DNA CODING FOR DECARBAMYLASE IMPROVED IN THERMOSTABILITY AND USE THEREOF

Abstract: A DNA fragment coding for a decarbamylase protein improved in thermostability as the result of replacement of at least one base of a DNA fragment coding for a decarbamylase protein originating in a microbe with another base and the resultant replacement of at least one of the corresponding amino acids; a process for producing the fragment; a vector containing the fragment; a transformant produced with the vector; a decarbamylase improved in thermostability; and a process for producing the decarbamylase. Also disclosed is a process for producing a D-α-amino acid which comprises decarbamoylating an N-carbamoyl-D-α-amino acid with a decarbamylase having a thermostable temperature of 65° or above in an aqueous medium.

Description: Technical Field The present invention relates to a DNA fragment, and more particularly, it relates to a DNA fragment coding for a thermostability-improved enzyme capable of converting D-N-carbamoyl-α-amino acids into the corresponding D-α-amino acids (hereinafter referred to as decarbamylase). The present invention also relates to a process for preparing the DNA fragment, a vector comprising the DNA fragment, a transformant obtained by transformation with the vector, a decarbamylase improved in thermostability and its production process, and an improved process for producing amino acids using the decarbamylase. The present invention further relates to a method for increasing, by one or more, the number of mutations in amino acids related to the thermostability of a decarbamylase. Background Art Optically active D-α-amino acids are important compounds as intermediates of medicaments, and in particular, D-phenylglycine, D-parahydroxyphenylglycine and the like, which are intermediates for the production of semi-synthetic penicillins and semisynthetic sepharospolines, are useful compounds from an industrial point of view. As the production process for such D-α-amino acids, a process is known in which these compounds are obtained by the removal of carbamoyl groups of the corresponding D-N-carbamoyl-α-amino acids, wherein the removal of carbamoyl groups is effected by a chemical method (the specification of the Japanese Patent Publication No. 58-4707) or by the utilization of enzyme reactions of microorganisms (the specifications of the Japanese Patent Publication Nos. 57-18793, 63-20520 and 1-48758, and the Japanese Patent Application No. 2-407922). However, because a mineral acid such as sulfuric acid is used at great amounts in the chemical method employed for the removal of carbamoyl groups, serious problems on the environment may occur in relation to the treatment thereof and the like. On the other hand, the method utilizing an enzyme reaction has disadvantages that the amount of enzyme produced is not satisfactory and there are some difficulties in its properties even if mass production is made possible, so that enzymes with reactivity to substrates and enzyme stability have not yet been found. In general, some enzymes have poor stability, and when such an enzyme is prepared, a stabilizing agent is added or a means of preventing inactivation such as treatment at low temperatures is employed. When an enzyme is used in an actual reaction at ordinary or high temperatures, attention will be directed to the stability of the enzyme. In particular case where an enzyme is used on an industrial scale, its stability often has an effect upon the product cost. Also, in particular case where an enzyme is used on an industrial scale, as a means of allowing the enzyme reaction to proceed with advantage, the enzyme is repeatedly used in the reaction as a so-called bioreactor such as an immobilized enzyme or immobilized bacterial cells, and even at this time, the stability of the enzyme will give a limitation to the number of use and have an important effect upon the product cost. In the present invention, attention is paid to the thermostability as an index of the enzyme stability and an improvement in the thermostability of an enzyme is achieved using a genetic engineering means, whereby the stability of this enzyme is increased and this makes it possible to produce a stabilized enzyme having excellent advantages for industrial applications. To solve such a problem, the present invention has an object to prepare a decarbamylase with high reactivity to D-N-carbamoyl-α-amino acids as the substrate and excellent stability by an improvement of currently available decarbamylases and to produce D-α-amino acids using this enzyme with high efficiency. Summary of the Invention That is, the present invention provides an improved process for producing D-α-amino acids, comprising converting D-N-carbamoyl-α-amino acids into the corresponding D-α-amino acids in an aqueous medium by the action of an enzyme and collecting the D-α-amino acids produced, said enzyme being produced by a transformant which has been obtained by the transformation of a host bacterial cell selected from the microorganisms of the genus Escherichia, Pseudomonas, Flavo-bacterium, Bacillus, Serratia, Corynebacterium or Brevi-bacterium with a gene which has been obtained by a process comprising subjecting a DNA fragment containing a decarbamylase gene to chemical or enzyme mutagenesis, introducing a recombinant DNA having a vector DNA ligated with this DNA fragment into a host cell and screening a strain capable of producing an enzyme improved in thermostability. Thus, the present invention also provides a DNA fragment for conducting the improved process for producing D-α-amino acids according to the present invention, its production process and an expression vector containing the fragment, a transformed microorganism obtained by transformation with the expression vector, as well as a decarbamylase improved in thermostability and its production process. The present invention further provides a method for increasing, by one or more, the number of mutations in amino acids related to the thermostability of decarbamylases. Brief Description of the Drawings The present invention will hereinafter be illustrated by reference to the accompanying drawings. Figure 1 is a graph showing the thermostable activity of a decarbamylase after thermal treatment, which is obtained by the use of a strain capable of producing the decarbamylase improved in thermostability according to the present invention. Figure 2 is a graph showing the results of stability obtained by a repeated continuous test on the thermostability using a resin having immobilized thereon a decarbamylase improved in thermostability according to the present invention. Figure 3 shows restriction maps of plasmids pKK233-2 and pKK NE. Figure 4 shows restriction maps of plasmids pAD108 and pAD1086. Figure 5 shows restriction maps of plasmids pAD402 and pAD4021. Figure 6 shows restriction maps of plasmids pAD429 and pAD455. Figure 7 is a graph showing the relationship between the reaction temperature of a decarbamylase produced by E. coli JM109 pAD108 or E. coli JM109 pAD455 and its activity. Figure 8 is a graph showing the stability against pH, of a decarbamylase produced by E. coli JM109 pAD416 or E. coli JM109 pAD455. Figure 9 shows a process for preparing vector pUC NT for the introduction of a foreign gene and vector pNT4553 for the expression of a decarbamylase improved in thermostability. Figure 10 shows a restriction map of plasmid pUC NT and pNT4553. Detailed Description of the Invention In general, it is known that the thermostability and stability of an enzyme are correlated with each other. It is also known that various enzymes produced by thermophiles are those which have high thermostability and also have high stability with almost no exception, and that enzymes having thermostability given by the substitution of amino acids according to a gene recombination technique also have excellent stability (Kagawa et al., Saibo Kogaku, vol. 7, 509-571 (1988)). As the method for making an enzyme thermostable, there can be employed a method in which an amino acid sequence of the enzyme protein is changed or a method in which the enzyme is modified by chemical treatment, enzyme reaction or the like. As the method for improving the thermostability by a change in the amino acid sequence of the enzyme protein, there can be employed a method in which bacterial cells are allowed to cause some mutation with a chemically mutagenic agent such as nitrosoguanidine (NTG) or by ultraviolet