Process for producing L-threonine

Abstract: A process is disclosed for producing L-threonine, the process involves culturing in a medium a microorganism of the genus Escherichia capable of producing L-threonine which has resistance to at least one of rifampicin, lysine, methionine, aspartic acid and homoserine, or a decreased ability to degrade L-threonine, accumulating L-threonine in the culture liquor and recovering L-threonine therefrom.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for producing L-threonine, which is an amino acid useful as a medicament (e.g., an amino acid preparation), an additive for animal feed, etc. 2. Description of the Prior Art Heretofore, various processes for producing L-threonine by fermentation have been known; for example, a process using a microorganism belonging to the genus Escherichia and having sensitivity to borrelidin (Japanese Published Examined Patent Application No. 6752/76), a process using a microorganism belonging to the genus Escherichia which requires diaminopimelic acid and methionine and of which threonine biosynthesis system is resistant to the feedback inhibition by threonine (Japanese Published Examined Patent Application No. 10037/81), a process using a microorganism belonging to the genus Serratia which is deficient in threonine-dehydrogenase and resistant to threonine metabolism-antagonist (Japanese Published Examined Patent Application No. 48195/77), a process using a microorganism belonging to the genus Corynebacterium and having resistance to α-amino-β-hydroxyvaleric acid and S-(2-aminoethyl)-L-cysteine, and a requirement for methionine (Japanese Published Unexamined Patent Application No. 19087/72), a process using a microorganism belonging to the genus Brevibacterium and having resistance to α-amino-β-hydroxyvaleric acid and S-(2-aminoethyl)-L-cysteine, and a requirement for leucine (Japanese Published Unexamined Patent Application No. 31093/75), a process using a microorganism belonging to the genus Brevibacterium and having resistance to α-amino-β-hydroxyvaleric acid and S-(2-aminoethyl)-L-cysteine, and requirements for L-isoleucine and L-lysine (Japanese Published Unexamined Patent Application No. 224684/83), etc. However, the known processes are still insufficient in efficiency of the production of L-threonine. It is therefore an object of the present invention to provide a process for producing L-threonine in higher yield and at low cost. SUMMARY OF THE INVENTION According to the present invention, L-threonine can be produced in high yield by using a microorganism of the genus Escherichia capable of producing L-threonine which has resistance to at least one of lysine, methionine, aspartic acid and homoserine. BRIEF DESCRIPTION OF THE DRAWING Fig. 1 is a graph showing relative molar concentrations of threonine, glycine and isoleucine during the course of degradation reactions thereof utilizing Escherichia coli ATCC 21530. In the figure, ―·― indicates the relative molar concentration of threonine (Thr), ―▴― that of glycine (Gly) produced by the degradation of threonine, ----▵---- that of glycine (Gly), and ―·―□―· that of isoleucine (Ile). DETAILED DESCRIPTION OF THE INVENTION In the present invention, any microorganism belonging to the genus Escherichia can be used, provided that it is capable of producing L-threonine and has resistance to at least one of lysine, methionine, aspartic acid and homoserine. Suitable strains resistant to lysine, methionine, aspartic acid or homoserine can be obtained, for example, by mutating an L-threonine-producing microorganism of the genus Escherichia in accordance with a conventional mutation technique, culturing the resulting mutants in a minimal medium containing not less than 10 g/l lysine, methionine, aspartic acid or homoserine, and then recovering the mutants capable of growth on said medium. Examples of the suitable strains include lysine-resistant Escherichia coli H-4435 (FERM BP-1094) (hereinafter referred to as H-4435), methionine-resistant Escherichia coli H-4436 (FERM BP-1095) (hereinafter referred to as H-4436), aspartic acid-resistant Escherichia coli H-4225 (FERM BP-1236) (hereinafter referred to as H-4225) and homoserine-resistant Escherichia coli H-4226 (FERM BP-1237) (hereinafter referred to as H-4226). (1) Strains H-4435 and H-4436 Escherichia coli ATCC 21530 was subjected to a conventional mutation treatment utilizing NTG (200 γ/ml, 30°C, 30 minutes). Then, the strain was smeared on a minimal medium [0.5% glucose, 0.1% (NH₄)₂SO₄, 0.7% K₂HPO₄, 0.2% KH₂PO₄, 0.1 g/l MgSO₄·7H₂O, 20 mg/l Fe₂(SO₄)₃, 50 mg/l diaminopimelic acid, 50 mg/l methionine and 2% agar, pH 7.2] containing 15 g/l lysine or methionine, and cultured at 30°C for 2 to 6 days to obtain colonies of lysine- or methionine-resistant mutants growable thereon. Fifty strains each of lysine-resistant mutants and methionine-resistant mutants were picked up and subjected to the L-threonine production test as shown in Example 1 to select the strains having an L-threonine-producing ability greater than that of the parent strain. Among the thus selected strains are H-4435 which is resistant to lysine and H-4436 which is resistant to methionine. These strains were deposited with the Fermentation Research Institute, Agency of Industrial Science and Technology, Japan (hereinafter referred to as FR) on June 28, 1986 under deposition numbers FERM BP-1094 and FERM BP-1095, respectively. (3) Strains H-4225 and H-4226 The aspartic acid-resistant strain, H-4225, and the homoserine-resistant strain, H-4226, were obtained in the same manner as in paragraph (2) above, except that 15 g/l aspartic acid or homoserine was used in place of 15 g/l lysine or methionine. These strains were deposited with the FRI on December 17, 1986 under deposition numbers FERM BP-1236 and FERM BP-1237, respectively. In the present invention, either a synthetic medium or a natural medium can be used so long as it contains carbon sources, nitrogen sources, inorganic salts, growth factors, and the like. As the carbon sources, carbohydrates such as glucose, fructose, molasses and starch hydrolyzate; and organic acids such as acetic acid, propionic acid, formic acid, fumaric acid and malic acid, etc. may be used. As the nitrogen sources, ammonia, ammonium chloride, ammonium sulfate, ammonium acetate, ammonium phosphate, peptone, meat extract, yeast extract, corn steep liquor, casein hydrolyzate, acid hydrolyzate of soy bean cake, various microbial cells, digest of microbial cells, etc., may be used. As the inorganic salts, potassium dihydrogenphosphate, potassium hydrogenphosphate, magnesium phosphate, magnesium sulfate, sodium chloride, ferrous sulfate, manganese sulfate, copper sulfate, calcium carbonate, etc. may be used. As the growth factors, amino acids (e.g., D,L-methionine), diaminopimelic acid, etc. maybe used. Culturing is carried out under aerobic conditions, e.g., by shaking culture, agitation submerged culture, etc. at a temperature of 20 to 40°C, preferably 25 to 35°C. The pH of the medium is in the range of 5 to 9, and is preferably maintained at around neutrality. The pH is adjusted with calcium carbonate, organic or inorganic acids, alkali solutions, ammonia, pH buffering agent, etc. A substantial amount of L-threonine is usually produced and accumulated in the resulting culture liquor after 2 to 7 days of culturing. After the completion of culturing, precipitates such as cells are removed from the culture liquor by means of centrifugation, etc. and L-threonine is recovered therefrom by means of ion exchange treatment, concentration, adsorption, salting-out or combinations thereof. The present invention is illustrated by the following Examples. Example 1 (not an example of the invention). L-threonine production test was carried out using H-4258 strain as the seed strain. The seed strain was cultured with shaking at 30°C for 16 hours in a seed medium (pH 7.4) having a composition of 2% glucose, 1% peptone, 1% yeast extract, 0.25% NaCl and 0.1 g/l diaminopimelic acid. The resultant seed culture (2 ml) was inoculated into a 250 ml-Erlenmeyer flask containing 20 ml of fermentation medium (pH 7.4) comprising 7% glucose, 1.4% (NH₄)₂SO₄, 0.2% KH₂PO₄, 0.1% MgSO₄·7H₂O, 300 γ/ml diaminopimelic acid, 100 γ/ml D,L-methionine, 0.2% corn steep liquor and 3% CaCO₃, and cultured with shaking at 30°C for 72 hours. L-threonine was produced in an amount of 18.6 g/l. As a control, the above procedure was repeated using the parent strain, ATCC 21530, whereby L-threonine was produced in an amount of 15.8 g/l. The L-threonine-containing fermentation broth (200 ml) obtained by the use of H-4258 strain was then subjected to centrifugation (3,000 rpm, 10 minutes) to remove microbial cells and other impurities. The supernatant obtained was passed through a column of H⁺-type Diaion SKI (highly acidic cation exchange resin manufactured by Mitsubishi Chemical Industries Ltd.) to adsorb L-threonine. The column was washed with water and eluted with 0.5 N aqueous ammonia. Fractions containing L-threonine were combined and concentrated under reduced pressure. Then, ethanol was added and the mixture was stored under cooling to give 2.1 g of L-threonine. Example 2 The same culturing procedure as in Example 1 was repeated, except that H-4435, H-4436 and ATCC 21530 strains were used as the seed strains in place of H-4258 strain. The results are shown in Table 1. Strain L-threonine Produced (g/l) H-4435 18.7 H-4436 18.9 ATCC 21530 15.8 The fermentation broth of the strain H-4435 was treated in the same manner as in Example 1 and, as a result, 2.1 g of L-threonine was obtained. Example 3 The same culturing procedure as in Example 1 was repeated except that H-4225, H-4226 and ATCC 21530 strains were used as the seed strains in place of H-4258 strain. The results are shown in Table 2. Strain L-threonine Produced (g/l) H-4225 19.1 H-4226 19.2 ATCC 21530 15.9 The fermentation broth of the strain H-4226 was treated in the same manner as in Example 1 and, as a result, 2.1 g of L-threonine was obtained.

