# Patent

## Title
Acid-resistant yeast cell with reduced FPS1 activity and method of producing lactate by using the yeast cell

## Abstract

Provided are an acid-resistant yeast cell with genetic modification and reduced Fps1 activity compared to that of a parent cell without the genetic modification; and a method of producing lactate by using the yeast cell.

## Background
RELATED APPLICATION
This application claims the benefit of Korean Patent Application No. 10-2014-0076636, filed on Jun. 23, 2014, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
INCORPORATION-BY-REFERENCE OF MATERIAL ELECTRONICALLY SUBMITTED
Incorporated by reference in its entirety herein is a computer-readable nucleotide/amino acid sequence listing submitted herewith and identified as follows: One 65,020 byte ASCII (Text) file named “720021.ST25.TXT,” created Jun. 17, 2015.
BACKGROUND
1. Field
The present disclosure relates to an acid-resistant yeast cell with reduced Fps1 activity and a method of producing lactate by using the yeast cell.
2. Description of the Related Art
Organic acids are widely used in a variety of industries. For example, lactate is an organic acid that is used in a variety of industrial fields, including food, pharmaceutical, chemical, and electronic industries. Lactate is a colorless, odorless, water-soluble, low-volatile material. Lactate is also not toxic to the human body, and is used as a flavoring agent, a sour taste agent, a preserving agent, or the like. Lactate is also used as a source of polylactic acid (PLA) that is an environmentally friendly, biodegradable plastic known as an alternate polymeric material.
Organic acids may be dissociated into hydrogen ions and their own negative ions at a higher pH than their own dissociation constant (pKa value), for example, under a neutral condition. Meanwhile, organic acids, for example, lactic acid, may be present in the form of free acid without an electromagnetic force at a pH lower than its own pKa value. An organic acid in the form of negative ions may not be permeable with respect to a cell membrane, but may be permeable with respect to the cell membrane when it is present in the form of free acid. Therefore, an organic acid in free acid form may flow into the cells from extracellular environments where the concentration of the organic acid is high, and thus lower an intercellular pH level. Meanwhile, an organic acid present as negative ions requires an additional isolation process involving the addition of a salt. Thus, a cell lacking acid-resistance may become inactive and die under acidic conditions, such as cells exposed to lactic acid during a lactate production process.
Therefore, there is a need for a microorganism with acid-resistance used in the production of lactate.
SUMMARY
Provided is an acid-resistant yeast cell comprising a genetic modification that reduces Fps1 activity in the acid-resistant yeast cell compared to a parent cell without the genetic modification.
Also provided is a method of producing lactate by culturing the yeast cell in a cell culture medium.
Further provided is a method of increasing the acid-resistance of a yeast cell, the method comprising deleting or disrupting expression of a polynucleotide that encodes the Fps1 in a yeast cell to increase the acid-resistance of the yeast cell.
Additional aspects will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented embodiments.



BRIEF DESCRIPTION OF THE DRAWINGS
These and/or other aspects will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings in which:
FIG. 1 illustrates a lactate production pathway of an acid-resistant yeast cell according to an embodiment of the present invention;
FIG. 2 is a vector map of a p416-CCW12p-LDH vector;
FIG. 3 is a vector map of a pUC19-HIS3 vector;
FIG. 4 is a vector map of a pUC19-CCW12p-LDH-HIS3 vector;
FIG. 5 is a vector map of a pUC57-ura3HA vector;
FIG. 6 is a vector map of a pUC57-CCW12-LDH-ura3HA vector; and
FIGS. 7A to 7E are a series of graphs illustrating cell growth of a Δfps1 strain and a Δnde1Δnde2 (C) strain at different concentrations of lactic acid in the cell culture medium.



