Patent Application
20100159541
The present invention relates to a gene involved in foam formation during microorganism culture, and further relates to an acetic acid bacterium which is bred by reducing or deleting the function of a protein encoded by the gene involved in foam formation and which thus has a suppressed foaming ability during culture and generates a larger amount of acetic acid, a method for producing vinegar using the acetic acid bacterium, and vinegar produced by the production method.
Foam formation during microorganism culture is a major concern in food industry and chemical industry where microorganisms are utilized. In many cases, foam is generated when microorganisms are cultured, especially when cultured under aeration and agitation. A foam layer is formed at the upper part of the culture tank and raises problems such as reduction of the working volume in the culture tank, or loss of the culture broth, compositional change of the broth and leakage of the microorganisms due to the outflow of the foam from the upper part of the tank. As such, foam formation causes problems including decrease in the production efficiency and deterioration in quality, and environmental problems. For this reason, it has been a critical object to suppress foam formation when obtaining products by an efficient culture of microorganisms. Decrease in the production efficiency due to foam formation has similarly been a problem as well in the vinegar production using an acetic acid bacterium.
Therefore, physical and chemical methods have been developed as a defoaming method. As a physical method, there has been methods including a mechanical method in which a shear force is applied to foam by such as an agitating blade to destroy the foam, a thermal method in which a liquid viscosity is decreased by heating to destabilize the foam, and an electric method in which foam is broken by such as energization, sparking and electric current. All of these methods, however, raise cost for introducing and using the equipments. In addition, defoaming effects brought by these methods have been insufficient.
Further, as a chemical method, there is a method in which an antifoaming agent is added. Compounds such as alcohols, esters, fatty acids and silicon are used as an antifoaming agent. However, while a defoaming method using an antifoaming agent is simple, there have been problems caused by some antifoaming agents such as decrease in the oxygen transfer rate which is important for microorganism growth and the material production, inhibition of the microorganism growth, and adverse influence on the isolation and purification steps.
On the other hand, although the foam formation mechanism during microorganism culture remains largely unknown, some genes and proteins involved in foam formation in eukaryotes have been found. One of those genes is the awal gene which is involved in foam formation and found in the yeast (see for example, Non-patent document 1). This gene encodes the glycosylphosphatidylinositol anchor protein which is a protein specific to eukaryotes and this protein is involved in the cell surface hydrophobicity, and foam formation is suppressed when the gene encoding this protein is disrupted.
Further, a protein called hydrophobin which is either hydrophobic or amphipathic was found in fungi, mushrooms, etc., and it has been found that foam formation is suppressed by disrupting a gene encoding the hydrophobin (see for example, Patent document 1).
In prokaryotes, however, the current situation was that the knowledge of genes or proteins involved in foam formation during culture had scarcely been obtained. For prokaryotes therefore, breeding of bacterial strains with less foam formation had been desired as a novel defoaming means replacing physical or chemical methods.
On the other hand, the presence of an intercellular signal communication system in which transcription of specific genes is controlled depending on the cell density has been recently elucidated in many bacteria. This system is called quorum-sensing system (a control system sensing a cell density) and is involved in the expression control for various functions such as bioluminescence, exoenzyme production, virulence expression, biofilm formation, and antibiotic production.
For example, two kinds of proteins are involved in the quorum-sensing system which has been found in many Gram negative bacteria such as Vibrio fischeri (see for example, Non-Patent document 2). These proteins are an acyl homoserine lactone synthase that synthesizes acyl homoserine lactone which is an intracellular signal molecule, and an acyl homoserine lactone receptor-type transcription factor that is a receptor of acyl homoserine lactone and that also functions as a transcription factor.
Acyl homoserine lactone produced by an acyl homoserine lactone synthase in a bacterial cell diffuses inside and outside the bacterial cell. As the concentration of acyl homoserine lactone is increased, it forms a complex with the acyl homoserine lactone receptor-type transcription factor in a bacterial cell to control the gene transcription.
The present inventor has already obtained two kinds of genes involved in the quorum-sensing system in an acetic acid bacterium, namely, a gene encoding the acyl homoserine lactone synthase (hereinafter may be referred to as orf1) and a gene encoding the acyl homoserine lactone receptor-type transcription factor (hereinafter may be referred to as orf2) and has demonstrated that the quorum-sensing system in an acetic acid bacterium is involved in the acetic acid production ability (see for example, Patent document 2).
Further, despite that the nexus between the quorum-sensing system and the foam formation during microorganism culture had been totally unknown, the present inventor obtained a gene involved in foam formation during culture of an acetic acid bacterium (hereinafter may be referred to as orf3) as a gene involved in the quorum-sensing system of an acetic acid bacterium, modified the gene, and demonstrated that foam formation during culture of an acetic acid bacterium can be suppressed as well as the ability of an acetic acid bacterium to produce acetic acid can be increased by reducing or deleting the function of a protein encoded by the gene (see for example, Patent document 3).
