Patent Publication Number: US-2019194671-A1

Title: Erythritol production in cyanobacteria

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for producing erythritol and to a cyanobacterial cell for the production of erythritol. 
     BACKGROUND OF THE INVENTION 
     Numerous biotechnological processes make use of genetically engineered organisms in order to produce bulk or fine chemicals, proteins or antibiotics. In many cases, increased production has been obtained by improved gene expression and by optimization of growth conditions. In most processes, the initial carbon-precursor has been and still is sugar (notably glucose, but many other mono- and polysaccharides are in use) or related organic substrates: solventogenesis (including butanol and ethanol) and organic acid production (e.g. lactic-, citric- or succinic acid) always starts from glucose, which makes it inefficient as the production process uses a high energy compound as input substrate. 
     Standard fermentation processes require a carbon source, for which plants and algal species are employed to reduce carbon dioxide via photosynthesis (using the energy of the sun) to the level of sugars and cell material. After harvesting, these end products are converted to ethanol by yeast fermentation (in the case of crops) or converted chemically to biofuels (in the case of algae). The overall energy conservation of these methods is highly inefficient and therefore demands large surface areas. In addition, the crop processes are rather labor-intensive, are demanding with respect to water consumption and affect food stock prices with adverse consequences for food supplies. A more remotely similar process is based on the conversion of solar energy into hydrogen. Also this process suffers from a severely decreased efficiency. 
     U.S. Pat. No. 6,699,696 describes a process of producing ethanol by feeding carbon dioxide to a cyanobacterial cell, especially a Synechococcus comprising a nucleic acid molecule encoding an enzyme enabling the cell to convert pyruvate into ethanol, subjecting said cyanobacterial cell to sun energy and collecting ethanol. This system has several drawbacks among others the expression system used is temperature sensitive which demands to adapt the production system for such regulation. 
     WO 2009/078712 describes a process of producing ethanol, propanol, butanol, acetone, 1,3-propanediol, ethylene or D-lactate and where appropriate intermediary compounds in the pathway leading to any of these organic compounds. The process is carried out by feeding carbon dioxide to a culture of cyanobacterial cells and subjecting the culture to light, wherein the cells are capable of expressing a nucleic acid molecule under the control of a regulatory system which responds to a change in the concentration of a nutrient in the culture which confers on the cell the ability to convert a glycolytic intermediate into the above-mentioned organic compounds and/or into intermediary compounds. 
     Erythritol is a four-carbon polyol (sugar alcohol) that is used as a sweetener in food and pharmaceutical industries. It is a naturally occurring substance, usually as a storage compound in seaweeds and fungi. Erythritol has roughly 65% of the sweetness of sucrose and is thus an attractive non-caloric substitute. 
     Erythritol is commercially produced via fermentation by various industries, such as Bolak Corporation (Whasung, Kyungki-do, Korea), Cargill Food &amp; Pharm Specialties (Blair, Nebr., USA), and Mitsubishi Chemical Corporation (Tokyo, Japan). Glucose from chemically and enzymatically hydrolyzed wheat and corn starches is used as a major carbon source to produce erythritol by the fermentation of yeast-like fungi such as  Torula  sp. and  Moniliella pollinis  (Moon et al., 2010). 
     However, there is still a need for an improved production process of erythritol, preferably without the need of expensive or complicated starting materials, such as sugar, and which process does not have the drawbacks of existing processes such as those described here above. 
     DESCRIPTION OF THE INVENTION 
     Surprisingly, it has now been demonstrated that erythritol can conveniently be produced in a cyanobacterial cell. In brief, the inventors of the present invention have arrived at a scalable process for the production of the specific polyol, erythritol. The invention combines metabolic properties of photoautotrophic and chemoorganotrophic microorganisms and is based on the employment of recombinant oxyphototrophs with high rates of conversion of Calvin cycle intermediates to a fermentative end product. Its novelty resides in the fact that its core chemical reactions use carbon dioxide as the sole carbon-containing precursor and light (preferably sunlight), as the sole energy source, to drive carbon dioxide reduction. Moreover, the cyanobacterial cell factory is more suitable for production of erythritol than other microorganism used in fermentation processes such as  E. coli  and yeasts, since the abundantly available co-factor in the cyanobacterial cell is NADPH, rather than NADH in most chemotrophic organisms used for fermentation. Production may be controlled by a nutrient- or light-sensitive promoter. Using a nutrient-sensitive promoter, production is controlled by a medium component and can start at the most appropriate time, such as at the highest possible cell density. A light-mediated promoter is controlled by light intensity. Whereas in current production processes for biochemicals, organisms are substrate (e.g., crops in ethanol production) or product (e.g., microalgae as biodiesel), here microorganisms are used as highly specialized catalysts for the conversion of carbon dioxide as a substrate to a valuable end product. These catalysts can be subjected to further optimization strategies through physical- and chemical systems-biology approaches. The biochemical background of cyanobacterial cells for the production of valuable compounds is more extensively described in WO 2009/078712, especially in example 1. The various aspects of the present invention are more extensively described below. 
     In a first aspect, the present invention relates to a cyanobacterial cell capable of expressing, preferably expressing, at least one functional enzyme selected from the group of enzymes consisting of a phosphatase and a reductase. Said cyanobacterial cell is herein further referred to as a cyanobacterial cell according to the present invention. The cyanobacterial cell according to the present invention is preferably capable of producing erythritol, more preferably producing erythritol. 
     The term “functional enzyme” is herein preferably defined in the context of a phosphatase as an enzyme from the group of Haloacid Dehalogenase-like phosphatases, with affinity for erythrose-4-phosphate and/or erythritol-4-phosphate, such as with a Km for erythrose-4-phosphate in the range of 0.001 to 50.0 mM, more preferably 0.001 to 10 mM, even more preferably 0.001 to 1 mM, even more preferably 0.001 to 0.1 mM, even more preferably 0.001 to 0.01 mM, even more preferably 0.001 to 0.005 mM. The term “functional enzyme” is herein preferably defined in the context of a reductase as an enzyme closely related to the family of aldose reductases and able catalyze the reduction of aldehydes and preferably able to reduce either erythrose into erythritol or, erythrose-4-phosphate into erythritol-4-phosphate. 
     A preferred cyanobacterial cell according to the invention is capable of expressing, preferably expressing, at least one functional enzyme selected from the group consisting of enzymes having activity of an erythrose-phosphatase, an erythritol-phosphatase and an erythrose reductase; more preferably of an erythrose-4-phosphate reductase and an erythritol-4-phosphate phosphatase, or of an erythrose-4-phosphate phosphatase and an erythrose reductase. The enzyme may be native or may be heterologous to the cyanobacterial cell. 
     In a cyanobacterial cell according to the present invention, the at least one functional enzyme is preferably selected from the group consisting of an erythrose-4-phosphate phosphatase from  Thermotoga maritima, Escherichia coli  or  Synechocystis  PCC6803 and an erythrose-4-phosphate reductase or erythrose reductase from  Saccharomyces cerevisiae, Candida magnoliae, Trichoderma reesei, Aspergillus niger  or  Penicillium chrysogenum.    
     In a cyanobacterial cell according to the present invention, the at least one functional enzyme preferably comprises or consists of a polypeptide that has an amino acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 2, SEQ ID NO: 4, SEQ ID NO: 6, SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 14 and SEQ ID NO: 16. 
     In a cyanobacterial cell according to the present invention, the at least one functional enzyme is preferably encoded by a polynucleotide that has an nucleic acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 5, SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11, SEQ ID NO: 13 and SEQ ID NO: 15. 
     In a cyanobacterial cell according to the present invention, the at least one functional enzyme preferably is pair of enzymes consisting of a phosphatase that has an amino acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 2, SEQ ID NO: 4 and SEQ ID NO: 6; and a reductase that has an amino acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 8, SEQ ID NO: 10, SEQ ID NO: 12, SEQ ID NO: 14 and SEQ ID NO: 16. 
     Preferred pairs of a phosphatases and a reductase are a pair selected from the group consisting of SEQ ID NO: 2 and SEQ ID NO: 8, SEQ ID NO: 2 and SEQ ID NO: 10; SEQ ID NO: 2 and SEQ ID NO: 12, SEQ ID NO: 2 and SEQ ID NO: 14, SEQ ID NO: 2 and SEQ ID NO: 16, SEQ ID NO: 4 and SEQ ID NO: 8, SEQ ID NO: 4 and SEQ ID NO: 10, SEQ ID NO: 4 and SEQ ID NO: 12, SEQ ID NO: 4 and SEQ ID NO: 14, SEQ ID NO: 4 and SEQ ID NO: 16, SEQ ID NO: 6 and SEQ ID NO: 8, SEQ ID NO: 6 and SEQ ID NO: 10, SEQ ID NO: 6 and SEQ ID NO: 12, SEQ ID NO: 6 and SEQ ID NO: 14, and SEQ ID NO: 6 and SEQ ID NO: 16; as well as a variants of these sequences with a sequence identity of at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% to the respective sequence. 
     In a cyanobacterial cell according to the present invention, the at least one functional enzyme preferably is pair of enzymes consisting of a phosphatase encoded by a polynucleotide that has an nucleic acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 1, SEQ ID NO: 3 and SEQ ID NO: 5; and a reductase encoded by a polynucleotide that has an nucleic acid sequence with at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% sequence identity with a sequence selected from the group consisting of SEQ ID NO: 7, SEQ ID NO: 9, SEQ ID NO: 11, SEQ ID NO: 13 and SEQ ID NO: 15. Preferred pairs of a phosphatases and a reductase are a pair encoded by a pair of sequences selected from the group consisting of SEQ ID NO: 1 and SEQ ID NO: 7, SEQ ID NO: 1 and SEQ ID NO: 9; SEQ ID NO: 1 and SEQ ID NO: 11, SEQ ID NO: 1 and SEQ ID NO: 13, SEQ ID NO: 1 and SEQ ID NO: 15, SEQ ID NO: 3 and SEQ ID NO: 7, SEQ ID NO: 3 and SEQ ID NO: 9, SEQ ID NO: 3 and SEQ ID NO: 11, SEQ ID NO: 3 and SEQ ID NO: 13, SEQ ID NO: 3 and SEQ ID NO: 15, SEQ ID NO: 5 and SEQ ID NO: 7, SEQ ID NO: 5 and SEQ ID NO: 9, SEQ ID NO: 5 and SEQ ID NO: 11, SEQ ID NO: 5 and SEQ ID NO: 13, and SEQ ID NO: 5 and SEQ ID NO: 15; as well as a variants of these sequences with a sequence identity of at least 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99% or 100% to the respective sequence. In the context of all embodiments of the present invention, the terms “a cyanobacterium”, “a cyanobacterium cell” and “a cyanobacterial cell” are used interchangeably and refer to a blue-green algae, a photosynthetic unicellular microorganism. Examples of cyanobacteria include the genera  Aphanocapsa, Anabaena, Nostoc, Oscillatoria, Synechococcus, Synechocystis, Gloeocapsa, Agmenellum, Scytonema, Mastigocladus, Arthrosprira, Haplosiphon . A preferred order of cyanobacteria is Chroococcales. A more preferred cyanobacterium genus is  Synechocystis . A more preferred species of this genus is a  Synechocystis  PCC6803 species. Even more preferably, the  Synechocystis  is a Pasteur Culture Collection (PCC) 6803  Synechocystis , which is a publicly available strain via ATCC for example. PCC 6803 has been stored at ATCC under ATCC27184. The phototrophic  Synechocystis  PCC 6803 is a fast growing cyanobacterium with no specific nutritional demands. Its physiological traits are well-documented: it is able to survive and grow in a wide range of conditions. For example,  Synechocystis  sp. PCC 6803 can grow in the absence of photosynthesis if a suitable fixed-carbon source such as glucose is provided. Perhaps most significantly,  Synechocystis  sp. PCC 6803 was the first photosynthetic organism for which the entire genome sequence was determined (available via the internet world wide web at kazusa.or.jp/cyano/cyano). In addition, an efficient gene deletion strategy (Shestakov S V et al., 2002; and Nakamura Y et al., 1999) is available for  Synechocystis  sp. PCC 6803, and this organism is furthermore easily transformable via homologous recombination (Grigirieva G A et al., 1982). 
