PROCESS FOR CONTROLLING CULTURE OF RECOMBINANTS

Abstract: Expression of a desired gene and production of a product from the desired gene can be obtained effectively by cultivating a microorgansim containing a hybrid plasmid comprising the desired gene, a vector and a promoter in its cell, and adding an inducer and a nutriment simultaneously to the culture medium at the time when the nutriment originally present in the culture medium is almost con­ sumed.

Description: BACKGROUND OF THE INVENTION This invention relates to a process for effectively cultivating microorganisms containing therein desired genes obtained from animals, plants and microorganisms so as to produce a product from the desired genes in large amounts. Recently, there has been developed a gene recombination technique for producing a large amount of a useful substance by using a host microorganism containing a hybrid plasmid obtained by inserting a gene encoding the useful substance into a vector plasmid of the host microorganism. By this technique, human interferon, insulin, etc. are produced. As the host microorganism, Escherichia coli is generally used. But a process for industrially producing the desired product in large amounts by using a host microorganism maintaining the desired gene has not been developed yet. Thus, it has been desired to develop a process for cultivating effectively the microorganism harboring hybrid plasmids. SUMMARY OF THE INVENTION It is an object of this invention to provide a process for increasing the production of a desired protein in a recombinant host containing a plasmid vector carrying the gene encoding the protein under the control of a promoter, characterized by culturing Escherichia coli, containing as promotor a trp promoter, a lac promoter or a tac promoter, and adding an inducer and a nutriment simultaneously to the culture medium at the time when the nutriment originally present in the culture is almost exhausted, wherein the consumption of nutriment is measured by a dissolved oxygen concentration. According to a special embodiment, the process carried through in using as the inducing substance 3-β-indolylacrylic acid or isopropyl-β-D-thiogalactoside. According to another special embodiment, the process is carried through wherein the nutriment is casamino acid, glucose, amino acids other than tryptophan, yeast extract, or a mixture thereof. According to a further embodiment, the process is carried through wherein the consumption of nitriment is carried out by adding a repressing substance to the culture medium at an initial stage of the culture. Finally, it is a preferred embodiment to use as the repressing substance tryptophan. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a structure of hybrid plasmid pTREZ1. Fig. 2 shows a base sequence at the downstream region of a trp promoter. Fig. 3 is a flow diagram showing the construction of hybrid plasmid pTREZ1. Fig. 4 is a graph showing a relationship between the adding amount of IA and the β-galactosidase produced amount. Fig. 5 is a schematic diagram showing one example of culture apparatus usable in this invention. Fig. 6 is a graph showing an influence of adding time of IA and casamino acid using a DO concentration as an indicator. Fig. 7 is a graph showing an influence of adding time of casamino acid using a DO concentration as an indicator. Fig. 8 is a graph showing a relationship between the amount of tryptophan and the β-galactosidase produced. DESCRIPTION OF THE PREFERRED EMBODIMENTS As the promoter for expressing the desired gene, there can be used a trp (tryptophan) promoter, a lac promoter (Nature, 281 , pp 544-548, 1979), a tac promoter (Proc. Natl. Acad. Sci. USA, 80 , pp 21-25, 1983), etc. As the hybrid plasmid obtained by ligating a trp promoter and β-galactosidase gene, there can be used hybrid plasmid pTREZ1 having the structure shown in Fig. 1. The hybrid plasmid pTREZ1 can be obtained from plasmid pTRE1 and pMC1403. The plasmid pTRE1 containing a trp promoter is obtained by inserting a DNA fragment of about 500 bp (base pairs) containing a promoter of trp operon of E . coli , trp L (leader peptide) and a part of end portion of trp E (anthranilate synthase) into the EcoRI site of plasmid pBR322 (Gene, 29 , pp 41-49, 1984). The direction of trp promoter is the direction of Tc r (a tetracycline resistance gene) of pBR322. The EcoRI site on the upstream side of trp promoter is removed by filling up with DNA polymerase I so as to leave only the EcoRI site on the downstream of the promoter. Fig. 2 shows the base