However, the whole picture of a family of genes involved in the quorum-sensing system of an acetic acid bacterium has not yet been sufficiently elucidated and it is possible that there are genes involved in foam formation during culture of an acetic acid bacterium other than the above genes. It has thus been expected that the gene can more strongly improve the acetic acid production ability.
Non-Patent Document 1: Journal of bioscience and bioengineering, Vol. 97, No. 1, pp. 14-18, 2004
Patent Document 1: Published Japanese translation of PCT international publication No. 2003-507056
The objects of the present invention are to provide a method for suppressing foaming ability of an acetic acid bacterium during culture by obtaining a novel gene involved in foam formation during culture of an acetic acid bacterium and reducing or deleting the function of a protein encoded by the gene, and further to provide a method for more efficiently producing vinegar containing a high concentration of acetic acid by using the acetic acid bacterium with suppressed foaming ability which is bred by the method, and vinegar produced by said production method.
The present inventor conducted a comprehensive microarray analysis on gene expression for the wild-type strain and a disrupted strain of the orf1 gene encoding the acyl homoserine lactone synthase which is one of the components of the quorum-sensing system (hereinafter may be referred to as orf1-disrupted strain). Consequently, the present inventor found a novel gene glyT, the transcript amount of which is decreased in the orf1-disrupted strain as compared to the wild-type strain, meaning that this novel gene glyT is thought to be under the control of the quorum-sensing system.
The present inventor then modified the glyT, obtained an acetic acid bacterium with significantly suppressed foam formation and significantly enhanced acetic acid fermentation ability by reducing or deleting the function of a protein encoded by glyT, and found that vinegar containing a high concentration of acetic acid can be more efficiently produced with the use of this acetic acid bacterium. The present invention was thus completed.
The present invention relates to the following.
(1) A protein shown by following (A), (B) or (C):
(A) a protein consisting of the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing;
(B) a protein which consists of an amino acid sequence wherein one or a few amino acids are substituted, deleted, inserted or added in the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing, and which is involved in foam formation during culture of an acetic acid bacterium;
(C) a protein which consists of an amino acid sequence having at least 85% or more identity to the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing, and which is involved in foam formation during culture of an acetic acid bacterium.
(2) A DNA encoding a protein shown by following (A), (B) or (C):
(A) a protein consisting of the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing;
(B) a protein which consists of an amino acid sequence wherein one or a few amino acids are substituted, deleted, inserted or added in the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing, and which is involved in foam formation during culture of an acetic acid bacterium;
(C) a protein which consists of an amino acid sequence having at least 85% or more identity to the amino acid sequence shown by SEQ ID NO: 2 in the sequence listing, and which is involved in foam formation during culture of an acetic acid bacterium.
(3) A DNA shown by following (A), (B), (C) or (D):
(A) a DNA consisting of the nucleotide sequence of nucleotide numbers 846 to 3794 in the nucleotide sequence shown by SEQ ID NO: 1 in the sequence listing;
(B) a DNA which hybridizes under stringent conditions to a DNA consisting of a sequence complementary to the nucleotide sequence of nucleotide numbers 846 to 3794 in the nucleotide sequence shown by SEQ ID NO: 1 in the sequence listing, and which encodes a protein involved in foam formation during culture of an acetic acid bacterium;
(C) a DNA which hybridizes under stringent conditions to a DNA consisting of a nucleotide sequence produced from a part of the nucleotide sequence of nucleotide numbers 846 to 3794 in the nucleotide sequence shown by SEQ ID NO: 1 in the sequence listing and having the function as a primer or a probe, and which encodes a protein involved in foam formation during culture of an acetic acid bacterium;
(D) a DNA which consists of a nucleotide sequence wherein one or a few nucleotides are substituted, deleted, inserted or added in the nucleotide sequence of nucleotide numbers 846 to 3794 in the nucleotide sequence shown by SEQ ID NO: 1 in the sequence listing, and which encodes a protein involved in foam formation during culture of an acetic acid bacterium.
(4) A method for producing an acetic acid bacterium with suppressed foaming ability, wherein the function of a protein which is encoded by the DNA according to (2) or (3) is reduced or deleted.
(5) An acetic acid bacterium with suppressed foaming ability which is obtained by the method according to (4).
(6) The acetic acid bacterium with suppressed foaming ability according to (5), wherein the acetic acid bacterium is Gluconacetobacter intermedius NCI1051ΔglyT (FERM BP-11068).
(7) A method for producing vinegar comprising culturing the acetic acid bacterium according to (5) or (6) in an alcohol-containing medium, and generating and accumulating acetic acid in the medium.
According to the present invention, a gene involved in foam formation during culture of an acetic acid bacterium and a protein encoded by the gene are provided. Further provided is a method for significantly suppressing foam formation during culture of an acetic acid bacterium by reducing or deleting the function of a protein encoded by the gene.
