# Patent

## Title
Method for producing basic substance

## Abstract

A method for producing a basic substance by fermentation comprising culturing a microorganism having an ability to produce the basic substance in a liquid medium contained in a fermentation tank to produce and accumulate the basic substance in the medium, wherein amount of sulfate and/or chloride ions used as counter ions of the basic substance is reduced by adjusting total ammonia concentration in the medium to be within a specific concentration range during at least a part of the total period of culture process.

## Background
This application is a continuation under 35 U.S.C. §120 of PCT/JP2005/018657, filed Oct. 7, 2005, and claims priority under 35 U.S.C. §119 to JP2004-295123, filed Oct. 7, 2004, both of which are incorporated by reference. The Sequence Listing filed electronically herewith is also hereby incorporated by reference in its entirety (File Name: US-328_Seq_List_Copy—1; File Size: 55 KB; Date Created: Apr. 9, 2007).


BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a technique for the microbial industry, more precisely, a method for producing a basic substance by fermentation. As basic substances which are able to be produced by fermentation, for example, L-lysine is useful as an additive in animal feed, and L-arginine and L-histidine are useful for pharmaceutical preparations such as infusions.
2. Brief Description of the Related Art
In the methods for producing basic substances by fermentation, microorganisms having an ability to produce a basic substance are cultured to produce and accumulate the basic substance in a medium, and the basic substance is collected from the medium. In such methods, the culture is performed as batch culture, feeding culture or continuous culture.
In such production of basic substances by fermentation, sulfate or chloride ions have been typically added to a medium as counter anions for an objective substance which dissociates into a cation in the medium in order to maintain pH of the medium at a neutral level (Japanese Patent Laid-open (Kokai) Nos. 5-30985 and 5-244969).
In many cases, basic substances are collected from a medium by ion exchange, when purification is required. For example, in the case of L-lysine, after fermentation broth is made weakly acidic, L-lysine is adsorbed on an ion exchange resin and then eluted from the resin with ammonium ions. The eluted L-lysine is used as it is as lysine base, or it can be crystallized with hydrochloric acid to form L-lysine hydrochloride.
When chloride ions are used as counter anions in the medium in the aforementioned purification of L-lysine, L-lysine hydrochloride can be obtained directly by concentrating the medium. However, since chloride ions corrode metal fermentation tanks etc., it is not preferable to make them exist in the medium in high concentration in actual production.
On the other hand, when the basic substance is not purified, the fermentation broth is concentrated as it is, or it is made weakly acidic with hydrochloric acid or sulfuric acid, followed by spray granulation. In this case, the residual components contain the counter anions added to the medium, and therefore the amount of the basic substance is reduced in the resulting fermentation product.
Japanese Patent Laid-open No. 2002-65287 (U.S. Patent Application No. 2002025564) discloses a method of utilizing, in the production of a basic amino acid by fermentation, carbonate and bicarbonate ions as counter anions of the basic amino acid to substitute for a part of sulfate or chloride ions. Carbonate and bicarbonate ions can be comparatively easily removed from the culture medium by making the pH of the medium acidic, or concentrating the medium, or both. The above-cited publication teaches a method of controlling the internal pressure in the fermentation tank so that it is positive during fermentation, or adding carbon dioxide gas or a mixed gas containing carbon dioxide to the medium, as a means for adding carbonate ions and bicarbonate ions to the medium. However, at typical medium conditions, such as a neutral pH, only a small amount of carbon dioxide gas dissolves, if at all. Therefore, to maintain the presence of bicarbonate and carbonate ions in the culture medium so that the effect of reducing the sulfate or chloride ion concentration is maintained, the culture must be performed at an alkaline pH. However, if pH becomes high, the bacterial growth rate and productivity of the objective substance are generally reduced.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for achieving both reduction of sulfate ions and chloride ions and efficient production of an objective substance in the production of a basic substance by fermentation using a microorganism having an ability to produce the objective basic substance and utilizing carbonate ions and bicarbonate ions as counter anions of the basic substance with avoiding reduction of growth rate of the microorganism or reduction of productivity of the objective substance.
When using coryneform or Escherichia bacteria to produce basic substances by fermentation, if pH becomes too high, the bacterial growth rate or the productivity of the objective substance is usually reduced. The inventors of the present invention found that the major factor causing this phenomenon is ammonia, which is added to the medium as a nitrogen source for production of the basic substance, for bacterial growth, or as a source of counter ions of the basic substance, and reduction of the growth rate of the microorganism or the productivity of the objective substance under a high pH condition could be markedly suppressed by performing the fermentation with controlling the total ammonia concentration to be within a suitable concentration range.
The present invention was accomplished on the basis of the aforementioned findings.
That is, the present invention provides the followings.
(1) A method for producing a basic substance by fermentation comprising culturing a microorganism having an ability to produce the basic substance in a liquid medium contained in a fermentation tank to produce and accumulate the basic substance in the medium, wherein amount of sulfate and/or chloride ions used as counter ions of the basic substance is reduced by adjusting total ammonia concentration in the medium to be within a specific concentration range during at least a part of the total period of culture process.
(2) The method according to (1), wherein the specific concentration range of the total ammonia concentration is a range satisfying the following conditions:
(A) concentration of ammonium ions in the medium is at such a level that the sum of the ion equivalents of bicarbonate ions and/or carbonate ions and other anions dissolved in the medium is larger than the ion equivalent of the basic substance ionized from the basic substance accumulated in the medium, and
(B) the total ammonia concentration in the medium is at a level not inhibiting the production of the basic substance by the microorganism, which is determined beforehand as follows:
the microorganism is cultured in the medium having various pH values and various total ammonia concentrations, productivity of the basic substance is measured at each pH value and each total ammonia concentration, and a total ammonia concentration providing 50% or more of productivity of the basic substance based on the productivity obtained under optimum conditions is determined for each pH value.
(3) The method according to (1), wherein the specific range of the total ammonia concentration is determined beforehand as follows:
(A′) the culture is performed in a medium which contains sulfate and/or chloride ions in an amount sufficient for performing the culture at pH 7.2 or lower as a source of counter ions of the objective basic substance, of which pH is maintained to be in the range between 6.5 to 7.2 by adding at least one of ammonia gas, aqueous ammonia and urea, and productivity of the basic substance is measured,
(B′) the culture is started in the same medium as that used in the step (A′) except that sulfate ions and/or chloride ions of medium components are lowered by a desired amount, and the culture is continued with various total ammonia concentrations during a period where it becomes impossible to maintain the pH of the medium to be 7.2 or lower due to the shortage of sulfate ions and/or chloride ions as counter ions of the basic substance caused by accumulation of the objective basic substance to determine a total ammonia concentration range providing 50% or more of productivity based on the productivity measured in the step (A′).
(4) The method according to (1), wherein the at least a part of the total period includes at least one of a period where the pH of the medium increases due to shortage of the counter ions caused with accumulation of the objective basic substance, and a period where the pH increases due to the addition of cations to the medium.
(5) The method according to (1), wherein the total ammonia concentration in the medium is adjusted by adding ammonia or urea to the medium when the activity of the microorganism is reduced or ceases as determined based on the indicators: dissolved oxygen concentration in the medium, consumption rate of carbon source in the medium, turbidity of the medium, productivity of the basic substance, and pH change in the medium observed.
(6) The method according to (1), wherein a medium having the same composition as that of a medium containing sulfate ions and/or chloride ions as a counter ion source of the basic substance in an amount sufficient for performing the culture at pH 7.2 or lower except that amount of sulfate ions and/or chloride ions is reduced by a desired amount is used as the medium, and
the at least a part of the total period is a period where pH of the medium cannot be maintained to be 7.2 or lower due to shortage of counter ions for the basic substance which has accumulated in the medium.
(7) The method according to (2), wherein the other anions are selected from sulfate ions, chloride ions, phosphate ions, and ionized organic acids.
(8) The method according to (2) or (7), wherein total amount of the other anions is 900 meq/l or lower.
(9) The method according to (1), wherein the total ammonia concentration in the medium is adjusted to be 200 mM or lower.
(10) The method according to (1), which comprises the step of proliferating the microorganism.
(11) The method according to (10), wherein the total ammonia concentration is not adjusted during the step of proliferating the microorganism.
(12) The method according to (1), wherein the basic substance is selected from L-lysine, L-arginine and L-histidine.
(13) The method according to (12), wherein the basic substance is L-lysine.
(14) The method according to (12), wherein the basic substance is L-arginine.
(15) The method according to (1), wherein the medium or a processed product thereof is heated after the fermentation to eliminate bicarbonate ions and carbonate ions.
(16) The method according to (1), wherein the microorganism is a coryneform bacterium or an Escherichia bacterium.
(17) A fermentation broth or fermentation product containing a basic substance, which is obtainable by the method according to (15).