rays, followed by screening, or a method in which a gene of interest is obtained by a gene recombination technique and then allowed to cause some chemical or enzyme mutation, followed by returning it into the bacterial cell and then screening. When a gene is obtained and allowed to cause some mutation, it is more effective to subject a DNA fragment coding for a decarbamylase protein to mutagenesis in the form of a single strand. To make the fragment into a single-stranded form, there can be employed, for example, a method for incorporating the fragment into phage particles. As the single-stranded DNA, there can be used either a DNA strand containing the codons corresponding to the amino acid sequence of a decarbamylase protein or the complementary strand thereof. Moreover, as to the mutation sites, there can be employed a method for introducing random mutations or a method for introducing site-specific mutations. As the agent for allowing a gene to cause some random mutation and thereby introducing some mutation into the amino acid sequence of an enzyme protein, there can be used hydroxylamine hydrochloride, sodium nitrite, formic acid, hydrazine or the like. For example, in the mutagenesis using hydroxylamine hydrochloride, a decarbamylase gene is first ligated with the double-stranded DNA of an M13 phage such as M13mp19, with which E. coli JM109 or the like is infected, and this bacterial culture is incubated to prepare phage particles for use in the mutagenesis. The mutation can be effected by the reaction using hydroxylamine hydrochloride in a concentration of 0.1 to 2 M, desirably 0.25 M, at a pH of 6.0 to 8.0, preferably 6.0, at a temperature of 37°C for a period of 1 to 24 hours. The phage particles undergoing such a mutagenic reaction are used for the infection of E. coli and recovered as a double-stranded recombinant DNA, followed by the transfer of such a mutant decarbamylase gene to a plasmid. In the mutagenesis using sodium nitrite, formic acid, hydrazine or the like, there can be basically used a method as described by R.M. Myers, et al. (Science, 229, 242-247 (1985)). This method can be conducted by the preparation of a single-stranded DNA from the recombinant M13 phage having a decarbamylase gene incorporated thereinto and the subsequent mutagenesis. The mutation with sodium nitrite can be effected by the reaction using sodium nitrite in a concentration of 0.5 to 2 M, preferably 1 M, at a pH on the acidic side, preferably pH 4.3, at a temperature of 4°C to 37°C, preferably 25°C, for a period of 1 minute to 5 hours, preferably 30 minutes. The mutation with formic acid can be effected by the reaction using 12 M formic acid at 4°C to 37°C, preferably 15°C, for a period of 2 to 10 minutes. The mutation with hydrazine can be effected by the treatment using hydrazine in a concentration of 20% to 60% at 25°C for 3 to 10 minutes. These mutant single-stranded DNAs are made into double-stranded forms using E. coli DNA polymerase Klenow fragment, Sequenase R and the like, followed by incorporation into a plasmid, which makes it possible to produce a mutant decarbamylase gene. As the method for causing some random mutation using enzyme reaction, there can be employed a method using PCR (polymerase chain reaction) (D.W. Leung et al., A Journal of Methods in Cell and Molecular Biology, 1, 11-15 (1989)) or the like. This is a method in which TaqDNA polymerase is used for the synthesis of a gene from a synthetic DNA (DNA primer) having the sequences at both ends of the gene, at which time a mutant decarbamylase gene can be prepared by causing an error in the gene synthesis using TaqDNA polymerase under the reaction conditions such as higher concentrations of MgCl 2 and substrates (dNTPs) as compared with ordinary reaction or an extremely lowered concentration of only one substrate in four kinds of substrates. As the method for introducing a mutation into a gene, which is specific to the site of a enzyme protein, there can be employed an in vitro method for introducing a site-specific mutation using an oligonucleotide, a method for replacing a cassette at the mutation site, a method using PCR or the like. The in vitro method for introducing a site-specific mutation using an oligonucleotide can be conducted by preparing a single-stranded DNA from a recombinant M13 phage having a decarbamylase gene incorporated thereinto, and making the DNA into a double-stranded form using a synthetic DNA primer containing a sequence, after the mutation, of a gene part to be allowed to cause some mutation and using an enzyme such as a DNA polymerase Klenow fragment, followed by introduction into E. coli. These reactions can readily be effected using a commercially available kit, for example, "Mutan™-K" or "Mutan™-G" available from Takara Shuzo, or "Oligonucleotide-directed in vitro mutagenesis System, version 2.0" from Amersham Japan. The method for replacing a cassette at the mutation site can be conducted by replacing the whole restriction enzyme fragment containing the site to be allowed to cause a mutation, with a synthetic DNA containing such a mutation. The method using a PCR reaction can be conducted according to the method of W. Ito, et al., Gene, 102, 67-70 (1991) or the like. In this method, PCR reaction is effected using a synthetic DNA primer containing the mutation and a synthetic DNA corresponding to the end of the gene, after which the resulting DNA fragment is allowed to hybridize with the full-length DNA of the gene, followed by another PCR reaction for the enzyme elongation of the fragment to its full length. In the present invention, the mutant decarbamylase gene prepared in the above-described manner is ligated to a plasmid vector such as pUC19 or pKK-233-2, and the resulting recombinant vector is incorporated into E. coli, such as JM109, followed by colony formation on a plate containing an antibiotic and the like. Then, this colony is replicated onto a sterilized filter paper, and the plate is stored. The filter paper, after drying, is immersed in a solution containing lysozyme and Triton X-100 or repeatedly put in and out of an acetone-dry ice bath (freeze and thaw), thereby causing lysis. Then, this filter paper is immersed in a water bath thermostated at a temperature such as 65°C or 70°C for a constant period of time, for example, 5 minutes, and then dried, after which this filter paper is immersed in a reaction solution capable of making color development by decarbamylase activity. As the reaction solution, there can be used, for example, those which contain a carbamyl-D-amino acid as the substrate and also contains phenol, 4-aminoantipyrine, D-amino acid oxidase and peroxidase. The colony exhibiting color development can be separated as a thermo-stability-acquiring strain. First, as an index indicating the thermostability of an enzyme, thermostable temperature is defined. The thermostable temperature is defined as the treatment temperature at which the activity of an enzyme is 50% inactivated by the thermal treatment for 10 minutes. As the source of a decarbamylase gene, microorganisms which are known to have a decarbamylase (e.g., microorganisms disclosed in the International Publication No. WO92/10579) can usually be used. The thermostable temperature is different with the respective decarbamylases of these microorganisms. To attain excellent stability tolerable to repeated use by an improvement in the thermostability, various properties other than thermostability should, of course, be taken into consideration, and it is desirable to use a decarbamylase exhibiting high thermostable temperature, for example, in the range of 60° to 63°C, from the first. From this point of view, a decarbamylase produced by Agrobacterium species KNK712 (FERM BP-1900) is preferred because of its relatively higher thermostable temperature of about 62°C. The decarbamylase from KNK712 and the DNA fragment coding for this enzyme have the amino acid sequence and the DNA sequence, respectively, shown in Sequence Listing, SEQ ID NO: 1. The decarbamylase improved in thermostability obtained by the mutagenesis of this decarbamylase gene was found to contain a replacement of 57-histidine, 203-proline or 236-valine with a different amino acid, as shown in Sequence Listing, SEQ ID NOs: 2-16. Therefore, amino acids at at least three positions are related to the thermostability. High thermostability can be obtained not only by the replacement of one amino acid but also by the combined