Claims: A process for producing L-threonine which comprises culturing in a medium a microorganism of the genus Escherichia capable of producing L-threonine which has resistance to at least one of lysine, methionine, aspartic acid and homoserine, accumulating L-threonine in the culture liquor, and recovering L-threonine therefrom, wherein said microorganism is Escherichia coli H-4435 (FERM BP-1094), H-4436 (FERM BP-1095), H-4225 (FERM BP-1236)or H-4226 (FERM BP-1237). A process according to claim 1, wherein said culturing is conducted at 20 to 40°C for 2 to 7 days. A biologically pure culture of the microorganism Escherichia coli having the identifying characteristics of a member selected from the group consisting of FERM BP-1094, FERM BP-1095, FERM BP-1236 and FERM BP-1237, which culture possesses an ability to produce L-threonine.

Inventors: FURUKAWA SATORU, KOTANI YUKINOBU, NAKANISHI TOSHIHIDE, OZAKI AKIO, SUGIMOTO MASAHIRO, FURUKAWA, SATORU, KOTANI, YUKINOBU, NAKANISHI, TOSHIHIDE, OZAKI, AKIO, SUGIMOTO, MASAHIRO
Applicants: KYOWA HAKKO KOGYO KK, KYOWA HAKKO KOGYO CO., LTD.
Application year: 1987
Main classification: C12P13
IPC codes: C12P13