DETAILED DESCRIPTION
Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. In this regard, the present embodiments may have different forms and should not be construed as being limited to the descriptions set forth herein. Accordingly, the embodiments are merely described below, by referring to the figures, to explain aspects of the present description. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. Expressions such as “at least one of,” when preceding a list of elements, modify the entire list of elements and do not modify the individual elements of the list.
As used herein, the term “activity reduction,” “reduced activity” and like terms in reference to an enzyme or a polypeptide denotes a cell, an isolated enzyme, or a polypeptide whose activity is lower than the activity measured in a comparable cell of the same type or the original polypeptide. Additionally, activity reduction may refer to a cell, an isolated enzyme, or a polypeptide having no activity. The activity of a subject enzyme or polypeptide of a genetically engineered cell may be reduced by any amount, such as reduced by about 5% or more, about 10% or more, about 15% or more, about 20% or more, about 30% or more, about 40% or more, about 50% or more, about 55% or more, about 60% or more, about 70% or more, about 75% or more, about 80% or more, about 85% or more, about 90% or more, about 95% or more, or about 100% as compared to the activity of an enzyme or polypeptide measured in a comparable cell of the same type that does not have a given genetic modification (e.g. a parent cell or “wild-type” cell). The activity of a specific enzyme or polypeptide in a subject cell may be about 5% or more, about 10% or more, about 15% or more, about 20% or more, about 30% or more, about 50% or more, about 60% or more, about 70% or more, or about 100% or more reduced than the activity of the enzyme or polypeptide in a parent cell. The reduced activity of the enzyme or polypeptide may be confirmed by using a commonly known method in the art.
Activity of the enzyme or polypeptide of the yeast cell may be reduced due to mutation such as deletion or disruption of a gene that encodes the enzyme or polypeptide. As used herein, the “deletion” or “disruption” of the gene includes situations where a part or a whole gene, or a part or a whole regulatory region of a promoter or terminator of the gene is mutated, substituted, deleted, or at least one base is inserted into the gene, such that the gene is not expressed or has a reduced amount of expression, or activity of the enzyme is removed or reduced even when the gene is expressed. The deletion or disruption of the gene may be caused by genetic engineering such as homologous recombination, mutation induction, or molecular evolution. When a cell includes a plurality of the same genes or at least two different polypeptide paralogs, at least one gene may be deleted or disrupted.
As used herein, the term “activity increase” or “increased activity” or like terms used in reference to an enzyme or a polypeptide refers to situations where an amount of an enzyme or a polypeptide is increased enough so as to increase the activity thereof. Additionally, the terms “activity increase” or “increased activity” or like terms denotes a cell or an isolated polypeptide that has been genetically modified such that a specified activity is increased as compared to the same activity measured in a comparable cell of the same type that has not been engineered (e.g., parent cell or “wild-type” cell) or the original polypeptide that has not been genetically engineered. The activity can be increased by any suitable amount. For instance, activity of a subject enzyme or polypeptide in a genetically engineered cell may be increased by about 5% or more, about 10% or more, about 15% or more, about 20% or more, about 30% or more, about 50% or more, about 60% or more, about 70% or more, or about 100% or more as compared to the activity of an enzyme or polypeptide measured in a comparable cell of the same type that does not contain the genetic modification (e.g., parent cell or “wild-type” cell). Also, the activity of a specific enzyme or polypeptide in a subject cell may be increased by about 5% or more, about 10% or more, about 15% or more, about 20% or more, about 30% or more, about 50% or more, about 60% or more, about 70% or more, or about 100% or more compared to the activity of the enzyme or polypeptide in a parent cell. A cell having increased activity of a polypeptide may be confirmed by using any method commonly known in the art.
An increase in the activity of the enzyme or polypeptide may be caused by an increase in expression of the enzyme or polypeptide, or an increase in a specific activity of the enzyme or polypeptide. An increase in specific activity may be due to enzyme engineering caused by mutation or random mutagenesis of specific amino acid of an active domain in the enzyme or polypeptide. An increase in the expression may be caused by introduction of a polynucleotide encoding a polypeptide into a cell, by an increased number of copies of the polynucleotide, or by mutation of a regulatory region of the polynucleotide. The mutation of a regulatory region of the polynucleotide may be caused by mutation of a promoter. The polynucleotide introduced from the outside of the cell or the polynucleotide having the increased number of copies may be an endogenous gene or an exogenous gene. The term “endogenous” may refer to a gene present in a genetic material contained within the microorganism. The term “exogenous” may refer to a gene introduced into a cell from the outside of the cell, and for example, the exogenous gene to be introduced into a host cell may be a homologous or heterologous with respect to the host cell. The term “heterologous” denotes that the gene is a foreign gene and is non-native to the host cell.
As used herein, the term “activity increase” or “increased activity” of an enzyme or a polypeptide includes the introduction of an activity into a cell that was not present prior to the genetic modification, e.g., an activity that is not in the parent cell by genetic modification.
The expression “increased copy number” includes an increase in copy number by introduction of an exogenous gene or by amplification of an endogenous gene, or both. The introduction of an exogenous gene may occur by using a vehicle such as a vector. The introduction may be a transient introduction, in which the gene is not integrated into the genome, or integration into the genome. The introduction may, for example, occur by introducing a vector inserted with a polynucleotide encoding a desired polypeptide into the cell and then replicating the vector in the cell or integrating the polynucleotide into the genome of the cell and then replicating the polynucleotide together with the replication of the genome.
As used herein, the gene modification may be performed by molecular biological methods known in the art (see, e.g., Roslyn M. Bill, Recombinant Protein Production in Yeast: Methods and Protocols (2012); Sambrook et al., Molecular cloning, A laboratory manual: Cold Spring Harbor Laboratory (1989); and R Daniel Gietz et al., Quick and easy yeast transformation using the LiAc/SS carrier DNA/PEG method: Nature protocols (2007)).
As used herein, the term “gene” denotes a polynucleotide that is expressed by at least one of transcription and translation to produce a gene product, including mRNA or other nucleic acid fragment capable of producing a protein, or the protein expressed therefrom. A gene may include a coding region as well as a regulatory sequence of a 5′-non coding sequence and a 3′-non coding sequence in addition to the coding region.
The terms “cell”, “strain”, or “microorganism” as used herein may be interchangeably used and may include bacteria, yeast, or fungi.