replacement of two or three amino acids. In the present invention, the thermostable temperature could be increased by at least 2°C, mostly about 5°C or higher, and in some cases, by 10°C or higher. For example, it is possible to obtain a decarbamylase improved in thermostability by the substitution of leucine or tyrosine for 57-histidine; of leucine, asparagine, glutamic acid, threonine or serine for 203-proline; or of alanine, threonine or serine for 236-valine, as the different amino acid (Sequence Listing, SEQ ID NOs: 2-30). As the DNA sequences coding for these decarbamylases improved in thermostability, there were obtained those which contain a substitution of TAT, CTT or CTA for CAT, i.e., 401- to 403-nucleotides corresponding to 57-histidine; of TCT, CTT, GAA, AAC or ACC for CCT, i.e., 839- to 841-nucleotides corresponding to 203-proline; of GCG, GCT, ACC, ACG, TCA, TCG or AGT for GTG, i.e., 938- to 940-nucleotides corresponding to 236-valine (Sequence Listing, SEQ ID NOs: 2-30). Once the thermostability-related amino acids are identified, the use of site-specific mutagenesis makes it possible to substitute many amino acids for these amino acids by the substitution of various bases. Thus, the DNA fragment coding for the decarbamylase improved in thermostability can be subjected to mutagenesis for a further improvement in the thermostability. In case of Agrobacterium species KNK712 (FERM BP-1900), the derivative having two or three amino acid mutations in three amino acids which have been found to have a relation to the thermostability, i.e., 57-histidine, 203-proline and 236-valine (multiple mutant) is produced, for example, as follows. In the gene structures of pAD108 (coding for a naturally occurring decarbamylase) and pAD402 (coding for a decarbamylase having Tyr substituted for 57-His), respectively, as shown in Figs. 4 and 5, SphI and SalI are close to each other in the vicinity of the decarbamylase gene; therefore, an NdeI restriction site is formed at the initiation codon of the decarbamylase gene by the PCR method. Thus, 57-amino acid is present on the NdeI-SacI DNA fragment of about 190 bp; 203-amino acid on the SalI-ClaI DNA fragment of about 170 bp; and 236-amino acid on the ClaI-SphI DNA fragment of about 75 bp. The multiple mutant is prepared by the replacement of these mutation-free DNA fragments with DNA fragments having some mutation in the thermostability-related amino acids. The DNA fragment having some amino acid mutation is prepared from the transformant or expression vector as obtained in Example 3 or 8. On the other hand, an NdeI-EcoRI DNA fragment of about 1.6 kb containing a decarbamylase gene (which is naturally occurring or contains one amino acid mutation) is obtained by cleaving pAD108, pAD402 or the like, which has been conferred an NdeI restriction site, and then incorporated into an appropriate vector having NdeI and EcoRI restriction sites, such as pKK NE (prepared from pKK233-2 shown in Fig. 3) described in Example 9 and shown in Fig. 3, thereby obtaining a plasmid (e.g., pAD 1086 shown in Fig. 4 from pAD108; and pAD4021 shown in Fig. 5 from pAD402). The DNA fragments of this plasmid, which have no mutation site for the above-mentioned three amino acids, are replaced with DNA fragments containing such amino acid mutation sites, thereby obtaining an expression vector for multiple mutants. In general, the multiple mutants thus prepared have improved thermostability in an additive manner depending upon the degree of thermostability improvement obtained by a single mutation. For the decarbamylase having two or more amino acid mutations in three kinds of amino acids, it is possible to select any combination of the substituted amino acids as described above. The above-described method is not limited to the case of Agrobacterium species KNK712 (FERM BP-1900), and it can be used, in general, for a further improvement in the thermostability of a decarbamylase enzyme. That is, according to the method for making an enzyme thermostable as described above, several kinds of decarbamylases improved in thermostability can be obtained by the replacement of an amino acid, from which the thermostability-related amino acids are identified, and still another replacement of this amino acid makes it possible to find other amino acids giving an improvement in the thermostability. Under the circumstances that DNA fragments corresponding to the respective thermostable decarbamylases have already been obtained, simultaneous mutations can be introduced into plural amino acid sites related to the thermostability improvement by the operations in the following steps 1) to 5) (multiple mutant), which afforded a decarbamylase additively improved in thermostability. 1) Restriction enzymes are found out, each of which can yield a DNA fragment comprising a DNA portion coding for one of the thermostability-related amino acids by cleavage for the DNA fragment. 2) A DNA fragment comprising a DNA portion coding for one amino acid mutation site is obtained from the DNA fragments each coding for a thermostable decarbamylase having at least one amino acid mutation or from the vectors comprising the fragments with the corresponding restriction enzyme in step 1). 3) The corresponding DNA fragment but having no amino acid mutation is removed by cleaving a DNA fragment coding for a decarbamylase or a vector comprising the fragment with the same restriction enzyme as described above. 4) The mutation-containing DNA fragment obtained in step 2) is incorporated into the remaining DNA fragment after the cleavage for removing the corresponding DNA fragment in step 3) or into the vector comprising the remaining fragment. 5) The operations in steps 2) to 4) are repeated, if necessary. According to this series of operations, it is possible to increase, by one or more, the number of mutations in the thermostability-related amino acids. In case of Agrobacterium species KNK712 (FERM BP-1900), as the vector in step 2), any vector can be used so long as it has been prepared by giving an NdeI restriction site in an expression vector which is pUC19 having incorporated thereinto one of the DNA fragments shown in Sequence Listing, SEQ ID NOs: 2-30. As the vector containing a decarbamylase gene in step 3), various vectors, including pAD4021 and pAD1086, can be used which are obtained using pKK NE in the same operations as for pAD4021 from the vector prepared by giving an NdeI restriction site in any vector selected from various examples of the expression vector as recited below, except for the vectors each containing a DNA fragment coding for a decarbamylase having already received an amino acid substitution for an improvement in the thermostability at every one of three thermostability-related sites (i.e., triple mutation-containing pAD426, pAD427, pAD454, pAD455 and pAD456). The DNA fragments obtained with restriction enzymes are NdeI-SacI, SalI-ClaI and ClaI-SphI in any case of step 2) or step 3). The preparation of DNA fragments each coding for a decarbamylase, expression vectors obtainable by incorporating each of the DNA fragments into a vector, and transformants obtainable by incorporating each of the expression vectors into a host cell can be conducted using a gene manipulation technique as described in, for example, International Publication No. WO92/10579. From the expression vector obtained by the incorporation of a DNA fragment shown in Sequence Listing, SEQ ID NOs: 2-34 into pUC19 or pKK NE, for example, the incorporation into pUC19 gives pAD402, pAD404, pAD406, pAD416, pAD428, pAD429, pAD431, pAD434, pAD435, pAD439, pAD441, pAD445, pAD447, pAD448, pAD450, pAD451, pAD452, pAD453, pAD454 or pAD456; the incorporation into pKK NE gives pAD421, pAD422, pAD423, pAD424, pAD425, pAD426, pAD427, pAD461 and pAD455; and the incorporation into pUC NT (Fig. 10, upper part) gives pNT4553 (Fig. 10, lower part). The replacement of amino acids at the thermostability-related sites makes no change in the above-described restriction sites for restriction enzymes; therefore, the expression vector obtained by incorporation into pUC19 has the same restriction map as that of pAD429 (Fig. 6, upper part) and the expression vector obtained by incorporation into pKK NE has the same restriction map as that of pAD455 (Fig. 6, lower part). These expression vectors were used for the transformation of Escherichia coli JM109 or HB101, which afforded the following transformants capable of producing a decarbamylase