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Construction of L-Threonine Overproducing Strains Of Agric. Biol Chem., 47 (10), 2329-2334, 1983 2329 Construction of L-Threonine Overproducing Strains of Escherichia coli K-12 Using Recombinant DNATechniques Kiyoshi Miwa, Takayasu Tsuchida, Osamu Kurahashi, Shigeru Nakamori, Konosuke Sano and HaruoMomose Central Research Laboratories ofAjinomoto Co., Inc., Kawasaki-ku, Kawasaki 210, Japan Received April 4, 1983 The threonine operon of Escherichia coli was cloned in plasmid pBR322, using a threonine producing mutant, /?IM4, as the DNAdonor. A recombinant plasmid, pAJ294, that contains the whole threonine operon was obtained. No. 29-4, a transformant of /?IM4 with pAJ294, had about eleven copies of pAJ294, five times higher homoserine dehydrogenase activity (coded by the thrA gene), and about three times higher threonine productivity than those of /?IM4. No. 29-4 produced 13.4g/liter of l-threonine from 30 g/liter of glucose in the culture medium. Threonine biosynthesis is known to be reg- mutant /?IM44) was used as the DNA donor and the ulated by two mechanisms in E. coli. One is recipient. Threonine auxotrophic mutants, No. 1 thrA, feedback inhibition by L-threonine of a bi- No. 43 thrB, No. 313 thrC, No. 44 thr (not identified) and functional enzyme, aspartokinase I (AK-I)- No. 255 thr (not identified), were derived from /?IM4 and used as the recipients. Plasmid pBR3225) was used as the homoserine dehydrogenase I (HDH-I), and vector. another is multivalent repression of the thre- onine biosynthetic enzymes by L-threonine Media. L medium6) and Davis'7) minimal medium were used. Amino acids (100 /ig/ml), thiamine-HCl (1 /ig/ml), plus L-isoleucine1} through the attenuation ampicillin (lOOjUg/ml) and tetracycline (20/ig/ml) were mechanism.2'3) Wehave previously construct- added when required. ET medium containing 3%glucose, ed threonine producing mutants of E. coli.4) 1% (NH4)2SO4, 0.2% KH2PO4, 0.1% MgSO4à"7H2O, These mutants have an altered AK-I-HDH-I 2ppm Fe2+, 2ppm Mn2+, 1/jg/ml thiamine-HCl, which is less sensitive to the feedback in- 300/zg/ml L-proline, 100 /xg/ml L-isoleucine, 100/ig/ml l- hibition by L-threonine, and also have an methionine and 2%CaCO3(sterilized separately) was used isoleucine auxotrophic mutation that may for l-threonine production. cause derepression of threonine genes under Preparation ofDNA.Chromosomal DNAwas prepared isoleucine-limiting conditions. from strain /?IM4 using the method of Saito and Miura8) In order to increase the productivity of with modifications as follows. Cells were cultured in 1 liter threonine by amplifying the threonine genes, of L medium at 37°C for 3hr to the exponential growth phase and then harvested by centrifugation. The packed weemployed recombinant DNA techniques. cells were resuspended in 25 ml of buffer containing 0.1 m In this paper we will describe the construction EDTAand 0.1m Tris-HCl (pH 9.0). SDSwas added to a of improved L-threonine producing strains final concentration of 1 %, and the lysate was incubated at 60°C for lOmin. After phenol extraction of the viscous and someof their properties. lysate, the DNAwas precipitated by adding 2 volumes of ethanol and spooled out on a glass rod. The DNAwas MATERIALS AND METHODS further purified by digestion with preheated pancreatic RNase and self-digested pronase, phenol extraction and Bacterial strains and plasmid. The bacterial strains used ethanol precipitation, and then dissolved in TENbuffer; were derived from E. coli K-12. Threonine overproducing 20mM Tris-HCl (pH 8.0)/20mM NaCl/1 mMEDTA. 2330 K. Miwa et al. Plasmid DNAwas prepared in essentially the same way RESULTS as described by Tanaka and Weisblum.9) Cloning of threonine genes Twenty-one fig of /?IM4 chromosomal DNA Enzymes.Restriction enzyme digestion and ligation of and 12fig ofpBR322 DNAwere digested with DNA were done as suggested by the suppliers. The Hindlll, mixed and ligated as described in Materials and Methods, then the ligation following commercial preparations of enzymes were used: EcoRl, Hindlll, Pstl, BamHl, T4 DNA ligase trophicmixture wasstrains,used Nos.to transform43, 44 threonineand 255. auxo-Pre- (Boehringer) and Sail (Miles). sumable transformants that grew on minimal L-methionineagar supplementedandwiththiamine-HClL-proline, L-isoleucine,were picked Homoserine dehydrogenase activity was assayed by the up and tested for ampicillin resistence, l- method of Truffa-Bachi et al.10) threonine excretion and the existence of plas- mids larger than the vector pBR322. Twenty- two transformants were obtained. All the plas- Transformation.Recipient strains were transformed mids obtained from them were almost the using the method of Lederberg et al.n) samesize, about 13kb. extractedTo confirmfromtheoneclone,of the plasmid22 transformantsDNAwas and used for re-transformation of a thrB mu- Determination