Further, according to the present invention, there is provided a method wherein vinegar containing a high concentration of acetic acid is more efficiently produced by significantly suppressing foam formation during culture, and vinegar containing a high concentration of acetic acid produced by such production method is also provided.
The present invention is explained in detail in the following.
A protein of the present invention is a protein involved in the acetic acid fermentation ability and foam formation. Specifically, it relates to a protein consisting of the amino acid sequence shown by SEQ ID NO: 2 (
The method of obtaining and preparing a protein of the present invention is not particularly limited and the protein may be any of an isolated naturally-occurring protein, chemically synthesized protein, or a recombinant protein prepared by a gene recombination technique. When obtaining a naturally-occurring protein of the present invention, the protein can be obtained from the cells expressing it by appropriately combining isolation and purification methods for proteins.
When preparing a protein of the present invention by chemical synthesis, the protein of the present invention can be synthesized according to a chemical synthesis method such as Fmoc method (fluorenylmethyloxycarbonyl method), tBOC method (t-butyloxycarbonyl method) or the like. A protein of the present invention can also be synthesized by utilizing various commercially-available peptide synthesizers based on the amino acid sequence information.
Further, when preparing a protein of the present invention by a gene recombination technique, a protein of the present invention can be prepared by introducing a DNA encoding the protein into a suitable expression system. Among these methods, it is preferred to prepare a protein of the present invention by a gene recombination technique which enables the preparation with a relatively easy operation at a large quantity.
When preparing a protein of the present invention by a gene recombination technique, known methods including anion- or cation-exchange chromatography; phosphocellulose chromatography; hydrophobic interaction chromatography; affinity chromatography; hydroxyapatite chromatography; and lectin chromatography may be employed after performing ammonium sulfate or ethanol precipitation and acid extraction to recover and purify the protein from the cell culture, where a high-speed liquid chromatography is preferably employed.
Particularly, the purified products of these proteins can be obtained with affinity chromatography using a column to which an antibody such as a monoclonal antibody against a protein of the present invention is bound, or a column to which a substance having affinity to the peptide tag is bound when a usual peptide tag has been added to a protein of the present invention.
Further, a protein consisting of an amino acid sequence wherein one or a few amino acids are substituted, deleted, inserted or added in the amino acid sequence shown by SEQ ID NO: 2 (
For example, a homologue of the DNA can be isolated from acetic acid bacteria belonging to the genus Acetobacter or the genus Gluconacetobacter, or from other acetic acid bacteria by screening a homologue of the DNA under appropriate conditions by a polymerase chain reaction (PCR reaction) that uses as a primer an oligonucleotide synthesized based on the nucleotide sequence shown by SEQ ID NO: 1 (
An oligonucleotide can be synthesized according to a conventional method using, for example, various commercially-available DNA synthesizers. Further, a PCR reaction can be performed according to a conventional method using a thermal cycler, Gene Amp PCR System 9700 manufactured byAppliedBiosystems, with the use of TaqDNA polymerase (Takara Bio Inc.) or KOD-Plus (Toyobo Co., Ltd.).
It is also possible to bind the above protein of the present invention with a marker protein and/or a peptide tag to provide a fusion protein. The marker protein is not particularly limited as long as it is a conventionally known marker protein. Specific examples of the marker protein include enzymes such as alkaline phosphatase and HRP, the Fc region of an antibody, and a fluorescent material such as GFP. Further, specific examples of the peptide tag include conventionally known peptide tags including epitope tags such as HA, FLAG, Myc; and affinity tags such as GST, maltose-binding protein, biotinated peptide, and oligohistidine. The fusion protein can be produced by a common method, and is useful for a purification of a protein of the present invention, a detection of a protein of the present invention and a quantitative determination of an antibody against a protein of the present invention by utilizing affinity between Ni-NTA and His tag. The fusion protein is also useful as a laboratory reagent in the field to which the present invention pertains.
A protein of the present invention can be confirmed to be a protein involved in foam formation during culture of an acetic acid bacterium by, for example, producing an acetic acid bacterium in which the function of the protein is reduced or deleted, culturing the acetic acid bacterium in a liquid medium containing alcohol under an aerobic condition, and then comparing the degree of foam formation during culture to that of the wild-type strain. An acetic acid bacterium in which the function of a protein of the present invention is reduced or deleted can be produced by transforming an acetic acid bacterium with a DNA comprising a modified gene which has been modified so as not to produce a normally functioning protein by, for example, deleting a partial sequence of a gene encoding the protein or by inserting a drug-resistant gene into the gene, then by disrupting a normal gene on the chromosome with the modified gene by a homologous recombination.
Further, examples of a DNA of the present invention include: a DNA encoding a protein consisting of the amino acid sequence shown by SEQ ID NO: 2 (
As above, a DNA encoding a protein involved in foam formation during culture of an acetic acid bacterium of the present invention may be a DNA that encodes a protein having deletion, substitution, insertion or addition of one or a few amino acids at a single site or plural sites, as long as the function of the encoded protein is not impaired.