improved in thermostability. E. coli JM109 pAD402 (FERM BP-3912), E. coli JM109 pAD404 (FERM BP-3913), E. coli JM109 pAD406 (FERM BP-3914), E. coli JM109 pAD416 (FERM BP-3915), E. coli JM109 pAD428, E. coli JM109 pAD429 (FERM BP-4035), E. coli JM109 pAD431, E. coli JM109 pAD434, E. coli JM109 pAD435, E. coli JM109 pAD439, E. coli JM109 pAD441, E. coli JM109 pAD445, E. coli JM109 pAD447, E. coli JM109 pAD448, E. coli JM109 pAD450, E. coli JM109 pAD421, E. coli JM109 pAD422, E. coli JM109 pAD423, E. coli JM109 pAD424 (FERM BP-4034), E. coli JM109 pAD425, E. coli JM109 pAD426, E. coli JM109 pAD427, E. coli JM109 pAD451, E. coli JM109 pAD452, E. coli JM109 pAD453, E. coli JM109 pAD461, E. coli JM109 pAD454, E. coli JM109 pAD455 (FERM BP-4036), E. coli JM109 pAD456, E. coli JM109 pAD468, E. coli JM109 pAD469, E. coli JM109 pAD470 or E. coli HB101 pNT4553 (FERM BP-4368). For these decarbamylases of the improved type, the amount of such an enzyme of interest produced by a transformant can be increased by incorporating the corresponding gene downstream the strong promotor of a vector. The transformant can be cultured on a conventional nutrient medium to express a transformant DNA which has been introduced thereinto. In case where the transformant DNA has been conferred with certain properties originating in the genetic or vector DNA, the medium may be supplemented with various agents depending upon the properties. The transformant thus produced can be obtained as an enzyme source only by culture preparation on a conventional medium; if necessary, various treatments may be employed for enzyme induction, such as the addition of hydantoin compounds, D-N-carbamoyl-α-amino acids, isopropyl-1-thio-β-D-galactoside (IPTG) or the like, and a temperature increase. The medium to be used for the culture preparation of a transformant may usually be a conventional medium containing carbon sources, nitrogen sources and inorganic ions. If organic micronutrients such as vitamins and amino acids are added thereto, preferred results are often obtained. As the carbon sources, carbohydrates such as glucose and sucrose, organic acids such as acetic acid, alcohols and the like are suitably used. As the nitrogen sources, ammonia gas, ammonia water, ammonium salts and the like are used. As the inorganic salts, phosphate ion, magnesium ion, potassium ion, ferric ion and the like may be used. The culture preparation may be conducted under aerobic conditions for 1 to 10 days while making an appropriate adjustment to the pH range of 4 to 8 and the temperature range of 25° to 45°C, which makes it possible to obtain desirable results. The enzyme produced by a transformant can be applied as a culture solution of the transformant, bacterial cells, treated bacterial cells, enzyme extracts from bacterial cells, immobilized bacterial cells or the like. As the bacterial cells, there can be used any one of the forms such as an untreated culture solution after completion of the culture preparation, bacterial cells separated from the culture solution and washed bacterial cells. As the treated bacterial cells, there can be used lyophilized bacterial cells, acetone-dried bacterial cells, toluene- or surfactant-contacted bacterial cells, lysozyme-treated bacterial cells, ultrasonicated bacterial cells, mechanically-disrupted bacterial cells, enzyme extracts having an enzyme activity to convert D-N-carbamoyl-α-amino acids into the corresponding D-α-amino acids by removal of the carbamoyl groups, which have been obtained from these treated bacterial cells; immobilization products of these bacterial cells; insolubilization products of the treated bacterial cells; immobilization products of the treated bacterial cells on a support for the immobilization of an enzyme protein (e.g., anion exchange resin), or the like. For the method for immobilization, reference is made to the specification of the Japanese Patent Laid-open Publication No. 63-185382. As the support to be used for immobilization, various anion exchange resins having various amine-, ammonium salt- or diethanolamine-type functional groups are suitable, such as phenol-formaldehyde anion exchange resins, for example, Duolite A568 and DS17186 (Rohm & Haas; registered trade names), and polystyrene resins, for example, Amberlite IRA935, IRA945, IRA901 (Rohm & Haas; registered trade names), Lewatatit OC1037 (Bayer; registered trade name) and Diaion EX-05 (Mitsubishi Chemical Inductries; registered trade name). In addition to these, any other support such as DEAE-cellulose can also be used. Further, to ensure firm and stable adsorption of an enzyme, a crosslinking agent is usually used, and a preferred example thereof is glutaraldehyde. As the enzyme to be used, there can be applied, in addition to purified enzymes, those which have been purified to various degrees, such as partially-purified enzymes, solutions containing disrupted bacterial cells and cell-free extracts. For the preparation of an immobilized enzyme, conventional methods can be used, which comprises, for example, allowing an enzyme to be adsorbed from an enzyme solution on a support and then subjecting the support to crosslinking treatment. D-N-carbamoyl-α-amino acids used as the substrate of an enzyme reaction in the present invention can be expressed by the formula: R-CH(NHCONH 2 )-COOH, and their forms to be subjected to the reaction in actual cases are as follows: in case where the enzyme to be used has strict stereoselectivity on the D-N-carbamoyl-α-amino acids, they can be used as the D-form or a mixture of the D- and L-forms; and in case where the enzyme has loose stereoselectivity because of its action even on L-carbamoylamino acids, or in case where it is used as an enzyme mixture capable of acting even on the L-amino acids, it is preferred that they are used only in the D-form for the production of α-amino acids in the D-form. The substituent R can be selected within a wide range as described in the specifications of the Japanese Patent Publication Nos. 57-18793, 63-20520 and 1-48758. In particular, to provide useful compounds from an industrial point of view, such as intermediates for medicaments, it is preferred that R is phenyl, hydroxy-substituted phenyl, alkyl, substituted alkyl, aralkyl or thienyl. When R is hydroxy-substituted phenyl, it may be substituted with one or more hydroxy groups at the o-, m- and/or p-positions, and the typical example thereof is p-hydroxyphenyl. The term alkyl refers to a group of 1 to 4 carbon atoms, the corresponding amino acid of which is D-alanine, D-valine, D-leucine, D-isoleucine or the like. The term substituted alkyl refers to an alkyl group of 1 to 4 carbon atoms, which is substituted with hydroxy, alkylthio, carboxyl, amino, phenyl, hydroxy-substituted phenyl, amide or the like, the corresponding amino acid of which is D-serine, D-threonine, D-methionine, D-cysteine, D-asparagine, D-glutamine, D-tyrosine, D-tryptophan, D-aspartic acid, D-glutamic acid, D-histidine, D-lysine, D-arginine, D-citrulline or the like. The term aralkyl refers to a group of 7 to 8 carbon atoms, such as benzyl or phenetyl, the corresponding amino acid of which is D-phenylalanine or the like. As the aqueous medium, there can be used water, buffer or those which contain an organic solvent such as ethanol. If necessary, various additives such as nutrients needed for the growth of microorganisms, antioxidants, surfactants, coenzymes, hydroxylamine and metals can also be added to the aqueous medium. In case where the bacterial cells of the above-described microorganism are cultured in an aqueous medium, during which the bacterial cells are brought into contact with D-N-carbamoyl-α-amino acids for the reaction, D-N-carbamoyl-α-amino acids and nutrients needed for the growth of the microorganism, such as carbon sources, nitrogen sources and inorganic ions, are contained in the aqueous medium used. If organic micronutrients such as vitamins and amino acids are added thereto, preferred results are often obtained. As the nitrogen sources, carbohydrates such as glucose and sucrose, organic acids such as acetic acid, alcohols and the like are suitably used. As the nitrogen sources, ammonia gas, ammonia water, ammonium salts and the like are used. As the inorganic salts, phosphate ion, magnesium ion, potassium ion, ferric ion and the like may be used. The culture preparation may be conducted under aerobic conditions for 1 to 10 days while making an appropriate adjustment to the pH