As used herein, the term “sequence identity” of a polypeptide or polynucleotide with respect to another polypeptide or polynucleotide refers to a degree of sameness in an amino acid residue or a base in a specific region of two sequences that are aligned to best match each other for comparison. The sequence identity is a value obtained via optimal alignment and comparison of the two sequences in the specific region for comparison, in which a partial sequence in the specific region for comparison may be added or deleted with respect to a reference sequence. The sequence identity represented in a percentage may be calculated by, for example, comparing two sequences that are aligned to best match each other in the specific region for comparison, determining matched sites with the same amino acid or base in the two sequences to obtain the number of the matched sites, dividing the number of the matched sites in the two sequences by a total number of sites in the compared specific regions (i.e., a size of the compared region), and multiplying a result of the division by 100 to obtain a sequence identity as a percentage. The sequence identity as a percentage may be determined using a known sequence comparison program, for example, BLASTP or BLASTN (NCBI), CLC Main Workbench (CLC bio), or MegAlignTM (DNASTAR Inc).
In identifying a polypeptide or polynucleotide with the same or similar function or activity with respect to various types of species, any various levels of sequence identity may be applied. In some embodiments, the sequence identity may be, for example, at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, at least 99%, or at least 100%.
As used herein, the term “parent cell” or “parent strain” refers to a cell or strain used for a subject genetic modification, i.e., the cell or strain prior to introduction of a given genetic modification that provides a modified cell or strain. The parent cell is the same as a subject modified cell with the exception of the genetic modification present in the subject cell, and thus may be a reference cell with respect to the genetic modification. Also, the term “genetic modification” may refer to situations where the constitution or a structure of a genetic material of a cell is artificially changed. The parent cell may not have the subject genetic modification, that is, for example, a genetic modification resulting in reducing Fps1 activity. The parent cell may be a parent yeast cell. Also, the term “wild-type” polypeptide or polynucleotide may be a polypeptide or a polynucleotide without specific genetic modification, and the specific genetic modification may result a genetically engineered polypeptide or polynucleotide. A wild-type polypeptide or polynucleotide may serve as a basis of comparison with a genetically modified polypeptide or polynucleotide.
As used herein, the term “lactate” is interpreted as including its anion form, a salt thereof, a solvate, a polymorph, or a combination thereof in addition to lactic acid itself. The salt may be, for example, an inorganic acid salt, an organic acid salt, or a metal salt. The inorganic acid salt may be a hydrochloride, bromate, phosphate, sulfate, or disulfate. The inorganic acid salt may be formate, acetate, propionate, lactate, oxalate, tartrate, malate, maleate, citrate, fumarate, besylate, camsylate, edisylate, trifluoroacetate, benzoate, gluconate, methansulfonate, glycolate, succinate, 4-toluenesulfonate, galacturonate, embonate, glutamate, or aspartate. The metal salt may be a calcium salt, a sodium salt, a magnesium salt, a strontium salt, or a potassium salt.
According to an aspect of the present invention, provided is an acid-resistant yeast cell that has a genetic modification that reduces Fps1 activity compared to that of a parent cell without the genetic modification.
In the yeast cell, a polynucleotide that encodes Fps1 may be modified, for example, deleted or disrupted. The Fps1 may be aquaglyceroporin. The Fps1 may be referred to as a glycerol channel protein, a glycerol transport polypeptide, a glycerol facilitator channel, or a glycerol uptake/efflux facilitator protein. Glycerol may be secreted to the outside of a cell through the Fps1. The Fps1 may be classified under TCDB 1.A.8.5.1 in the transporter classification system provided by Transport Classification Database (M. Saier; U of CA, San Diego). A Fps1 protein (Fps1p) may have an amino acid sequence having at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with an amino acid sequence of SEQ ID NO: 1. A polynucleotide that encodes the Fps1 protein may have a polynucleotide sequence that encodes an amino acid having at least 95% sequence identity with an amino acid sequence of SEQ ID NO: 1 or may have a polynucleotide sequence of SEQ ID NO: 2.
The term “acid-resistant”, “acid-tolerant”, “acid tolerating”, “acid-resistance”, and “acid tolerance” may be used interchangeably.
The yeast cell with an acid-resistant property of may have a better growth rate under an acid condition compared to the growth of the parent cell. The acid condition may be an organic acid, an inorganic acid, or a combination thereof. The organic acid may be an organic acid having 1 to 20 carbon atoms. The organic acid may be acetic acid, lactic acid, propionic acid, 3-hydroxypropionic acid, butyric acid, 4-hydroxybutyric acid, succinic acid, fumaric acid, malic acid, oxalic acid, adipic acid, or a combination thereof. The yeast cell better under a pH condition in a range of about 2.0 to about 7.0 (below pH 7.0), for example, pH in a range of about 2.0 to about 5.0, about 2.0 to about 4.5, about 2.0 to about 4.0, about 2.0 to about 3.8, about 2.5 to about 3.8, about 3.0 to about 3.8, about 2.0 to about 3.0, about 2.0 to about 2.7, about 2.0 to about 2.5, or about 2.5 to about 3.0 compared to that of a yeast cell in which Fps1 activity is not reduced. The growth rate may be measured by counting microorganism colonies or measuring the optical density (OD) of the colonies. The yeast cell may have increased growth rate as measured by OD compared to that of a yeast cell in which Fps1 activity is not reduced.
The yeast cell with an acid-resistant property of may have a higher survival rate under an acid condition compared to that of a parent cell. The acid condition may be an acid condition including an organic acid, an inorganic acid, or a combination thereof. The organic acid may be an organic acid having 1 to 20 carbon atoms. The organic acid may be acetic acid, lactic acid, propionic acid, 3-hydroxypropionic acid, butyric acid, 4-hydroxybutyric acid, succinic acid, fumaric acid, malic acid, oxalic acid, adipic acid, or a combination thereof. The yeast cell may have a higher survival rate under a pH condition in a range of about 2.0 to about 7.0 (below pH 7.0), for example, pH in a range of about 2.0 to about 5.0, about 2.0 to about 4.5, about 2.0 to about 4.0, about 2.0 to about 3.8, about 2.5 to about 3.8, about 3.0 to about 3.8, about 2.0 to about 3.0, about 2.0 to about 2.7, about 2.0 to about 2.5, or about 2.5 to about 3.0 compared to that of a yeast cell in which Fps1 activity is not reduced (e.g., a parent cell).
Also, the yeast cell with an acid-resistant property may have higher (greater) metabolization ability under an acid condition compared to that of a parent cell. The metabolization may mean chemical transformations (e.g., enzyme-catalyzed reactions) within the yeast cell. The enzyme-catalyzed reactions may allow the yeast cell to grow, reproduce, and respond to their environment such as acidic conditions. The metabolization ability may be determined by measuring consumption of a nutrient such as hexose (e.g., glucose) or pentose, or production of metabolite such as lactate in the cell. The acid condition may be an acid condition including an organic acid, an inorganic acid, or a combination thereof. The organic acid may be an organic acid having 1 to 20 carbon atoms. The organic acid may be acetic acid, lactic acid, propionic acid, 3-hydroxypropionic acid, butyric acid, 4-hydroxybutyric acid, succinic acid, fumaric acid, malic acid, oxalic acid, adipic acid, or a combination thereof. The yeast cell may have higher metabolization ability under a pH condition in a range of about 2.0 to about 7.0 (below pH 7.0), for example, pH in a range of about 2.0 to about 5.0, about 2.0 to about 4.5, about 2.0 to about 4.0, about 2.0 to about 3.8, about 2.5 to about 3.8, about 3.0 to about 3.8, about 2.0 to about 3.0, about 2.0 to about 2.7, about 2.0 to about 2.5, or about 2.5 to about 3.0 compared to that of a yeast cell in which Fps1 activity is not reduced. Here, a degree of “metabolization ability” may be measured by a nutrition uptake rate per cell, for example, a glucose uptake rate per cell. Also, a degree of “metabolization ability” may be measured by a product secretion rate per cell, for example, a carbon dioxide secretion rate per cell.