sequence in the trp promoter downstream region. By connecting a foreign gene at the EcoRI site in trp E polypeptide gene, it is possible to express the foreign gene in the form fused with 8 amino acids at the N-terminal side of trp E polypeptide. On the other hand, as the β-galactosidase gene, plasmid pMC1403 (J. Bacteriol, 143 , pp 971-980, 1980) can be used. Plasmid pMC1403 is obtained by inserting β-galactosidase gene (lac Z + lac Y) of 6.2 kb (kilobase pairs) between the EcoRI site and the SalI site of pBR322. Fig. 3 shows a process for constructing plasmid pTREZ1 wherein the β-galactosidase gene is connected at the downstream of trp promoter. The hybrid plasmid pTREZ1 of 10.21 kb can be constructed by ligating the DNA fragment of 4.21 kb containing the trp promoter cut from the plasmid pTRE1 and the β-galactosidase gene of 6.2 kb cut from the plasmid pMC1403 with T₄ DNA ligase. As the microorganism harboring the hybrid plasmid comprising a desired gene, a vector and a promoter in its cell and having the ability to express the desired gene, there can be used various E . coli , e.g. M182, HB101, C600, X1776, DH1, etc. Among them, a recombinant for producing β-galactosidase obtained by introducing the hybrid plasmid pTREZ1 having the structure mentioned above into E . coli M182 which is a strain lacking in β-galactosidase gene on a chromosome (deposited in the FRI as FERM BP-816) and a recombinant for producing β-galactosidase obtained by introducing the hybrid plasmid pTREZ1 into E . coli HB101 (deposited in the FRI as FERM BP-815) are used under the control of trp promoter and can produce β-galactosidase in a remarkably larger amount than known methods by effectively applying the function of the promoter. Microorganisms other than E . coli such as yeast, Bacillus subtilis , etc. can also be used as the host microorganism, when suitable promoters are used and desired genes are introduced thereinto. According to this invention, the microorganism containing a hybrid plasmid comprising a desired gene, a vector and a promoter in its cell and having the ability to express the desired gene, for example, a microorganism for producing β-galactosidase, is used for expressing the desired gene so as to produce its product, for example, β-galactosidase, with high efficiency. In order to control the culture of the recombinant, the recombinant is supplied to a culture medium for cultivation, and an inducer and a nutiment are added to the culture medium simultaneously at a time when the nutriment in the culture medium is almost consumed. As the inducer, 3-β-indolylacrylic acid (IA) for the trp promoter, isopropyl-β-D-thiogalactoside (IPTG) for the lac promoter and tac promoter, etc. can be used. As the nutriment, there can be used casamino acid which is a mixture of amino acids, glucose, amino acids other than tryptophan, yeast extract, and a mixture thereof. Among them, the use of casamino acid is preferable. The time when almost all the nutriment in the culture medium is consumed can be judged by a point when the microorganism concentration shows a maximum value, a point when the concentration of glucose becomes almost zero, a point when the concentration of dissolved oxygen (DO) in the culture medium is changed from a decrease to an increase, and the like. In the case of the hybrid plasmid containing a trp promoter, it is known that a gene is expressed by adding 3-β-indolylacrylic acid (IA) as an inducer during the culture (Nature, vol 291, pp 503-506, 1981). That is, since a repressor which suppress the transcription of the gene is inactivated by IA, the synthesis of mRNA by RNA polymerase begins. In the case of trp promoter, when the inducer IA is added, the promoter begins to function. It is very important in the production of the product of desired gene when the inducer should be added. As to the adding time of IA, a method of the addition at one hour from the beginning of the culture is disclosed in Nature, vol. 29, 503-506, 1981 and a method of using the microorganism concentration as an indicator is disclosed in Japanese Patent Unexamined Publication No. 141796/83. But these methods are insufficient for industrial culture. One method of this invention is to add the inducer such as IA using the change of DO concentration as an indicator in order to make possible the addition of the inducer suitable for physiologically active state of the microorganism. When microorganisms are cultivated under constant conditions of agitation speed and air flow rate, the DO concentration decreases with the