A DNA encoding a protein that is substantially the same as a protein having the function of a protein involved in foam formation during culture of an acetic acid bacterium can be obtained by alteration of a nucleotide sequence, such modification being a deletion, substitution, insertion, addition, or inversion of an amino acid at a particular site by means of, for example, site-directed mutagenesis. Further, an altered DNA such as the above can also be obtained by a conventionally known mutagenic treatment. Still further, it is possible to obtain a DNA that encodes substantially the same protein from acetic acid bacteria in general, or particularly from species, strains, mutants or variants of the genus Acetobacter or the genus Gluconacetobacter, because it is generally known that an amino acid sequence of a protein and a nucleotide sequence encoding the same are slightly different among the species, strains, mutants or variants.
The above “amino acid sequence wherein one or a few amino acids are substituted, deleted, inserted or added” means, for example, an amino acid sequence wherein any number of amino acids, for example, 1 to 20, preferably 1 to 15, more preferably 1 to 10, and still more preferably 1 to 5 amino acids, are substituted, deleted, inserted or added.
Further, the above “nucleotide sequence wherein one or a few nucleotides are substituted, deleted, inserted or added” means, for example, a nucleotide sequence wherein any number of nucleotides, for example, 1 to 20, preferably 1 to 15, more preferably 1 to 10, and still more preferably 1 to 5 nucleotides are substituted, deleted, inserted or added.
For example, these DNAs consisting of a nucleotide sequence comprising a substitution, deletion, insertion, or addition of one or a few nucleotides (mutated DNAs) can also be prepared by any method known to a skilled person in the art, such as chemical synthesis, genetic engineering technique, and mutagenesis, as stated above. Specifically, mutated DNAs can be obtained by introducing a mutation into the DNA consisting of the nucleotide sequence shown by SEQ ID NO: 1 (
The site-directed mutagenesis, one of the genetic engineering techniques, is useful as it is a technique that allows an introduction of a specific mutation into a specific site, and can be performed according to the method described in Molecular Cloning: A laboratory Manual, 2nd Ed., Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., 1989, or in Current Protocols in Molecular Biology, Supplement 1-38, John Wiley & Sons (1987-1997), or the like. An expression of this mutated DNA using an appropriate expression system provides a protein consisting of an amino acid sequence comprising a substitution, deletion, insertion or addition of one or a few amino acids.
The above “amino acid sequence having at least 85% or more identity to the amino acid sequence shown by SEQ ID NO: 2 (
The above “under stringent conditions” refers to a condition under which a so-called specific hybrid is formed while a non-specific hybrid is not formed. Specific examples include a condition under which DNAs sharing 50% or more, preferably 70% or more identity hybridize each other, while DNAs with the lower identity do not hybridize; or a hybridization condition at a salt concentration corresponding to 1×SSC (1-fold-concentration SSC solution comprises 150 mM NaCl and 15 mM sodium citrate) and 0.1% SDS, or 0.1×SSC and 0.1% SDS at 65° C., which are washing conditions for a usual southern hybridization.
Further, the above “DNA that hybridizes under stringent conditions” means a DNA that can be obtained by using a method such as a colony hybridization, plaque hybridization or southern-blot hybridization using nucleic acids such as DNA or RNA as a probe. Specifically exemplified is a DNA that can be identified by conducting a hybridization at 65° C. in the presence of 0.7 to 1.0 M NaCl using a filter on which a colony- or plaque-derived DNA or a fragment thereof is immobilized, and washing the filter under the condition of 65° C. with about 0.1 to 2-fold-concentration SSC solution.
Hybridization can be performed according to the method described in Molecular Cloning, 2nd Ed. or the like. Examples of the DNA that can hybridize under stringent conditions include a DNA having an identity above a certain level to a nucleotide sequence of a DNA used as a probe. A DNA having an identity of, for example, 60% or more, preferably 70% or more, more preferably 80% or more, still more preferably 90% or more, particularly preferably 95% or more, and most preferably 98% or more, can be exemplified advantageously.
A method of obtaining or preparing a DNA of the present invention is not particularly limited. The DNA of interest can be isolated by preparing an appropriate probe or primer based on the nucleotide sequence information shown by SEQ ID NO: 1 (
A DNA of the present invention can be obtained, for example, by preparing a cDNA library according to a common method from acetic acid bacteria belonging to the genus Acetobacter or the genus Gluconacetobacter, and then selecting from this library the desired clone by using an appropriate probe specific to a DNA of the present invention. Further, isolation of total RNA, isolation and purification of mRNAs, acquisition and cloning of cDNAs and the like from these acetic acid bacteria can all be performed according to a common method. Examples of the method of screening a DNA of the present invention from a cDNA library include the methods commonly used by a skilled person in the art such as a method described in Molecular Cloning, 2nd ed.