     “Capable of producing erythritol” preferably means herein that detectable amounts of erythritol can be detected in a culture of a cyanobacterial cell according to the present invention cultured, under conditions conducive to the production of erythritol, preferably in the presence of light and dissolved carbon dioxide and/or bicarbonate ions, during at least 1 day using a suitable assay for detecting erythritol. A preferred concentration of said dissolved carbon dioxide and/or bicarbonate ions is, the natural occurring concentration at neutral to alkaline conditions (pH 7 to 9) being approximately 1 mM. This is equivalent to 0.035% of carbon dioxide in ambient air. A more preferred concentration of carbon dioxide and/or bicarbonate ions is higher than this natural occurring concentration. Preferably, the concentration of bicarbonate ions is at least 0.5 mM, 0.6 mM, 0.7 mM, 0.8 mM, 0.9 mM, 1 mM, 2 mM, 5 mM, 10 mM, 15 mM, 20 mM, 25 mM, 30 mM, 35 mM, 40 mM, 45 mM, 50 mM, 60 mM, 70 mM, 80 mM, 90 mM or 100 mM. A preferred method to increase the carbon dioxide and/or bicarbonate ions in solution is by enrichment with carbon dioxide, preferably waste carbon dioxide from industrial plants, sparged into the culture broth. The concentration of carbon dioxide is preferably increased to at least 0.04%, 0.05%, 0.1%, 0.15%, 0.2%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%. 
     Preferably, erythritol is thus detected in a cyanobacterial cell according to the present invention and/or in its culture broth, wherein said cyanobacterial cell is cultured under conditions conducive to the production of erythritol, preferably the conditions include culturing in the presence of sunlight and carbon dioxide during at least 1 day using a given assay for the intermediary compound. 
     The erythritol produced within the cyanobacterial cell according to the invention may spontaneously diffuse into the culture broth. Assays for the detection of erythritol are, but not limited to, High Performance Liquid Chromatography (HPLC), Gas Chromatography (GC), Gas Chromatography-Mass Spectrometry (GC-MS), or Liquid Chromatography-Mass Spectrometry (LC-MS). A preferred assay for the detection of erythritol is High Performance Liquid Chromatography (HPLC). A detectable amount for erythritol is preferably at least 0.05 mM under said culture conditions and using said assay. Preferably, a detectable amount is at least 0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM, 0.5 mM, 1 mM, 2 mM, 3 mM, 4 mM, 5 mM, 6 mM, 7 mM, 8 mM, 9 mM, 10 mM, 15 mM, 20 mM, 25 mM, 30 mM, 35 mM, 40 mM, 45 mM, 50 mM, or at least 100 mM. 
     Preferably, a cyanobacterial cell according to the present invention comprises at least one nucleic acid molecule comprising or consisting of a polynucleotide encoding at least one of the at least one functional enzyme as defined here above. Accordingly, a preferred cyanobacterial cell according to the invention comprises at least one nucleic acid molecule comprising or consisting of a polynucleotide encoding at least one of the at least one functional enzyme as defined here above. 
     The at least one functional enzyme as defined here above is encoded by a polynucleotide. In all embodiments according to the invention, each encoding polynucleotide may be present on a separate nucleic acid molecule. Alternatively, the encoding polynucleotides may be present on a single nucleic acid molecule. 
     A preferred cyanobacterial cell according to the invention is a cyanobacterial cell wherein the at least one functional enzyme is encoded by a nucleic acid molecule comprising or consisting of a polynucleotide wherein said nucleic acid molecule is preferably present in the cyanobacterial cell as an episomal entity, preferably said episomal entity is a plasmid, more preferably a self-replicating plasmid. The episomal entity and plasmid can be any episomal entity and plasmid known to the person skilled in the art or can be based on any episomal entity and plasmid known to the person skilled in the art and modified to comprise any nucleic acid and/or polynucleotide described herein. 
     Another preferred cyanobacterial cell according to the invention is a cyanobacterial cell wherein the at least one functional enzyme is encoded by a nucleic acid molecule comprising or consisting of a polynucleotide wherein said nucleic acid molecule is preferably integrated in the cyanobacterial genome, preferably via homologous recombination. 
     A cyanobacterial cell according to the present invention may comprise a single but preferably comprises multiple copies of each nucleic acid molecule. 
     A preferred cyanobacterial cell according to the present invention is a cyanobacterial cell, wherein a polynucleotide encoding the at least one functional enzyme is under control of a regulatory system which responds to a change in the concentration of a nutrient when culturing said cyanobacterial cell. 
     A promoter that may be used for the expression of a polynucleotide encoding the at least one functional enzyme may be foreign to the polynucleotide, i.e. a promoter that is heterologous to the polynucleotide encoding the at least one functional enzyme to which it is operably linked. Although a promoter preferably is heterologous to the polynucleotide to which it is operably linked, it is also possible that a promoter is native to the cyanobacterial cell according to the present invention. Preferably, a heterologous (to the nucleotide sequence) promoter is capable of producing a higher steady state level of a transcript comprising a coding sequence (or is capable of producing more transcript molecules, i.e. mRNA molecules, per unit of time) than is a promoter that is native to the coding sequence. A suitable promoter in this context includes both a constitutive and an inducible natural promoter as well as an engineered promoter. A promoter used in a cyanobacterial cell according to the present invention may be modified, if desired, to affect its control characteristics. A preferred promoter for constitutive expression is a Ptrc, as is further outlined below in the next paragraph. The Ptrc promoter is an artificial promoter, which is constructed as a chimera of the  E. coli  trp operon and lacUV5 promoters (Brosius et al, J Biol Chem 1985). The promoter is thus regulated by the Lac repressor, LacI. In  Synechocystis , the Lad regulated repression and induction does not function efficiently, but the Ptrc promoter does show high constitutive expression levels in the absence of Lad (Huang H-H, Camsund D, Lindblad P, Heidorn T: Design and characterization of molecular tools for a Synthetic Biology approach towards developing cyanobacterial biotechnology.  Nucleic Acids Res  2010, 38:2577-2593). It was 
     The cyanobacterial cell according to the present invention can conveniently be used for the production of erythritol. 
     Accordingly, in a second aspect, the present invention relates to a process for producing erythritol comprising culturing a cyanobacterial cell according to the present invention, preferably a cyanobacterial cell as defined in the first aspect of the present invention, under conditions conducive to the production of erythritol and, optionally, isolating and/or purifying the erythritol from the culture broth. 
     Usually a process is started with a culture (also named culture broth) of cyanobacterial cells having an optical density measured at 730 nm of approximately 0.2 to 2.0 (OD730=0.2 to 2) as measured in any conventional spectrophotometer with a measuring path length of 1 cm. Usually the cell number in the culture doubles every 20 hours. A preferred process takes place in a tank with a depth of 30-50 cm exposed to sun light. In a preferred process, the number of cells increases until the source of ammonium is exhausted or below a given value as earlier explained herein, subsequently the production of erythritol will start. Preferably, the light used is natural. A preferred natural light is sunlight. Daylight (or sunlight) may have an intensity ranged between approximately 500 and approximately 1500 μEinstein/m 2/s. In another preferred embodiment, the light used is artificial. Such artificial light may have an intensity ranged between approximately 70 and approximately 800 μEinstein/m 2/s. Preferably, the cells are continuously under the light conditions as specified herein. However, the cells may also be exposed to high light intensities (such as e.g. daylight/sunlight) as defined elsewhere herein for a certain amount of time, after which the cells are exposed to a lower light intensity as defined elsewhere herein for a certain amount of time, and optionally this cycle is repeated. In a preferred embodiment, the cycle is the day/night cycle. 
     In a preferred process, erythritol is separated from the culture broth. This may be realized continuously with the production process or subsequently to it. Separation may be based on any separation method known to the person skilled in the art. 
     Definitions 
     “Sequence identity” or “identity” in the context of amino acid- or nucleic acid-sequence is herein defined as a relationship between two or more amino acid (peptide, polypeptide, or protein) sequences or two or more nucleic acid (nucleotide, polynucleotide) sequences, as determined by comparing the sequences. In the art, “identity” also means the degree of sequence relatedness between amino acid or nucleotide sequences, as the case may be, as determined by the match between strings of such sequences. Within the present invention, sequence identity with a particular sequence preferably means sequence identity over the entire length of said particular polypeptide or polynucleotide sequence. The sequence information as provided herein should not be so narrowly construed as to require inclusion of erroneously identified bases. The skilled person is capable of identifying such erroneously identified bases and knows how to correct for such errors. 
     “Similarity” between two amino acid sequences is determined by comparing the amino acid sequence and its conserved amino acid substitutes of one peptide or polypeptide to the sequence of a second peptide or polypeptide. In a preferred embodiment, identity or similarity is calculated over the whole SEQ ID NO as identified herein. “Identity” and “similarity” can be readily calculated by known methods, including but not limited to those described in Computational Molecular Biology, Lesk, A. M., ed., Oxford University Press, New York, 1988; Biocomputing: Informatics and Genome Projects, Smith, D. W., ed., Academic Press, New York, 1993; Computer Analysis of Sequence Data, Part I, Griffin, A. M., and Griffin, H. G., eds., Humana Press, New Jersey, 1994; Sequence Analysis in Molecular Biology, von Heine, G., Academic Press, 1987; and Sequence Analysis Primer, Gribskov, M. and Devereux, J., eds., M Stockton Press, New York, 1991; and Carillo, H., and Lipman, D., SIAM J. Applied Math., 48:1073 (1988). 
     Preferred methods to determine identity are designed to give the largest match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Preferred computer program methods to determine identity and similarity between two sequences include e.g. the GCG program package (Devereux, J., et al., Nucleic Acids Research 12 (1): 387 (1984)), BestFit, BLASTP, BLASTN, and FASTA (Altschul, S. F. et al., J. Mol. Biol. 215:403-410 (1990). The BLAST X program is publicly available from NCBI and other sources (BLAST Manual, Altschul, S., et al., NCBI NLM NIH Bethesda, Md. 20894; Altschul, S., et al., J. Mol. Biol. 215:403-410 (1990). The well-known Smith Waterman algorithm may also be used to determine identity. 