The yeast cell may belong to Saccharomyces genus, Kluyveromyces genus, Candida genus, Pichia genus, Issatchenkia genus, Debaryomyces genus, Zygosaccharomyces genus, Shizosaccharomyces genus, or Saccharomycopsis genus. Saccharomyces genus may be, for example, S. cerevisiae, S. bayanus, S. boulardii, S. bulderi, S. cariocanus, S. cariocus, S. chevalieri, S. dairenensis, S. ellipsoideus, S. eubayanus, S. exiguus, S. florentinus, S. kluyveri, S. martiniae, S. monacensis, S. norbensis, S. paradoxus, S. pastorianus, S. spencerorum, S. turicensis, S. unisporus, S. uvarum, or S. zonatus. 
The yeast cell may produce lactate. The yeast cell may have an activity of a polypeptide that converts pyruvate into lactate. In the yeast cell, the activity of a polypeptide that converts pyruvate into lactate may be increased compared to a parent cell. The yeast cell may have an exogenous gene encoding a polypeptide that converts pyruvate into lactate or a mutant of the gene. The polypeptide that converts pyruvate into lactate may be a lactate dehydrogenase (LDH). The LDH may be an NAD(P)-dependent enzyme. Also, the LDH may be stereo-specific and may produce only L-lactate, only D-lactate, or both L-lactate and D-lactate. The NAD(P)-dependent enzyme may be an enzyme that is classified under EC 1.1.1.27 that is related to production of L-lactate or EC 1.1.1.28 that is related to production of D-lactate.
In the yeast cell having a capability of producing lactate, an activity of LDH may be increased. The yeast cell may include a gene encoding at least one LDH, and the gene may be exogenous. A polynucleotide may be derived from bacteria, yeasts, fungi, mammals or reptiles. The polynucleotide may be a polynucleotide that encodes at least one LDH selected from the group consisting of Lactobacillus helveticus, L. bulgaricus, L. johnsonii, L. plantarum, Pelodiscus sinensis japonicus, Ornithorhynchus anatinus, Tursiops truncatus, Rattus norvegicus, Xenopus laevis, and Bos Taurus. An LDH derived from Pelodiscus sinensis japonicus, an LDH derived from Ornithorhynchus anatinus, an LDH derived from Tursiops truncatus, and an LDH derived from Rattus norvegicus may each include an amino acid sequence having a sequence identity of about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more with amino acids of SEQ ID NOS: 3, 4, 5, and 6. For example, a polynucleotide encoding the LDH may be a polynucleotide encoding an amino acid sequence with a sequence identity of about 95% or more with an amino acid sequence of SEQ ID NOS: 3, 4, 5, and 6. In some embodiments, a polynucleotide encoding the LDH may have a polynucleotide sequence encoding an amino acid sequence with about 95% or more with an amino acid sequence of SEQ ID NOS: 3, 4, 5, and 6 or a polynucleotide sequence of SEQ ID NO: 7.
A polynucleotide encoding the LDH may be included in a vector. The vector may include a replication origin, a promoter, a LDH-encoding polynucleotide, and/or a terminator. The replication origin may include a yeast autonomous replication sequence (ARS). The yeast ARS may be stabilized by a yeast centrometric sequence (CEN). The promoter may be a constitutive promoter. For example, the promoter may be selected from the group consisting of a cytochrome c (CYC) promoter, a transcription elongation factor (TEF) promoter, a glyceraldehyde-3-phosphate dehydrogenase (GPD) promoter, and an alcohol dehydrogenase (ADH) promoter. The CYC promoter, the TEF promoter, the GPD promoter, and the ADH promoter may each have a nucleotide sequence of SEQ ID NOS: 19, 20, 21, 22, and 23. The terminator may be selected from the group consisting of a terminator of a gene encoding a phosphoglycerate kinase 1 (PGK1), a terminator of a gene encoding a cytochrome c 1 (CYC1), and a terminator of a gene encoding a galactokinase 1 (GAL1). The CYC1 terminator may have a nucleotide sequence of SEQ ID NO: 24. The vector may further include a selection marker. The LDH-encoding polynucleotide may be included in a specific location of a yeast cell genome. The specific location of the yeast cell genome may include a locus of a gene to be deleted and disrupted, such as pyruvate decarboxylase (PDC) and cytochrome-c oxidoreductase 2 (CYB2). When the LDH-encoding polynucleotide functions to produce an active protein in a cell, the polynucleotide is considered as “functional” in a cell.
The yeast cell may include a polynucleotide that encodes one LDH or a polynucleotide that encodes multiple LDH copies, e.g., 2 to 10 copies. The yeast cell may include a polynucleotide that encodes multiple LDH copies into, for example, 1 to 8, 1 to 7, 1 to 6, 1 to 5, 1 to 4, or 1 to 3 copies. When the yeast cell includes the polynucleotide encoding multiple LDHs, each polynucleotide may include copies of the same LDH polynucleotide or copies of polynucleotides encoding at least two different LDHs. The multiple copies of the polynucleotide encoding exogenous LDHs may be included in the same locus or multiple loci in a genome of a host cell, and the promoter or terminator of each copy of the polynucleotide may be identical to or different from each other.
In some embodiments, in the yeast cell, activity of a polypeptide that converts pyruvate into acetaldehyde, a polypeptide that converts lactate into pyruvate, a polypeptide that converts dihydroxyacetone phosphate (DHAP) into glycerol-3-phosphate, an external mitochondrial NADH dehydrogenase, or a combination thereof may be reduced compared to a parent cell.
In the yeast cell, a gene encoding the polypeptide that converts pyruvate into acetaldehyde may be deleted or disrupted. The polypeptide that converts pyruvate into acetaldehyde may be an enzyme that is classified under EC 4.1.1.1. The polypeptide that converts pyruvate to acetaldehyde may be a pyruvate decarboxylase, that is PDC1. The polypeptide that converts pyruvate to acetaldehyde may include an amino acid sequence having a sequence identity of about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more with an amino acid sequence of SEQ ID NO: 8. The gene encoding the polypeptide that converts pyruvate to acetaldehyde may be a polynucleotide encoding an amino acid sequence with a sequence identity of about 95% or more with respect to an amino acid sequence of SEQ ID NO: 8, or may have a polynucleotide sequence of SEQ ID NO: 9. The gene may be pdc1.
In the yeast cell, a gene encoding the polypeptide that converts lactate into pyruvate may be deleted or disrupted. The polypeptide that converts lactate into pyruvate may be a cytochrome c-dependent enzyme. The polypeptide that converts lactate into pyruvate may be an enzyme that is classified under EC 1.1.2.4 that acts on D-lactate or EC 1.1.2.3 that acts on L-lactate. The polypeptide that converts lactate into pyruvate may be lactate cytochrome c-oxidoreductase, for example, a CYB2 (CAA86721.1), a CYB2A, a CYB2B, or a DLD1. The polypeptide that converts lactate into pyruvate may include an amino acid sequence having a sequence identity of about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more with an amino acid sequence of SEQ ID NO: 10. The gene encoding the polypeptide that converts lactate into pyruvate may be a polynucleotide sequence encoding an amino acid sequence having a sequence identity of about 95% or more with an amino acid sequence of SEQ ID NO: 10, or may include a polynucleotide sequence of SEQ ID NO: 11.