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the results of culture for L-lysine production performed by using a conventional medium and the culture method. FIG. 1a shows the growth curve over time at two different pH values. FIG. 1b shows the lysine accumulation over time at two different pH values. FIG. 1c shows the ammonia concentration over time at two different pH values.
FIG. 2 shows the results of culture for L-lysine production performed in a medium with a limited ammonium concentration. FIG. 2a shows the growth curve over time at three different pH values. FIG. 2b shows the lysine accumulation over time at three different pH values. FIG. 2c shows the ammonia concentration over time at three different pH values.
FIG. 3 shows the results of culture for L-lysine production performed by controlling only the total ammonia concentration and not controlling pH. FIG. 3a shows the growth curve over time. FIG. 3b shows the lysince accumulation over time. FIG. 3c shows the ammonia concentration over time.
FIG. 4 shows changes in total ammonia concentration (4a) and pH (4b) over time in a conventional medium and a medium without ammonium sulfate and ammonium chloride.
FIG. 5 shows the changes in growth (5a), total ammonia concentration (5b), pH (5c), and remaining sugar amount (5d) over time in L-arginine fermentation in a medium with a limited ammonium concentration.
FIG. 6 shows the results of a culture for L-arginine production in a medium with a limited ammonium concentration.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereafter, the present invention will be explained in detail.
The method of the present invention is a method for producing a basic substance by fermentation, which comprises culturing a microorganism which is able to produce the basic substance in a liquid medium contained in a fermentation tank to produce and accumulate the basic substance in the medium. The method of the present invention is characterized in that amount of sulfate ions and/or chloride ions used as counter ions of the basic substance is reduced by adjusting total ammonia concentration in the medium to be within a specific concentration range during at least a part of the total period of culture process. That is, the method of the present invention is a method for producing the basic substance in the medium in which sulfate ions and chloride ions are reduced by using such a total ammonia concentration that the total ammonia is secured in an amount required for the growth of the microorganism or the production of the objective substance as a nitrogen source, and growth of the microorganism or the production of the objective substance is not inhibited.
Examples of the specific range of the total ammonia concentration include a range satisfying the following conditions:
(A) the concentration of ammonium ions in the medium is at such a level that the sum of the ion equivalents of bicarbonate and/or carbonate ions, and other anions dissolved in the medium is larger than the ion equivalent of the basic substance ionized from the basic substance accumulated in the medium, and
(B) the total ammonia concentration in the medium is at a level not inhibiting the production of the basic substance by the microorganism, which is determined beforehand as follows:
the microorganism is cultured in the medium having various pH values and various total ammonia concentrations, productivity of the basic substance is measured at each pH value and each total ammonia concentration, and a total ammonia concentration providing 50% or more of productivity of the basic substance based on the productivity obtained under optimum conditions is determined for each pH value.
Furthermore, in another embodiment of the present invention, the specific range of the total ammonia concentration is determined beforehand as follows.
(A′) (Procedure 1: Evaluation of fermentation result under neutral condition)
The culture is performed in a medium which contains an amount of sulfate and/or chloride ions which is sufficient for performing the culture at pH 7.2 or lower as a source of counter ions of the objective basic substance, of which pH is maintained to be in the range between 6.5 to 7.2 by adding at least one of ammonia gas, aqueous ammonia and urea, and productivity of the basic substance is measured,
(B′) (Procedure 2: Evaluation of fermentation results with reduced amount of sulfate ions and chloride ions at various ammonium concentrations)
The culture is started in the same medium as that used in Procedure 1 (step A′) described above except that sulfate and/or chloride ions of medium components are lower by a desired amount, and the culture is continued with various total ammonia concentrations during a period where it becomes impossible to maintain the pH of the medium to be 7.2 or lower due to shortage of sulfate ions and/or chloride ions as counter ions of the basic substance caused with accumulation of the objective basic substance to determine a total ammonia concentration range providing 50% or more of productivity based on the productivity measured in the step (A′).
Moreover, in another embodiment of the present invention, when the specific range of total ammonia concentration is not determined beforehand, the total ammonia concentration can be adjusted to be within the predetermined range. Specifically, the total ammonia concentration in the medium is adjusted by adding ammonia or urea to the medium when activity of the microorganism is reduced or ceases as determined on the basis of dissolved oxygen concentration in the medium, consumption rate of carbon source in the medium, turbidity of the medium, productivity of the basic substance, and the pH change in the medium observed as indexes. The medium has the same composition as that of a medium containing sulfate ions and/or chloride ions as a counter ion source of the basic substance in an amount sufficient for performing the culture at pH 7.2 or lower except that amount of sulfate ions and/or chloride ions is reduced by a desired amount. Examples of the at least a part of the total period include a period when the pH of the medium cannot be maintained to be 7.2 or lower due to shortage of counter ions for the basic substance which has accumulated in the medium.
Examples of the other anions include chloride ions, sulfate ions, phosphate ions, ions of organic acids (acetic acid, lactic acid, succinic acid etc.), and so forth. Furthermore, bicarbonate ions and/or carbonate ions dissolved in the medium function as counter anions of the basic substance.
In the present invention, an ion equivalent is a value obtained by multiplying the molar concentration of each ion by the ion's valence, and it is represented in a unit of eq/l. That is, the ion equivalent of 1 mM of a monovalent ion is 1 meq/l, and the ion equivalent of 1 mM of a divalent ion is 2 meq/l.
The aforementioned total ammonia concentration is adjusted in order to make the total ammonia exist in the medium in an amount required for growth of the microorganism or the production of the basic substance, and at a concentration not inhibiting the production of the basic substance by the microorganism, and the medium is thereby automatically adjusted to a pH suitable for dissolving bicarbonate ions and/or carbonate ions required as counter anions of the basic substance.
In the present invention, “the total ammonia” means the sum of non-dissociated ammonia (NH3) and ammonium ions (NH4+). When adjusting the total ammonia concentration, non-dissociated ammonia or ammonium ions may be measured, or the both may be measured.
Typically, ammonium sulfate and ammonium chloride are added to the medium as sources of counter anions of the basic substance and source of nitrogen, in general. Moreover, since ammonia and urea are typically used to adjust the pH of the medium, high concentration of ammonia and ammonium ions are present in the medium. When reducing the amount of ammonium sulfate or ammonium chloride in order to reduce the amount of sulfate or chloride ions added to the medium, a nitrogen source such as ammonia is supplied in an amount corresponding to the amount to be reduced. For such an operation, it has been necessary to develop a method for supplying ammonia, which takes into consideration the balance between cations including those produced by bacteria and increasing with progress of the culture such as those of the objective basic substance, cations which ionize from added ammonia, cations added to the medium such as sodium and potassium ions, and so forth, and anions increasing in the medium due to generation by respiration of bacteria or addition to the medium. If this balance is not maintained, the fermentation will not progress, because the ammonia concentration will become unduly high, or the pH will become excessively high, or conversely, ammonia could become depleted. According to the present invention, development of a method for adding ammonia for adjusting the total ammonia concentration to be within a specific range can enable favorable maintenance of the aforementioned balance of cations and anions, and thus favorable growth of a microorganism and favorable generation of a basic substance can be realized even under a condition that the amount of sulfate ions and chloride ions present in the medium is reduced.
The total ammonia concentration in the medium is adjusted by adding at least one of ammonia gas, ammonia solution and urea to the medium so that the total ammonia concentration in the medium is at an acceptable level. Furthermore, an ammonium salt such as ammonium chloride or ammonium sulfate may also be added, unless detrimental to the effect of the invention. Moreover, an ammonium salt containing bicarbonate ion or carbonate ion as a counter ion, which can be easily removed as a gas after completion of the culture, may also be used. The total ammonia concentration can be adjusted by using measured values of ammonium ion or ammonia concentration in the medium or exhaust gas as an index. Moreover, it is also possible to adjust the total ammonia concentration by determining beforehand a pH providing an acceptable total ammonia concentration when pH is adjusted with ammonia and adding ammonia so that such a pH can be obtained. In such a case, pH determined as described above may be changed during the culture, if needed.
Moreover, the total ammonia concentration in the medium can also be adjusted by adding ammonia or urea to the medium when the activity of the microorganism is reduced or ceases as determined on the basis of dissolved oxygen concentration in the medium, consumption rate of carbon source in the medium, turbidity of the medium, productivity of the basic substance, and the change in pH in the medium observed as indexes. That is, if the nitrogen source in the medium runs short or is depleted, proliferation of the microorganism or the activity of the microorganism, such as production of an objective substance is reduced or ceases. Activity of a microorganism usually appears as consumption of dissolved oxygen and a carbon source in a medium, increase of turbidity of medium, production of an objective substance, and reduction in the pH of the medium due to the consumption of ammonia or the release of carbon dioxide by respiration. Therefore, when activity of a microorganism is reduced or ceases, the concentration of dissolved oxygen in a medium increases when aeration and stirring rates per unit time are constant, and pH of a medium increases due to a decrease in the consumption of ammonia and secretion of carbon dioxide. Furthermore, the consumption rate of a carbon source, increasing rate of turbidity of a medium and the production rate of an objective substance are reduced. Therefore, when stagnation of activity of a microorganism is observed on the basis of these items used as indexes under a state that medium components other than a nitrogen source are sufficient, the nitrogen source runs short or has been depleted. If this occurs, ammonia or urea is added to the medium in an amount which is required for the growth of the microorganism or the production of the objective substance. By repeating this procedure, the total ammonia concentration in the medium is maintained to be within a specific range as a result. If the culture is performed with adding urea to the medium, urea is utilized by the microorganism, and ammonia is released into the medium. If the addition of ammonia or urea is repeated as described above, the pH of the medium gradually increases. The amount of ammonia or urea added at each time point may be, for example, 300 mM, preferably 200 mM, more preferably 100 mM, expressed as the final concentration of total ammonia in the medium. Alternatively, ammonia or urea may be added so that the pH increases by 0.3 or less, preferably 0.15 or less, more preferably 0.1 or less, after addition of ammonia or urea.
The dissolved oxygen concentration in the medium can be measured, for example, by using a dissolved oxygen electrode.
Whether the sum of the ion equivalents of bicarbonate ions and/or carbonate ions and the other anions, which are all dissolved in the medium is higher than the ion equivalent of the basic substance which has accumulated in the medium can be confirmed by measuring the concentrations of bicarbonate ions, carbonate ions and other anions as well as the concentration of the basic substance. Moreover, the above conditions can also be fulfilled by conducting a preliminary experiment to determine the pH and/or the addition amount of ammonia which satisfies the aforementioned conditions, and performing the culture at the pre-determined pH and/or addition of pre-determined amount of ammonia.
In the present invention, the pH of the culture may or may not be constant. Moreover, when the pH of the medium is controlled, it may be controlled by using pH itself as an index, or indirectly by controlling the total ammonia concentration without directly controlling the pH. Furthermore, if ammonia or urea is added using the activity of the microorganism as an index as described above, the total ammonia concentration in the medium is adjusted so that it is within an appropriate concentration range, and the pH gradually increases with the accumulation of the basic substance. Moreover, if the culture is performed with controlling the total ammonia concentration to be within a specific range, the pH changes as a result of change of accumulation balance of various cations and anions in the medium. Whichever means is chosen, the total ammonia concentration in the medium is adjusted to be within a specific concentration range as a result, and thus the amount of sulfate ions and/or chloride ions used as counter ions of the basic substance can be reduced.
In the present invention, the expression “not inhibiting production of a basic substance” means that the microorganism used for the present invention grows favorably, and the basic substance is favorably produced. When the growth of the microorganism is insufficient, or when the basic substance is not efficiently produced in spite of favorable growth of the microorganism, it is considered that production of the basic substance is inhibited.
Specifically, the microorganism used for the present invention is cultured at various pH levels and the total ammonia concentrations of the medium, productivities of the basic substance accumulated in the medium are measured, and the total ammonia concentrations which results in production of the basic substance at a rate of preferably 50% or more, more preferably 70% or more, particularly preferably 90% or more, as compared to the amount of the basic substance obtainable under optimal conditions, for example, conventionally used general conditions at a neutral pH, at each pH value are considered to be concentrations “not inhibiting production of the basic substance”. In the present invention, “productivity” refers to the yield, the production rate or the total amount produced. The “yield” refers to production amount of the basic substance based on the carbon source present in the medium which is able to be consumed, and the “production rate” refers to a production amount per unit time. Moreover, when the term “production amount” or “amount produced” is solely used, it refers to the amount of the basic substance which is accumulated in the medium once the carbon source is completely consumed.