     Preferred parameters for polypeptide sequence comparison include the following: Algorithm: Needleman and Wunsch, J. Mol. Biol. 48:443-453 (1970); Comparison matrix: BLOSSUM62 from Hentikoff and Hentikoff, Proc. Natl. Acad. Sci. USA. 89:10915-10919 (1992); Gap Penalty: 12; and Gap Length Penalty: 4. A program useful with these parameters is publicly available as the “Ogap” program from Genetics Computer Group, located in Madison, Wis. The aforementioned parameters are the default parameters for amino acid comparisons (along with no penalty for end gaps). Preferred parameters for nucleic acid comparison include the following: Algorithm: Needleman and Wunsch, J. Mol. Biol. 48:443-453 (1970); Comparison matrix: matches=+10, mismatch=0; Gap Penalty: 50; Gap Length Penalty: 3. Available as the Gap program from Genetics Computer Group, located in Madison, Wis. Given above are the default parameters for nucleic acid comparisons. 
     Optionally, in determining the degree of amino acid similarity, the skilled person may also take into account so-called “conservative” amino acid substitutions, as will be clear to the skilled person. Conservative amino acid substitutions refer to the interchangeability of residues having similar side chains. For example, a group of amino acids having aliphatic side chains is glycine, alanine, valine, leucine, and isoleucine; a group of amino acids having aliphatic-hydroxyl side chains is serine and threonine; a group of amino acids having amide-containing side chains is asparagine and glutamine; a group of amino acids having aromatic side chains is phenylalanine, tyrosine, and tryptophan; a group of amino acids having basic side chains is lysine, arginine, and histidine; and a group of amino acids having sulphur-containing side chains is cysteine and methionine. Preferred conservative amino acids substitution groups are: valine-leucine-isoleucine, phenylalanine-tyrosine, lysine-arginine, alanine-valine, and asparagine-glutamine. Substitutional variants of the amino acid sequence disclosed herein are those in which at least one residue in the disclosed sequences has been removed and a different residue inserted in its place. Preferably, the amino acid change is conservative. Preferred conservative substitutions for each of the naturally occurring amino acids are as follows: Ala to ser; Arg to lys; Asn to gln or his; Asp to glu; Cys to ser or ala; Gln to asn; Glu to asp; Gly to pro; His to asn or gln; Ile to leu or val; Leu to ile or val; Lys to arg; gln or glu; Met to leu or ile; Phe to met, leu or tyr; Ser to thr; Thr to ser; Trp to tyr; Tyr to trp or phe; and, Val to ile or leu. 
     A polynucleotide is represented by a nucleotide sequence. A polypeptide is represented by an amino acid sequence. A nucleic acid construct is defined as a polynucleotide which is isolated from a naturally occurring gene or which has been modified to contain segments of polynucleotides which are combined or juxtaposed in a manner which would not otherwise exist in nature. Optionally, a polynucleotide present in a nucleic acid construct is operably linked to one or more control sequences, which direct the production or expression of said peptide or polypeptide in a cell or in a subject. 
     Polynucleotides described herein may be native or may be codon optimized. Codon optimization adapts the codon usage for an encoded polypeptide towards the codon bias of the organism where the polypeptide is to be produced in. Codon optimization generally helps to increase the production level of the encoded polypeptide in the host cell, such as in the preferred host herein: Cyanobacterium  Synechocystis . Many algorithms are available to the person skilled in the art for codon optimization. A preferred method is the “guided random method based on a Monte Carlo alogorithm available via the internet world wide web genomes.urv.es/OPTIMIZER/(P. Puigbò, E. Guzmán, A. Romeu, and S. Garcia-Vallve. Nucleic Acids Res. 2007 July; 35 (Web Server issue): W126-W131). 
     A nucleotide sequence encoding an enzyme expressed or to be expressed in a cyanobacterial cell according to the invention or a promoter used in a cell according to the invention may be defined by its capability to hybridize with a nucleotide sequence such as SEQ ID NO: 1, 3, 5, 7, 9, 11 or 13, respectively, under moderate, or preferably under stringent hybridization conditions. Stringent hybridization conditions are herein defined as conditions that allow a nucleic acid sequence of at least about 25, preferably about 50 nucleotides, 75 or 100 and most preferably of about 200 or more nucleotides, to hybridize at a temperature of about 65° C. in a solution comprising about 1 M salt, preferably 6×SSC or any other solution having a comparable ionic strength, and washing at 65° C. in a solution comprising about 0.1 M salt, or less, preferably 0.2×SSC or any other solution having a comparable ionic strength. Preferably, the hybridization is performed overnight, i.e. at least for 10 hours and preferably washing is performed for at least one hour with at least two changes of the washing solution. These conditions will usually allow the specific hybridization of sequences having about 90% or more sequence identity. 
     Moderate conditions are herein defined as conditions that allow a nucleic acid sequences of at least 50 nucleotides, preferably of about 200 or more nucleotides, to hybridize at a temperature of about 45° C. in a solution comprising about 1 M salt, preferably 6×SSC or any other solution having a comparable ionic strength, and washing at room temperature in a solution comprising about 1 M salt, preferably 6×SSC or any other solution having a comparable ionic strength. Preferably, the hybridization is performed overnight, i.e. at least for 10 hours, and preferably washing is performed for at least one hour with at least two changes of the washing solution. These conditions will usually allow the specific hybridization of sequences having up to 50% sequence identity. The person skilled in the art will be able to modify these hybridization conditions in order to specifically identify sequences varying in identity between 50% and 90%. 
     As used herein the term “heterologous sequence” or “heterologous nucleic acid” is one that is not naturally found operably linked as neighboring sequence of said first nucleotide sequence. As used herein, the term “heterologous” may mean “recombinant”. “Recombinant” refers to a genetic entity distinct from that generally found in nature. As applied to a nucleotide sequence or nucleic acid molecule, this means that said nucleotide sequence or nucleic acid molecule is the product of various combinations of cloning, restriction and/or ligation steps, and other procedures that result in the production of a construct that is distinct from a sequence or molecule found in nature. 
     “Operably linked” is defined herein as a configuration in which a control sequence is appropriately placed at a position relative to the nucleotide sequence coding for the polypeptide of the invention such that the control sequence directs the production/expression of the peptide or polypeptide of the invention in a cell and/or in a subject. 
     “Operably linked” may also be used for defining a configuration in which a sequence is appropriately placed at a position relative to another sequence coding for a functional domain such that a chimeric polypeptide is encoded in a cell and/or in a subject. 
     Expression will be understood to include any step involved in the production of the peptide or polypeptide including, but not limited to, transcription, post-transcriptional modification, translation, post-translational modification and secretion. 
     As used herein, the term “promoter” refers to a nucleic acid fragment that functions to control the transcription of one or more nucleic acid molecules, located upstream with respect to the direction of transcription of the transcription initiation site of the nucleic acid molecule, and is structurally identified by the presence of a binding site for DNA-dependent RNA polymerase, transcription initiation sites and any other DNA sequences, including, but not limited to transcription factor binding sites, repressor and activator protein binding sites, and any other sequences of nucleotides known to one of skill in the art to act directly or indirectly to regulate the amount of transcription from the promoter. A “constitutive” promoter is a promoter that is active under most environmental and developmental conditions. An “inducible” promoter is a promoter that is active under environmental or developmental regulation. 
     For expression of an enzyme in a cyanobacterial cell according to the inventions, as well as for additional genetic modification of a cyanobacterial cell according to the invention, the cell can be transformed with a nucleic acid or nucleic acid construct described herein by any method known to the person skilled in the art. Such methods are e.g. known from standard handbooks, such as Sambrook and Russel (2001) “Molecular Cloning: A Laboratory Manual (3 rd  edition), Cold Spring Harbor Laboratory,  Cold Spring Harbor Laboratory Press , or F. Ausubel et al, eds., “Current protocols in molecular biology”, Green Publishing and Wiley Interscience, New York (1987). Methods for transformation and genetic modification of cyanobacterial cells are known from e.g. U.S. Pat. No. 6,699,696 or 4,778,759. 
     When a nucleic acid construct is used for expression of an enzyme in a cyanobacterial cell according to the invention, a selectable marker may be present in the nucleic acid construct comprising a polynucleotide encoding the enzyme. The term “marker” refers herein to a gene encoding a trait or a phenotype which permits the selection of, or the screening for, a cyanobacterial cell containing the marker. A marker gene may be an antibiotic resistance gene whereby the appropriate antibiotic can be used to select for transformed cells from among cells that are not transformed. Preferably however, a non-antibiotic resistance marker is used, such as an auxotrophic marker (URA3, TRP1, LEU2). A preferred cyanobacterial cell according to the invention, e.g. transformed with a nucleic acid construct, is marker gene free. Methods for constructing recombinant marker gene free microbial host cells are described in (Cheah et al., 2013) and are based on the use of bidirectional markers. Alternatively, a screenable marker such as Green Fluorescent Protein, lacZ, luciferase, chloramphenicol acetyltransferase, beta-glucuronidase may be incorporated into a nucleic acid construct according to the invention allowing to screen for transformed cells. 
     Optional further elements that may be present in a nucleic acid construct according to the invention include, but are not limited to, one or more leader sequences, enhancers, integration factors, and/or reporter genes, intron sequences, centromers, telomers and/or matrix attachment (MAR) sequences. A nucleic acid construct according to the invention can be provided in a manner known per se, which generally involves techniques such as restricting and linking nucleic acids/nucleic acid sequences, for which reference is made to the standard handbooks, such as Sambrook and Russel (2001) “Molecular Cloning: A Laboratory Manual (3 rd  edition), Cold Spring Harbor Laboratory,  Cold Spring Harbor Laboratory Press.    
     Methods for inactivation and gene disruption in a cyanobacterial cell are well known in the art (see e.g. Shestakov S V et al, (2002), Photosynthesis Research, 73: 279-284 and Nakamura Y et al, (1999), Nucleic Acids Res. 27:66-68). 
     In this document and in its claims, the verb “to comprise” and its conjugations is used in its non-limiting sense to mean that items following the word are included, but items not specifically mentioned are not excluded. In addition, reference to an element by the indefinite article “a” or “an” does not exclude the possibility that more than one of the element is present, unless the context clearly requires that there be one and only one of the elements. The indefinite article “a” or “an” thus usually means “at least one”. 
     The word “about” or “approximately” when used in association with a numerical value (e.g. about 10) preferably means that the value may be the given value (of 10) more or less 0.1% of the value. 
     The sequence information as provided herein should not be so narrowly construed as to require inclusion of erroneously identified bases. The skilled person is capable of identifying such erroneously identified bases and knows how to correct for such errors. In case of sequence errors, the sequence of the enzymes obtainable by expression of the genes as represented by SEQ ID NO&#39;s 1, 3, 5, 7, 9, 11, 13 and 15 containing the enzyme encoding polynucleotide sequences should prevail. 
     All patent and literature references cited in the present specification are hereby incorporated by reference in their entirety. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Sequences 
               