range of 4 to 8 and the temperature range of 25°C to 45°C, which makes it possible that D-N-carbamoyl-α-amino acids are converted only into D-α-amino acids with high efficiency. In contrast, when the reaction of an untreated culture solution of the above-described microorganism, cultured bacterial cells, treated bacterial cells, enzyme extracts, immobilization products of bacterial cells, insolubilization products of bacterial cells or immobilization products of enzyme proteins is effected in an aqueous medium containing D-N-carbamoyl-α-amino acids dissolved or suspended therein, the reaction system may be allowed to stand or stirred for a time, while the temperature is adjusted to an appropriate range of 10°C to 80°C and the pH is kept in the range of 4 to 9.5. In this manner, after the lapse of 5 to 100 hours, D-α-amino acids are produced in great amounts and accumulated in the aqueous medium. The D-N-carbamoyl-α-amino acids may be added in separated portions according as the reaction proceeds. The D-α-amino acids produced can be separated and purified by a conventional separation method. The D-α-amino acids obtained herein can be expressed by the formula: R-CHNH 2 -COOH (wherein R is as defined above). The following will describe the typical embodiments of the present invention. The detection and quantitative analysis of D-α-amino acids produced were conducted by high performance liquid chromatography (HPLC) or thin layer chromatography (TLC). Example 1 Mutagenesis of Agrabacterium species KNK712 decarbamylase gene with hydroxylamine Plasmid pAD108 having a KNK712 decarbamylase gene was digested with restriction enzymes HindIII and EcoRI, which was mixed and ligated with the digest of an M13mp18 double-stranded DNA with HIndIII and EcoRI. This was transformed into E. coli JM109, which was mixed with 2 ml of H top agar medium (10 g/l bactotrypton, 8 g/l NaCl, 8 g/l bactoagar) containing 100 mM isopropyl-β-D-thiogalactopyranoside (IPTG) and 0.2% 5-bromo-4-chloro-3-indolyl-β-D-galactoside (X-Gal) added both in an amount of 40 µl, and then plated on an H agar plate (10 g/l bactotrypton, 8 g/l NaCl, 12 g/l bactoagar), followed by incubation at 37°C and separation of a white plaque. This recombinant phage was cultured on 100 ml of 2YT medium (16 g/l bactotrypton, 10 g/l bactoyeast extract, 5 g/l NaCl), and a PEG-NaCl (20% polyethylene glycol 6000, 2.5 M NaCl) solution was added to the supernatant at a 1/5 volume, so that phage particles were precipitated and collected by centrifugation. These recombinant phage particles were treated in NH 2 OH having a final concentration of 0.25 M (pH 6.0) at 37°C for 1 to 8 hours. The phage particles were precipitated with a PEG-NaCl solution, and then dissolved in sterilized water, with which E. coli JM109 was infected. The microorganism was cultured on 800 ml of 2YT medium, and a double-stranded DNA having some mutation was prepared by the alkaline-SDS method and CsCl ultracentrifugation method. This was digested with HindIII and EcoRI, and then incorporated into pUC19, which was transformed into E. coli JM109, and then plated on a 2YT agar plate containing 50 µg/ml ampicillin. Thus, about 15,000 E. coli transformants each having a mutant decarbamylase gene were obtained. Example 2 Mutagenesis of Agrobacterium species KNK712 decarbamylase gene with nitrous acid The recombinant phage particles obtained by incorporating a KNK712 decarbamylase gene into M13mp18 as prepared in Example 1 were cultured on 800 ml of 2YT medium for preparation, and then subjected to phenol extraction and ethanol precipitation, which afforded a single-stranded phage DNA. This single-stranded DNA was treated with NaNO 2 having a final concentration of 0.9 M (pH 4.3) at 25°C for 30 minutes, allowing a gene to cause some mutation. This was made into a double-stranded form by the use of Sequenase R ver. 2.0 (United States Biochemical) and AMV reverse transcriptase (Life Science). This was digested with restriction enzymes HindIII and EcoRI, and incorporated into pUC19, which was transformed into E. coli JM109 and then plated on a 2YT plate (containing ampicillin). Thus, about 7600 E. coli transformants each having a mutant decarbamylase gene were obtained. Example 3 Screening of decarbamylase-producing strain improved in thermostability The colony of the E. coli transformants each having a mutant decarbamylase gene on the plate were replicated onto a filter paper (Toyo Roshi, 5C, 83 mm), which was soaked with 1.5 ml of lytic solution (20 mM Tris·HCl (pH 7.5), 10 mM EDTA, 2 mg/ml lysozyme, 1% Triton X-100), followed by a reaction at 37°C for 30 minutes, washing with water and drying. This filter paper was immersed in hot water at 65°C for thermal treatment, and after drying, it was soaked with 1 ml of color development reaction solution (30 mM K-phosphate buffer (pH 7.4), 0.3% carbamyl-D-phenylglycine, 0.25% phenol, 10 mg/ml D-amino acid oxidase (Sigma), 2.36 µg/ml peroxidase (derived from horseradish, CALZYME Lab.), 0.1 mg/ml 4-aminoantipyrine), followed by a reaction at 37°C for 30 minutes. Colonies corresponding to the red-color developed spots were separated, as the strain improved in thermostability, from the original plate. From 27,000 mutants as prepared by mutagenesis with hydroxylamine in Example 1 and 7600 mutants as prepared by mutagenesis with nitrous acid in Example 2, twelve and seven mutants improved in thermostability were obtained, respectively. Example 4 Thermostability evaluation of decarbamylase improved in thermostability A shaken culture containing one of four decarbamylase-producing strains improved in thermostability as obtained in Example 3 (three mutants obtained by mutagenesis with hydroxylamine and one mutant obtained by mutagenesis with nitrous acid), together with recombinant E. coli JM109 pAD108 (FERM BP-3184) having a KNK712 decarbamylase gene, was prepared in 10 ml of 2YT liquid medium (containing 50 µg/ml ampicillin and 1 mM IPTG) and incubated overnight. After harvested, the bacterial cells were washed with 0.1 M K-phosphate buffer (pH 7.0), and suspended in 1 ml of the same buffer, after which the suspension was disrupted with an ultrasonic disrupting apparatus (Tomy Seiko, model UR-20P) and the residue was removed by centrifugation. One part of this crude enzyme solution was subjected to thermal treatment at a temperature such as 55°C, 60°C, 65°C, 70°C and 75°C for 10 minutes, and some denatured insolubilized protein was removed by centrifugation. Then, the decarbamylase activity before and after the thermal treatment were measured. In the measurement, using 1% carbamyl-D-hydroxy-phenylglycine as the substrate, the reaction was effected in 0.1 M K-phosphate buffer (pH 7.0) at 40°C for 20 minutes, followed by protein denaturation with 5% TCA and removal thereof, after which the amount of D-hydroxyphenylglycine produced was determined by high performance liquid chromatography. The results are shown in Fig. 1. Example 5 Gene analysis of decarbamylase improved in thermostability The gene analysis was conducted for the mutant decarbamylase improved in thermostability, to presume the mutation sites of the decarbamylase protein. The plasmid having a gene for the decarbamylase improved in thermostability was reacted in a programmable incubator (ASTEC, model PC-700) using Taq Dye Deoxy TM Terminator Cycle Sequencing Kit (Applied Biosystems), followed by removal of excess Dye Deoxy using Bio Spin 30 (BIO-RAD). This sample was subjected to electrophoresis and data analysis with DNA sequencer model 373A (Applied Biosystems). As the result, the mutation sites were found as shown in Table 1. In every cases, the thermostability was improved by one amino acid mutation. Mutant Thermostable temperature(°C) DNA mutation Amino acid mutation (712) 61.8 none none Escherichia coli JM109 pAD402 67.3 401C→T 57His→Tyr JM109 pAD404 68.0 840C→T 203Pro→Leu JM109 pAD406 66.5 839C→T 203Pro→Ser JM109 pAD416 71.4 939T→C 236Val→Ala Example 6 Immobilization of decarbamylase improved in thermostability on resin A shaken culture containing one of four decarbamylase-producing strains improved in thermostability, together with recombinant E. coli JM109 pAD108 (FERM BP-3184) having a KNK712 decarbamylase gene, was prepared in 1 liter of 2YT medium (containing 50 µg/ml ampicillin and 1 mM IPTG) and incubated overnight. After harvested, the bacterial cells were washed with 0.1 M K-phosphate buffer (pH 7.0), and suspended in 100 ml of the same buffer, after which the