Alternatively, the microorganism used for the present invention is cultured under optimal conditions, for example, conventionally used general conditions at a neutral pH, and productivity of the basic substance which has accumulated in the medium is measured. Then, the culture is performed in a medium having the same composition except that amount of sulfate ions and/or chloride ions is reduced by a desired amount, and the productivity of the basic substance is measured. In this case, there is a period where the pH of the medium will increase due to the shortage of sulfate ions and/or chloride ions as the counter ions with accumulation of the objective basic substance. For that period, the culture is performed with maintaining the total ammonia concentration to be within the specific concentration range. As for the range within which the concentration is controlled, the culture is performed with various concentrations within the range of 1 to 500 mM, and concentrations within a range providing a productivity of the basic substance of preferably 50% or more, more preferably 70% or more, particularly preferably 90% or more, of the productivity obtainable under optimal conditions are determined to be concentrations “not inhibiting production of the basic substance”. Examples of the medium used for the aforementioned “conventionally used general conditions at a neutral pH” include a medium containing sulfate ions and/or chloride ions in an amount sufficient for performing the culture at pH 7.2 or lower.
The desired amount by which the sulfate ions and/or chloride ions are reduced is not particularly limited, so long as objective productivity of the basic substance can be obtained.
The total ammonia concentration which is defined as “not inhibiting the production of the basic substance” can also be determined, for example, as follows. The microorganism used for the present invention is cultured at various pH levels and the total ammonia concentration of the medium, and the amount of the basic substance which accumulates in the medium are measured. The accumulated amount of the basic substance which is obtained under various conditions are compared with the amount accumulated under the optimum conditions. Thus, the total ammonia concentration which does not inhibit the production of the basic substance can be determined. The optimum conditions are defined as conditions of culture using sufficient counter ions at a neutral pH as in the typically used general conditions at a neutral pH.
Furthermore, another method for determining the total ammonia concentration which is defined as “not inhibiting production of the basic substance” is, for example, as follows. The microorganism used for the present invention is cultured under optimal conditions, for example, typically used general conditions at a neutral pH, and productivity of the basic substance which accumulates in the medium is measured. Then, the culture is performed in a medium having the same composition except that sulfate ions and/or chloride ions is reduced by a desired amount, and the productivity is examined. In this case, there is a period where pH of the medium will increase due to shortage of sulfate ions and/or chloride ions as the counter ions with accumulation of the objective basic substance. For that period, the culture is performed with maintaining the total ammonia concentration to be with in the specific concentration range. As for the range within which the concentration is controlled, the culture is performed with various concentrations within the range of 1 to 500 mM, and the productivities obtained thereby are compared to that under the optimum conditions.
The concentration which is defined as “not inhibiting production of the basic substance” includes, for example, a concentration allows production of the basic substance preferably at 50% or more, more preferably 70% or more, particularly preferably 90% or more, as compared to the productivity of the basic substance under optimal conditions. Specifically, the total ammonia concentration in the medium is, for example, preferably 300 mM or less, more preferably 200 mM or less, particularly preferably 100 mM or less. The degree to which ammonia dissociates is reduced as the pH increases. Non-dissociated ammonia is more toxic to bacteria as compared with ammonium ion. Therefore, the upper limit of the total ammonia concentration also depends on the pH of the medium. That is, as the pH of the medium increases, the acceptable total ammonia concentration becomes lower. Therefore, as for the aforementioned total ammonia concentration which is defined as “not inhibiting production of the basic substance”, a total ammonia concentration range acceptable for the highest pH during the culture may be regarded as the total ammonia concentration range throughout the culture.
On the other hand, the total concentration of ammonia as a nitrogen source, which is required for growth of the microorganism and production of the basic substance, is not particularly limited, so long as the productivity of the objective substance provided by the microorganism is not reduced due to a shortage of the nitrogen source during the culture, and it may be appropriately determined. For example, the ammonia concentration is measured over time during the culture, and when the ammonia in the medium is depleted, a small amount of ammonia may be added to the medium. Although the concentration after the addition of ammonia is not particularly limited, it is, for example, preferably 1 mM or higher, more preferably 5 mM or higher, particularly preferably 10 mM or higher, in terms of the total ammonia concentration.
The method of the present invention may include a culture step which is primarily for proliferating the microorganism having an ability to produce a basic substance, and a culture step which is primarily for allowing the microorganism to produce the basic substance. Furthermore, in the method of the present invention, proliferation of the microorganism and production of the basic substance may be performed in parallel. Furthermore, besides such culture as described above, which may also be called main fermentation, main culture or the like, a preculture may also be independently performed.
In the present invention, in addition to adjusting the total ammonia concentration in the medium as described above, an operation facilitating dissolution of bicarbonate ions and/or carbonate ions in the medium may also be performed. Examples of such an operation include controlling the pressure in the fermentation tank during the culture so that it is positive, supplying carbon dioxide gas or a mixed gas containing carbon dioxide gas to the medium, limiting the aeration in the fermentation tank so that bicarbonate ions and/or carbonate ions are dissolved in the medium, increasing the pH of the medium by adding cations other than ammonium ions such as sodium ions and potassium ions to the medium, and so forth.
To make the pressure in the fermentation tank positive, for example, the pressure of the air supply to the fermentation tank may be made higher than the pressure of the exhaust. By making the pressure in the fermentation tank higher, carbon dioxide gas generated by the fermentation dissolves in the culture medium and produces bicarbonate ions or carbonate ions. Specifically, the pressure in the fermentation tank may be 0.13 to 0.3 MPa, preferably 0.15 to 0.25 MPa.
Furthermore, carbon dioxide gas may be dissolved in the culture medium by supplying carbon dioxide gas or a mixed gas containing carbon dioxide gas into the medium. Alternatively, by limiting aeration to the fermentation tank, carbon dioxide gas generated by the fermentation can also dissolves in the medium. A suitable aeration rate can be determined by, for example, measuring the amount of bicarbonate ions or carbonate ions in the medium, or measuring the pH and ammonia concentration of the medium. When carbon dioxide gas is supplied to the medium, for example, pure carbon dioxide gas or a mixed gas containing 5% by volume or more of carbon dioxide gas may be bubbled into the medium. The aforementioned methods for dissolving bicarbonate ions and/or carbonate ions in the medium may be used independently or as a combination of two or more.
The operation of adjusting the total ammonia concentration in the medium and the operation of facilitating dissolution of bicarbonate ions and/or carbonate ions in the medium if needed may be performed during at least a part of the total period of culture process.
Although the “at least a part of the total period” is not particularly limited so long as desired productivity is obtained, it may be specifically, for example, 1/10 or more, preferably 1⁄5 or more, of the total culture process of the main culture. More specifically, examples of the period include a period where the pH of the medium increases due to the shortage of the counter ions such as sulfate ions and/or chloride ions, with accumulation of the objective basic substance, or a period where the pH of the medium increases due to addition of cations, or both of these periods.
The medium used for the present invention is not particularly limited, so long as at least the total ammonia concentration can be made to be within the aforementioned range by the operation of adjusting the total ammonia concentration, and a medium containing organic and inorganic nutrients such as a carbon source and a nitrogen source and other trace amount nutrients may be suitably used depending on the microorganism to be used.
Any carbon source can be used, as long as it can be consumed by the microorganism, and examples include saccharides such as saccharose, glucose, fructose, molasses and starch hydrolysate, organic acids such as acetic acid, alcohols such as ethanol, and hydrocarbons such as methane.
Examples of the nitrogen source include inorganic substances such as ammonia, protein hydrolysates, yeast extract, and so forth. Examples of the trace amount nutrients include amino acids, vitamins, and trace metal elements.
Examples of anions other than bicarbonate ions and/or carbonate ions which are present in the medium include chloride ions, sulfate ions, phosphate ions, ionized organic acids, hydroxide ions, and so forth. The sum of the ion equivalents of these other ions is usually 900 meq/l or less, preferably 700 meq/l or less, more preferably 500 meq/l or less, still more preferably 300 meq/l or less, particularly preferably 200 meq/l or less.
One of the objects of the present invention is to reduce the amount of sulfate ions and/or chloride ions used, and the ion equivalent of sulfate ions or chloride ions, or the sum of ion equivalents of these ions present in the medium is usually 700 meq/l or less, preferably 500 meq/l or less, more preferably 300 meq/l or less, still more preferably 200 meq/l or less, particularly preferably 100 meq/l or less.
The fermentation scheme is not particularly limited, and may be a batch culture in which medium is not fed, a feeding culture in which the medium is fed after the charged sugar is consumed, a continuous culture in which the medium is extracted when the volume of the medium exceeds the volume acceptable for a fermentation tank, a cell recycle method in which bacterial cells are recycled, and so forth. The culture temperature may be appropriately determined depending on the chosen microorganism. It is usually 25 to 45° C., preferably 30 to 40° C. Furthermore, it is preferable to stir sufficiently so that sufficient oxygen is present during the fermentation.
The culture for producing the objective basic substance is specifically performed, for example, as follows. A medium containing typical medium components is prepared, but most if not all of the ammonium salts such as ammonium sulfate and ammonium chloride are eliminated. A microorganism which has been separately cultured is inoculated into this medium, and cultured while controlling the total ammonia concentration to be within a range suitable for the chosen microorganism, which is determined as described above. The ammonia concentration in the medium in the fermentation tank or the sampled medium can be measured by using, for example, a commercially available ion meter or the like. By using the measured values as an index, the total ammonia concentration can be controlled. To maintain the total ammonia concentration within the predetermined concentration range, ammonia gas, aqueous ammonia or urea may be added to the medium. The total ammonia concentration in the medium can also be indirectly measured by measuring the ammonia concentration in the exhaust gas from the fermentation tank using a common ammonia electrode.
Furthermore, in the present invention, the total ammonia concentration in the medium can be adjusted by the following method using the pH of the medium as an index, as described above.
The culture is performed in a medium which has the same composition as a medium containing sulfate ions and/or chloride ions in an amount sufficient to maintain the culture at pH 7.2 or lower, except that amount of sulfate ions and/or chloride ions is reduced by a desired amount at various pH levels, wherein the pH level is changed by adding at least any one of ammonia gas, aqueous ammonia and urea, and
the culture is continued while maintaining the total ammonia concentration in the medium so that it is within the preferred concentration range by adding at least any one of ammonia gas, aqueous ammonia and urea to the medium based on indicators such as change in the dissolved oxygen concentration in the medium, the change in the consumption rate of the carbon source in the medium, the change in the turbidity of the medium, the pH change in the medium, or the like in an indirect manner during a period where pH of the medium cannot be maintained at 7.2 or lower due to shortage of counter ions to the basic substance which has accumulated in the medium.
Examples of the basic substance produced by the method of the present invention include basic amino acids, specifically, L-lysine, L-arginine and L-histidine. Among these, L-lysine is preferred.
The microorganism which is able to produce a basic substance is not particularly limited, and any microorganism can be chosen so long as it can produce the basic substance by fermentation. In particular, a microorganism which favorably produces the basic substance even at a high pH of medium, if the total ammonia concentration of the medium is low, is preferably chosen. Examples of such a microorganism include bacteria belonging to coryneform bacteria, genus Escherichia, Serratia, or Bacillus. 
Coryneform bacteria and Escherichia bacteria will be explained hereinafter, however, the microorganism used for the method of the present invention is not limited to these bacteria.
The coryneform bacteria used for the present invention include Corynebacterium bacteria and those bacteria having been previously classified into the genus Brevibacterium but have been re-classified into the genus Corynebacterium (Int. J. Syst. Bacteriol., 41, 255 (1981)), and further include bacteria belonging to the genus Brevibacterium, which is extremely close to the genus Corynebacterium. Specific examples include the following:
Corynebacterium acetoacidophilum 
Corynebacterium acetoglutamicum 
Corynebacterium alkanolyticum 
Corynebacterium callunae 
Corynebacterium glutamicum 
Corynebacterium lilium (Corynebacterium glutamicum)
Corynebacterium melassecola 
Corynebacterium thermoaminogenes 
Corynebacterium efficiens 
Corynebacterium herculis 
Brevibacterium divaricatum (Corynebacterium glutamicum)
Brevibacterium flavum (Corynebacterium glutamicum)
Brevibacterium immariophilum 
Brevibacterium lactofermentum (Corynebacterium glutamicum)
Brevibacterium roseum 
Brevibacterium saccharolyticum 
Brevibacterium thiogenitalis 
Brevibacterium album 
Brevibacterium cerinum 
Microbacterium ammoniaphilum 
Specifically, the following strains are included:
Corynebacterium acetoacidophilum ATCC 13870
Corynebacterium acetoglutamicum ATCC 15806
Corynebacterium alkanolyticum ATCC 21511
Corynebacterium callunae ATCC 15991
Corynebacterium glutamicum ATCC 13020, ATCC 13032, ATCC 13060
Corynebacterium lilium (Corynebacterium glutamicum) ATCC 15990
Corynebacterium melassecola ATCC 17965
Corynebacterium thermoaminogenes AJ12340 (FERM BP-1539)
Corynebacterium herculis ATCC 13868
Brevibacterium divaricatum (Corynebacterium glutamicum) ATCC 14020
Brevibacterium flavum (Corynebacterium glutamicum) ATCC 13826, ATCC 14067
Brevibacterium immariophilum ATCC 14068
Brevibacterium lactofermentum (Corynebacterium glutamicum) ATCC 13665, ATCC 13869
Brevibacterium roseum ATCC 13825
Brevibacterium saccharolyticum ATCC 14066