            
           
           
               
               
               
            
               
                 SEQ 
                   
                   
               
               
                 ID 
                 Gene/ 
                   
               
               
                 NO 
                 Polypeptide 
                 Sequence 
               
               
                   
               
               
                  1 
                 phosphatase 
                 ATGGCTATTAAACTCATTGCTATCGATATGGATGGCAC 
               
               
                   
                 from  E. coli   
                 CCTTCTGCTGCCCGATCACACCATTTCACCCGCCGTTA 
               
               
                   
                   
                 AAAATGCGATTGCCGCAGCTCGCGCCCGTGGCGTGAA 
               
               
                   
                   
                 TGTCGTGCTAACGACGGGTCGCCCGTATGCAGGTGTG 
               
               
                   
                   
                 CACAACTACCTGAAAGAGCTGCATATGGAACAGCCGG 
               
               
                   
                   
                 GCGACTACTGCATTACTTATAACGGCGCGCTGGTACA 
               
               
                   
                   
                 GAAGGCCGCTGATGGTAGCACCGTGGCGCAAACTGCT 
               
               
                   
                   
                 CTCAGCTATGACGACTATCGTTTCCTGGAAAAACTCTC 
               
               
                   
                   
                 TCGCGAAGTCGGTTCTCATTTCCACGCCCTGGACCGCA 
               
               
                   
                   
                 CCACGCTGTACACCGCCAACCGTGATATCAGCTACTA 
               
               
                   
                   
                 CACGGTGCATGAATCCTTCGTTGCCACCATTCCGCTGG 
               
               
                   
                   
                 TGTTCTGCGAAGCGGAGAAAATGGACCCCAATACCCA 
               
               
                   
                   
                 GTTCCTGAAAGTGATGATGATTGATGAACCCGCCATC 
               
               
                   
                   
                 CTCGACCAGGCTATCGCGCGTATTCCGCAGGAAGTGA 
               
               
                   
                   
                 AAGAGAAATATACCGTGCTGAAAAGTGCGCCGTACTT 
               
               
                   
                   
                 CCTCGAAATCCTCGATAAACGCGTTAACAAAGGTACG 
               
               
                   
                   
                 GGGGTGAAATCACTGGCCGACGTGTTAGGTATTAAAC 
               
               
                   
                   
                 CGGAAGAAATCATGGCGATTGGCGATCAGGAAAACG 
               
               
                   
                   
                 ATATCGCAATGATTGAATATGCAGGCGTCGGTGTGGC 
               
               
                   
                   
                 GATGGATAACGCTATTCCTTCAGTGAAAGAAGTGGCG 
               
               
                   
                   
                 AACTTTGTCACCAAATCTAACCTTGAAGATGGCGTGG 
               
               
                   
                   
                 CGTTTGCTATTGAGAAGTATGTGCTGAATTAA 
               
               
                   
               
               
                  2 
                 phosphatase 
                 MAIKLIAIDMDGTLLLPDHTISPAVKNAIAAARARGVNV 
               
               
                   
                 from  E. coli   
                 VLTTGRPYAGVHNYLKELHMEQPGDYCITYNGALVQKA 
               
               
                   
                   
                 ADGSTVAQTALSYDDYRFLEKLSREVGSHFHALDRTTLY 
               
               
                   
                   
                 TANRDISYYTVHESFVATIPLVFCEAEKMDPNTQFLKVM 
               
               
                   
                   
                 MIDEPAILDQAIARIPQEVKEKYTVLKSAPYFLEILDKRV 
               
               
                   
                   
                 NKGTGVKSLADVLGIKPEEIMAIGDQENDIAMIEYAGVG 
               
               
                   
                   
                 VAMDNAIPSVKEVANFVTKSNLEDGVAFAIEKYVLN 
               
               
                   
               
               
                  3 
                 phosphatase 
                 GTGTCAATCAAGTTAGTAGTATTGGACATTGATGGCA 
               
               
                   
                 from 
                 CCATCGCCGGAGTATCCAATCAAATTAACCCGTCAGT 
               
               
                   
                 
                   Synechocystis 
                 
                 GGTGAAAACCATTCACCAGGTACAGAGCCGGGGTATC 
               
               
                   
                 sp. PCC 
                 CAAGTGGCGTTGGCCACTGGCCGTATGTTTAGTTCTGC 
               
               
                   
                 6803 
                 TCTACGGTTCCATCAGACCATTCAATCAACCCTGCCTT 
               
               
                   
                   
                 TGATTAGTTACAACGGTGCCCTAACCAAGCATCCCCA 
               
               
                   
                   
                 CACTGGTGCTGTTTTAAGGGAAAAACCCCTGCCCCCG 
               
               
                   