In the yeast cell, a gene encoding the polypeptide that converts DHAP into glycerol-3-phosphate may be deleted or disrupted. The polypeptide that converts DHAP into glycerol-3-phosphate may be a cytosolic glycerol-3-phosphate dehydrogenase and may be an enzyme that catalyzes reduction of DHAP to glycerol-3-phosphate by using oxidation of NADH to NAD+. The polypeptide may be classified under EC 1.1.1.8. The cytosolic glycerol-3-phosphate dehydrogenase may be GPD1. The cytosolic glycerol-3-phosphate dehydrogenase may include an amino acid sequence having a sequence identity of about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more with an amino acid sequence of SEQ ID NO: 12. A gene encoding the cytosolic glycerol-3-phosphate dehydrogenase may include a polynucleotide sequence encoding an amino acid sequence having a sequence identity of about 95% or more with an amino acid sequence of SEQ ID NO: 12, or may include a polynucleotide sequence of SEQ ID NO: 13.
In the yeast cell, a gene encoding the external mitochondrial NADH dehydrogenase may be deleted or disrupted. The external mitochondrial NADH dehydrogenase may be an enzyme that is classified under EC. 1.6.5.9 or EC. 1.6.5.3. The external mitochondrial NADH dehydrogenase may be a type II NADH:ubiquinone oxidoreductase. The external mitochondrial NADH dehydrogenase may be located on the outer surface of the inner mitochondrial membrane facing a cytoplasm. The external mitochondrial NADH dehydrogenase may be an enzyme catalyzing oxidation of cytosolic NADH to NAD+. The external mitochondrial NADH dehydrogenase may re-oxidize cytosolic NADH formed by a glycolysis process. The external mitochondrial NADH dehydrogenase may provide cytosolic NADH to a mitochondrial respiratory chain. The external mitochondrial NADH dehydrogenase may be NDE1, NDE2, or a combination thereof. The external mitochondrial NADH dehydrogenase may be distinguished from an internal mitochondrial NADH dehydrogenase NDI1 that is present and functions inside mitochondria. The external mitochondrial NADH dehydrogenase may include an amino acid sequence having a sequence identity of about 60% or more, about 70% or more, about 80% or more, about 90% or more, about 95% or more, about 96% or more, about 97% or more, about 98% or more, or about 99% or more with an amino acid sequence of SEQ ID NO: 14 or 16. For example, NDE1 and NDE2 may each have amino acid sequences of SEQ ID NO: 14 and SEQ ID NO: 16. The gene encoding the external mitochondrial NADH dehydrogenase may be a polynucleotide sequence encoding an amino acid sequence having a sequence identity of about 95% or more with an amino acid sequence of SEQ ID NO: 14 or 16, or may have a polynucleotide sequence of SEQ ID NO: 15 or 17. In some embodiments, NDE1 may include a polynucleotide sequence of SEQ ID NO: 15, and NDE2 may include a polynucleotide sequence of SEQ ID NO: 17.
In the yeast cell, Fps1 activity is reduced, and a gene encoding a polypeptide that converts pyruvate into acetaldehyde, a gene encoding a polypeptide that converts lactate into pyruvate, a gene encoding a polypeptide that converts DHAP into glycerol-3-phosphate, a gene encoding an external mitochondrial NADH dehydrogenase, or a combination thereof are deleted or disrupted, and a gene encoding a polypeptide that converts pyruvate into lactate is included or additionally introduced to a genome of the yeast cell. The yeast cell may be Saccharomyces cerevisiae. 
According to another aspect of the present invention, a composition for producing lactate is provided, wherein the composition includes the yeast cell and a cell culture medium.
According to another aspect of the present invention, a method of producing lactate is provided, wherein the method includes culturing the yeast cell in a cell culture medium, whereby the yeast cell produces lactate.
The culturing of the yeast cell may be performed in a suitable medium under suitable culturing conditions known in the art. One of ordinary skill in the art may suitably change a culture medium and culturing conditions according to the microorganism selected. A culturing method may be batch culturing, continuous culturing, or fed-batch culturing. The yeast cell is as defined above.
The culture medium may include various carbon sources, nitrogen sources, and trace elements.
The carbon source may be, for example, carbohydrate such as glucose, sucrose, lactose, fructose, maltose, starch, or cellulose; fat such as soybean oil, sunflower oil, castor oil, or coconut oil; fatty acid such as palmitic acid, stearic acid, or linoleic acid; alcohol such as glycerol or ethanol; organic acid such as acetic acid, and/or a combination thereof. The culturing may be performed by having glucose as the carbon source. The nitrogen source may be an organic nitrogen source such as peptone, yeast extract, beef stock, malt extract, corn steep liquor (CSL), or soybean flour, or an inorganic nitrogen source such as urea, ammonium sulfate, ammonium chloride, ammonium phosphate, ammonium carbonate, and ammonium nitrate, or a combination thereof. The culture medium is a supply source of phosphorus and may include, for example, potassium dihydrogen phosphate, dipotassium phosphate, and corresponding sodium-containing salt thereof, and a metal salt such as magnesium sulfate or iron sulfate. Also, amino acid, vitamin, a suitable precursor, or the like may be included in the culture medium. The culture medium or individual component may be added to a culture medium solution in a batch or continuous manner.
Also, pH of the culture medium solution may be adjusted by adding a compound such as ammonium hydroxide, potassium hydroxide, ammonia, phosphoric acid, and sulfuric acid to the culture medium solution by using a suitable method during the culturing process. Also, an antifoaming agent such as fatty acid polyglycol ester may be used during the culturing process to inhibit the generation of bubbles.
The yeast cell may be cultured under an aerobic, microaerobic, or anaerobic condition. In some embodiments, the microaerobic condition may refer to a culturing condition in which oxygen is dissolved in the medium at a lower level of an oxygen concentration than an oxygen concentration in the atmosphere. The lower level of an oxygen concentration may be about 0.1% to about 10%, about 1% to about 9%, about 2% to about 8%, about 3% to about 7%, or about 4% to about 6% of the oxygen concentration in the atmosphere. Also, the microaerobic condition may include maintaining a dissolved oxygen (DO) concentration of the medium in a range of about 0.9 ppm to about 3.6 ppm, for example, about 0.9 ppm to about 2.6 ppm, about 0.9 ppm to about 1.6 ppm, about 1.0 ppm to about 3.6 ppm, about 2.0 ppm to about 3.6 ppm, or about 3.0 ppm to about 3.6 ppm. A temperature for the culturing may be in a range of, for example, about 20° C. to about 45° C. or about 25° C. to about 45° C. A period of time for the culturing may be continued until a desired amount of lactate is obtained. The method of producing lactate may include collecting or isolating lactate from the culture.
The collecting of lactate from the culture may be performed by using a separation and purification method known in the art. The collecting of lactate may be performed by centrifugation, ion-exchange chromatography, filtration, precipitation, extraction, distillation, or combination thereof. For example, the culture may be centrifuged to separate biomass, and a supernatant thus obtained may be separated by ion-exchange chromatography.
The yeast cell according to an aspect of the present invention may be acid-resistant, and lactate may be produced at a high concentration and a high yield.
Lactate may be produced at a high concentration and a high yield by using the method of producing lactate according to another aspect of the present invention.
Hereinafter, the present invention is described in greater detail with reference to embodiments. However, the embodiments are for illustrative purposes only and do not limit the scope of the present invention.
Example 1. Manufacture of Strain and Manufacture of Expression Vector for Effective Production of Lactate
1.1 Manufacture of Saccharomyces cerevisiae Strain in which Pdc1, Cyb2, and Gpd1 Genes are Deleted and an Exogenous Ldh Gene Introduced