Brevibacterium thiogenitalis ATCC 19240
Brevibacterium ammoniagenes (Corynebacterium ammoniagenes) ATCC 6871
Brevibacterium album ATCC 15111
Brevibacterium cerinum ATCC 15112
Microbacterium ammoniaphilum ATCC 15354
Examples of the Escherichia bacteria include Escherichia coli. When Escherichia coli is bred by using genetic engineering techniques, the E. coli K12 strain and derivatives thereof, i.e., E. coli MG1655 strain (ATCC No. 47076), W3110 strain (ATCC No. 27325), and so forth, may be chosen. The E. coli K12 strain was isolated at Stanford University in 1922, and is a lysogenic bacterium of λ phage. In addition, it is a highly versatile strain having the F-factor, for which genetic recombinants can be created by conjugation or the like. Furthermore, the genomic sequence of the E. coli K12 strain has been determined, and the genetic information is publicly available. The E. coli K12 strain and derivatives thereof may be obtained from American Type Culture Collection (ATCC, Address: P.O. Box 1549, Manassas, Va. 20108, United States of America).
Examples of coryneform bacteria which are able to produce L-lysine include S-(2-aminoethyl)cysteine (abbreviated as “AEC” hereinafter) resistant mutant strains, mutant strains which require an amino acid such as L-homoserine for growth (Japanese Patent Publication (Kokoku) Nos. 48-28078 and 56-6499), mutant strains with resistance to AEC and which further require an amino acid such as L-leucine, L-homoserine, L-proline, L-serine, L-arginine, L-alanine and L-valine (U.S. Pat. Nos. 3,708,395 and 3,825,472), L-lysine producing mutant strains with resistance to DL-α-amino-ε-caprolactam, α-amino-lauryllactam, aspartic acid analogue, sulfa drug, quinoid and N-lauroylleucine, L-lysine producing mutant strains with resistance to oxaloacetate decarboxylase or a respiratory tract enzyme inhibitor (Japanese Patent Laid-open Nos. 50-53588, 50-31093, 52-102498, 53-9394, 53-86089, 55-9783, 55-9759, 56-32995, 56-39778, Japanese Patent Publication Nos. 53-43591 and 53-1833), L-lysine producing mutant strains which require inositol or acetic acid (Japanese Patent Laid-open Nos. 55-9784 and 56-8692), L-lysine producing mutant strains that are susceptible to fluoropyruvic acid or a temperature of 34° C. or higher (Japanese Patent Laid-open Nos. 55-9783 and 53-86090), L-lysine producing mutant strains of Brevibacterium or Corynebacterium bacteria with resistance to ethylene glycol (U.S. Pat. No. 333,455), and so forth.
Specific examples include, for example, the Brevibacterium lactofermentum ATCC 31269, Brevibacterium flavum ATCC 21475, and Corynebacterium acetoglutamicum ATCC 21491 strains.
Furthermore, the Brevibacterium lactofermentum ATCC 13869/pVK-C*, plysE strain described in the examples is also a preferred L-lysine producing coryneform bacterium. This strain was obtained by incorporating a plasmid pVK-C* containing the gene coding for aspartokinase which is desensitized to feedback inhibition by L-lysine and L-threonine (lysC*) and a plasmid plysE (U.S. Patent Application No. 2003113899) containing the lysE gene which is homologous to the gene which promotes secretion of L-lysine known for the Corynebacterium bacteria (International Patent Publication 9723597A2) into the ATCC 13869 strain, which is a wild type strain of Brevibacterium lactofermentum. 
The lysC* gene can be isolated from, for example, the L-lysine producing mutant strain AJ3463 (FERM P-1987) (see Japanese Patent Publication No. 51-34477) which is generated by mutagenesis of the ATCC 13869 strain. The AJ3463 strain was deposited at International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (formerly National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Address: Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Mar. 22, 1973, and assigned accession number FERM P-1987. Furthermore, a lysC* gene fragment can also be isolated from the Brevibacterium lactofermentum AJ12691 strain which contains a plasmid p399AK9B containing the gene. The AJ12691 strain was deposited at International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology on Apr. 10, 1992, and assigned accession number FERM P-12918. Then, it was converted to an international deposit under the provisions of the Budapest Treaty on Feb. 10, 1995, and assigned accession number FERM BP-4999. The plasmid p399AK9B (U.S. Pat. No. 5,766,925) was obtained by inserting a DNA fragment enabling autonomous replication of the plasmid in Corynebacterium bacteria into a plasmid p399AK9 which was obtained by inserting lysC derived from the AJ3463 strain into the cloning vector pHSG399 (see Takeshita, S et al, Gene (1987), 61, 63-74).
In the aforementioned desensitized aspartokinase, the alanine residue at position 279 of the α-subunit and the alanine residue at position 30 the β-subunit of the wild-type aspartokinase are each replaced with a threonine residue. The α-subunit and the β-subunit are both encoded in the same frame of the lysC gene. The nucleotide sequence of the lysC* gene and the amino acid sequence of the α-subunit of the desensitized aspartokinase are shown in the sequence listing as SEQ ID NOS: 5 and 6, respectively, and the nucleotide sequence of the same gene and the amino acid sequence of the β-subunit of the desensitized aspartokinase are shown as SEQ ID NOS: 7 and 8, respectively.
The lysE gene of coryneform bacteria can be obtained by PCR (polymerase chain reaction, see White, T. J. et al., Trends Genet., 5, 185 (1989)) using primers based on the reported nucleotide sequence (GenBank accession X96471), for example, primers shown as SEQ ID NOS: 3 and 4, and a chromosomal DNA of coryneform bacterium as the template. A nucleotide sequence of a DNA fragment containing the Corynebacterium glutamicum lysG and lysE genes (GenBank accession X96471) is shown as SEQ ID NO: 10, and the amino acid sequence of the LysE protein encoded by this gene is shown as SEQ ID NO: 9. LysG is encoded by a complementary sequence corresponding to the nucleotide numbers 1723 to 2352 in SEQ ID NO: 8.
The DNAs coding for the α-subunit, β-subunit and LysE protein of aspartokinase include DNAs coding for proteins that may include deletions, substitutions, insertions or additions of one or several amino acid residues at one or several positions in each protein, provided that the activities of the proteins are not lost. Although the number of amino acid residues meant by the term “several” may vary depending on the positions in the three dimensional structures of the proteins and types of amino acid residues, it is preferably 2 to 30, more preferably 2 to 20, particularly preferably 2 to 10, for each protein. This is based on the following reasons. That is, it is because some amino acids are highly homologous to each other, and the differences among such amino acids do not greatly affect the three dimensional structures and activities of proteins. Therefore, each protein may be one having a homology of 50% or more, preferably 70% or more, more preferably 90% or more, particularly preferably 95% or more, to the amino acid residues of SEQ ID NO: 6, 8 or 10 and having the activity of aspartokinase or LysE protein.
Such modification of the proteins as described above is a conservative mutation that maintains the activity of each protein. The substitution is a change in which at least on residue in an amino acid sequence is removed, and another residue is inserted there. Examples of substitution of an amino acid residue for an original amino acid residue considered as a conservative substitution include substitution of ser or thr for ala, substitution of gln, his or lys for arg, substitution of glu, gln, lys, his or asp for asn, substitution of asn, glu or gln for asp, substitution of ser or ala for cys, substitution of asn, glu, lys, his, asp or arg for gln, substitution of asn, gln, lys or asp for glu, substitution of pro for gly, substitution of asn, lys, gln, arg or tyr for his, substitution of leu, met, val or phe for ile, substitution of ile, met, val or phe for leu, substitution of asn, glu, gin, his or arg for lys, substitution of ile, leu, val or phe for met, substitution of trp, tyr, met, ile or leu for phe, substitution of thr or ala for ser, substitution of ser or ala for thr, substitution of phe or tyr for trp, substitution of his, phe or trp for tyr, and substitution of met, ile or leu for val.
A DNA coding for substantially the same protein as the protein having the amino acid sequence as shown in SEQ ID NOS: 6, 8 or 10 can be obtained by modifying the nucleotide sequence coding for the amino acid sequence as shown in SEQ ID NOS: 6, 8 or 10 by using, for example, site-specific mutagenesis, so that substitution, deletion, insertion or addition of one or several amino acid residues occurs. Such a modified DNA can be obtained in a conventional manner by treating with a regent or under conditions which cause a mutation. Examples of such a treatment include treating the DNA coding for the protein of the present invention with hydroxylamine, ultraviolet ray irradiation of a microorganism containing the DNA, treating with a regent such as N-methyl-N′-nitro-N-nitrosoguanidine or nitrous acid.
A DNA coding for such a modified protein as described above can also be obtained by isolating a DNA which is able to hybridize with the lysC gene, lysE gene or a portion of these genes under stringent conditions and still encodes a protein having aspartokinase activity or the activity of the LysE protein. The term “stringent conditions” includes a condition when a so-called specific hybrid is formed, and non-specific hybrid is not formed. The stringent conditions include, for example, conditions under which DNAs having high homology to each other, for example, DNAs having a homology of not less than 70%, preferably not less than 80%, more preferably not less than 90%, particularly preferably not less than 95%, are able to hybridize. The stringent conditions also include typical washing conditions of Southern hybridization, i.e., 1×SSC, 0.1% SDS, preferably 0.1×SSC, 0.1% SDS, at 60° C.
Examples of L-lysine producing bacteria belonging to the genus Escherichia include mutants having resistance to L-lysine analogues. The L-lysine analogue inhibits growth of Escherichia bacteria, but this inhibition is fully or partially eliminated when L-lysine coexists in a medium. Examples of L-lysine analogues include oxalysine, lysine hydroxamate, (S)-(2-aminoethyl)-L-cysteine (AEC), γ-methyllysine, α-chlorocaprolactam, and so forth. Mutants having resistance to these lysine analogues can be obtained by subjecting Escherichia microorganisms to a conventional artificial mutation treatment. Specific examples of bacterial strains used for producing L-lysine include E. coli AJ11442 (FERM BP-1543, NRRL B-12185; see Japanese Patent Laid-open No. 56-18596 and U.S. Pat. No. 4,346,170) and E. coli VL611 strains. The AJ11442 strain was deposited at International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (formerly National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Address: Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on May 1, 1981, and assigned accession number FERM P-5084. Then, this deposit was converted to an international deposit under the provisions of the Budapest Treaty on Oct. 29, 1987, and assigned accession number FERM BP-1543. In these microorganisms, feedback inhibition of aspartokinase by L-lysine is desensitized.
Furthermore, for example, bacteria with enhanced expression of a gene coding for an enzyme involved in L-lysine biosynthesis other than desensitized aspartokinase may also used as a preferred L-lysine producing bacteria. Examples of such an enzyme include enzymes involved in the diaminopimelate pathway, such as dihydrodipicolinate synthase, dihydrodipicolinate reductase, diaminopimelate decarboxylase, diaminopimelate dehydrogenase (International Patent Publication WO96/40934 for all of the foregoing enzymes), phosphoenolpyruvate carboxylase (Japanese Patent Laid-open No. 60-87788), aspartate aminotransferase (Japanese Patent Publication No. 6-102028), diaminopimelate epimerase (Japanese Patent Laid-open No. 2003-135066), and aspartate semialdehyde dehydrogenase (International Patent Publication WO00/61723), enzymes involved in the aminoadipate pathway, such as homoaconitate hydratase (Japanese Patent Laid-open No. 2000-157276), and so forth.
Specific examples of E. coli strains having L-lysine producing ability include the E. coli W3110(tyrA)/pCABD2 strain (International Patent Publication WO95/16042) and so forth. The E. coli W3110(tyrA)/pCABD2 strain was obtained by introducing the plasmid pCABD2 containing genes encoding the L-lysine biosynthesis system enzymes into W3110(tyrA), which is a tyrA deficient strain of E. coli (it was designated as AJ12604, deposited at International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (formerly National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, Address: Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Jan. 28, 1991, and assigned accession number FERM P-11975, and then the deposit was converted to an international deposit under the provisions of the Budapest Treaty on Sep. 26, 1991, and assigned accession number FERM BP-3579).
The plasmid pCABD2 contains a gene coding for a mutant dihydrodipicolinate synthase, wherein the histidine residue at position 118 is mutated to a tyrosine residue, and feedback inhibition by L-lysine is desensitized, a gene coding for a mutant aspartokinase III, wherein threonine residue at position 352 is mutated to an isoleucine residue, and feedback inhibition by L-lysine is desensitized, and genes coding for dihydrodipicolinate reductase and diaminopimelate dehydrogenase.
Furthermore, the E. coli W3110(tyrA) strain can be obtained as described below. That is, many strains obtained by introducing a plasmid into the W3110(tyrA) strain are disclosed in European Patent Laid-open Publication No. 488424/1992. For example, a strain obtained by introducing a plasmid pHATerm was designated as E. coli W3110(tyrA)/pHATerm strain, and deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology, and assigned accession number FERM BP-3653. The W3110(tyrA) strain can be obtained by, for example, eliminating the plasmid pHATerm from that E. coli W3110(tyrA)/pHATerm strain. Elimination of the plasmid can be performed in a conventional manner.
Furthermore, the WC196 strain (see International Patent Publication WO96/17930) can also be used as an L-lysine producing strain of E. coli. The WC196 strain was bred by imparting AEC (S-(2-aminoethyl)cysteine) resistance to the W3110 strain derived from E. coli K-12. This strain was designated E. coli AJ13069, and deposited at the National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology (presently International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology (Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan)) on Dec. 6, 1994, and assigned accession number FERM P-14690. Then, the deposit was converted to an international deposit under the provisions of the Budapest Treaty on Sep. 29, 1995, and assigned accession number FERM BP-5252.
The microorganism usable for the present invention may have decreased activity of an enzyme that catalyzes a reaction for generation of compounds other than L-lysine via pathway which branches off of the biosynthetic pathway of L-lysine, or an enzyme which down regulates L-lysine production, or may be deficient in such an enzyme. Illustrative examples of the enzyme involved in L-lysine production include homoserine dehydrogenase, lysine decarboxylase (cadA, ldcC) and malic enzyme. Strains in which the activities of these enzymes are decreased or deficient are described in International Patent Publications WO95/23864, WO96/17930, WO2005/010175, and so forth.
To reduce or eliminate enzymatic activities, genes encoding the enzymes on a chromosome may be mutated by a common mutagenesis method so that intracellular activities of the enzymes are reduced or eliminated. For example, this can be achieved by using genetic recombination to eliminate these genes coding for the enzymes on a chromosome or to modify an expression control sequence, such as a promoter or the Shine-Dalgarno (SD) sequence. It can also be achieved by introducing an amino acid substitution (missense mutation), introducing a stop codon (nonsense mutation), introducing a frame shift mutation adding or deleting one or two nucleotides into coding regions for the enzymes on the chromosome, or deleting a part of the genes (Journal of Biological Chemistry, 272:8611-8617 (1997)). The enzymatic activities can also be decreased or eliminated by constructing a gene which encodes a mutant enzyme, wherein the coding region is deleted, and replacing the wild type gene on the chromosome by homologous recombination or the like with the mutated gene, or introducing a transposon or IS factor into the gene.