multiplication, and thus the glucose concentration in a substrate decreases due to the consumption by the microorganisms. Then, there takes place a rapid increase in the DO concentration at the time when the substrate is consumed. When the inducer such as IA and a nutriment which can be a synthesis material for the desired product such as β-galactosidase are added simultaneously to the culture medium at that time, the desired product such as β-galactosidase can be produced in large amounts. That is, by the addition of IA and the nutriment at the time of termination of multiplication of the microorganism due to the consumption of substrate, it becomes possible to begin the production of β-galactosidase at the maximum microorganism concentration. The amount of the inducer to be added together with the nutriment is preferably 10 to 30 µg/ml, more preferably 15 µg/ml. Fig. 4 shows a relationship between the β-galactosidase produced amount and the adding amount of IA. As is clear from Fig. 4, the best result is obtained when 15 µg/ml of IA is added. This means that the addition of 10 to 30 µg/ml of IA together with the nutriment during the culture gives satisfactory results. As the nutriment, casamino acid is most effective as shown in Table 1. When glucose is used, the production of β-galactosidase is lowered. Since casamino acid is a mixture of amino acids, the same results can be obtained by adding amino acids other than tryptophan (e.g. glycine, glutamic acid, alanine, etc.) or a mixture thereof. The produced amount increases when a larger amount of casamino acid is added, but it requires a longer culture time. In the above, the explanation was made as to the hybrid plasmid containing a trp promoter, but the same thing can be applied to the case of hybrid plasmids containing a lac promoter or a tac promoter and isopropyl-β-D-thiogalactoside (IPTG) being added as an inducer. The above-mentioned culture controlling process, that is, a process for increasing the production of desired gene production by adding the inducer and the nutriment simultaneously to the culture medium at the time of the increase of dissolved oxygen (DO) concentration, is not known in conventional culture processes of recombinants and ordinary microorganisms, and is found for the first time by the present inventors. Fig. 5 shows one example of apparatus suitable for practicing the present invention. In Fig. 5, a culture medium and seed microorganism are placed in a culture tank 1, wherein a recombinant is cultivated while blowing air into the culture tank 1 through a pipe 12 and stirring a culture solution with a stirrer 2. The dissolved oxygen concentration of the culture solution is measured by using a dissolved oxygen sensor 11 step up to the culture tank 1 and a dissolved oxygen meter 10. The measured data are input to an electronic computer for controlling 3 to watch the change of dissolved oxygen concentration. When the dissolved oxygen concentration increases rapidly, signals are input to fixed displacement pumps 6 and 7 so as to supply a nutriment such as casamino acid from a substrate tank 4 and an inducer such as IA from an adding tank 5 in predetermined amounts at the same time to the culture tank 1. The cultivation is continued for a predetermined time, the microorganisms in the culture solution are recovered, and the desired useful substance such as β-galactosidase produced in the microorganisms is separated and purified. In Fig. 5, numerals 8 and 9 are pipes. As a means for consuming the nutriment present in the culture medium from the beginning, it is possible to add a repressing substance to the culture medium at the initial stage of the culture. That is, the expression of genes is controlled by a gene region called a "promoter-operator". In the case of trp promoter, an RNA polymerase is linked to a trp promoter and transferred to the downstream of the promoter to transcript the DNA base sequence so as to synthesize RNA. A polypeptide is synthesized by translation of the information of the synthesized RNA by a ribosome. On the other hand, when the tryptophan concentration in a cell increases, tryptophan binds to a repressor to activate it, and the repressor binds to a region called an "operator". As a result, the RNA polymerase cannot bind to the promoter portion. The production of polypeptide terminates due to no synthesis of RNA. In such a case, tryptophan acts as a repressing substance. This invention also provides a process for producing a polypeptide, e.g., β-galactosidase, effectively by controlling