Since the nucleotide sequence of a DNA of the present invention has already been elucidated, the DNA can be obtained by a PCR reaction using the oligonucleotide synthesized based on the nucleotide sequence as a primer or by hybridization using the oligonucleotide synthesized based on the nucleotide sequence as a probe, with the use of, for example, the genomic DNA of the acetic acid bacterium, Gluconacetobacter intermedius NCI1051 (FERM BP-10767), as a template. The chromosomal DNA can be obtained by a common method (e.g., a method disclosed in Japanese Laid-Open Patent Application No. 60-9489).
An oligonucleotide can be synthesized according to a common method using, for example, various commercially-available DNA synthesizers. Further, a PCR reaction can be performed according to a common method using a thermal cycler, Gene Amp PCR System 9700 manufactured by Applied Biosystems, with the use of TaqDNA polymerase (Takara Bio Inc.), KOD-Plus (Toyobo Co., Ltd.), etc.
A DNA of the present invention can be obtained by alteration of a nucleotide sequence, such alteration being a deletion, substitution, insertion or addition of amino acids at a specific site by means of, for example, site-directed mutagenesis. Further, an altered DNA such as the above can also be obtained by a conventionally known mutagenic treatment.
Still further, it is possible to obtain a DNA that encodes substantially the same protein from general acetic acid bacteria, particularly from species, strains, mutants or variants of the genus Acetobacter or the genus Gluconacetobacter, because it is generally known that an amino acid sequence of a protein and a nucleotide sequence encoding the same are slightly different among the species, strains, mutants or variants.
It is also possible to obtain a DNA encoding a protein that is substantially the same as the above protein, specifically by isolating a DNA which hybridizes under stringent conditions to a DNA comprising a nucleotide sequence described in the nucleotide sequence shown by SEQ ID NO: 1 (FIG. 4) in the sequence listing and which encodes a protein involved in foam formation during culture of an acetic acid bacterium from acetic acid bacteria belonging to the genus Acetobacter or the genus Gluconacetobacter, or mutated acetic acid bacteria belonging to the genus Acetobacter or the genus Gluconacetobacter, or from spontaneous mutants or variants thereof.
A mutant gene or homologous gene of the present invention consisting of a DNA encoding a protein consisting of an amino acid sequence wherein one or a few amino acids are substituted, deleted, inserted or added in the amino acid sequence shown by SEQ ID NO: 2 (
It is also possible to obtain a DNA encoding a protein that is substantially the same as the above protein by isolating a DNA which hybridizes under stringent conditions to a probe prepared from the nucleotide sequence shown by SEQ ID NO: 1 (
An acetic acid bacterium for use in the present invention is not particularly limited and is exemplified by a bacterium belonging to such as the genus Acetobacter or the genus Gluconacetobacter having alcohol oxidation ability, and the specific examples include the following.
Namely, examples of the acetic acid bacterium belonging to the genus Gluconacetobacter include Gluconacetobacter intermedius, Gluconacetobacter xylinus, Gluconacetobacter europaeus, Gluconacetobacter diazotrophicus and Gluconacetobacter entanii, and more specifically include Gluconacetobacter xylinus IFO3288, Gluconacetobacter europaeus DSM6160, Gluconacetobacter diazotrophicus ATCC49037, Acetobacter altoacetigenes MH-24, and Gluconacetobacter intermedius NCI1051 (FERM BP-10767).
Further, examples of the acetic acid bacterium belonging to the genus Acetobacter include Acetobacter aceti, and more specifically include Acetobacter aceti No. 1023 and Acetobacter aceti IFO3283.
A DNA of the present invention can be confirmed to be a DNA encoding a protein involved in foam formation during culture of an acetic acid bacterium by, for example, producing an acetic acid bacterium in which the DNA is deficient, culturing the acetic acid bacterium in a liquid medium containing alcohol under an aerobic condition, and then comparing the degree of foam formation during culture to that of the wild-type strain. An acetic acid bacterium in which the DNA of the present invention is deficient can be produced by transforming an acetic acid bacterium with a DNA comprising a modified gene prepared by, for example, deleting a partial sequence of the DNA or by inserting a drug-resistant gene into the DNA sequence, and disrupting a normal gene on the chromosome with the modified gene by a homologous recombination.
The method of the present invention for suppressing foam formation during culture of an acetic acid bacterium by reducing or deleting the function of a protein encoded by the gene involved in foam formation during culture of an acetic acid bacterium (i.e., a gene encoding a protein involved in foam formation during culture of an acetic acid bacterium) includes a method which comprises culturing an acetic acid bacterium under the physical conditions that inhibits the expression of the gene encoding a protein involved in foam formation during culture of the acetic acid bacterium or the activity of a protein encoded by the gene.