suspension was disrupted with an ultrasonic disrupting apparatus (BRANSON, Sonifier model 250) and the residue was removed by centrifugation to yield a crude enzyme solution. To this, Duolite A-568 (Rohm & Haas) equilibrated with 0.1 M K-phosphate buffer (pH 7.0) was added at a ratio of 1 g resin to 40 mg protein, and the mixture was stirred under a nitrogen seal at 4°C for 20 hours to ensure the adsorption. This enzyme-adsorbing resin was washed with 0.1 M K-phosphate buffer (pH 7.0) and 10 mM dithiothreitol (DTT), and reacted in 0.2% glutaraldehyde and 0.1 M K-phosphate buffer (pH 7.0) at 4°C for 10 minutes for the protein crosslinking, which afforded an immobilized enzyme-containing resin. The activities of the resins thus obtained are shown in Table 2. Table 2 Mutant E.coli JM109 pAD402 E.coli JM109 pAD404 E.coli JM109 pAD406 E.coli JM109 pAD416 712 Activity (u/g resin) 12.4 9.4 10.0 14.6 9.6 Example 7 Repeated continuous reaction using immobilized decarbamylase The mutant decarbamylase improved in thermostability was evaluated by the repeated continuous reaction using the immobilized enzyme-containing resin as obtained in Example 6. The reaction was effected (100 ml of reaction mixture) using 50 units of the immobilized decarbamylase and 3% carbamyl-D-HPG as the substrate at 40°C with stirring under a stream of nitrogen gas while the pH was adjusted to 7.0. The sample was taken for the activity measurement after 10 and 60 minutes, and the reaction was continued for 23.5 hours in total. The reaction mixture was removed by suction, after which another fresh reaction mixture was charged and allowed to react in the same manner as described above; such an operation was repeated 15 times, and the change in the activity of the immobilized decarbamylase was examined. The results are shown in Fig. 2. All the decarbamylases improved in thermostability were found to have improved stabilities when used in the reaction as the immobilized enzyme-containing resin, as compared with those before the mutagenesis. Example 8 Amino acid replacement at thermostability-related sites Various amino acids were substituted for three sites found to be related to thermostability, i.e., histidine which is 57-amino acid, proline which is 203-amino acid, and valine which is 236-amino acid, to prepare various derivatives. The preparation of these derivatives was conducted by the use of a method utilizing polymerase chain reaction (PCR) (W. Ito, et al., Gene, 102, 67-70 (1991)). For each of the thermostability-related sites, various synthetic primers were prepared with a mixed solution of A, T, G and C by the DNA synthesizer model 391 (Applied Biosystems), said primers having about 10-base sequence portions which were the same as the complimentary gene sequences on both sides of the gene portion corresponding to the amino acid at each site and also having any combination of A, T, G and C in the 3-base portion corresponding to the amino acid to be replaced, so that any kind of amino acid was incorporated into this 3-base portion after the replacement. With the use of pAD108 as the template, and using this primer and M13RV primer (Takara Shuzo), the PCR was effected by the programmable incubator model PC-700 (ASTEC). A DNA fragment containing the full-length decarbamylase gene fragment (1785 bases) and short-length sequences on both sides and having a mutation at every HindIII restriction sites so as not to be cleaved was prepared by the PCR reaction using pAD108 and MUTF3 and M13M4 primers (both, Takara Shuzo). These two kinds of PCR products were mixed together, and heated at 94°C for 10 minutes, after which the mixture was gradually cooled for annealing, which afforded a combined form of these two kinds of DNAs. Using Taq DNA polymerase, single-stranded DNA portions were made into double-stranded forms. Using this DNA and M13M4 and M13RV primers, the PCR reaction was effected, and the DNA produced was simultaneously digested with HindIII and EcoRI. The digest was ligated with pUC19 which had been digested in the same manner as described above, and transformed into E. coli. According to the method as described in Example 3, only the strains improved in thermostability were screened, and it was found that derivatives having a substitution of leucine for 57-histidine, derivatives having a substitution of asparagine, glutamic acid, threonine, alanine, isoleucine or histidine for 203-proline, and derivatives having a substitution of threonine or serine for 236-valine had improved thermostability, as shown in Tables 3 and 4. Mutant Thermostable temperature(°C) DNA mutation Amino acid mutation (712) 61.8 none none Escherichia coli JM109 pAD434 67.5 402A→T 57His→Leu 403T→A JM109 pAD435 67.0 402A→T 57His→Leu JM109 pAD431 67.0 839C→A 203Pro→Asn 840C→A 841T→C JM109 pAD429 70.0 839C→G 203Pro→Glu 840C→A 841T→A JM109 pAD445 67.5 839C→A 203Pro→Thr 841T→C JM109 pAD468 67.7 839C→G 203Pro→Ala JM109 pAD469 67.2 839C→A 203Pro→Ile 840C→T JM109 pAD470 65.2 840C→A 203Pro→His Mutant Themostable temperature(°C) DNA mutation Amino acid mutation (712) 61.8 none none Escherichia coli JM109 pAD428 70.0 939T→C 236Val→Ala 940G→C JM109 pAD439 71.5 939T→C 236Val→Ala 940G→T JM109 pAD441 71.5 938G→A 236Val→Ser 939T→G 940G→T JM109 pAD447 72.0 938G→T 236Val→Ser 939T→C 940G→A JM109 pAD448 72.0 938G→T 236Val→Ser 939T→C JM109 pAD450 69.5 938G→A 236Val→Thr 939C→C Example 9 Multiple mutation at thermostability-related site Derivatives (multiple mutants) having a combination of two or three amino acid mutations which had been found to improve the thermostability, at three sites which had been found to be related to the thermostability, i.e., histidine which is 57-amino acid, proline which is 203-amino acid, and valine which is 236-amino acid, were prepared as follows. The multiple mutants were prepared by replacing the mutation-free DNA fragments of a native gene with the restriction enzyme-digested DNA fragments each containing some mutation site of a mutant decarbamylase gene which had been improved in thermostability by a single amino acid substitution, as obtained in Example 3 or 8. First, because pAD108, pAD402 and other plasmids, which had the same restriction sites, each have two SalI and two SphI restriction sites for use in the replacement of the above-described DNA fragments, it is necessary to make these sites into a single restriction site. Therefore, as a new vector for gene incorporation, pKK NE was prepared, into which a decarbamylase gene fragment was incorporated, thereby constructing an expression vector to be used for the DNA fragment replacement. Plasmid pKK NE was prepared as follows. Plasmid pKK233-2 (Pharmacia) shown in Fig. 3 was digested with EcoRI and NdeI, and subjected to agarose gel electrophoresis to separate a 2.7-kb DNA fragment, after which the cohesive end of the DNA fragment was changed to the blunt end with a DNA blunting kit (Takara Shuzo), followed by ligation and transformation into E. coli JM109. The NcoI restriction site of the vector thus prepared was changed to the NdeI restriction site using the PCR method. The resulting plasmid was digested with HindIII, and the cohesive end thereof was changed to the blunt end, followed by ligation with pEcoRI linker (Takara Shuzo), which afforded pKK NE as shown in Fig. 3. Next, into the initiation codon portion of the decarbamylase gene in plasmid pAD108 or pAD402, the Nael restriction site was generated by the PCR method. The resulting NdeI-EcoRI DNA fragment of 1.6 kb was then incorporated into pKK NE, which afforded expression vectors pAD1086 (derived from pAD108; shown in Fig. 4) and pAD4021 (derived from pAD402; shown in Fig. 5). The 57-amino acid mutation is positioned on the NdeI-SacI DNA fragment of about 190 bp; the 203-amino acid mutation on the SalI-ClaI DNA fragment of about 170 bp (this DNA fragment is hereinafter referred to as fragment A); and the 236-amino acid mutation on the ClaI-SphI DNA fragment of about 75 bp (hereinafter referred to as fragment B). Therefore, fragment A of pAD4021 was removed, into which site the same fragment of pAD404 or pAD406 was incorporated, resulting in pAD421 or pAD422, respectively. Also, fragment B of pAD1086, pAD4021, pAD421 or pAD422 was replaced with the same fragment of pAD416, resulting in pAD4161, pAD423, pAD426 or pAD427, respectively. Further, fragment A of pAD4161 was replaced with the same fragment of pAD404, pAD406 or pAD429, resulting in pAD424, pAD425 or pAD461, respectively; fragment A of pAD402 