For example, the following methods may be employed to introduce a mutation which causes a decrease in the activities of the aforementioned enzymes or eliminates the activities by genetic recombination. The objective gene on a chromosome can be replaced with a mutant gene which cannot produce an enzyme that normally functions by modifying a partial sequence of the objective gene to prepare the mutant gene, and transforming a coryneform bacterium with a DNA containing the mutant gene to cause recombination between the mutant gene and the gene on the chromosome. Such site-specific mutagenesis based on gene substitution using homologous recombination has been already established, and methods using linear DNA, methods using plasmids containing a temperature-sensitive replication origin (Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, pp. 6640-6645; U.S. Pat. No. 6,303,383; Japanese Patent Laid-open No. 05-007491) and so forth, are known. Furthermore, such site-specific mutagenesis based on gene substitution using homologous recombination as described above can also be performed with a plasmid which is not able to replicate in the host.
Furthermore, microorganisms which have been modified so that expression amount of the L-lysine and L-arginine secretion gene, ybjE, is increased can also be used for the present invention (International Patent Publication WO2005/073390).
Examples of L-lysine producing bacteria belonging to the genus Serratia include Serratia bacteria transformed with a DNA coding for dihydrodipicolinate synthase which has a mutation that desensitizes feedback inhibition by L-lysine, and Serratia bacteria containing aspartokinase which is desensitized to feedback inhibition by L-lysine (International Patent Publication WO96/41871).
Examples of coryneform bacteria which produce L-arginine include wild-type strains of coryneform bacteria: coryneform bacteria resistant to certain agents including sulfa drugs, 2-thiazolealanine, α-amino-β-hydroxyvaleric acid and so forth: coryneform bacteria exhibiting auxotrophy for L-histidine, L-proline, L-threonine, L-isoleucine, L-methionine or L-tryptophan in addition being resistant to 2-thiazolealanine (Japanese Patent Laid-open No. 54-44096); coryneform bacteria resistant to ketomalonic acid, fluoromalonic acid, or monofluoroacetic acid (Japanese Patent Laid-open No. 57-18989); coryneform bacteria resistant to argininol (Japanese Patent Laid-open No. 62-24075); coryneform bacteria resistant to X-guanidine (X represents a derivative of fatty acid or aliphatic chain, Japanese Patent Laid-open No. 2-186995) and so forth. Furthermore, the coryneform bacteria which are deficient in the L-arginine repressor (U.S. Patent Application No. 20020045233), and the coryneform bacteria with increased glutamate dehydrogenase activity (European Patent Publication Laid-open No. 1057893) are also suitable strains for L-arginine production.
Specifically, the examples include the Brevibacterium flavum AJ11169 (FERM BP-6892), Corynebacterium glutamicum AJ12092 (FERM BP-6906), Brevibacterium flavum AJ11336 (FERM BP-6893), Brevibacterium flavum AJ11345 (FERM BP-6894), and Brevibacterium lactofermentum AJ12430 (FERM BP-2228) strains. The AJ11169 and AJ12092 strains are resistant to 2-thiazolealanine (Japanese Patent Laid-open No. 54-44096). The AJ11336 strain is resistant to argininol and sulfadiazine (Japanese Patent Publication No. 62-24075). The AJ11345 strain is resistant to arginino, 2-thiazolealanine and sulfaguanidine, and is auxotrophic for histidine (Japanese Patent Publication No. 62-24075). The AJ12430 strain is resistant to octylguanidine and 2-thiazolealanine (Japanese Patent Laid-open No. 2-186995).
The Corynebacterium glutamicum AJ12092 was deposited at National Institute of Bioscience and Human-Technology, Agency of Industrial Science and Technology (presently International Patent Organism Depositary, National Institute of Advanced Industrial Science and Technology, Tsukuba Central 6, 1-1, Higashi 1-Chome, Tsukuba-shi, Ibaraki-ken, 305-8566, Japan) on Dec. 6, 1994, and assigned accession number FERM P-12092. Then, the deposit was converted to an international deposit under the provisions of the Budapest Treaty on Oct. 1, 1999, and assigned accession number FERM BP-6906.
Examples of Escherichia bacteria which are able to produce L-arginine include E. coli transformed with the argA gene (see Japanese Patent Laid-open No. 57-5693), and E. coli strain 237 (Russian Patent Application No. 2000117677), which is an L-arginine producing derivative of mutant strain which is able to assimilate an acetic acid. The 237 strain was deposited at the Russian National Collection of Industrial Microorganisms (VKPM), GNII Genetika (Address: Russia, 117545, Moscow, 1 Dorozhnyproezd, 1) on Apr. 10, 2000, and assigned number VKPM B-7925. The deposit was converted to an international deposit under the provisions of the Budapest Treaty on May 18, 2001. The E. coli strain 382 is a mutant which is resistant to feedback inhibition by L-arginine (Japanese Patent Laid-open No. 2002-017342), which is a derivative of the 237 strain, and can also be employed. The E. coli 382 strain was deposited at Russian National Collection of Industrial Microorganisms (VKPM) with a number VKPM B-7926 on Apr. 10, 2000, and the deposit was converted to an international deposit under the provisions of the Budapest Treaty on May 18, 2001.
Examples of Serratia bacteria which are able to produce L-arginine include Serratia marcescens which is unable to decompose L-arginine and is resistant to an arginine antagonist and canavanine, and is auxotorophic for lysine (see Japanese Patent Laid-open No. 52-8729).
Examples of coryneform bacteria which are able to produce L-histidine include microorganisms belonging to the genus Brevibacterium which are resistant to a thiamin antagonist, specifically, Brevibacterium lactofermentum FERM P-2170, FERM P-2316, FERM P-6478, FERM P-6479, FERM P-6480 and FERM P-6481 strains (Japanese Patent Laid-open Publication No. 59-63194). Furthermore, the examples include mutant strains belonging to the genus Brevibacterium or Corynebacterium which are resistant to polyketides and L-histidine producing ability, specifically, the FERM P-4161, FERM P-7273, FERM P-8371, FERM P-8372 and ATCC 14067 strains.
Examples of Escherichia bacteria which are able to produce L-histidine include mutant strains belonging to the genus Escherichia which are resistant to a histidine analogue, for example, the E. coli R-344 strain, and Escherichia bacteria transformed with L-histidine synthesis system enzyme genes isolated from the strain R-344. Specifically, the examples include the E. coli NRRL-12116, NRRL-12118, NRRL-12119, NRRL-12120 and NRRL-12121 strains (Japanese Patent Laid-open No. 56-5099).
Examples of Bacillus bacteria able to produce L-histidine include mutant strains belonging to the genus Bacillus which are resistant to a histidine analogue, and Bacillus bacteria transformed with a gene obtained from these mutant strains which are involved in resistance to histidine antagonist. Specifically, the examples include the FERM BP-218, FERM BP-224 and FERM BP-219 strains (Japanese Patent Laid-open No. 58-107192).
The fermentation broth or the processed product thereof containing the basic substance obtained by the present invention will contain carbonate ions or bicarbonate ions as counter anions for the dissociated basic substance. These carbonate ions or bicarbonate ions are emitted as carbon dioxide gas when the culture medium is heated or concentrated, or if the pH of the medium is lowered by adding a strong acid such as hydrochloric acid. The relative amount of the basic substance among the solid components in the fermentation broth is thus increased.
According to the present invention, by using bicarbonate ions, carbonate ions or the like in place of chloride ions and sulfate ions, the amount of the chloride ions can be reduced even to a level not causing corrosion of equipments, or sulfate ions can be reduced. Furthermore, after the fermentation, bicarbonate ions and carbonate ions can be replaced with chloride ions only by adding hydrochloric acid to the medium, lysine hydrochloride can be obtained only by further concentrating the medium without using ion exchange, and further, crystals of lysine hydrochloride can be directly separated.
In the present invention, the “fermentation product” includes concentrate and dried product obtained from the fermentation broth, and products obtained by processing the fermentation broth or dried product thereof.
EXAMPLES
Hereafter, the present invention will be explained more specifically with reference to the following examples.
Example 1
Construction of L-Lysine Producing Coryneform Bacterium
A gene coding for desensitized aspartokinase and a gene coding for a lysine secretion factor were introduced into a wild coryneform bacterium to prepare an L-lysine producing bacterium.
(1) Acquisition of Gene Coding for Desensitized Aspartokinase
A gene (lysC*) coding for aspartokinase (Ask*) which is desensitized to feedback inhibition by L-lysine and L-threonine was isolated by PCR from an L-lysine producing mutant strain, AJ3463 (FERM P-1987, see Japanese Patent Publication No. 51-34477) derived from the Corynebacterium glutamicum (Brevibacterium lactofermentum) ATCC 13869 strain by mutagenesis.
The AJ3463 strain was cultured in CM-Dex medium, and chromosomal DNA was extracted from the obtained cells by a typical method (Biochem. Biophys. Acta., 72, 619-629 (1963)). By using this chromosomal DNA as a template with an oligonucleotide ASK-F (SEQ ID NO: 1) for introducing a restriction enzyme BamHI site at the 5′ end of the objective DNA fragment and an oligonucleotide ASK-R (SEQ ID NO: 2) for introducing a restriction enzyme KpnI site at the 3′ end of the objective DNA fragment as primers for PCR, a gene DNA fragment containing lysC* as the objective gene was amplified. For amplification, a cycle consisting of a denaturation step at 98° C. for 10 seconds, an annealing step at 55° C. for 30 second and an extension step at 72° C. for 2 minutes was repeated 25 times. Enzyme, Pyrobest DNA Polymerase (Takara Shuzo), was used according to the manufacturer's instructions.
The amplified DNA fragment was purified by a phenol/chloroform treatment and ethanol precipitation, and then digested with the restriction enzymes BamHI and KpnI. The obtained reaction mixture was developed by agarose gel electrophoresis, the band containing the lysC* gene was excised, and the gene fragment was purified by conventional methods.
A shuttle vector for E. coli and Corynebacterium glutamicum, pVK7 (see U.S. Pat. No. 6,004,773), was separately treated with the restriction enzymes BamHI and KpnI in a similar manner, and ligated to the aforementioned lysC* fragment. Competent cells of the E. coli JM109 strain (Takara Shuzo) were transformed with the ligation reaction mixture according to the manufacturer's protocol, and several kanamycin-resistant colonies were selected.
The pVK7 was constructed by ligating a cryptic plasmid of Brevibacterium lactofermentum, pAM330, to a vector for E. coli, pHSG299 (Kmr, see Takeshita, S. et al., Gene, 61, 63-74, (1987)) as follows (see Japanese Patent Laid-open No. 11-266881, International Patent Publication WO99/07853). pAM330 was prepared from the Brevibacterium lactofermentum ATCC 13869 strain. pHSG299 was digested with Avail (Takara Shuzo), which had been blunt-ended with T4 DNA polymerase, then digested with HindIII (Takara Shuzo), and ligated to pAM330 blunt-ended with T4 DNA polymerase. Thus, pVK7 was obtained. pVK7 is autonomously replicable in cells of E. coli and Brevibacterium lactofermentum, and contains a multiple cloning site derived from pHSG299, lacZ′, and a kanamycin resistance gene as a marker.
Plasmid DNAs were extracted from the kanamycin-resistant colonies obtained as described above in a conventional manner, and the plasmid containing the objective lysC* gene was designated pVK-C*.
(2) Acquisition of a Gene Coding for Lysine Secretion Factor Lyse
By using chromosomal DNA from Brevibacterium lactofermentum ATCC 13869 as a template, the lysE gene was isolated by PCR (see U.S. Patent Application No. 2003113899). The lysE gene has been known to function in Corynebacterium bacteria to promote secretion of L-lysine (International Patent Publication 9723597A2). The chromosomal DNA of the strain was prepared in the same manner as described above.
LysE-F (SEQ ID NO: 3) and LysE-R (SEQ ID NO: 4) were used as the primers. The PCR was performed using Pyrobest (Takara Shuzo) with a heat treatment at 94° C. for 90 seconds, and the following cycle was repeated 30 times: denaturation at 94° C. for 20 seconds, annealing at 55° C. for 30 seconds and extension reaction at 72° C. for 60 seconds. Then the reaction was incubated at 72° C. for 10 minutes. A DNA fragment of the predicted size was obtained by this reaction. This DNA fragment was purified, and then cloned into cloning vector pCR2.1 (Invitrogene) according to the manufacturer's protocol. Competent cells of the E. coli JM109 strain (Takara Shuzo) were transformed with the ligation reaction mixture according to the manufacturer's protocol, and several ampicillin-resistant colonies were selected. Plasmid DNAs were extracted from these colonies, and the plasmid having the desired structure was designated pCRlysE.
Then, pCRlysE was digested with the restriction enzymes BamHI and XbaI, and subjected to agarose gel electrophoresis to obtain a fragment containing the lysE gene. A shuttle vector for E. coli and Corynebacterium glutamicum, pKC (see U.S. Patent application No. 2003113899), was separately treated with the restriction enzymes BamHI and KpnI in a similar manner, and subjected to agarose gel electrophoresis to obtain a gene fragment containing the chloramphenicol resistance gene. This gene was purified and then ligated to the aforementioned lysE fragment. By using this ligation reaction mixture, competent cells of the E. coli JM109 strain (Takara Shuzo) were transformed according to the manufacturer's protocol, and several chloramphenicol-resistant colonies were chosen. Plasmids were prepared from the colonies obtained as described above to obtain the LysE expression plasmid, plysE.
pKC4 was prepared as follows. A plasmid pHK4 (see Japanese Patent Laid-open No. 5-7491) having a replication origin derived from the already obtained plasmid pHM1519, which is autonomously replicable in coryneform bacteria (Agric. Biol. Chem., 48, 2901-2903 (1984)), was digested with the restriction enzymes BamHI and KpnI to obtain a gene fragment containing the replication origin. The obtained fragment was blunt-ended with a DNA Blunting Kit (Takara Shuzo), and inserted at the KpnI site of pHSG399 (Takara Shuzo) by ligation using a KpnI Linker (Takara Shuzo). Competent cells of the E. coli JM109 strain (Takara Shuzo) were transformed with this ligation reaction mixture according to the manufacturer's protocol, and several chloramphenicol-resistant colonies were selected. Plasmids were prepared from the colonies obtained as described above to obtain pKC4.
(3) Construction of L-Lysine Producing Coryneform Bacterium
The above-described two plasmids, pVK-C* and plysE, were introduced into the Brevibacterium lactofermentum ATCC 13869 strain by electroporation. The electroporation was performed by using Gene Pulser (BIO-RAD). The distance between the electrodes in the cuvette was 0.1 cm and the electric pulse application conditions were 25 μF, 200Ω and 1.8 kV. The strains containing the plasmids were selected on a CM-Dex agar plate (see below for the composition of the medium) containing 5 μg/l of chloramphenicol and 25 μg/l of kanamycin. The strain containing the plasmid was cultured overnight at 31.5° C. with shaking in the CM-Dex liquid medium containing 5 μg/l of chloramphenicol and 25 μg/l of kanamycin. The culture was performed in 3 ml of the culture medium in a test tube with shaking.
The CM-Dex medium was prepared as follows. All the components listed in Table 1 were mixed, adjusted to pH 7.5 with KOH, and then sterilized by autoclaving at 120° C. for 20 minutes. In the agar medium, agar was added to a final concentration of 20 g/L.







