/Visible dual wavelength UV (210 nm) detector.
A. niger wild type mycelium was cultivated on meso-galactaric acid and the utilization of meso-galactaric acid was monitored using HPLC (data not shown). It was indeed confirmed that A. niger is capable of catabolizing meso-galactaric acid. In addition, we observed that oxalic acid was produced in the cultivation. For the RNA sequencing, total RNA was extracted after 0, 5 and 18 hours and sequenced. The results from the RNA sequencing are presented in
We selected seven putative genes based on their induction on meso-galactaric acid and relevant enzyme prediction for carbohydrate metabolism (
The non-homologous end joining (NHEJ) pathway is the predominant mechanism for DNA repair in A. niger. Thus the frequency of homologous recombination is typically low in transformations. Previously, CRISPR/Cas9 mediated gene deletions were described in several Aspergillus species by using the AMA plasmid expressing both Cas9 protein and sgRNA (Nodvig et al., 2015). Due to the poor availability of characterized RNA polymerase (RNA pol) III promoters, sgRNA was expressed under a RNA pol II promoter which requires the use of additional ribozyme structures to release a functional sgRNA. In A. niger, transformation of the plasmid without donor DNA resulted in successful gene disruptions via short deletions by the NHEJ mediated repair. In the present study, we used in vitro synthesized sgRNAs. This approach has been described earlier in the filamentous fungi Trichoderma reesei (Liu et al., 2015), Penicillium chrysogenum (Pohl et al., 2016) and Aspergillus fumigatus (C. Zhang et al., 2016) but not in Aspergillus niger. We also decided to use deletion cassettes as donor DNA. This approach allowed double selection resulting in high frequencies of correct deletions. The use of donor DNA allows easier screening of the genotypes by colony PCR which would not necessary detect short deletions in the genome resulting from NHEJ pathway repair without donor DNA.
The resulting mutant strains from the gene deletions were tested for meso-galactaric acid consumption in liquid cultivations (
The protein 39114 has a predicted function of α-aminoadipate-semialdehyde dehydrogenase (EC 1.2.1.31) also known as α-aminoadipate reductase (AAR) (Napora-Wijata et al., 2014). The proteins 1090836 and 1121140 have predicted functions of aldo/keto reductase and FAD-dependent oxidoreductase, respectively. Enzymatic activities of these proteins also remain unknown.
2.4. Engineering A. niger for Meso-Galactaric Acid Production
The genes with the ID 39114, 1090836 and 1121140 were deleted from the A. niger strain ΔgaaA-udh. The strain ΔgaaA-udh has a disrupted pathway for D-galacturonic acid catabolism (deletion of gaaA); however, introduction of udh regenerated the catabolism of D-galacturonic acid with only minor production of meso-galactaric acid (Mojzita et al., 2010). The strain was uracil autotroph (+pyrG) and we decided to use the same deletion cassettes containing pyrG selectable marker which were used in the initial gene deletions from ΔpyrG strain. This time we combined the deletion cassette with the Cas9 plasmid and in vitro synthesized sgRNA. Consequently, the selection pressure was only for the Cas9 plasmid but not for the donor DNA. Nevertheless, about 1 out of 10 screened colonies revealed the correct gene deletion.
Next the resulting strain ΔgaaA-Δ39114-udh was tested for meso-galactaric acid production in shake flask cultivations on D-galacturonic acid (
We also wanted to investigate the consolidated bioprocess for the production directly from pectin-rich biomass. Processing waste from orange juice industry was used as substrate in submerged cultivations (
Napora-Wijata, K., Strohmeier, G. A., Winkler, M., 2014. Biocatalytic reduction of carboxylic acids. Biotechnol. J. 9, 822-843. doi:10.1002/biot.201400012
Zhang, C., Meng, X., Wei, X., Lu, L., 2016. Highly efficient CRISPR mutagenesis by microhomology-mediated end joining in Aspergillus fumigatus. Fungal Genet. Biol. 86, 47-57. doi:10.1016/j.fgb.2015.12.007
| Number | Date | Country | Kind |
|---|---|---|---|
| 20165425 | May 2016 | FI | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FI2017/050378 | 5/19/2017 | WO | 00 |