Saccharomyces cerevisiae CEN.PK2-1D (MATαura3-52; trp1-289; leu2-3, 112; his3Δ1; MAL2-8C; SUC2, EUROSCARF accession number: 30000B) was used as a lactate production strain, and Saccharomyces cerevisiae CEN.PK2-1D Δpdc1::ldh Δcyb2::ldhΔgpd1::ldh (KCTC 12415BP), in which a pyruvate decarboxylase (pdc1) gene that is a main enzyme of alcohol fermentation, a NAD-dependent glycerol-3-phosphate dehydrogenase (gpd1) gene that is a main enzyme of glycerol biosynthesis, and a L-lactate cytochrome-c oxidoreductase 2 (cyb2) gene that is a lactate decomposition enzyme are deleted and disrupted and a lactate dehydrogenase gene is introduced at a location of each of the genes, was used to block production pathways of main by-products such as ethanol and glycerol.
1.1.1 Preparation of L-LDH Overexpression Vector
A CCW12 promoter PCR fragment obtained by performing PCR with a genomic DNA of Saccharomyces cerevisiae CEN.PK2-1D as a template and primers of SEQ ID NOS: 25 and 26 was digested with SacI and XbaI, and the resultant was introduced into a p416-GPD (http://www.atcc.org/products/all/87360.aspx) vector, from which a GPD promoter was digested with SacI and XbaI, thereby producing a p416-CCW12p vector for overexpression of L-ldh.
Then, PCR was performed using a genomic DNA of L-ldh gene (SEQ ID NO: 7) derived from Pelodiscus sinensis japonicus as a template and primers of SEQ ID NOs: 27 and 28. The PCR fragment thus obtained and the p416-CCW12p prepared as described above were digested with BamHI and SalI, and ligated, thereby producing p416-CCW12p-LDH, which is a L-ldh expression vector. The L-ldh expression vector had a yeast autonomous replication sequence/a yeast centrometric sequence of SEQ ID NO: 18, a CCW12 promoter of SEQ ID NO: 22, and a CYC1 terminator of SEQ ID NO: 24. The L-ldh expression vector included a polynucleotide of SEQ ID NO: 3 encoding L-ldh derived from Pelodiscus sinensis japonicus. FIG. 2 is a view illustrating a p416-CCW12p-LDH vector.
1.1.2 Preparation of Introduction Vector for L-Ldh Gene into a Genome
In order to increase production of lactate by enhancing redox balance or engineering glycolysis pathways, L-ldh was additionally introduced into a genome of a strain of KCTC12415BP. In order to introduce the L-ldh gene into the genome of KCTC12415BP, a gene-introduction vector was prepared as follows.
In this regard, PCR was performed by using the prepared p416-CCW12p-LDH as a template and primers of SEQ ID NOS: 29 and 30, the resulting PCR fragment and a pUC19-HIS3 (Appl Environ Microbiol. 2002 May; 68(5):2095-100, FIG. 3) vector prepared in advance were digested with SacI and ligated, producing a pUC19-CCW12p-LDH-HIS3 (FIG. 4). PCR was performed by using the pUC19-CCW12p-LDH-HIS3 as a template and primers of SEQ ID NOS: 31 and 32 to prepare a cassette to be inserted to a location of TRP1. The expression cassette including L-ldh may be inserted to a locus of TRP1 gene, and in this case, as the TRP1 gene is deleted, the L-ldh may be inserted to the locus. A mutation strain to which L-ldh is inserted was prepared as follows.
A KCTC12415BP strain was plated onto a YPD agar plate (including 10 g/L of yeast extract, 20 g/L of peptone, and 20 g/L of glucose, and 20 g/L of agar) and incubated for 24 hours at 30° C., and then, a colony obtained therefrom was inoculated in about 10 ml of a YPD liquid medium and cultured for about 18 hours at 30° C. The sufficiently grown culture solution was inoculated in about 50 ml of a YPD liquid medium contained in a 250 ml-flask at a concentration of 1% (v/v) and incubated in an incubator at a rate of about 230 rpm and at 30° C.
After about 4 to 5 hours, when the OD600 reached about 0.5, the culture was centrifuged at a rate of about 4,500 rpm for about 10 minutes to harvest cells, and the cells were resuspended in a lithium acetate solution at a concentration of about 100 mM. Then, the cells were harvested by performing centrifugation at a rate of about 4,500 rpm for about 10 minutes, resuspended in a lithium acetate solution at a concentration of about 1 M including about 15% of glycerol.
In order to express L-ldh at the same time deleting TRP1, the cassette including L-ldh was mixed with 50% of polyethylene glycol and a single stranded carrier DNA, added to 100 μl of the resuspended competent cell, and reacted in a water tub for about 1 hour at 42° C., and then, the culture solution was spread on a histidine-free minimal agar plate (YSD, containing 6.7 g/L of yeast nitrogen base without amino acids, 1.4 g/L of yeast synthetic drop-out without histidine (Sigma-Aldrich: Cat. no. Y1751), 20 g/L glucose, and 20 g/L of agar) and grown for about 24 hours or more at 30° C. Ten colonies (mutant strains) grown on the plate were selected, patched onto the fresh YSD (−his) minimal agar plate, and at the same time, inoculated into a YSD (−his) liquid medium to isolate the genomic DNA from the above mutant strains by using a commonly used kit (Gentra Puregene Cell kit, Qiagen, USA). In order to confirm deletion of TRP1, PCR was performed by using the genomic DNA of the isolated mutant strain as a template and primers of SEQ ID NOS: 33 and 34, and then, electrophoresis was performed on the obtained PCR product, and thus insertion of the L-ldh expression cassette was confirmed. As a result, Δtrp1::ldh strain (KCTC12415BP Δtrp1::ldh) was obtained.
1.2 Preparation of Saccharomyces cerevisiae Strain in which Nde1 and Nde2 are Deleted
1.2.1 Preparation of Nde1 Gene Deletion Cassette
In order to prepare the nde1 gene deletion cassette, PCR was performed by using the prepared pUC57-ura3HA (FIG. 5) as a template and primers of SEQ ID NOs: 35 and 36.
1.2.2 Preparation of Saccharomyces cerevisiae Strain in which Nde1 is Deleted
A mutant strain prepared by deleting nde1 from the Δtrp1::ldh strain (KCTC12415BPΔtrp1::ldh) was manufactured as follows.
A Δtrp1::ldh strain (KCTC12415BPΔtrp1::ldh) was spread on a YPD agar plate (10 g/L of yeast extract, 20 g/L of peptone, 20 g/L of glucose, and 20 g/L of agar)) and incubated for 24 hours at 30° C., and then, a colony obtained therefrom was inoculated in about 10 ml of a YPD liquid medium and cultured for about 18 hours at 30° C. The sufficiently grown culture solution was inoculated in about 50 ml of a YPD liquid medium contained in a 250 ml-flask at a concentration of 1% (v/v) and incubated in an incubator at a rate of about 230 rpm and at 30° C. After about 4 to 5 hours, when the OD600 reached about 0.5, the culture was centrifuged at a rate of about 4,500 rpm for about 10 minutes to harvest cells, and the cells were resuspended in a lithium acetate solution at a concentration of about 100 mM. Then, the cells were harvested by performing centrifugation at a rate of about 4,500 rpm for about 10 minutes, resuspended in a lithium acetate solution at a concentration of about 1 M including about 15% of glycerol.
In order to delete NDE1 gene, the prepared NDE1 gene deletion cassette was mixed with 50% of polyethylene glycol and a single stranded carrier DNA, added to 100 μl of the resuspended competent cell, and reacted in a water tub for about 1 hour at 42° C., and then, the culture solution was spread on a uracil-free minimal agar plate (YSD, containing 6.7 g/L of yeast nitrogen base without amino acids, 1.4 g/L of yeast synthetic drop-out without uracil, 20 g/L glucose, and 20 g/L of agar) and grown for about 24 hours or more at 30° C. Ten colonies (mutant strains) grown on the plate were selected, patched onto the fresh YSD (−ura) minimal agar plate, and at the same time, inoculated into a YSD (−ura) liquid medium to isolate the genomic DNA from the above mutant strains by using a commonly used kit (Gentra Puregene Cell kit, Qiagen, USA). In order to confirm deletion of nde1, PCR was performed by using the genomic DNA of the isolated mutant strain as a template and primers of SEQ ID NOS: 37 and 38, and then, electrophoresis was performed on the obtained PCR product. As a result, KCTC12415BPΔtrp1::ldhΔnde1+ura3 was obtained.