TABLE 1





Composition of CM-Dex medium (per 1 L)



















Glucose
5
g



Polypeptone
10
g



Yeast extract
10
g



KH2PO4
1
g



MgSO4•7H2O
0.4
g



FeSO4•7H2O
0.01
g



MnSO4•4H2O or 5H2O
0.01
g



Urea
3
g



Mameno (soy bean protein hydrolysate,
1.2
g



in terms of nitrogen weight)



Biotin
10
μg







(Filled to 1 L with sterilized water)






As described above, an L-lysine producing coryneform bacterium, ATCC 13869/pVK-C*, plysE was obtained.
Example 2
Growth of L-Lysine Producing Bacterium in an Alkaline Medium, and Effect of the Total Ammonia Concentration on L-Lysine Production
By using the L-lysine producing bacterium constructed in Example 1, the total ammonia concentration not inhibiting productivity of L-lysine in an alkaline medium was investigated.
First, a conventional culture method was used. That is, a medium (medium B) obtained by adding ammonium sulfate was added to the medium A (composition was shown in Table 2) in an amount of 55% (w/w) based on glucose was used. The pH of the medium was maintained constant with ammonia gas during the culture, to perform L-lysine fermentation. The pH was controlled to be 7.0 or 8.0.







TABLE 2





Composition medium A (per 1 L)



