It is also effective to reduce or delete the function by modifying a gene involved in foam formation during culture of an acetic acid bacterium. It is also effective to induce a mutation in a region of the gene where it is involved in the expression of the gene so as to inhibit the expression thereof. As a method to modify a gene, a method in which mutation is induced to the gene by a physical treatment or by using a chemical mutagenic agent is effective. As these methods for inducing mutation, methods conventionally practiced for acetic acid bacteria are effective. Examples of such conventional method include a method for inducing mutation by subjecting an acetic acid bacterium to an ultraviolet irradiation or treating an acetic acid bacterium with N-methyl-N′-nitro-N-nitrosoguanidine (NTG) or with a mutagenic agent usually used for mutagenic treatment such as nitrous acid.
Since acetic acid bacteria are known as bacteria that tend to mutate spontaneously, an acetic acid bacterium with suppressed foam formation during culture of an acetic acid bacterium can also be obtained by isolating from the nature an acetic acid bacterium having a gene in which expression or function of the above enzyme has been spontaneously mutated. Further, since these genes have already been obtained and the nucleotide sequences thereof have also been elucidated, it is also effective to subject these genes to recombination for introducing mutagenesis, then to introduce the mutated gene into the original acetic acid bacterium, and to reduce or delete the function of the gene of the original acetic acid bacterium by employing such as a homologous recombination.
For example, a method is effective wherein the method comprises: transforming an acetic acid bacterium with a DNA comprising a modified gene that has been modified so as not to produce a normally functioning protein involved in foam formation during culture of an acetic acid bacterium by deleting a partial sequence of the gene or by inserting a drug-resistant gene into the gene; and disrupting a normal gene on the chromosome with the modified gene by a homologous recombination.
Further, when a gene involved in foam formation during culture of an acetic acid bacterium of the present invention is controlled by the quorum-sensing system, it is also possible to reduce or delete the function of a protein encoded by the gene involved in foam formation during culture of an acetic acid bacterium by reducing or deleting the function of the quorum-sensing system. For example, an acetic acid bacterium with suppressed foam formation during culture of an acetic acid bacterium wherein the function of the acyl homoserine lactone synthase and/or the acyl homoserine lactone receptor-type transcription factor is reduced or deleted can be prepared by disrupting the gene of acyl homoserine lactone synthase (orf1) and/or the gene of acyl homoserine lactone receptor-type transcription factor (orf2) .
Transformation of an acetic acid bacterium may be conducted by such as a calcium chloride method (see for example, Agric. Biol. Chem., Vol. 49, p. 2091, 1985), and an electroporation method (see for example, Proc. Natl. Acad. Sci. U.S.A., Vol. 87, pp. 8130-8134, 1990).
As above, foam formation during culture of an acetic acid bacterium can be suppressed in an acetic acid bacterium belonging to the genus Acetobacter or the genus Gluconacetobacter having the alcohol oxidation ability by altering a protein involved in foam formation during culture of an acetic acid bacterium by reducing or deleting the function as described above so that the protein does not function normally.
The acetic acid bacterium with suppressed foam formation during culture of an acetic acid bacterium of the present invention is exemplified by Gluconacetobacter intermedius NCI1051ΔglyT (FERM BP-11068) .
A conventionally known method is employed for the method of producing vinegar of the present invention except that the function of a protein involved in foam formation during culture of an acetic acid bacterium is reduced or deleted so that the protein does not function normally, and culturing the acetic acid bacterium in an alcohol-containing medium to generate and accumulate acetic acid in the medium. Namely, an acetic acid bacterium may be cultured basically under the conditions where acetic acid fermentation can be performed. Specifically, the culture of an acetic acid bacterium may be conducted similarly to that in a production method of vinegar that employs a conventional method for acetic acid fermentation.
As for an alcohol-containing medium, any medium suffices as long as it is a medium used for acetic acid fermentation. Those alcohol-containing media may be used that contain a carbon source, nitrogen source, inorganic substance, etc. other than an alcoholic component such as ethanol, and contain an appropriate amount of nutrient source required for growth of a bacterial strain in use, if necessary. A medium may be either a synthetic medium or a natural medium. Examples of the carbon source include various carbohydrates including glucose and sucrose, and various organic acids. As a nitrogen source, a natural nitrogen source such as peptone, degradation product of microbial cells or the like can be used.
Further, the culture is performed under an aerobic condition such as in a static culture, shaking culture, aeration-agitation culture and the like. The culture temperature is usually in the range of 25 to 35° C., preferably at 30° C. The pH of medium is generally within the range of 2.5 to 7, preferably within the range of 2.7 to 6.5, and the pH can also be adjusted with various acids, various bases, buffers or the like. Generally, a 1- to 21-day culture can accumulate a high concentration of acetic acid in the medium. A high acidity vinegar with suppressed foam formation during culture of an acetic acid bacterium can be produced more efficiently by the above method for producing vinegar of the present invention.
The present invention is specifically explained in the following with reference to the examples. The technical scope of the present invention, however, will not be limited to these exemplifications.