                   
                 GCGATCGCCTTGGAAATTTTGGACCATTTTGAGCGACC 
               
               
                   
                   
                 GGAACTGGAACCCCATCTTGATATCCACTGCTATTACA 
               
               
                   
                   
                 ACGACCAGCTCCATGTGCGGCATATCACCCCAGAAAC 
               
               
                   
                   
                 CCATGTTTATATGGAAAGGTCCGGTGCCATGGCCCAA 
               
               
                   
                   
                 GCTAGCGGCGATCTACGCTCAATTATTGAATTGGGTA 
               
               
                   
                   
                 GCACCACCAAAATGTTAGCCATCAGTCGCAATGCTCC 
               
               
                   
                   
                 CCTCATGGCCCAGTTGATGGCGGAAATGGGTCAAAAA 
               
               
                   
                   
                 CTCCAGGGCCAAGCCGTGCATCTGACCCAATCCACCG 
               
               
                   
                   
                 AGATTTACTTTGAAGTCACCCACGCCGAAGCCACCAA 
               
               
                   
                   
                 AGGCCTGGCCCTGCAACATTTAGCTGAAGACGTGTTA 
               
               
                   
                   
                 GGGCTTGATCCCCAAGAAGTTTTGGCGATCGGAGACA 
               
               
                   
                   
                 ATTTTAACGACGTGGAAATGCTGAAATATGCCGGAGT 
               
               
                   
                   
                 GGGGGTAGCCATGGGTAATGCTCCCCCGGAAGTGCAA 
               
               
                   
                   
                 AAGGTGGCAGACTGGGTAACGGCGGACGTGGAAGCC 
               
               
                   
                   
                 GATGGAGTGTCCCAAGCCTTGGCTAGGTTCTGCCTAG 
               
               
                   
                   
                 ATTCAACCCTAGCACTCTGTTAA 
               
               
                   
               
               
                  4 
                 phosphatase 
                 MSIKLVVLDIDGTIAGVSNQINPSVVKTIHQVQSRGIQVA 
               
               
                   
                 from 
                 LATGRMFSSALRFHQTIQSTLPLISYNGALTKHPHTGAVL 
               
               
                   
                 
                   Synechocystis 
                 
                 REKPLPPAIALEILDHFERPELEPHLDIHCYYNDQLHVRHI 
               
               
                   
                 sp. PCC 
                 TPETHVYMERSGAMAQASGDLRSIIELGSTTKMLAISRN 
               
               
                   
                 6803 
                 APLMAQLMAEMGQKLQGQAVHLTQSTEIYFEVTHAEAT 
               
               
                   
                   
                 KGLALQHLAEDVLGLDPQEVLAIGDNFNDVEMLKYAGV 
               
               
                   
                   
                 GVAMGNAPPEVQKVADWVTADVEADGVSQALARFCLD 
               
               
                   
                   
                 STLALC 
               
               
                   
               
               
                  5 
                 phosphatase 
                 ATGGAAGCGGTGATTTTCGACATGGATGGAGTGCTCA 
               
               
                   
                 from 
                 TGGACACAGAGCCTCTCTACTTCGAAGCTTACAGAAG 
               
               
                   
                 
                   Thermotoga 
                 
                 AGTCGCGGAAAGCTATGGAAAACCTTACACGGAGGAT 
               
               
                   
                 
                   maritima 
                 
                 CTCCACAGGAGAATAATGGGAGTTCCTGAAAGAGAAG 
               
               
                   
                 MSB8 
                 GTCTTCCCATCCTCATGGAAGCTCTGGAGATAAAAGA 
               
               
                   
                   
                 TTCTCTGGAGAACTTCAAAAAGAGGGTCCACGAAGAA 
               
               
                   
                   
                 AAAAAGCGCGTTTTCTCTGAGCTTCTCAAGGAAAATC 
               
               
                   
                   
                 CGGGTGTAAGAGAGGCGCTCGAGTTCGTAAAGAGCAA 
               
               
                   
                   
                 AAGAATAAAACTCGCGCTCGCAACCTCCACACCACAG 
               
               
                   
                   
                 CGAGAAGCGCTGGAGAGATTGAGAAGACTCGATCTCG 
               
               
                   
                   
                 AAAAGTACTTCGACGTCATGGTGTTCGGTGATCAGGT 
               
               
                   
                   
                 GAAGAACGGAAAGCCTGATCCAGAGATATACCTTCTC 
               
               
                   
                   
                 GTTCTGGAAAGGTTGAATGTGGTCCCAGAGAAGGTTG 
               
               
                   
                   
                 TGGTCTTCGAAGACTCAAAGAGCGGTGTTGAAGCCGC 
               
               
                   
                   
                 AAAAAGCGCCGGCATAGAAAGAATCTATGGAGTCGTT 
               
               
                   
                   
                 CACTCTTTGAACGACGGTAAAGCGCTTCTTGAAGCGG 
               
               
                   
                   
                 GTGCGGTTGCTCTGGTGAAACCCGAGGAAATCCTGAA 
               
               
                   
                   
                 CGTTCTCAAAGAGGTTCTTTAA 
               
               
                   
               
               
                  6 
                 phosphatase 
                 MEAVIFDMDGVLMDTEPLYFEAYRRVAESYGKPYTEDL 
               
               
                   
                 from 
                 HRRIMGVPEREGLPILMEALEIKDSLENFKKRVHEEKKR 
               
               
                   
                 
                   Thermotoga 
                 
                 VFSELLKENPGVREALEFVKSKRIKLALATSTPQREALER 
               
               
                   
                 
                   maritima 
                 
                 LRRLDLEKYFDVMVFGDQVKNGKPDPEIYLLVLERLNV 
               
               
                   
                 MSB8 
                 VPEKVVVFEDSKSGVEAAKSAGIERIYGVVHSLNDGKAL 
               
               
                   
                   
                 LEAGAVALVKPEEILNVLKEVL 
               
               
                   
               
               
                  7 
                 aldose 
                 ATGCCTGCTACTTTACATGATTCTACGAAAATCCTTTC 
               
               
                   
                 reductase 
                 TCTAAATACTGGAGCCCAAATCCCTCAAATAGGTTTA 
               
               
                   
                 from 
                 GGTACGTGGCAGTCGAAAGAGAACGATGCTTATAAGG 
               
               
                   
                 
                   Saccharomyces 
                 
                 CTGTTTTAACCGCTTTGAAAGATGGCTACCGACACATT 
               
               
                   
                 
                   cerevisiae 
                 
                 GATACTGCTGCTATTTACCGTAATGAAGACCAAGTCG 
               
               
                   
                 S288c 
                 GTCAAGCCATCAAGGATTCAGGTGTTCCTCGGGAAGA 
               
               
                   
                   
                 AATCTTTGTTACTACAAAGTTATGGTGTACACAACACC 
               
               
                   
                   
                 ACGAACCTGAAGTAGCGCTGGATCAATCACTAAAGAG 
               
               
                   
                   
                 GTTAGGATTGGACTACGTAGACTTATATTTGATGCATT 
               
               
                   
                   
                 GGCCTGCCAGATTAGATCCAGCCTACATCAAAAATGA 
               
               
                   
                   
                 AGACATCTTGAGTGTGCCAACAAAGAAGGATGGTTCT 
               
               
                   
                   
                 CGTGCAGTGGATATCACCAATTGGAATTTCATCAAAA 
               
               
                   
                   
                 CCTGGGAATTAATGCAGGAACTACCAAAGACTGGTAA 
               
               
                   
                   
                 AACTAAGGCCGTTGGAGTCTCCAACTTTTCTATAAATA 
               
               
                   
                   
                 ACCTGAAAGATCTATTAGCATCTCAAGGTAATAAGCT 
               
               
                   
                   
                 TACGCCAGCTGCTAACCAAGTCGAAATACATCCATTA 
               
               
                   
                   
                 CTACCTCAAGACGAATTGATTAATTTTTGTAAAAGTAA 
               
               
                   
                   
                 AGGCATTGTGGTTGAAGCTTATTCTCCGTTAGGTAGTA 
               
               
                   
                   
                 CCGATGCTCCACTATTGAAGGAACCGGTTATCCTTGAA 
               
               
                   
                   
                 ATTGCGAAGAAAAATAACGTTCAACCCGGACACGTTG 
               
               
                   
                   
                 TTATTAGCTGGCACGTCCAAAGAGGTTATGTTGTCTTG 
               
               
                   
                   
                 CCAAAATCTGTGAATCCCGATCGAATCAAAACGAACA 
               
               
                   
                   
                 GGAAAATATTTACTTTGTCTACTGAGGACTTTGAAGCT 
               
               
                   
                   
                 ATCAATAACATATCGAAGGAAAAGGGCGAAAAAAGG 
               
               
                   
                   
                 GTTGTACATCCAAATTGGTCTCCTTTCGAAGTATTCAA 
               
               
                   
                   
                 GTAA 
               
               
                   
               
               
                  8 
                 aldose 
                 MPATLHDSTKILSLNTGAQIPQIGLGTWQSKENDAYKAV 
               
               
                   
                 reductase 
                 LTALKDGYRHIDTAAIYRNEDQVGQAIKDSGVPREEIFVT 
               
               
                   
                 from 
                 TKLWCTQHHEPEVALDQSLKRLGLDYVDLYLMHWPAR 
               
               
                   
                 
                   Saccharomyces 
                 
                 LDPAYIKNEDILSVPTKKDGSRAVDITNWNFIKTWELMQ 
               
               
                   