Also, in order to delete an additional gene by using the gene deletion vector, URA3 gene, which is a selection marker used for deletion of nde1, was deleted by using an URA3 pop-out method. That is, KCTC12415BPΔtrp1:ldhΔnde1+ura3 was inoculated in about 10 ml of a YPD liquid medium and cultured for about 18 hours at 30° C., was spread on a 5-FOA agar plate (YSD, containing 6.7 g/L of yeast nitrogen base without amino acids, 1.4 g/L of yeast synthetic drop-out, 20 g/L glucose, 1 μg/L of 5-fluoroorotic acid, and 20 g/L agar) and grown for about 24 hours or more at 30° C. Ten colonies (URA3 pop-out strains) grown on the 5-FOA plate were selected, patched onto the fresh 5-FOA agar plate, and at the same time, inoculated into a YPD liquid medium to isolate the genomic DNA from the above strains by using a commonly used kit (Gentra Puregene Cell kit, Qiagen, USA). In order to confirm deletion of URA3, PCR was performed by using the genomic DNA of the isolated URA3 pop-out strain as a template and primers of SEQ ID NOS: 37 and 38, and then, electrophoresis was performed on the obtained PCR product. As a result, Δnde1 strain (KCTC12415BP Δtrp1::ldhΔnde1) was obtained.
1.2.3 Preparation of Nde2 Gene Deletion Cassette
In order to prepare the nde2 gene deletion cassette, PCR was performed by using the prepared pUC57-ura3HA (FIG. 5) as a template and primers of SEQ ID NOS: 39 and 40.
1.2.4 Preparation of Saccharomyces cerevisiae Strain in which Nde1 and Nde2 are Deleted
A mutant strain prepared by deleting nde2 from the Δnde1 strain (KCTC12415BPΔtrp1::ldhΔnde1) was manufactured as follows.
A competent cell was obtained by treating the Δnde1 strain with a lithium acetate solution in the same manner as in Example 1.2.2.
In order to delete nde2 gene, the nde2 gene deletion cassette prepared in Example 1.2.3 was mixed with 50% of polyethylene glycol and a single stranded carrier DNA, added to 100 ul of the resuspended competent cell, and reacted in the same manner performed in Example 1.2.2, and thus colonies were obtained on a YSD (−ura) minimal agar plate. Ten colonies (mutant strains) grown on the plate were selected, patched onto the fresh YSD (−ura) minimal agar plate, and at the same time, inoculated into a YSD (−ura) liquid medium to isolate the genomic DNA from the above mutant strains by using a commonly used kit (Gentra Puregene Cell kit, Qiagen, USA). In order to confirm deletion of nde2, PCR was performed by using the genomic DNA of the isolated mutant strain as a template and primers of SEQ ID NOS: 41 and 42, and then, electrophoresis was performed on the obtained PCR product. As a result, KCTC12415BPΔtrp1::ldhΔnde1Δnde2+ura3 was obtained.
Also, in order to delete an additional gene by using the gene deletion vector, URA3 gene, which is a selection marker used for deletion of nde2, was deleted from Saccharomyces cerevisiae KCTC12415BP Δtrp1:ldhΔnde1Δnde2+ura3 in the same manner as in Example 1.2.2. In order to confirm deletion of URA3, PCR was performed by using a genomic DNA of the separated URA3 pop-out strain as a template and primers of SEQ ID NOs: 41 and 42, and then, electrophoresis was performed on the obtained PCR product. As a result, a Δnde1Δnde2 strain (KCTC12415BPΔtrp1::ldhΔnde1Δnde2) was obtained.
Example 2. Preparation of Saccharomyces Cerevisiae Strain in which Fps1 is Deleted
2.1 Preparation of Fps1 Gene Deletion Cassette
In order to delete fps1 gene by using a homogenous recombination method, PCR was performed by using the pUC57-ura3HA shown in FIG. 5 as a template and primers (fps1_del_F, fps1_del_R) of SEQ ID NOS: 43 and 44 to prepare a fps1 gene deletion cassette.
2.2 Preparation of Saccharomyces cerevisiae Strain in which Fps1 is Deleted
A mutant strain prepared by deleting fps1 from the Δnde1Δnde2 strain (KCTC12415BPΔtrp1::ldhΔnde1Δnde2) was manufactured as follows.
A competent cell was obtained by treating the Δnde1Δnde2 strain with a lithium acetate solution in the same manner as in Example 1.2.2.
In order to delete fps1 gene, the fps1 gene deletion cassette prepared in Example 2.1 was mixed with 50% of polyethylene glycol and a single stranded carrier DNA, added to 100 μl of the resuspended competent cell, and reacted in the same manner performed in Example 1.2.2, and thus colonies were obtained on a YSD (−ura) minimal agar plate. Ten colonies (mutant strains) grown on the plate were selected, patched onto the fresh YSD (−ura) minimal agar plate, and at the same time, inoculated into a YSD (−ura) liquid medium to isolate the genomic DNA from the above mutant strains by using a commonly used kit (Gentra Puregene Cell kit, Qiagen, USA). In order to confirm deletion of fps1, PCR was performed by using the genomic DNA of the isolated mutant strain as a template and primers (fps1_F, fps1_R) of SEQ ID NOs: 45 and 46, and then, electrophoresis was performed on the obtained PCR product. As a result, KCTC12415BPΔtrp1::ldhΔnde1Δnde2Δfps1+ura3 was obtained.
Also, in order to delete an additional gene, URA3 gene, which is a selection marker used for deletion of fps1, was deleted from Saccharomyces cerevisiae KCTC12415BPΔtrp1:ldhΔnde1Δnde2Δfps1+ura3 by using the URA3 pop-out method in the same manner as in Example 1.2.2. In order to confirm deletion of URA3, PCR was performed by using a genomic DNA of the separated URA3 pop-out strain as a template and primers of SEQ ID NOS: 45 and 46, and then, electrophoresis was performed on the obtained PCR product. As a result, a Δfps1 strain (KCTC12415BPΔtrp1::ldhΔnde1Δnde2Δfps1) was obtained.
Example 3. Measurement of Acid-Resistant Property of Strain in which Fps1 Gene is Deleted
Each of the Δnde1Δnde2 strain and the Δfps1 strain prepared in Examples 1 and 2 were spread on a YPD agar plate and grown for about 24 hours or more at 30° C., inoculated into 50 ml of YPD including about 0 to about 30 g/L of lactic acid and 40 g/L of glucose, and incubated in an anaerobic condition for about 48 hours or more at 30° C. Periodically, the culture was obtained from the flask during the culturing, and a cell concentration was measured at OD600 nm.
FIGS. 7A to 7E illustrate cell growth of the Δfps1 strain and Δnde1Δnde2 strain (C) according to a concentration of lactic acid included in the medium. As shown in FIGS. 7A to 7E, when an amount of the included lactic acid increases, an OD600 value of the Δfps1 strain is higher than that of the Δnde1Δnde2 strain (C). These results demonstrate that a Fps1 gene-deleted yeast cell allows an amount of glycerol to be increased in the cell and provides better acid-resistant property and improved growth under an acid condition compared to a yeast cell in which a Fps1 gene is not deleted.
Example 4. Production of Lactate Using a Fps1-Deleted Strain
Each of the Δnde1Δnde2 strain and the Δfps1 strain prepared in Examples 1 and 2 were spread on a YPD agar plate and grown for about 24 hours or more at 30° C., inoculated into 50 ml of a YPD liquid medium including 40 g/L of glucose, and incubated in an aerobic condition for about 16 hours or more at 30° C. Fermentation was performed by quantifying an amount of a cell concentration in 50 ml of the strain culture solution when its absorbance is 5.0 at 600 nm by using a spectrophotometer, centrifuging the quantified result, removing the supernatant, resuspending the cell, and re-inoculating the cell into 50 ml of a fresh YPD liquid medium including 80 g/L of glucose. Conditions for the fermentation included maintaining a rate of about 90 rpm of a stirring incubator for about 24 hours or more at 30° C. During the fermentation, samples were periodically obtained from the flask, and the obtained samples were centrifuged at a rate of about 13,000 rpm, and then, a concentration of lactate of the supernatant was analyzed by using a liquid chromatography (HPLC).
As shown in Table 1, an OD600 value of the Δfps1 strain was greater than that of the Δnde1Δnde2 strain (15.22 vs. 15.62); L-lactate producing capability of the Δfps1 strain was increased compared to the Δnde1Δnde2 strain (33.9 g/L vs 35.3 g/L); and yield of the Δfps1 strain was increased compared to the Δnde1Δnde2 strain (43.4% vs. 46.6%). Thus, lactate production and yield may increase in a fps1 gene-deleted strain. Also, the fps1 gene-deleted strain may have an improved metabolic pathway as well as a high cell growth rate in a high lactate concentration, and thus lactate production and yield may increase in the strain.