The present inventor has so far demonstrated that the quorum-sensing system is controlling foam formation during culture of an acetic acid bacterium, but it has been unknown as to what gene is controlled by the quorum-sensing system and what gene is involved in foam formation during culture of an acetic acid bacterium. Therefore, a microarray analysis was conducted to identify a gene which is under the control of the quorum-sensing system and to search a gene involved in foam formation during culture of an acetic acid bacterium.
Specifically, RNA was extracted from Gluconacetobacter intermedius NCI1051 (hereinafter may be referred to as wild-type strain) and Gluconacetobacter intermedius NCI1051Δorf1 (hereinafter may be referred to as orf1-disrupted strain), and the expression levels thereof were compared by microarray analysis employing a unicolor method (see for example, “Lab Manual, DNA chip and realtime PCR, Kodansha Limited, 2005”). Here, Gluconacetobacter intermedius NCI1051 is deposited under the Budapest Treaty with the International Patent Organism Depositary at the National Institute of Advanced Industrial Science and Technology (Central 6, 1-1 Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan) under the Accession Number FERM BP-10767, and Gluconacetobacter intermedius NCI1051Δorf1, in which a gene encoding the acyl homoserine lactone synthase (orf1) is disrupted, that is one of the components of the quorum-sensing system, is deposited under the Budapest Treaty with the same depositary under the Accession Number FERM BP-10768.
First, the wild-type strain and the orf1-disrupted strain were cultured at 30° C. and 120 spm by a shaking culture in a medium (100 ml) containing 20 ethanol, 3% glucose, 0.5% yeast extract, 0.3% polypeptone, and 1% Celluclast 1.5 L (Novozymes) using a Sakaguchi flask of 500 ml volume. RNA was extracted using RNeasy (QIAGEN) from the culture broth of at the logarithmic phase and the early stationary phase of the both strains. Quality of the RNA was confirmed by Bioanalyzer (Agilent).
Each RNA thus extracted was used as a template to synthesize cDNA using a reverse transcriptase and random primers, and the terminal end of the cDNA was labeled with biotin. On the other hand, a microarray arranged with specific probes corresponding to all the genes on the genome was designed and generated. Subsequently, the biotin-labeled cDNA synthesized beforehand was hybridized to the microarray according to a common method and the expression levels of all the genes were quantified. Hybridization was conducted for each of a sample derived from the wild-type strain and a sample derived from the orf1-disrupted strain. The expression data were normalized using Robust Multi-chip Analysis (RMA) and employed as the expression intensity of each gene.
As a result of comparison of the expression intensity obtained by the above-mentioned microarray analysis for each gene of the wild-type strain and the orf1-disrupted strain, there was found a gene in which the transcript amount is significantly decreased in the orf1-disrupted strain as compared to the wild-type strain. This gene (hereinafter may be referred to as glyT) had the nucleotide sequence of nucleotide numbers 846 to 3794 in SEQ ID NO: 1 (
As a result of the homology search by BLAST (http://blast.ncbi.nlm.nih.gov/Blast.cgi), the protein encoded by glyT showed 44% identity at the amino acid level to each of the proteins presumed to be glycosyltransferase in Gluconacetobacter diazotrophicus PALS and Gluconobacter oxydans 621H. However, function of these proteins has not been analyzed at all so far, and it has been totally unknown that the proteins are involved in foam formation during culture nor that the proteins are involved in the acetic acid fermentation ability of acetic acid bacteria.
Further, transcript analysis was conducted for glyT by S1 nuclease mapping method (see for example, Journal of bacteriology, Vol. 180, pp. 2515-2521, 1988) in order to confirm the results of the microarray analysis. The result confirmed that the transcript amount of glyT was significantly decreased in the orf1-disrupted strain as compared to the wild-type strain, and it was confirmed that glyT is a gene which is under the control of the quorum-sensing system.
To investigate whether or not glyT, that was found in Example 1 to be under the control of the quorum-sensing system, is involved in foam formation during culture of an acetic acid bacterium, a glyT-disrupted strain was prepared from Gluconacetobacter intermedius NCI1051 by the following procedures.
First, a DNA fragment of 2.3 kb including a part of glyT (nucleotide numbers 1 to 2252 in SEQ ID NO: 1 in the sequence listing (
The DNA fragment 1 was ligated by a ligation reaction to the plasmid 1 which was digested with the restriction enzyme EcoRV (TAKARA BIO INC.). Escherichia coli JM109 strain was transformed with the DNA thus prepared. Transformants were selected on a LB agar medium supplemented with 100 μg/ml ampicillin. The plasmid pUCΔglyT for disrupting glyT was recovered from the selected ampicillin-resistant transformant according to a common method.
The plasmid pUCΔglyT for disrupting glyT obtained as above was used to transform Gluconacetobacter intermedius NCI1051 by electroporation method (see for example, Proc. Natl. Acad. Sci. U.S.A., Vol. 87, pp. 8130-8134, 1990).