the trp promoter. For example, when E . coli M182 containing hybrid plasmid pTREZ1 deposited in the FRI as FERM BP-816 is cultivated in a culture medium containing a trace of or no tryptophan, for example, in a M9-casamino acid culture medium, β-galactosidase is produced from the initial stage of culture. In such a case, since the multiplication of the microorganisms is slight, the produced amount of β-galactosidase per culture solution is low. In order to increase the produced amount of β-galactosidase, a small amount of tryptophan which is a repressing substance is added at the initial stage of culture to suppress the action of trp promoter and to lower the production of β-galactosidase so as to multiply the microorganisms. The amount of tryptophan to be added is preferably 2.5 to 10 µg/ml, more preferably about 5 µg/ml. When the content of nutriment in the culture medium is remarkably lowered or becomes zero, an inducer such as IA which initiates the action of trp promoter is added to the culture solution together with a nutriment so as to activate the trp promoter and so as to make E . coli utilize the nutriment added to the culture solution. Thus, β-galactosidase is produced in large amounts. The culture process applying the trp promoter expression controlling mechanism and attaining separately the microorganism multiplication and the β-galactosidase production is not known in this art. This invention is illustrated by way of the following Examples. Example 1 Microorganism: E . coli M182 containing hybrid plasmid pTREZ1 Culture medium: M9-casamino acid culture medium (NH₄Cl 1 g, Na₂HPO₄ 6 g, KH₂PO₄ 3 g, NaCl 5 g, MgSO₄·7H₂O 0.1 g, CaCl₂·2H₂O 15 mg, glucose 5 g, casamino acid 2.5 g, distilled water 1 liter, pH 7.0). In order to multiply only E . coli M182 containing plasmid pTREZ1, 50 µg/ml of ampicillin (Ap) was added to the culture medium. Culture conditions: E . coli M182 containing plasmid pTREZ1 was inoculated into 100 ml of M9-casamino acid culture medium placed in 20 shaking flasks having a working volume of 500 ml and incubated overmight at 37°C using a shaking incubator at 115 oscillations per minute with a 7-cm stroke. The cultivated microorganisms are recovered by centrifugation and suspended in 50 ml of the M9-casamino acid culture medium. It was inoculated into 2 liters of M9-casamino acid culture medium in a 5-liter jar fermenter as seed microorganisms to start the culture. Two drops of antifoam agent solution (Lion Co., Ltd., 1705-W) were added. The culture was conducted at a temperature of 37°C and pH 7.2, with the agitation speed of 800 r.p.m., and air flow rate of 2 liters/min for 6 hours. Results: Since the dissolved oxygen (DO) concentration was rapidly increased after about 1.5 hours' culture as shown in Fig. 6, 15 µg/ml of IA and 2.5 mg/ml of casamino acid were added simultaneously. The β-galactosidase produced at the time of simultaneous addition of IA and casamino acid was 16.6 U/ml, but increased to 21.8 U/ml which value is about 1.3 times as large as the value of 16.6 U/ml after 6 hours' culture. After the addition of IA and casamino acid, the microorganism concentration hardly increased; it seems that almost all the casamino acid added is consumed for the production of β-galactosidase. From the above-mentioned results, it is clear that the β-galactosidase produced amount can remarkably be increased by the simultaneous addition of IA and casamino acid using the DO concentration as an indicator. Comparative Example 1 Microorganism: the same as Example 1 Culture medium: the same as Example 1 Culture conditions: The procedures of Example 1 were repeated except for adding only 2.5 mg/ml of casamino acid at the time of increase of the DO concentration. Results: Since the DO concentration was rapidly increased after about 1.5 hours' culture as shown in Fig. 7, 2.5 mg/ml of casamino acid alone was added to the culture solution. In this case, the β-galactosidase produced and the microorganism concentration were hardly increased after the addition of casamino acid. This means that the addition of IA is necessary for improving the production of β-galactosidase. Example 2 On a plate of L-broth culture medium (tryptone 10 g, yeast extract 5 g, glucose 1 g, NaCl 5 g, city water 1 liter, pH 7.2) containing 50 µg/ml of ampicillin, E . coli M182 containing plasmid pTREZ1 (deposited in the FRI as FERM BP-816) was plated out. A part of the colony produced was inoculated into 50 ml of culture solution obtained by adding 50 µg/ml of ampicillin to M9-casamino acid culture