or pAD423 was replaced with the same fragment of pAD429, resulting in pAD451 or pAD455 (shown in Fig. 6), respectively; fragment B of pAD402, pAD429 (shown in Fig. 6), pAD451 or pAD421 was replaced with the same fragment of pAD447, resulting in pAD452, pAD453, pAD454 or pAD456, respectively. The expression vectors prepared in the above-described manner were separately transformed into E. coli JM109, from which the extracts of disrupted bacterial cells were prepared and examined for the thermostability. As shown in Tables 5 and 6, for the multiple mutants, it was found that the thermostability was additively improved according to the degree of an improvement in the thermostability attained by a single mutation. For the decarbamylase produced by E. coli JM109 pAD455 (FERM PB-4036) having highest thermostability, it was found that the thermostability was improved even by about 19°C, as compared with the decarbamylase before the mutagenesis. Mutant Amino acid mutation Thermostable temperature(°C) (712) none 61.8 Escherichia coli JM109 pAD421 57His→Tyr 71.5 203Pro→Leu JM109 pAD422 57His→Tyr 70.0 203Pro→Ser JM109 pAD423 57His→Tyr 75.4 236Val→Ala JM109 pAD424 203Pro→Leu 77.5 236Val→Ala JM109 pAD425 203Pro→Ser 75.9 236Val→Ala JM109 pAD426 57His→Tyr 78.8 203Pro→Leu 236Val→Ala JM109 pAD427 57His→Tyr 77.8 203Pro→Ser 236Val→Ala Mutant Amino acid mutation Thermostable temperature(°C) (712) none 61.8 Escherichia coli JM109 pAD451 57His→Tyr 74.0 203Pro→Glu JM109 pAD452 57His→Tyr 75.3 236Val→Ser JM109 pAD453 203Pro→Glu 76.0 236Val→Ser JM109 pAD461 203Pro→Glu 79.0 236Val→Ala JM109 pAD454 57His→Tyr 80.4 203Pro→Glu 236Val→Ser JM109 pAD455 57His→Tyr 80.8 203Pro→Glu 236Val→Ala JM109 pAD456 57His→Tyr 78.5 203Pro→Leu 236Val→Ser Example 10 Reaction temperature characteristics of decarbamylase improved in thermostability A shaken culture containing a thermostability-improved decarbamylase-producing strain, E coli JM109 pAD108 (FERM BP-3184) or E. coli JM109 pAD455 (FERM BP-4036), was prepared in 10 ml of 2YT medium and incubated overnight. After harvested, the bacterial cells were washed with 0.1 M K-phosphate buffer (pH 7.0), and then suspended in 1 ml of the same buffer, after which the suspension was disrupted with an ultrasonic disrupting apparatus (Tomy Seiko, model UR-20P) and the residue was removed by centrifugation to yield a clude enzyme solution. This crude enzyme solution was 10-fold diluted with a solution obtained by the addition of 5 mM dithiothreitol to the same buffer. Several 1 ml substrate solutions (1% carbamyl-D-p-hydroxyphenylglycine, 0.1% K-phosphate buffer (pH 6.5)) were kept at various temperatures of 30°C to 85°C for 3 minutes, respectively, to which 100 µl of the diluted crude enzyme solution (or when the temperature was in the range of 50°C to 80°C for higher thermostability (the temperature was in the range of 50°C to 65°C for the crude enzyme solution of E. coli JM109 pAD108), 100 µl of the enzyme solution which had been further 2- to 3-fold diluted) was added, followed by a reaction at the respective temperatures for 20 minutes. Then, the reaction was allowed to stop by adding 250 µl of 20% trichloroacetic acid solution, followed by centrifugation, after which the supernatant was analyzed by high performance liquid chromatography (Nakalai tesque, Cosmosil 5C18-AR column). Taking the activity at 40°C as 100%, the relative activity at each temperature is shown in Fig. 7. As can be seen from Fig. 7, the enzyme produced by E. coli JM109 pAD455 exhibits the highest activity at a temperature around 75°C and has extremely improved stability, indicating that it was improved into an enzyme capable of acting at higher temperatures, as compared with the enzyme produced by E. coli JM109 pAD108. Example 11 pH stability of decarbamylase improved in thermostability A shaken culture containing a thermostability-improved decarbamylase-producing strain, E. coli JM109 pAD416 (FERM BP-3915) or E. coli JM109 pAD455 (FERM BP-4036), was prepared in 10 ml of 2YT medium and incubated overnight. After harvested, the bacterial cells were washed with 10 mM K-phosphate buffer (pH 7.0; containing 0.5 mM dithiothreitol), and suspended in 1 ml of the same buffer, after which the suspension was disrupted with a small-sized ultrasonic disrupting apparatus and the residue was removed by centrifugation to yield a clude enzyme solution. Various buffers for the respective pHs were prepared from 0.1 M K-phosphate buffer (pH 5.5, 6, 7, 8), Tris·HCl buffer (pH 7.5, 8, 9) and sodium carbonate buffer (pH 9, 10, 11). To 800 µl of each of these buffers, 200 µl of the crude enzyme solution was added, and the mixture was incubated at 40°C for 12.5 hours. Then, 100 µl of the mixture was added to 1 ml of substrate solution (1% carbamyl-D-p-hydroxyphenylglycine, 0.1% K-phosphate buffer (pH 7.0)), followed by a reaction at 40°C, after which the reaction mixture was analyzed in the same manner as described in Example 10. Taking the activity at pH 7.0 as 100%, the relative activity of the treated sample at each pH is shown in Fig. 8. Example 12 Preparation and expression of expression vector for decarbamylase improved in thermostability (1) Preparation of vector tor expression of foreign gene First, a vector was prepared for the incorporation of a thermostability-improved decarbamylase gene and expression thereof (Fig. 9). From pUC19, HindIII-Cfr10I fragment of 1.3 kb was prepared by the PCR method, at which time as the PCR primer, a primer having a sequence designed so that an NdeI restriction site was formed at the initiation codon site of a lacZ gene inside the HindIII restriction site and the sequence could be further digested with HindIII and Cfr10I, was used for the reaction. This DNA fragment obtained by the PCR was digested with HindIII and Cfr10I, followed by ligation with 1.4-kb fragment obtained by removal of the corresponding 1.3-kb fragment from pUC19, resulting in plasmid pUC·Nde which corresponds pUC19 having an NdeI restriction site added thereto. Next, from pTrc99A (commercially available from Pharmacia), an EcoRI-EcoRV fragment of 0.6 kb was prepared by the PCR method, at which time as the PCR primer, a primer having a sequence designed so that an NdeI restriction site was substituted for the NcoI restriction site inside the EcoRI restriction site and the sequence could be further digested with EcoRI and EcoRV, was used for the reaction. This DNA fragment obtained by the PCR was digested with EcoRI and EcoRV, followed by ligation with 3.6-kb fragment obtained by removal of the corresponding 0.6-kb fragment from pTrc99A, resulting in plasmid pTrcNde which corresponds pTrc99A having an NdeI restriction site added thereto. Then, a 2.0-kb fragment obtained by the digestion of pUC Nde with NdeI and SspI was ligated with a 0.6-kb fragment obtained by the digestion of pTrc Nde with NdeI and SspI, which afforded expression vector plasmid pUCNT for the expression of foreign genes (Fig. 10). (2) Incorporation of thermostability-improved decarbamylase gene into vector From pAD455 having a gene for one kind of decarbamylases improved in thermostability, only the gene portion coding for the decarbamylase was prepared by the PCR method, at which time as the PCR primer, a primer having a sequence designed so that an NdeI restriction site was formed in the initiation codon portion of the gene and a PstI restriction site was made just after the termination codon of the gene, was used for the reaction. A 0.9-kb DNA fragment obtained by this PCR was digested with NdeI and PstI, followed by ligation with pUCNT which was also digested with NdeI and PstI, which afforded expression vector plasmid pNT4553 (Fig. 10) for the decarbamylase improved in thermostability. (3) Expression of thermostability-improved decarbamylase expression vector The pNT4553 as prepared above was transformed into Escherichia coli HB101 by the calcium chloride method. A shaken culture of this transformant E. coli HB101 pNT4553 (FERM BP-4368) was prepared on a 2YT medium and incubated at 37°C for 16 hours. After the bacterial cells were harvested, a crude enzyme solution was prepared by the method as shown in (Example 10), and the decarbamylase activity was measured to be 5.6 unites/ml per culture solution. According to the present invention, DNA fragments each coding for a decarbamylase protein improved in thermostability can be obtained, from which a decarbamylase having high reactivity to D-N-carbamoyl-α-amino acids and excellent stability