Glucose
100
g



Yeast extract
10
g



KH2PO4
1
g



MgSO4•7H2O
1
g



Vitamin B1 hydrochloride
2
mg



Biotin
0.5
mg



Nicotinamide
5
mg



FeSO4•7H2O
10
mg



MnSO4•4H2O or 5H2O
10
mg



10% GD-113 (antifoaming agent)
0.05
mL










Specifically, the culture was performed as follows. The aforementioned strain was inoculated into 3 ml of the CM-Dex liquid medium and cultured overnight at 31.5° C. with shaking, and 200 μl of the medium was uniformly spread onto CM-Dex agar medium. The culture was performed overnight at 31.5° C. as a stationary culture. Then, one third of the L-lysine producing bacterial cells which grew on the agar medium on one plate were inoculated to 300 ml of medium B in a jar fermenter and cultured. During the culture, the medium was aerated with 300 ml per minute of filter-sterilized air, the stirring rate was maintained at 700 rpm, and the temperature of the medium was maintained at 31.5° C. The results are shown in FIG. 1.
As a result, at pH 7.0, 17.4 g/L of L-lysine was accumulated, and the production rate was 0.725 g/L/hr. On the other hand, at pH 8.0, the growth of cells was almost not existent, and fermentation did not progress (FIG. 1).
Then, fermentation was performed using the L-lysine producing bacteria in a medium without ammonium sulfate.
The aforementioned strain was inoculated into 3 ml of the CM-Dex liquid medium and cultured overnight at 31.5° C. with shaking, and 200 μl of the medium was uniformly spread onto the CM-Dex agar medium and left overnight at 31.5° C. 300 ml of medium A (without ammonium sulfate) was placed in a jar fermenter, and the pH was adjusted to 7.8, 8.2, or 8.9 by bubbling ammonia gas through the medium. One third of the L-lysine producing bacterial cells which grew on the agar medium on one plate were inoculated into the medium, and cultured. During the culture, the medium was aerated with 300 ml per minute of filter-sterilized air, the stirring rate was maintained at 700 rpm, and the temperature of the medium was maintained constant at 31.5° C. During the culture, a constant pH was maintained by bubbling ammonia gas through the medium. As a result, it was confirmed that, at pH 8.9, after the total ammonia concentration in the medium exceeded 100 mM, growth and L-lysine production, in particular, were strongly inhibited.
In the aforementioned culture method, carbon dioxide gas generated by the L-lysine producing bacteria dissolved in the medium as carbonate ions or bicarbonate ions, which results in a lowered pH as the culture progresses. Therefore, the amount of added ammonia necessary to control the pH at the predetermined level increases. Furthermore, the pH level to which the medium had been adjusted became higher, the concentrations of dissolved carbonate ions and bicarbonate ions became higher, and therefore the concentration of the ammonia added in order to adjust the pH to the predetermined value became higher.
Non-dissociated ammonia easily penetrates cells resulting in cellular damage the cells. Since a higher pH results in a lower amount of dissociation of ammonia, the growth of bacteria was inhibited as the pH increases, even if the total ammonia concentration is maintained at a constant level. Therefore, it was concluded that at pH 8.9 or lower, if the total ammonia concentration is controlled to be low, for example, at 100 mM or lower, inhibition of bacterial growth and L-lysine accumulation is not significant.
Then, culture for L-lysine production was performed while the total ammonia concentration in the medium was controlled at 100 mM or lower at pH 7.8, 8.2, and 8.9.
The aforementioned strain was inoculated into 3 ml of the CM-Dex liquid medium and cultured overnight at 31.5° C. with shaking, and 200 μl of the medium was uniformly spread onto the CM-Dex agar medium and left overnight at 31.5° C. 300 ml of medium A (without ammonium sulfate) was placed in a jar fermenter, and the pH was adjusted to 7.5, 7.8, 8.2, or 8.9 by bubbling ammonia gas through the medium. One third of the L-lysine producing bacterial cells which grew on the agar medium on one plate were inoculated into the medium, and cultured. During the culture, the medium was aerated with 300 ml per minute of filter-sterilized air, the stirring rate was maintained at 700 rpm, and the temperature of the medium was maintained at 31.5° C. During the culture, the pH was maintained at each of the predetermined levels with 6 N potassium hydroxide instead of ammonia.
The total ammonia concentration in the medium was measured by using an ammonia electrode and an ion meter (Orion). The medium was periodically sampled, and the total ammonia concentration controlled to within 0 to 100 mM by adding 10% aqueous ammonia solution as required. Furthermore, by monitoring the dissolved oxygen concentration in the medium, a sharp increase in the dissolved oxygen concentration when ammonia was depleted was detected. When this occurred, a 10% aqueous ammonia solution was added to prevent continuous depletion of ammonia in the medium. The results are shown in FIG. 2.
As a result, favorable growth and L-lysine production were observed at all pH levels from 7.8 to 8.9. Compared with the fermentation performed at pH 7.0 using the conventional culture method, the production rate of 115% or higher was observed.
Example 3
Production of L-Lysine
In this example, L-lysine fermentation was performed by controlling only the total ammonia concentration, but not controlling the pH. The range of the total ammonia concentration was maintained at 100 mM or lower. This range was chosen based on the results in Example 2.
The aforementioned strain was inoculated into 3 ml of the CM-Dex liquid medium and cultured overnight at 31.5° C. with shaking, and 200 μl of the medium was uniformly spread onto the CM-Dex agar medium and left overnight at 31.5° C. 300 ml of medium A (without ammonium sulfate) was placed in a jar fermenter, and the total ammonia concentration of the medium was adjusted to 23.8 mM by bubbling ammonia gas through the medium. One third of the L-lysine producing bacterial cells which grew on the agar medium on one plate were inoculated into the medium, and cultured. During the culture, the medium was aerated with 300 ml per minute of filter-sterilized air, the stirring rate was maintained at 700 rpm, and the temperature of the medium was maintained at 31.5° C. The total ammonia concentration was measured periodically, and an appropriate amount of 10% aqueous ammonia was added to the medium as required so that the total ammonia concentration was maintained between 0 to 100 mM. As a result, 15.9 g/L of lysine was accumulated, and L-lysine fermentation progressed (FIG. 3).
Example 4
Construction of L-Lysine Producing E. coli Bacterium
<1> Construction of Strain in which the cadA and ldcC Genes Coding for Lysine Decarboxylase are Disrupted
A lysine decarboxylase deficient strain was constructed first. Lysine decarboxylases are encoded by the cadA gene (Genbank Accession No. NP-418555, SEQ ID NO: 15), and the ldcC gene (Genbank Accession No. NP—414728, SEQ ID NO: 17) (see International Patent Publication WO96/17930). In this example, the WC196 strain was used as the parent strain.
The cadA and ldcC genes coding for lysine decarboxylase deleted by using the method developed first by Datsenko and Wanner called “Red-driven integration” (Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, pp. 6640-6645), and an excision system derived from λ phage (J. Bacteriol., 2002 Sep., 184 (18):5200-3, Interactions between integrase and excisionase in the phage lambda excisive nucleoprotein complex, Cho E H, Gumport R I, Gardner J F). According to the “Red-driven integration” method, a PCR product obtained using synthetic oligonucleotide primers in which a part of the objective gene is designed on the 5′ side and a part of an antibiotic resistance gene is designed on the 3′ side, can be used to obtain a gene-disrupted strain in one step. Furthermore, by using the excision system derived from λ phage in combination, the antibiotic resistance gene which had been incorporated into the gene-disruption strain can be eliminated (Japanese Patent Laid-open No. 2005-058227).
(1) Disruption of the cadA Gene
The plasmid pMW118-attL-Cm-attR (Japanese Patent Laid-open No. 2005-058827) was used as a template in PCR. pMW118-attL-Cm-attR was obtained by inserting the attL and attR genes which are the attachment sites of λ phage, and the cat gene which is an antibiotic resistance gene into pMW118 (Takara Bio). The order of insertion is attL-cat-attR.
PCR was performed with the synthetic oligonucleotide primers shown as SEQ ID NOS: 11 and 12 which have sequences corresponding to both ends of attL and attR at the 3′ ends of the primers, and a sequence corresponding to a portion of the objective cadA gene at the 5′ ends of the primers.
The amplified PCR product was purified on agarose gel and introduced by electroporation into the E. coli WC169 strain which contains plasmid pKD46 which is temperature sensitive replicable. The plasmid pKD46 (Proc. Natl. Acad. Sci. USA, 2000, vol. 97, No. 12, pp. 6640-6645) contains a DNA fragment of 2154 nucleotides in total of λ phage containing genes coding for Red recombinase of the λ Red homologous recombination system (λ, β, exo genes) controlled by arabinose-inducible ParaB promoter (GenBank/EMBL Accession No. J02459, nucleotides at positions 31088 to 33241). The plasmid pKD46 is required to incorporate the PCR product into the chromosome of the WC196 strain.
Competent cells for the electroporation were prepared as follows. That is, the E. coli WC196 strain cultured overnight at 30° C. in the LB medium containing 100 mg/L of ampicillin was diluted 100 times with 5 mL of the SOB medium (Molecular Cloning: A Laboratory Manual, 2nd Edition, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (1989)) containing ampicillin (20 mg/L) and L-arabinose (1 mM). The cells in the diluted culture were grown at 30° C. with aeration until the OD600 reached about 0.6, and then the culture was concentrated 100 times and washed three times with 10% glycerol so that the cells could be used for electroporation. The electroporation was performed by using 70 μl of the competent cells and about 100 ng of the PCR product. After the electroporation, the cells were added to 1 mL of the SOC medium (Molecular Cloning: A Laboratory Manual, 2nd Edition, Sambrook, J. et al., Cold Spring Harbor Laboratory Press (1989)), cultured at 37° C. for 2.5 hours, and then cultured at 37° C. as plate culture on L agar medium containing 25 mg/L of Cm (chloramphenicol), and Cm-resistant recombinants were selected. Then, in order to remove the pKD46 plasmid, the cells were subcultured twice at 42° C. on the L agar medium containing Cm, and ampicillin resistance of the colonies was examined to obtain an ampicillin-sensitive strain without pKD46.
Deletion of the cadA gene in the mutant identified on the basis of the chloramphenicol resistance gene was confirmed by PCR. The obtained cadA deficient strain was designated WC196ΔcadA::att-cat strain.
Then, in order to remove the att-cat gene which had been introduced into the cadA gene, a helper plasmid pMW-intxis-ts (Japanese Patent Laid-open No. 2005-058827) was used. pMW-intxis-ts carries a gene coding for integrase (Int) and a gene coding for excisionase (Xis) of λ phage, and is temperature sensitive replicable.
Competent cells of the WC196ΔcadA::att-cat strain obtained above were prepared in a conventional manner, transformed with the helper plasmid pMW-intxis-ts, and cultured on L-agar medium containing 50 mg/L of ampicillin at 30° C. as a plate culture, and ampicillin-resistant strains were selected.
Then, in order to remove the pMW-intxis-ts plasmid, the selected strains were subcultured twice on L-agar medium at 42° C., and ampicillin resistance and chloramphenicol resistance of the obtained colonies were examined to obtain a chloramphenicol- and ampicillin-sensitive cadA-disrupted strain without att-cat and pMW-intxis-ts. This strain was designated WC196ΔcadA.
(2) Deletion of the ldcC Gene from the WC196ΔcadA Strain
The ldcC gene was deleted from the WC196ΔcadA strain according to the aforementioned method using the primers of SEQ ID NOS: 13 and 14 as primers for disruption of ldcC. A cadA- and ldcC-disrupted strain, WC196ΔcadAΔldcC, was thereby obtained.
<2> Introduction of Plasmid for Lys Production into WC196ΔcadAΔldcC Strain
The WC196ΔcadAΔldcC strain was transformed with plasmid pCABD2 for Lys production carrying the dapA, dapB and lysC genes (International Patent Publication WO01/53459) in a conventional manner to obtain WC196ΔcadAΔldcC/pCABD2 strain (WC196LC/pCABD2).
Example 5
L-Lysine Production Using E. coli 
This example shows an example of the present invention applied to the production of L-lysine by E. coli. In this example, L-lysine was produced by fermentation without adding ammonium sulfate and ammonium chloride, which are generally added to the media for the purpose of supplying nitrogen and counter ions for L-lysine in L-lysine production by fermentation. Specifically, the culture was performed without controlling the pH, but controlling the ammonia concentration in the medium. The range within which the ammonia concentration in the medium should be controlled was examined beforehand. As a result, it was confirmed that the total ammonia concentration is preferably in the range of 50 to 100 mM. Therefore, in the practical main culture, the total ammonia concentration was controlled to be 100 mM or lower by bubbling ammonia gas. Furthermore, when the total ammonia concentration decreased to 50 mM, it was controlled with ammonia gas to maintain that concentration.
WC196ΔcadAΔldcC/pCABD2 was used for lysine production. 300 ml of the L-lysine production medium for E. coli shown in Table 3 placed in a jar fermenter was used. The total ammonia concentration was adjusted to 95 mM by bubbling ammonia gas To this medium, cells obtained by culturing the L-lysine producing strain on the entire surface of LB agar medium containing 20 μg/L of streptomycin and culturing it at 37° C. for 24 hours were inoculated. The amount of the inoculated cells corresponded to the cells grown on three plates of the agar medium. The culture was performed with a temperature of the medium maintained at 37° C., aeration of 50 ml per minute of filter-sterilized air, and a stirring rate of 700 rpm. When the dissolved oxygen concentration in the medium decreased to 20% saturation, the aeration rate was changed to 100 mL per minute. The feed solution for E. coli shown in Table 4 was appropriately added dropwise to the medium so that glucose is not depleted, and the concentration thereof does not become 30 g/L or higher in the medium. Finally, when 36 g of glucose was consumed, the culture was terminated. As a result, the culture could be favorably performed so that all the added glucose was consumed after 33 hours, 13.4 g of L-lysine was accumulated, and the production rate of L-lysine was 1.2 g/L/hr. The yield of this production was 37%.
As a control, results are shown for L-lysine production performed with the same strain by adding ammonium sulfate, and not controlling the total ammonia concentration, but controlling the pH, similar to common production methods for basic amino acids. The same strain was inoculated in a similar manner to a medium consisting of the L-lysine production medium for E. coli shown in Table 3 added with 13 g/L of ammonium sulfate, and cultured while controlling the pH to be constant at 6.7 by appropriately bubbling ammonia gas. The culture temperature, aeration rate, and stirring rate were the same as those described above. In this case, the feed solution for E. coli added with 112.5 g/L of ammonium sulfate (containing ammonium sulfate, Table 5) was added instead of the feed solution for E. coli shown in Table 4, so that that glucose is not depleted, and glucose concentration should not become 30 g/L or higher in the medium, and finally 36 g of glucose was consumed. As a result, all the added glucose was consumed after 33 hours, 14.7 g of L-lysine was accumulated, and the production rate of L-lysine was 1.3 g/L/hr. The yield of this production was 40%.
All the lysine concentrations described above are shown in terms of lysine hydrochloride. Furthermore, changes in the total ammonia concentration and pH during the cultures are shown in FIG. 4. From comparison of these results, it was confirmed that, when the method of the present invention was used, L-lysine production by fermentation could be performed without adding ammonium sulfate or ammonium chloride at a production rate of about 92%, a yield of about 93% and an L-lysine production amount of about 91% compared with those obtained in the common fermentative production in which ammonium sulfate was added.







TABLE 3





Composition of L-lysine production medium for E. coli (per 1 L)



















glucose
30
g



KH2PO4
1
g



MgSO4•7H2O
1.2
g



Mameno (soy bean protein hydrolysate,
0.77
g



in terms of nitrogen weight)



FeSO4•7H2O
30
mg



MnSO4•4H2O or 5H2O
30
mg



p-aminobenzoic acid
2
mg



L-threonine
300
mg



DL-methionine
200
mg



cystine
150
mg



betaine
3.5
g



GD-113 (antifoaming agent)
0.05
mL










Glucose and FeSO4.7H2O were weighed as a portion A, the other components were weighed as a portion B, and the portion A as it was and the portion B adjusted to pH 5.0 were separately sterilized by autoclaving at 115° C. for 10 minutes, and then mixed. 20 μg/L of streptomycin was added to the medium prior to use.