TABLE 1





Strain
Characteristic
OD600
LA (g/L)
Yield (%)







Δnde1Δnde2
KCTC12415BPΔtrp1::ldhΔnde1Δnde2
15.22
33.9
43.4


Δfps1 strain
KCTC12415BPΔtrp1::ldhΔnde1Δnde2Δfps1
15.62
35.3
46.6





50 ml flask, culturing time: about 25 hours, and LA: lactate






Accession Number: KCTC 12415BP was deposited at the Korean Collection for Type Cultures (KTCT) on May 30, 2013.
It should be understood that the exemplary embodiments described therein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each embodiment should typically be considered as available for other similar features or aspects in other embodiments.
While one or more embodiments of the present invention have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.
All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.
The use of the terms “a” and “an” and “the” and “at least one” and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The use of the term “at least one” followed by a list of one or more items (for example, “at least one of A and B”) is to be construed to mean one item selected from the listed items (A or B) or any combination of two or more of the listed items (A and B), unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.
Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

## Claims

1. A lactic acid-resistant yeast cell comprising a genetic modification that reduces aquaglyceroporin (Fps1) activity in the acid-resistant yeast cell compared to that of a parent cell without the genetic modification wherein the yeast cell comprises a polypeptide that converts pyruvate to lactic acid.

2. The acid-resistant yeast cell of claim 1, wherein the genetic modification is a deletion or disruption mutation of a polynucleotide that encodes the Fps1.

3. The acid-resistant yeast cell of claim 1, wherein the Fps1 is a polypeptide having at least 95% sequence identity with SEQ ID NO: 1.

4. The acid-resistant yeast cell of claim 1, wherein the polynucleotide that encodes the Fps1 has a polynucleotide sequence that encodes an amino acid sequence having at least 95% sequence identity with SEQ ID NO: 1, or that has at least 95% sequence identity to SEQ ID NO: 2.

5. The acid-resistant yeast cell of claim 1, wherein the acid-resistant yeast cell is resistant under a pH condition of about 2 to about 4.5.

6. The acid-resistant yeast cell of claim 1, wherein the yeast cell is Saccharomyces genus, Kluyveromyces genus, Candida genus, Pichia genus, Issatchenkia genus, Debaryomyces genus, Zygosaccharomyces genus, Shizosaccharomyces genus, or Saccharomycopsis genus.

7. The acid-resistant yeast cell of claim 1, wherein the yeast cell is Saccharomyces cerevisiae.

8. The acid-resistant yeast cell of claim 1, wherein the yeast cell comprises a polynucleotide that encodes a lactate dehydrogenase.

9. The acid-resistant yeast cell of claim 1, wherein the polypeptide that converts pyruvate to lactate has at least 95% sequence identity with SEQ ID NO: 3.

10. The acid-resistant yeast cell of claim 8, wherein expression or activity of the lactate dehydrogenase is increased in the acid-resistant yeast cell compared to a parent cell.

11. The acid-resistant yeast cell of claim 1, wherein activity of a polypeptide that converts pyruvate to acetaldehyde, a polypeptide that converts lactate to pyruvate, a polypeptide that converts dehydroxyacetone phosphate (DHAP) to glycerol-3-phosphate, an external mitochondrial NADH dehydrogenase, or a combination thereof is reduced in the acid-resistant yeast cell compared to a parent cell.

12. The acid-resistant yeast cell of claim 11, wherein the yeast cell comprises an exogenous polynucleotide encoding lactate dehydrogenase.

13. The acid-resistant yeast cell of claim 1, wherein the acid-resistant yeast cell comprises a deletion or disruption mutation of a gene encoding a polypeptide that converts pyruvate to acetaldehyde, a gene encoding a polypeptide that converts lactate to pyruvate, a gene encoding a polypeptide that converts dehydroxyacetone phosphate (DHAP) to glycerol-3-phosphate, a gene encoding an external mitochondrial NADH dehydrogenase, or a combination thereof.

14. The acid-resistant yeast cell of claim 11, wherein the polypeptide that converts pyruvate to acetaldehyde, the polypeptide that converts lactate to pyruvate, the polypeptide that converts dehydroxyacetone phosphate (DHAP) to glycerol-3-phosphate, and the external mitochondrial NADH dehydrogenase each comprise an amino acid sequence having at least 95% of sequence identity with SEQ ID NO: 8, 10, 12, 14, or 16, respectively.

15. A method of producing lactate, wherein the method comprises culturing the acid-resistant yeast cell of claim 1 in a cell culture medium, whereby the yeast cell produces lactate.

16. The method of claim 15, wherein the method further comprises collecting lactate from the cell culture medium.

17. The method of claim 15, wherein the culturing of the yeast cell is performed under a microaerobic or anaerobic condition.

18. The method of claim 15, wherein the culturing of the yeast cell is performed under a pH condition of about 2 to about 7.

19. A method of increasing the acid-resistance of a yeast cell, the method comprising deleting or disrupting expression of a polynucleotide that encodes the Fps1 in a yeast cell to increase the acid-resistance of the yeast cell.

20. The acid-resistant yeast cell of claim 1, wherein the cell produces more lactic acid compared to the parent cell.