                 
                   cerevisiae 
                 
                 ELPKTGKTKAVGVSNFSINNLKDLLASQGNKLTPAANQV 
               
               
                   
                 S288c 
                 EIHPLLPQDELINFCKSKGIVVEAYSPLGSTDAPLLKEPVI 
               
               
                   
                   
                 LEIAKKNNVQPGHVVISWHVQRGYVVLPKSVNPDRIKT 
               
               
                   
                   
                 NRKIFTLSTEDFEAINNISKEKGEKRVVHPNWSPFEVFK 
               
               
                   
               
               
                  9 
                 aldose 
                 ATGTCTTCGACCTACACCCTTACTCGCCTGTCTGCGCC 
               
               
                   
                 reductase 
                 TTCAATGGTGCTCAACAGTGGCTCCCAGATCCCTGCCG 
               
               
                   
                 from 
                 TTGGCTATGGACTCTGGAAACAGCAGGGCAGCGAGGC 
               
               
                   
                 
                   Candida 
                 
                 CAAGGACTCCGTGCGCTGCGCCATCGAGTCTGGCTAC 
               
               
                   
                 
                   magnoliae 
                 
                 CGTCACCTTGACTGTGCAACCGCTTACCAGAACCACA 
               
               
                   
                   
                 AAGAGGTCGGCCAAGCTATTCGTGAGGCCGGCGTGCC 
               
               
                   
                   
                 TCGCGACGAACTGTGGATCACGTCCAAGGTTTGGGGC 
               
               
                   
                   
                 ACGCACTTCGACAACCCTGAAGAGGGACTTGACGACA 
               
               
                   
                   
                 TTCTCGAGGAGCTCGGTGTCGAATACCTGGACCTGCTA 
               
               
                   
                   
                 CTCCTCCACCTGCCAGTCGCGTTCAAGCGGAACCCGG 
               
               
                   
                   
                 AGGACCCGAAGCAGCTGCGCGGCCTTCCTGTGGACCA 
               
               
                   
                   
                 CGACATGAAGTACGCCGACGTGTGGGCGCGCATGGAG 
               
               
                   
                   
                 AAGCTGCCCAAGTCGAAGGTGCGGAACATTGGTGTGT 
               
               
                   
                   
                 CGAACCTCACGGTGAGGGCGCTGGATGAGCTTTTGCA 
               
               
                   
                   
                 GACGGCGAAGGTGACTCCGGCCGTGAACCAGGTCGAG 
               
               
                   
                   
                 ATGCACCCGAACCTGCCTCAGAAGAAGCTGCTCGACT 
               
               
                   
                   
                 ACTGCAAGTCGAAGGGCATTGTTGTGCAGGCATACAG 
               
               
                   
                   
                 CCCTCTGGCTCAGGGCCAGCACGAGAACCCAGTTGTC 
               
               
                   
                   
                 ACAGACATCGCCGACGACCTCGGCGTCTCGCCGGCGC 
               
               
                   
                   
                 AAGTCGTGCTTTCGTGGGGCGCCTTGCGCGGCACGAA 
               
               
                   
                   
                 CATTCTTCCCAAATCCTCGACGCCCTCGCGTATTCGCG 
               
               
                   
                   
                 AGAACCTCGAACTCATCCAGCTTAGCGACGACCACAT 
               
               
                   
                   
                 GAGGCGTATTGACGCGCTAGCAAGACGGTGA 
               
               
                   
               
               
                 10 
                 aldose 
                 MSSTYTLTRLSAPSMVLNSGSQIPAVGYGLWKQQGSEA 
               
               
                   
                 reductase 
                 KDSVRCAIESGYRHLDCATAYQNHKEVGQAIREAGVPR 
               
               
                   
                 from 
                 DELWITSKVWGTHFDNPEEGLDDILEELGVEYLDLLLLH 
               
               
                   
                 
                   Candida 
                 
                 LPVAFKRNPEDPKQLRGLPVDHDMKYADVWARMEKLP 
               
               
                   
                 
                   magnoliae 
                 
                 KSKVRNIGVSNLTVRALDELLQTAKVTPAVNQVEMHPN 
               
               
                   
                   
                 LPQKKLLDYCKSKGIVVQAYSPLAQGQHENPVVTDIAD 
               
               
                   
                   
                 DLGVSPAQVVLSWGALRGTNILPKSSTPSRIRENLELIQLS 
               
               
                   
                   
                 DDHMRRIDALARR 
               
               
                   
               
               
                 11 
                 aldose 
                 ATGTCTTCCGGAAGGACCGTCACCCTCAACACCGGCT 
               
               
                   
                 reductase 
                 ACAAGATCCCCCAGATCGGCTACGGCACCTGGCAGGC 
               
               
                   
                 from 
                 CGCTCCCGGCGAGGTCGGCGCTGGTGTCTTTGAGGCC 
               
               
                   
                 
                   Hypocrea 
                 
                 CTCAAGGTTGGCTACCGCCACCTCGACCTGGCCAAGG 
               
               
                   
                 
                   jecorina 
                 
                 TCTACGGCAACCAGAAGGAGGTTGGTGAGGGCATCAA 
               
               
                   
                   
                 GAAGGCTCTTGCTGAGGTCCCCGGCCTGAAGCGCGAG 
               
               
                   
                   
                 GATATTTTCATCACCTCCAAGCTGTGGAACAACTCCCA 
               
               
                   
                   
                 CAAGCCCGAGGACGTCGAGCCCGCTCTCGACGACACC 
               
               
                   
                   
                 CTGGCCGAGCTTGGCCTCGACTACCTTGACCTCTACCT 
               
               
                   
                   
                 CATCCACTGGCCCGTTGCCTTTGCTCCCGGCGCCGACC 
               
               
                   
                   
                 TCTTCCCCAAGTCCGAGGACGGCTCCGAGGTGCAGCT 
               
               
                   
                   
                 CAACCAGAATGTGTCCATTGTCCAGACCTGGAAGGCC 
               
               
                   
                   
                 ATGACCGAGCTGCCCAAGTCCAAGGTCCGCTCCGTCG 
               
               
                   
                   
                 GTGTCTCCAACTTTACCATTGAGCACCTCGACGCCGTC 
               
               
                   
                   
                 ATCGAGGCCACCGGCGTCGTCCCCGCCGTCAACCAGA 
               
               
                   
                   
                 TCGAGCGCCACCCCCGCCTCCCCAACCAGCCCCTGATC 
               
               
                   
                   
                 GACTACTGCGCCAAGAAGGGCATCATCATCACCGCCT 
               
               
                   
                   
                 ACTCCGCCTTTGGCAACAACACAAAGGGCCTGCCCCT 
               
               
                   
                   
                 GCTCGTCAGCTCCGACGAGGTCAAGGCCGTCGCCGAC 
               
               
                   
                   
                 AACCTGTCCAAGAAGCAGGGCAAGACCGTCACTCCCG 
               
               
                   
                   
                 CCCAGGTCATCCTCGCCTGGTCCCAGATTGGTGGCCAC 
               
               
                   
                   
                 ACCGTCATTCCCAAGTCCGTCACCAAGGCGCGCATTG 
               
               
                   
                   
                 CGGAGAACTTCCAGGAGGTTGAGCTGGATGACGAGGC 
               
               
                   
                   
                 CATTGCTGCGCTGAACAAGTTGGGCGAGAAGCCTCAG 
               
               
                   
                   
                 CGGTTCAACATTCCTTACACCTACAAGCCTAGGTGGA 
               
               
                   
                   
                 ACATTAACCTGTTCAACACCGAGGAGGAGAAGGCCGC 
               
               
                   
                   
                 TGCCCACACTGCTGTCATCAAGCTGTAA 
               
               
                   
               
               
                 12 
                 aldose 
                 MSSGRTVTLNTGYKIPQIGYGTWQAAPGEVGAGVFEAL 
               
               
                   
                 reductase 
                 KVGYRHLDLAKVYGNQKEVGEGIKKALAEVPGLKREDI 
               
               
                   
                 from 
                 FITSKLWNNSHKPEDVEPALDDTLAELGLDYLDLYLIHW 
               
               
                   
                 
                   Hypocrea 
                 
                 PVAFAPGADLFPKSEDGSEVQLNQNVSIVQTWKAMTELP 
               
               
                   
                 
                   jecorina 
                 
                 KSKVRSVGVSNFTIEHLDAVIEATGVVPAVNQIERHPRLP 
               
               
                   
                   
                 NQPLIDYCAKKGIIITAYSAFGNNTKGLPLLVSSDEVKAV 
               
               
                   
                   
                 ADNLSKKQGKTVTPAQVILAWSQIGGHTVIPKSVTKARI 
               
               
                   
                   
                 AENFQEVELDDEAIAALNKLGEKPQRFNIPYTYKPRWNI 
               
               
                   
                   
                 NLFNTEEEKAAAHTAVIKL 
               
               
                   
               
               
                 13 
                 aldose 
                 ATGTCTCTCGGAAAGAAGGTTACTCTCAACTCCGGTGC 
               
               
                   
                 reductase 
                 TCAGATCCCCCAGCTGGGATTTGGTACCTGGCAGTCTG 
               
               
                   
                 from 
                 CCCCCGGTCAGGTCGGTGATGCCGTCTACGAGGCCTT 
               
               
                   
                 
                   Aspergillus 
                 
                 GAAGGCCGGCTACCGCCACTTGGATCTGGCTACTATCT 
               
               
                   