TABLE 4





Composition of feed solution for E. coli (per 1 L)



















glucose
561
g



GD-113
7
μl



KH2PO4
1.48
g



L-thr
0.44
g










The components were sterilized by autoclaving at 120° C. for 20 minutes.
20 μg/L of streptomycin was added to the medium prior to use.







TABLE 5





Composition of feed solution for E. coli


containing ammonium sulfate (per 1 L)



















glucose
561
g



GD-113
7
μl



KH2PO4
1.48
g



L-thr
0.44
g



(NH4)2SO4
112.5
g/L










The components were sterilized by autoclaving at 120° C. for 20 minutes.
20 μg/L of streptomycin was added to the medium prior to use.
Example 6
Production of L-Arginine
This example shows an example of the present invention applied to L-arginine production by a coryneform bacterium. Corynebacterium glutamicum AJ12092 (FERM BP-6906) was used as the L-arginine-producing strain.
First, as a control, results are shown for L-arginine production performed with the same strain by adding ammonium sulfate, and not controlling the total ammonia concentration, but controlling the pH, similar to a common methods for production of a basic amino acid. A medium for L-arginine production having the composition shown in Table 6, with the addition of 65 g/L of ammonium sulfate, Also, the glucose concentration was changed to 40 g/L. 300 ml of this medium was placed in a jar fermenter, and the pH was controlled to be 7.0 by bubbling ammonia gas. To this medium, two plates of cells obtained by culturing the Corynebacterium glutamicum AJ12092 strain on the entire surface of CM-Dex agar medium at 31.5° C. for 24 hours were inoculated. The culture was performed at a temperature of the medium maintained at 31.5° C. with aeration of 150 ml per minute of filter-sterilized air and stirring at a rate of 700 rpm. Furthermore, during the culture, the pH was controlled to be 6.9 by adding a 6 N KOH solution which had been separately sterilized. As the culture progresses, the glucose concentration decreases. In order to maintain the glucose concentration at 30 to 40 g/L, a separately sterilized glucose solution of 692 g/L was appropriately added. The culture was performed for 54 hours. As a result, 23.4 g/L of L-arginine was accumulated, the production yield of L-arginine was 26.7% of the consumed glucose, and the production rate was 0.43 g/L/hr. The amount of glucose consumed during the culture was 29.1 g per batch.
Production of L-arginine when ammonium sulfate is not added to the medium, and while controlling only the total ammonia concentration, but not the pH was performed. The results are shown hereinafter. 300 ml of the L-arginine production medium having the composition shown in Table 6 but not containing ammonium sulfate was placed in a jar fermenter, and the total ammonia concentration was adjusted to 12.6 mM by bubbling ammonia gas. To this medium, two plates of cells of the L-arginine producing strain cultured in the same manner as that of the control were inoculated. The culture was performed in the same manner as that of the control with maintaining the temperature at 31.5° C., aerating 150 ml per minute of filter-sterilized air, and maintaining the stirring rate at 700 rpm. By measuring the total ammonia concentration of the medium periodically or using an ammonia concentration controlling apparatus, the total ammonia in the medium was controlled so that it was at various levels during the culture. As a result, it was confirmed that the total ammonia concentration in the medium controlled to be about 20 mM by adding ammonia gas as required provided favorable results. The culture performed with controlling the total ammonia concentration in the medium to be about 20 mM based on the above result favorably progressed, 24.2 g/L of L-arginine was accumulated after 51 hours, and thus L-arginine fermentation was attained (FIG. 4). The glucose consumed during the culture was 35.1 g per batch, the production yield of L-arginine was 20.6% of the consumed glucose, and the production rate was 0.47 g/L/hr. Furthermore, the pH of the medium increased from 7.92 at the start of the culture to 8.02 at the end of the culture.
From the comparison of these results with those of the control experiment, it was demonstrated that the L-arginine production by fermentation could be performed without adding ammonium sulfate or ammonium chloride at a yield of about 77% and a production rate higher by about 9% compared with those obtained in the common fermentative production in which ammonium sulfate was added.







TABLE 6





Composition of L-arginine production medium (per 1 L)



















glucose
150
g



KH2PO4
1
g



MgSO4•7H2O
0.4
g



Mameno (soy bean protein hydrolysate,
0.23
g



in terms of nitrogen weight)



vitamin B1 hydrochloride
0.5
mg



biotin
0.5
mg



FeSO4•7H2O
10
mg



MnSO4•4H2O or 5H2O
10
mg



GD-113 (antifoaming agent)
0.05
mL










The medium was adjusted to pH 7.0 with potassium hydroxide aqueous solution, up to 1 L, and sterilized by autoclaving at 115° C. for 10 minutes.
Explanation of Sequence Listing
SEQ ID NO: 1: Primer sequence for cloning of lysC gene
SEQ ID NO: 2: Primer sequence for cloning of lysC gene
SEQ ID NO: 3: Primer sequence for cloning of lysE gene
SEQ ID NO: 4: Primer sequence for cloning of lysE gene
SEQ ID NO: 5: Nucleotide sequence of lycC* gene and amino acid sequence of α-subunit of inhibition-desensitized asp artokinase
SEQ ID NO: 6: Amino acid sequence of α-subunit of inhibition-desensitized asp artokinase
SEQ ID NO: 7: Nucleotide sequence of lycC* gene and amino acid sequence of 3-subunit of inhibition-desensitized asp artokinase
SEQ ID NO: 8: Amino acid sequence of β-subunit of inhibition-desensitized asp artokinase
SEQ ID NO: 9: Nucleotide sequence of lysE gene and amino acid sequence of LysE protein
SEQ ID NO: 10: Amino acid sequence of LysE protein
SEQ ID NO: 11: Primer for disruption of cadA gene
SEQ ID NO: 12: Primer for disruption of cadA gene
SEQ ID NO: 13: Primer for disruption of ldc gene
SEQ ID NO: 14: Primer for disruption of ldc gene
SEQ ID NO: 15: Nucleotide sequence of cadA gene and amino acid sequence of lysine decarboxylase
SEQ ID NO: 16: Amino acid sequence of lysine decarboxylase (cadA)
SEQ ID NO: 17: Nucleotide sequence of ldc gene and amino acid sequence of lysine decarboxylase
SEQ ID NO: 16: Amino acid sequence of lysine decarboxylase (ldc)
INDUSTRIAL APPLICABILITY
According to the present invention, a basic substance can be produced by fermentation even at a high pH, which enables reduction of the amounts of industrial raw materials such as ammonium sulfate, without substantially degrading the performances essentially obtained in the conventional common culture methods such as productivity.
Although the fermentation broth obtained by the method of the present invention contains carbonate ions and/or bicarbonate ions, these are easily emitted into air by heating, and therefore a fermentation broth or fermentation product with a large amount of the basic substance present as a solid can be obtained. Furthermore, when purification is needed, if an acid stronger than carbonic acid is added to the fermentation broth, carbonate can be easily replaced with the stronger acid without performing ion exchange, which is usually performed in the conventional production methods. Furthermore, crystals of lysine hydrochloride can be directly obtained by concentrating the fermentation broth.
While the invention has been described in detail with reference to preferred embodiments thereof, it will be apparent to one skilled in the art that various changes can be made, and equivalents employed, without departing from the scope of the invention. Each of the aforementioned documents is incorporated by reference herein in its entirety.

## Claims

1. A method for producing a basic amino acid by fermentation comprising culturing a bacterium having an ability to produce the basic amino acid in a liquid medium contained in a fermentation tank to produce and accumulate the basic amino acid in the medium, wherein the amount of sulfate and/or chloride ions which function as counter ions of the basic substance is reduced by adjusting the total ammonia concentration in the medium to during a period selected from the group consisting of:
A) when the pH of the medium increases due to a shortage of the counter ions caused by accumulation of the basic amino acid, and
B) when the pH increases due to the addition of cations to the medium;
wherein the concentration of ammonium ions in the medium is at such a level that the sum of the ion equivalents of bicarbonate ions and/or carbonate ions and other anions dissolved in the medium is larger than the ion equivalent of the basic amino acid ionized from the basic amino acid which has accumulated in the medium, and
wherein said total ammonium concentration in the medium is at a level which does not inhibit the production of the basic amino acid by the bacterium, and wherein said concentration is determined beforehand as follows:
a) the bacterium is cultured in the medium at various pH values and various total ammonia concentrations, but within the range of 1 to 500 mM,
b) productivity of the basic amino acid is measured at each pH value and each total ammonia concentration, and
c) a total concentration providing 50% or more of productivity of the basic amino acid based on the productivity obtained under optimum conditions is determined for each pH value.

2. The method according to claim 1, wherein the range of the total ammonia concentration is determined beforehand as follows:
(A′) the culture is performed in a medium which contains sulfate and/or chloride ions in an amount sufficient for performing the culture at pH 7.2 or lower as a source of counter ions of the basic amino acid, of which pH is maintained to be in the range between 6.5 to 7.2 by adding at least one of ammonia gas, aqueous ammonia and urea, and productivity of the basic substance amino acid is measured,
(B′) the culture is started in the same medium as that used in the step (A′) except that sulfate and/or chloride ions of medium components are lowered by a desired amount, and the culture is continued with various total ammonia concentrations during a period where it becomes impossible to maintain the pH of the medium to be 7.2 or lower due to the shortage of sulfate ions and/or chloride ions as counter ions of the basic substance amino acid caused by accumulation of the objective basic substance amino acid to determine a total ammonia concentration range providing 50% or more of productivity based on the productivity measured in the step (A′).

3. The method according to claim 1, wherein the total ammonia concentration in the medium is adjusted by adding ammonia or urea to the medium when the activity of the bacterium is reduced or ceases as determined based on the indicators: dissolved oxygen concentration in the medium, consumption rate of carbon source in the medium, turbidity of the medium, productivity of the basic amino acid, and pH change in the medium.

4. The method according to claim 1, wherein a medium having the same composition as that of a medium containing sulfate ions and/or chloride ions as a counter ion source of the basic amino acid in an amount sufficient for performing the culture at pH 7.2 or lower except that amount of sulfate ions and/or chloride ions is reduced by a desired amount is used as the medium, and
the at least a part of the total period is a period where pH of the medium cannot be maintained to be 7.2 or lower due to shortage of counter ions for the basic amino acid which has accumulated in the medium.

5. The method according to claim 1, wherein the other anions are selected from the group consisting of sulfate ions, chloride ions, phosphate ions, ionized organic acids, and combinations thereof.

6. The method according to claim 1, wherein total amount of the other anions is 900 meq/l or lower.

7. The method according to claim 1, wherein the total ammonia concentration in the medium is adjusted to be 200 mM or lower.

8. The method according to claim 1, which comprises the step of proliferating the bacterium.

9. The method according to claim 8, wherein the total ammonia concentration is not adjusted during the step of proliferating the bacterium.

10. The method according to claim 1, wherein the basic amino acid is selected from the group consisting of L-lysine, L-arginine and L-histidine.

11. The method according to claim 10, wherein the basic amino acid is L-lysine.

12. The method according to claim 10, wherein the basic amino acid is L-arginine.

13. The method according to claim 1, wherein the medium or a processed product thereof is heated after the fermentation to eliminate bicarbonate ions and carbonate ions.

14. The method according to claim 1, wherein the bacterium is a coryneform bacterium or an Escherichia bacterium.

15. The method according to claim 1, wherein the bacterium is Escherichia coli.