medium (NH₄Cl 1 g, Na₂HPO₄ 6 g, KH₂PO₄ 3 g, NaCl 5 g, MgSO₄·7H₂O 0.1 g, CaCl₂·2H₂O 15 mg, glucose 5 g, casamino acid 2.5 g, distilled water 1 liter, pH 7.0) placed in a shaking flask with a cotton plug. The culture was carried out at 37°C while shaking at 120 oscillations per minutes for 15 hours. After the stoppage of the culture, the resulting solution was subjected to centrifugation with 10000 r.p.m. for 5 minutes. The collected microorganisms here suspended in the above-mentioned M9-casamino acid culture medium and placed in three shaking flasks with a cotten plug in an amount of 50 ml. To these shaking flasks, tryptophan was added in amounts of 2.5 µg/ml, 5 µg/ml and 10 µg/ml, respectively. On the other hand, the same culture medium as mentioned above except for adding no tryptophan was placed in a shaking flask for comparison. After the addition of tryptophan or no addition of tryptophan, the culture was carried out at 37°C with shaking at 120 oscillations per minute for 2 hours, and then 15 µg/ml of IA and 2.5 mg/ml of casamino acid as a nutriment were added, followed by culture for an additional 3 hours. After the culture, to each 1 ml of culture medium was added 15 µl of toluene and the cultures were shaken at 37°C for 30 minutes. After shaking, the culture medium was diluted 10 to 100 times in order to measure the produced amount of β-galactosidase. One ml of the solution was taken out and shaked at 30°C for 5 minutes, followed by addition of 3.5 ml of 0.2 M phosphate buffer (pH 7.25) and 0.5 ml of 0.01 M O-nitrophenyl-β-D-galactopyranoside (ONPG) and 10 minutes' shaking. After sampling 1 ml of the solution and stopping the reaction by placing it in 1 M Na₂CO₃, 8 ml of water was added thereto so as to make the whole amount 10 ml. The absorbance of this sample was measured at 420 nm by using a spectrophotometer. The amount of O-nitrophenol (ONP) produced was calculated from the standard curves of ONT. In this case, the β-galactosidase activity for decomposing 1 µmole of ONPG per minute was defined as one unit (U). The results are shown in Fig. 8. At 2 hours from the beginning of culture (curve 1), the β-galactosidase produced amount is decreased with an increase of the adding amount of tryptophan, since the action of trp promoter is suppressed by the addition of tryptophan. But as shown in the curve 2 wherein IA and casamino acid are added after 2 hours' culture followed by 3 hours' additional culture, the β-galactosidase produced shows a maximum value at the addition of 5 µg/ml of tryptophan, the value being 10% as large as the case of adding no tryptophan. As mentioned above, the addition of tryptophan which is a repressing substance for a trp promoter at an initial stage of the culture, followed by the addition of an inducer and a nutriment at the time when almost the nutriment initially present in the culture medium is consumed, is effective for remarkably improving the production of β-galactosidase. According to this invention, the suppression and induction of the production of desired products can be conducted as desired in the culture of recombinants, and thus industrial production of the desired products by using the microorganisms harboring hybrid plasmids can be carried out with high efficiency.

Claims: Process for increasing the production of a desired protein in a recombinant host containing a plasmid vector carrying the gene encoding the protein under the control of a promoter, characterized by culturing Escherichia coli, containing as promotor a trp promoter, a lac promoter or a tac promoter, and adding an inducer and a nutriment simultaneously to the culture medium at the time when the nutriment originally present in the culture medium is almost exhausted, wherein the consumption of nutriment is measured by a dissolved oxygen concentration. The process according to claim 1, wherein the inducing substance is 3-β-indolylacrylic acid or isopropyl-β-D-thiogalactoside. The process according to claim 1, wherein the nutriment is casamino acid, glucose, amino acids other than tryptophan, yeast extract, or a mixture thereof. The process according to claim 1, wherein the consumption of nutriment is carried out by adding a repressing substance to the culture medium at an initial stage of the culture. The process according to claim 4, wherein the repressing substance is tryptophan.

Inventors: FUJIWARA, KIYOSHI, KURIHARA, TOSHIHARU, MASUDA, KEIKO, ODAWARA, YOJI, SHIMIZU, NORIO
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: C12P21
IPC codes: C12N9, C12P21, C12N1, C12N15