                 
                   niger 
                 
                 ACCAGAACCAGCGCGAGGTTGCTGAGGGCATCAAGAG 
               
               
                   
                   
                 AGCCTACAAGGACGTCCCTGGCCTCAAGCGTGAGGAC 
               
               
                   
                   
                 ATCTTCATCACCTCCAAGCTGTGGAACTCCCAGCACGA 
               
               
                   
                   
                 CCCCGCCGTTGTTGAGAAGGCTCTGGATGAGTGCCTTG 
               
               
                   
                   
                 CTGAGCTCGAGCTCGACTACCTCGATCTCTACCTCGTC 
               
               
                   
                   
                 CACTGGCCCGTTTCCTTCACCACCGGCTCCGAGTTGTT 
               
               
                   
                   
                 CCCCCTCGTCAAGGACAGCTCCGTTGAGGGCGGTGAT 
               
               
                   
                   
                 GTCGTGATCAACGACGACATCTCCATCGTCGACACCT 
               
               
                   
                   
                 GGAAGGCCATGACCCAGCTCCCCAAGAGCAAGGCCCG 
               
               
                   
                   
                 CACCGTCGGTGTCTCCAACCACATGATCCCTCACCTCG 
               
               
                   
                   
                 AGGCCATCATCAACGCCACCGGCGTTGTCCCCGCCGTT 
               
               
                   
                   
                 AACCAGATCGAGCGCCACCCCGTTCTCCAGAGCAACG 
               
               
                   
                   
                 AGCTCATCGAATACTGCCAGAAGAAGGGCATCCACGT 
               
               
                   
                   
                 GACCGCCTACTCTGCCTTCGGCAACAACGGCTTCGGC 
               
               
                   
                   
                 GTCCCCCTCCTCGTCACCCGCCCCGAAGTCAAGGAAG 
               
               
                   
                   
                 TCGCTGAGTCCGCCTCCAAGCGCCTCGGCACCACCGTC 
               
               
                   
                   
                 ACCCCTGCCCAGGTCATCCTGGCCTGGTCCCAGGTCGG 
               
               
                   
                   
                 CGGCCACAGTGTCATCCCCAAGTCGGTGACGCCGTCC 
               
               
                   
                   
                 CGCATCCATGAGAACTTCAAGGAGGTGGAGCTCACTC 
               
               
                   
                   
                 CCGAGGAAATCGCCAAGGTGTCCGAGCTGGGCAAGGA 
               
               
                   
                   
                 CCGCAGACGCTACAACACTCCTTACGTTGCTAACACG 
               
               
                   
                   
                 CCTCGCTGGGATATCGACATCTTCGGTGAGGAGGAGG 
               
               
                   
                   
                 AGAAGCCTGCTGGTCATAAGGTGATTGTTTAA 
               
               
                   
               
               
                 14 
                 aldose 
                 MSLGKKVTLNSGAQIPQLGFGTWQSAPGQVGDAVYEAL 
               
               
                   
                 reductase 
                 KAGYRHLDLATIYQNQREVAEGIKRAYKDVPGLKREDIF 
               
               
                   
                 from 
                 ITSKLWNSQHDPAVVEKALDECLAELELDYLDLYLVHW 
               
               
                   
                 
                   Aspergillus 
                 
                 PVSFTTGSELFPLVKDSSVEGGDVVINDDISIVDTWKAMT 
               
               
                   
                 
                   niger 
                 
                 QLPKSKARTVGVSNHMIPHLEAIINATGVVPAVNQIERHP 
               
               
                   
                   
                 VLQSNELIEYCQKKGIHVTAYSAFGNNGFGVPLLVTRPE 
               
               
                   
                   
                 VKEVAESASKRLGTTVTPAQVILAWSQVGGHSVIPKSVT 
               
               
                   
                   
                 PSRIHENFKEVELTPEEIAKVSELGKDRRRYNTPYVANTP 
               
               
                   
                   
                 RWDIDIFGEEEEKPAGHKVIV 
               
               
                   
               
               
                 15 
                 aldose 
                 ATGTCTCTCGGAAAGAAAGTCACTCTCAACACCGGCC 
               
               
                   
                 reductase 
                 ACCAGATCCCCCAGCTGGGCTTTGGTACCTGGCAGTCT 
               
               
                   
                 from 
                 GCCCCTGGCCAGGTCGGCGAGGCTGTCTATGAGGCCC 
               
               
                   
                 
                   Penicillium 
                 
                 TGAAGGCTGGTTACCGCCACCTGGATTTGGCAACTATC 
               
               
                   
                 
                   chrysogenum 
                 
                 TACCAGAACCAGCGCGAGGTCGCTGAGGGCATCAAGC 
               
               
                   
                   
                 GTGCTTATAAGGATGTCCCCGGTCTGAAGCGCGAGGA 
               
               
                   
                   
                 TCTCTTTATTACCTCCAAGTTGTGGAACAGCCAGCACC 
               
               
                   
                   
                 GCCCCGAGGTTGTCGAGGCCTCCTTGGATGCTTGCCTT 
               
               
                   
                   
                 GCTGAGCTCGAGTTGGATTATCTTGACCTTTACCTTGT 
               
               
                   
                   
                 TCACTGGCCCGTTGCCTTCCAGAAGGGCGATTCATACT 
               
               
                   
                   
                 TCCCGCTTGTTGCCAACAGCCCCGTCGAGGGTGGTGA 
               
               
                   
                   
                 CGTGATCATTGACGATGGCGTCTCCATCGTGGACACCT 
               
               
                   
                   
                 GGAAGGCCATGACCCAGCTCCCCAAGAACAAGGCTCG 
               
               
                   
                   
                 CTCCGTCGGTGTCTCCAACCACAAGATTGAGCATCTCG 
               
               
                   
                   
                 AGGCTCTCATTAAAGGCACCGGTGTCGTCCCTGCCGCC 
               
               
                   
                   
                 AACCAGATTGAGCGCCACCCCGTGCTCCAGAGCAACG 
               
               
                   
                   
                 ACCTGATTGAGTACTGCCAACAGAAGGGAATTCACGT 
               
               
                   
                   
                 TACTGCTTACTCCGCATTTGGTAACAACATGCTCGGCA 
               
               
                   
                   
                 TTCCTCTGCTCATCACCCGCCCCGAGGTCAAGGAAGTT 
               
               
                   
                   
                 GCCGAGTCTGTTGCCAAGCGCACTGGCCAGGAAGTCA 
               
               
                   
                   
                 GCCCCGCACACGTCATTCTCGCCTGGTCTCAGGTCGGT 
               
               
                   
                   
                 GGACACAGTGTCATCCCCAAGTCGGTCACGCCTTCGC 
               
               
                   
                   
                 GCATTCGCGACAACTTCAAGGAGATCGAACTCACTCC 
               
               
                   
                   
                 CGAGGAGGTCGAGAAGGTCAGCGCTCTGGGCCAGAAC 
               
               
                   
                   
                 CGGCAGCGATACAACACACCTTACACTGCCAACAAGC 
               
               
                   
                   
                 CTCGTTGGGACATTGATATCTTCGGCGAGCCCGAGGA 
               
               
                   
                   
                 GAAGCCCGCTGGTCACAAGGTCATCCTGAGTGTTTAA 
               
               
                   
               
               
                 16 
                 aldose 
                 MSLGKKVTLNTGHQIPQLGFGTWQSAPGQVGEAVYEAL 
               
               
                   
                 reductase 
                 KAGYRHLDLATIYQNQREVAEGIKRAYKDVPGLKREDL 
               
               
                   
                 from 
                 FITSKLWNSQHRPEVVEASLDACLAELELDYLDLYLVH 
               
               
                   
                 
                   Penicillium 
                 
                 WPVAFQKGDSYFPLVANSPVEGGDVIIDDGVSIVDTWKA 
               
               
                   
                 
                   chrysogenum 
                 
                 MTQLPKNKARSVGVSNHKIEHLEALIKGTGVVPAANQIE 
               
               
                   
                   
                 RHPVLQSNDLIEYCQQKGIHVTAYSAFGNNMLGIPLLITR 
               
               
                   
                   
                 PEVKEVAESVAKRTGQEVSPAHVILAWSQVGGHSVIPKS 
               
               
                   
                   
                 VTPSRIRDNFKEIELTPEEVEKVSALGQNRQRYNTPYTAN 
               
               
                   
                   
                 KPRWDIDIFGEPEEKPAGHKVILSV 
               
               
                   
               
            
           
         
       
     
    
    
     
       FIGURE LEGENDS 
         FIG. 1 . 
       Catabolic pathways for formation of erythritol in a cyanobacterium including the enzymes involved. 
         FIG. 2 . 
       Colony PCR to confirm the correct insertion of the plasmid into  Synechocystis . Lane 1: DNA ladder; lane 2: negative control; lane 3:  Synechocystis  strain SAW030 with plasmid conferring erythritol production. 
         FIG. 3 . 
       Growth and level of erythritol production in the supernatant of SAW030 and wild type  Synechocystis.    
         FIG. 4 . 
       Toxicity assay for erythritol. 
         FIG. 5 . 
       HPLC data showing the level of erythritol production measured in the supernatant of different  Synechocystis  erythritol producing strains. 
     