ofplasmid copy number. The copy num- tant, No.43. The transformants were selected ber of the plasmid was determined by the CsCl-EtBr inedby Ampr,for theirand fiveThr~phenotype.of them wereAllfurtherof the exam-five density gradient centrifugation method. Cells were cul- wereconcludedfound toto bebe aThr+,pBR322-thrthus the recombinantplasmid was tured in 3ml of minimal mediumsupplemented with plasmid. casamino acid (0.5%), thiamine-HCl (1 jug/ml), [3H]- thymidine (10fid; 1.6/im) and deoxyadenosine (200/im) ProductionThreonineof threonineproductivitiesby Thr+oftransformantsthe Thr+ at 30°C for 16hr, harvested by centrifugation, and transformants were examined by culti- vation in ET medium (Fig. 1). Among 22 washed twice with TEN buffer. The packed cells were re- suspended in 0.5 ml of TEN buffer containing 2 mg/ml of lysozyme and 50mMEDTA.After this suspension was incubated at 37°C for 50 min, preheated pancreatic RNase (to a final concentration of 50/xg/ml) was added and incubated for a further 30 min. Then self-digested pronase (1 mg/ml) and Sarkosyl (0.6%) were added and the mix- ture was incubated at 37°C for 2 hr. Using a small portion of this lysate, the total radioacivity (5% ice-cold TCA precipitable) was determined. Then covalently closed cir- cular plasmid fraction was separated by CsCl-EtBr gra- dient centrifugation in a Hitachi PR65Tangle rotor at 38,000 rpm for 40hr, and its radioactivity was measured. Cultivations for L-threonine production. A loopful of cells grown at 30°C for 24hr on L agar (supplemented with ampicillin or tetracycline when necessary) was in- oculated into 20ml of ET medium in a 500ml flask, and cultured at 30°C for 3 days with shaking. The amount ofl- threonine in the culture broth was determined by a microbioassay with Leuconostoc mesenteroides ATCC 8042 and with an amino acid analyser. Agarose gel electrophoresis. Electrophoresis was carried out using 0.8% agarose gel with a Tris-acetate buffer system as described by Sharp et al.12) I I Halo test. Excretion of L-threonine from colonies was I simply determined by the halo formation test. Strain No. 2 3 4 5 43, a threonine auxotrophic mutant, was used as the L-Threonine (g/liter) indicator. It was spread on minimal mediumagar plates, and strains to be tested were spotted on the plate. After Fig. 1. Distribution of Threonine Productivities among incubation at 30°C for 1 ~2 days the satellite growth of the Thr+ Transformants. the indicator strain around the colonies was observed. Twenty-two Thr+ transformants were cultured in ET mediumat 30°C for 70hr. The arrow in the figure shows the amount of L-threonine produced by strain /?IM4. Threonine Producing Strains of E. coli 2331 transformants, strain No. 29 had the highest result was 9.0g/liter of L-threonine accumu- productivity; 5.4 g/liter of L-threonine, while lated by No. 29-4. /JIM4 produced 2.9 g/liter. Threonine production by recombinant strain Transformation of f$IM4 with the pBR322-thr No. 29-4 recombinant plasmid By checking the effects of components of ET In order to improve the productivity of mediumon L-threonine production of No. 29- threonine, the pBR322-^r recombinant plas- 4, the following ET-II medium was devised; mid was introduced into the parental thre- 3% glucose, 1% (NH4)2SO4, 0.1% KH2PO4, onine producing strain, /JIM4. The recombi- 0.1% MgSO4-7H2O, 2ppm Fe2+, 2ppm nant plasmid DNAwas prepared from trans- Mn2+, lmg/ml L-aspartate, 450/ig/ml l- formant No. 29 described above, and was proline, 100/xg/ml L-isoleucine, 100/xg/ml l- used to transform /?IM4 to Ampr. About 70% methionine, 1 /ig/ml thiamine-HCl, 2mg/ml of the transformants showed higher l- yeast RNA and 4% CaCo3 (pH 7.0). threonine productivities than /?IM4 as checked Figure 3 shows the time course of l- by the halo formation test. From those high threonine production by strain No. 29-4. After producers, 14 transformants were picked up cultivation for 72 hr, 13.4g/liter of threonine and cultured in ET medium (Fig. 2). The best was accumulated in the culture broth. Amino acids in the broth were assayed using an amino acid analyser (Table I). By-production of 1 2 it J ln en *« 2- " * i Ln | I I I I I I I 0 2 4 6 8 10 L-Threonine (g/liter) Fig. 2. Distribution of Threonine Productivities among 24 48 Plasmid.the Transformants of /?IM4, Harboring the pBR322-thr mediumThe 14attransformants30°C for 72hr.(seeThetext)arrowswere(1) culturedand (2) in theET andfigurestrainshowNo.the29,amountsrespectively.of L-threonine produced by /?IM4 Culture time (hr) Fig. 3. Time Course of L-Threonine Production by Recombinant Strain No. 29-4. Strain No. 29-4 was cultured in 20ml of ET-II medium in flasks. -#-, L-threonine accumulated, -O-, residual glucose. Table I. Accumulation of Amino Acids by Strain No. 29-4 and j?IM4 Strain No. 29-4 and /?IM4 were cultured at 30°C in ET-II medium for 72hr.
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