Transformants were selected on a YPG medium (3% glucose, 0.5% yeast extract, 0.3% polypeptone) supplemented with 100 μg/ml kanamycin. The kanamycin-resistant transformant grown on the selection medium was confirmed by southern hybridization to be a strain in which glyT has been disrupted by insertion of the kanamycin-resistant gene into glyT.
One of the strains among the obtained transformants was named as Gluconacetobacter intermedius NCI1051ΔglyT (hereinafter may be referred to as glyT-disrupted strain) and deposited under the Budapest Treaty with the International Patent Organism Depositary at the National Institute of Advanced Industrial Science and Technology (Central 6, 1-1 Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, Japan). The Accession Number is FERM BP-11068 and the date of depositary is Dec. 2, 2008.
Further, in order to confirm that the phenotype of the glyT-disrupted strain is attributed to the disruption of glyT gene, a complementary plasmid pGlyT comprising glyT gene was introduced into the glyT-disrupted strain to produce a glyT complementary strain.
First, the plasmid pGlyT comprising glyT gene was produced as follows. Specifically, plasmid 1 mentioned above was digested with the restriction enzymes EcoRI and ApaI (TAKARA BIO INC.) to prepare a DNA fragment including the 5′ side region of glyT (hereinafter may be referred to as DNA fragment 2) .
Next, a DNA fragment including the 3′ side region of glyT was amplified by PCR method using primer 3 (see SEQ ID NO: 5 in the sequence listing (
The DNA fragments 2 and 3 were ligated by a ligation reaction to the shuttle vector pMV24 between Escherichia coli and acetic acid bacteria which was digested with the restriction enzymes EcoRI and XbaI (TAKARA BIO INC.). The DNA thus prepared was used to transform Escherichia coli JM109 strain. Transformants were selected on a LB agar medium supplemented with 100 μg/ml ampicillin. The complementary plasmid pGlyT was recovered from the selected ampicillin-resistant transformant according to a common method.
The complementary plasmid pGlyT thus obtained was used to transform Gluconacetobacter intermedius NCI1051ΔglyT by electroporation method.
Transformants were selected on a YPG medium (3% glucose, 0.5% yeast extract, 0.3% polypeptone) supplemented with 100 μg/ml ampicillin. A plasmid was extracted from the ampicillin-resistant transformant grown on the selection medium and it was confirmed that pGlyT had been introduced. The obtained transformant was used as a glyT complementary strain.
The glyT-disrupted strain obtained in Example 2 was compared to the wild-type strain and the glyT complementary strain for the aspect of foam formation during culture. Specifically, a shaking culture was carried out at 30° C. and 120 spm in a medium (100 ml) containing 2% ethanol, 3% glucose, 0.50 yeast extract, 0.3% polypeptone, and 1% Celluclast 1.5 L (Novozymes) by using a 500-ml Sakaguchi flask.
The result demonstrated that foam formation was significantly suppressed in the glyT-disrupted strain as compared to the wild-type strain (
Next, the aspect of foam formation during culture was also compared in a mini-jar fermenter between the glyT-disrupted strain and the wild-type strain. An aeration-agitation culture was carried out at 30° C., 500 rpm and 1 L/min in a medium (1.5 L) containing 3% ethanol, 3% glucose, 0.5% yeast extract, 0.3% polypeptone, 100 μg/ml ampicillin, 1% Celluclast 1.5 L (Novozymes) and 0.01% antifoaming agent by using a 3-liter mini-jar fermenter (Bioneer 300, 3 L; B. E. MARUBISHI Co. Ltd.). The ethanol concentration in the medium was controlled at 2% during culture.
The aspect of foam formation in the mini-jar fermenter was shown in
The glyT-disrupted strain obtained in Example 2 was compared to the wild-type strain for the acetic acid fermentation ability. Specifically, an aeration-agitation culture was carried out at 30° C., 500 rpm and 1 L/min in a medium (1.5 L) containing 3% ethanol, 3% glucose, 0.5% yeast extract, 0.3% polypeptone, 1% Celluclast 1.5 L (Novozymes) and 0.01% antifoaming agent by using a 3-liter mini-jar fermenter (Bioneer 300, 3 L; B. E. MARUBISHI Co. Ltd.). The ethanol concentration in the medium was controlled at 20 during culture. Time course of the acetic acid fermentation is shown in
As is clear from
According to the present invention, a gene involved in foam formation during culture of an acetic acid bacterium and a protein encoded by the gene are provided. Further provided is an acetic acid bacterium that is capable of significantly reducing foam formation during culture and producing a larger amount of acetic acid by reducing or deleting the function of a protein encoded by the gene which is involved in foam formation during culture of an acetic acid bacterium, thereby a method is provided for suppressing foam formation during culture and a method for significantly enhancing the acetic acid fermentation ability using the acetic acid bacterium. The methods thus enable a more efficient production of vinegar containing a high concentration of acetic acid by using an acetic acid bacterium.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-327795 | Dec 2008 | JP | national |