    
    
     EXAMPLES 
     The present invention is further described by the following examples which should not be construed as limiting the scope of the invention. 
     Unless stated otherwise, the practice of the invention will employ standard conventional methods of molecular biology, virology, microbiology or biochemistry. Such techniques are described in Sambrook et al. (1989) Molecular Cloning, A Laboratory Manual (2 nd  edition), Cold Spring Harbor Laboratory, Cold Spring Harbor Laboratory Press; in Sambrook and Russell (2001)  Molecular Cloning: A Laboratory Manual , Third Edition, Cold Spring Harbor Laboratory Press, NY; in Volumes 1 and 2 of Ausubel et al. (1994)  Current Protocols in Molecular Biology, Current Protocols , USA; and in Volumes I and II of Brown (1998)  Molecular Biology LabFax , Second Edition, Academic Press (UK);  Oligonucleotide Synthesis  (N. Gait editor);  Nucleic Acid Hybridization  (Hames and Higgins, eds.). 
     Example 1. Cloning Strategy 
     We have introduced a specific two enzyme catabolic pathway into a cyanobacterial cell to produce erythritol. 
     Two catabolic pathways for the formation of erythritol from erythrose-4-phosphate have been reported in literature ( FIG. 1 ). The pathway of erythritol formation has been best studied in yeast, in which erythrose-4-phosphate (E4P) is first dephosphorylated to d-erythrose, and then reduced to erythritol. In bacteria, erythrose-4-phosphate is described to be reduced to erythritol-4-phosphate first, but the enzymes involved are unknown (Veiga-da-Cunha M, Santos H, Van Schaftingen E: Pathway and regulation of erythritol formation in  Leuconostoc  oenos. J Bacteriol 1993, 175:3941-3948). 
     Phosphatase: 
     of the group of Haloacid Dehalogenase-like phosphatases, with affinity for erythrose-4-phosphate or erythritol-4-phosphate. Such phosphatases usually have a quite broad substrate specificity, but for example YidA (derived from  Escherichia coli ) has a quite attractive Km for erythrose-4-phosphate. 
     Reductase: 
     Closely related to the family of aldose reductases and can usually catalyze the reduction of several aldehydes. Should be able to reduce either erythrose into erythritol or erythrose-4-phosphate into erythritol-4-phosphate. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Characteristics of phosphatases 
               
            
           
           
               
               
               
            
               
                   
                 D-erythrose-4-P 
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                 Km 
                 Kcat 
                 Vmax (umol/ 
                 SEQ 
                   
               
               
                 gene 
                 donor organism 
                 (mM) 
                 (s−1) 
                 min/mg protein) 
                 ID NO: 
                 ref 
               
               
                   
               
               
                 TM1254 
                 
                   Thermotoga maritima 
                 
                 0.152 
                 — 
                 2.63 
                 5, 6 
                 Kunetsova et 
               
               
                   
                 MSB8 
                   
                   
                   
                   
                 al, 2005 
               
               
                 YidA 
                 
                   Escherichia coli 
                 
                 0.019 
                 19 
                 — 
                 1, 2 
                 Kunetsova et 
               
               
                   
                   
                   
                   
                   
                   
                 al, 2006 
               
               
                 sll1524 
                 
                   Synechocystis 
                 
                 — 
                 — 
                 — 
                 3, 4 
               
               
                   
                 PCC6803 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Characteristics of reductases 
               
            
           
           
               
               
               
               
            
               
                   
                 D-erythrose 
                 NADPH 
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                   
                 SEQ ID 
                 Km 
                 Kcat 
                   
                 Kcat 
                   
               
               
                 gene 
                 donor organism 
                 NO: 
                 (mM) 
                 (s−1) 
                 Km 
                 (s−1) 
                 ref 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 ErCm 
                 
                   Candida magnolia 
                 
                  9, 10 
                 8.5 
                 7.6 
                 0.016 
                 48 
                 (Lee et al, 
               
               
                   
                 JH110 
                   
                   
                   
                   
                   
                 2010) 
               
               
                 Gcy1p 
                 
                   Saccharomyces 
                 
                 7, 8 
                 3.4 
                 — 
                 — 
                 — 
                 Ookura et al, 
               
               
                   
                 
                   cerevisiae 
                 
                   
                   
                   
                   
                   
                 2007 
               
               
                 GLD1 
                 
                   Hypocrea jecorina 
                 
                 11, 12 
                 0.016-0.134 
                 530-36.5 
                 — 
                 — 
                 Jovanović et 
               
               
                   
                 ( Trichoderma reesei ) 
                   
                   
                   
                   
                   
                 al, 2013 
               
               
                 ALR1 
                 
                   Aspergillus niger 
                 
                 13, 14 
                 0.139 
                 25 
                   
                   
                 Jovanović et 
               
               
                   
                   
                   
                   
                   
                   
                   
                 al, 2013 
               
               
                 Pc20g15580 
                 
                   Penicillium 
                 
                 15, 16 
                 ? 
                 ? 
               
               
                   
                 
                   chrysogenum 
                 
               
               
                   
               
            
           
         
       
     
     Example 2. Biochemical Background of a Cyanobacterial Cell According to the Present Invention 
     The genes encoding the phosphatase TM_1254, derived from  Thermotoga maritima  MSB8 (Kuznetsova et al., 2005), and the erythrose reductase Gcy1p, derived from  Saccharomyces cerevisiae  (Ookura and Kasumi, 2007) were codon-optimized for expression in  Synechocystis  and obtained through chemical synthesis. These genes were each cloned with a trc promoter into a RSF1010-based conjugative plasmid pVZ. Introduction of the phosphatase-encoding gene, in combination with a gene encoding one of the erythrose reductases (via a conjugative plasmid) should allow the transconjugant  Synechocystis  strain to produce erythritol from erythrose-4-phosphate. These strains were tested by colony PCR to confirm the presence of the plasmid ( FIG. 2 ).  FIG. 2  depicts SAW030 in the third lane with a band of ˜2200 bp, representing the phosphatase and reductase, whereas the second lane shows wildtype  Synechocystis  as a negative control. 
     Example 3. Production of Erythritol by a Cyanobacterial Cell 
     Mutant cultures obtained in example 2 were selected for presence of the plasmid by growth on agar plates containing 20 μg/ml of kanamycine. A selected mutant was named  Synechocystis  SAW030. This SAW030 mutant was inoculated in BG-11 medium supplemented with 10 mM TES-buffer-NaOH (pH=8.0) and 20 ug/ml kanamycine and grown to stationary phase within several days (OD of 1.5). An aliquot of the initial culture was used to inoculate 100 ml BG-11 supplemented with 10 mM TES-buffer-NaOH (pH=8.0) and with 10 μg/ml kanamycine to an OD of 0.1. The culture was incubated at low light intensity (˜40 μE), 30° C. and shaking at 120 rpm. After every few days, an 800 μl sample was taken for measurement of optical density (A730) and determination of erythritol concentration in the supernatant. With the help of standard concentrations of erythritol, the concentration of erythritol in the culture was determined via an HPLC method ( FIG. 3 ). In conclusion, erythritol production increases in time (at least up to 35 days) to a concentration of at least 550 uM (60 mg/L) in the extracellular medium. 
     Example 4. Resistance to Erythritol of  Synechocystis  PCC6803 
       Synechocystis  PCC6803 was inoculated in 10 ml BG-11 supplemented with 10 mM TES-buffer-NaOH (pH=8.0) and with 10 μg/ml kanamycine at an OD of 0.2. The culture was incubated at low light intensity (˜40 μE), 30° C. and shaking at 120 rpm. It was clearly shown ( FIG. 4 ) that up to a concentration of 10 gr/L erythritol, cultures are not affected with respect to growth-rate. 
     Example 5. Biochemical Background and Production of Erythritol of Several Additional Cyanobacterial Cells According to the Present Invention 
     The genes encoding phosphatases TM_1254 and YidA, and the erythrose reductases Gcy1p, GLD1, ALR1 and Pc20g15580 were codon-optimized for expression in  Synechocystis  and obtained by chemical synthesis. These genes were each cloned with a trc promoter into a vector containing homologous regions targeting for genome integration at the slr0168 gene. Introduction of the phosphatase-encoding gene, in combination with a gene encoding one of the erythrose reductases (via natural transformation and homologous recombination) allows the transformant  Synechocystis  strains to produce erythritol from erythrose-4-phosphate. Mutant cultures were selected for by growth on agar plates containing 20 μg/ml of kanamycine. The resulting strains were tested by colony PCR to confirm the presence of the desired genes in the genome. From the strains obtained, strains comprising TM1254 and GLD1, YidA and GLD1, and TM1254 and Gcy1p were selected for further analysis. 
     These strains were inoculated in BG-11 medium supplemented with 25 mM CAPSO-buffer-NaOH (pH=9.0) and 20 ug/ml kanamycine and grown to stationary phase within several days (OD of 1.5). An aliquot of the initial culture was used to inoculate 100 ml BG-11 supplemented with 25 mM CAPSO-buffer-NaOH (pH=9.0) and 20 ug/ml kanamycine to an OD of 0.1. The culture was incubated at low light intensity (˜40 μE), 30° C. and shaking at 120 rpm. After 23-30 days of culture, an 800 μl sample was taken for measurement of optical density (A730) and for determination of erythritol concentration in the supernatant. Using standard concentrations of erythritol, the concentration of erythritol in the culture was determined using HPLC ( FIG. 5 ). In conclusion, erythritol production was detected in the extracellular medium of each of the tested strains; strain TM1254/GLD1 produced 0.05 mM erythritol (retention 15.75 min.), strain YidA/GLD1 produced 0.09 mM erythritol (retention 15.75 min.), and strain TM1254/Gcy1p produced 0.1 mM erythritol (retention 15.72 min.). These results are clearly in the same magnitude as strain SAW030 in example 3. 
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