can be produced, and with the use of this enzyme, D-α-amino acids can be produced with high efficiency. "Sequence Listing" SEQ ID NO: 1 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Agrobacterium species STRAIN: KNK712 (FERM BP-1900) SEQ ID NO: 2 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD402 (FERM BP-3912) SEQ ID NO: 3 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD404 (FERM BP-3913) SEQ ID NO: 4 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD406 (FERM BP-3914) SEQ ID NO: 5 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD416 (FERM BP-3915) SEQ ID NO: 6 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD428 SEQ ID NO: 7 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD429 SEQ ID NO: 8 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD431 SEQ ID NO: 9 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD434 SEQ ID NO: 10 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD435 SEQ ID NO: 11 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD439 SEQ ID NO: 12 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD441 SEQ ID NO: 13 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD445 SEQ ID NO: 14 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD447 SEQ ID NO: 15 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD448 SEQ ID NO: 16 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD450 SEQ ID NO: 17 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD421 SEQ ID NO: 18 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD422 SEQ ID NO: 19 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD423 SEQ ID NO: 20 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD424 (FERM BP-4034) SEQ ID NO: 21 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD425 SEQ ID NO: 22 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD426 SEQ ID NO: 23 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD427 SEQ ID NO: 24 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD451 SEQ ID NO: 25 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD452 SEQ ID NO: 26 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD453 SEQ ID NO: 27 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD461 SEQ ID NO: 28 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD454 SEQ ID NO: 29 SEQUENCE LENGTH: 1559 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD455 (FERM BP-4036) SEQ ID NO: 30 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD456 SEQ ID NO: 31 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD468 SEQ ID NO: 32 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD469 SEQ ID NO: 33 SEQUENCE LENGTH: 1785 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: JM109 pAD470 SEQ ID NO: 34 SEQUENCE LENGTH: 926 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Genomic DNA ORIGINAL SOURCE ORGANISM: Escherichia coli STRAIN: HB101 pNT4553 (FERM BP-4368)

Claims: A decarbamylase derived from Agrobacterium species KNK712 (FERM BP-1900) having an improved thermostability containing a substitution of another amino acid for at least one amino acid selected from 57-histidine, 203-proline and 236-valine as shown in SEQ ID No. 1. A decarbamylase according to claim 1 wherein at least 57-histidine is replaced with another amino acid. A decarbamylase according to claim 1 wherein another amino acid is tyrosine or leucine. A decarbamylase according to claim 1 wherein at least 203-proline is replaced with another amino acid. A decarbamylase according to claim 4 wherein another amino acid is leucine, glutamic acid, serine, asparagine, threonine, alanine, isoleucine or histidine. A decarbamylase according to claim 1 wherein at least 236-valine is replaced with another amino acid. A decarbamylase according to claim 6 wherein another amino acid is alanine, threonine or serine. A decarbamylase having an amino acid sequence as shown in Sequence Listing, SEQ ID NOs: 2-34. A DNA fragment coding for a decarbamylase protein according to any one of claims 1 to 8. A DNA fragment according to claim 9 coding for a decarbamylase protein in which at least one of cytosine, adenine and thymine (CAT) of a base sequence coding for histidine which is the amino acid of amino acid number 57 is replaced with another base so that the histidine is replaced with tyrosine or leucine. A DNA fragment according to claim 9 coding for a decarbamylase protein in which at least one of cytosine, cytosine and thymine (CCT) of a base sequence coding for proline which is the amino acid of amino acid number 203 is replaced with another base so that the proline is replaced with serine, leucine, asparagine, glutamic acid, threonine, alanine, isoleucine or histidine. A DNA fragment according to claim 9 coding for a decarbamylase protein in which at least one of guanine, thymine and guanine (GTG) of a base sequence coding for valine which is the amino acid of amino acid number 236 is replaced with another base so that the valine is replaced with alanine, serine or threonine. A DNA fragment according to claim 10 wherein the base sequence CAT of claim 10 is replaced with TAT, CTT or CTA. A DNA fragment according to claim 11 wherein the base sequence CCT of claim 11 is replaced with TCT, CTT, AAC, GAA, ACC, GCT, ATT or CAT. A DNA fragment according to claim 12 wherein the base sequence GTG of claim 12 is replaced with GCG, GCT, ACC, ACG, TCA, TCG or AGT. An expression vector comprising a DNA fragment according to claims 9 to 15. A transformant microorganism transformed with an expression vector according to claim 16. A transformant microorganism, wherein transformant microorganism is E coli, JM109 pAD402 (FERM BP-3912), E. coli JM109 pAD404 (FERM BP-3913), E. coli JM109 pAD406 (FERM BP-3914), E. coli JM109 pAD416 (FERM BP-3915), E. coli JM109 pAD429 (FERM BP-4035), E. coli JM109 pAD424 (FERM BP-4034), E. coli JM109 pAD455 (FERM BP-4036) or E. coli HB101 pNT4553 (FERM BP-4368). A process for producing a decarbamylase improved in thermostability wherein the decarbamylase derived from Agrobacterium species KNK712 (FERM BP-1900) improved in thermostability is a decarbamylase containing a substitution of another amino acid for at least one amino acid selected from 57-histidine, 203-proline and 236-valine as shown in SEQ ID No. 1, comprising the step of culturing a microorganism transformed with a recombinant plasmid containing a DNA fragment coding for said decarbamylase. A process according to claim 19 wherein another amino acid is tyrosine or leucine for 57-histidine, leucine, serine, asparagine, glutamic acid, alanine, isoleucine, histidine or threonine for 203-proline, and alanine, threonine or serine for 236-valine. A process according to any one of claims 19 and 20, wherein the transformant microorganism is E. coli JM109 pAD402 (FERM BP-3912), JM109 pAD404 (FERM BP-3913), JM109 pAD406 (FERM BP-3914), JM109 pAD416 (FERM BP-3915), JM109 pAD429 (FERM BP-4035), JM109 pAD424 (FERM BP-4034), JM109 pAD455 (FERM BP-4036), or E. coli HB101 pNT4553 (FERM BP-4368). A process for producing a D-α-amino acid, characterized in that an N-carbamyl-D-α-amino acid is converted into the corresponding D-α-amino acid in an aqueous medium by the action of a decarbamylase with a thermostable temperature of 65°C or higher, derived from Agrobacterium species KNK712 (FERM BP-1900) containing a substitution of another amino acid for at least one amino acid selected from 57-histidine, 203-proline and 236-valine as shown in SEQ ID No. 1 and the resulting D-α-amino acid is collected. A production process according to claim 22 wherein another amino acid is tyrosine or leucine for 57-histidine, leucine, serine, asparagine, glutamic acid, alanine, isoleucine, histidine or threonine for 203-proline, and alanine, threonine or serine for 236-valine. A production process according to claim 22 wherein the decarbamylase is allowed to act in an immobilized form. A production process according to claim 24 wherein the immobilized decarbamylase is repeatedly used. A production process according to any one of claims 22 to 25 wherein the microorganism producing the decarbamylase with a thermostable temperature of 65°C or higher is a transformant microorganism of E. coli, wherein the transformant microorganism is E. coli JM109 pAD402 (FERM BP-3912), JM109 pAD404 (FERM BP-3913), JM109 pAD406 (FERM BP-3914), JM109 pAD416 (FERM BP-3915), JM109 pAD429 (FERM BP-4035), JM109 pAD424 (FERM BP-4034), JM109 pAD455 (FERM BP-4036), or E. coli HB101 pNT4553 (FERM BP-4368).

Inventors: IKENAKA YASUHIRO, NANBA HIROKAZU, TAKAHASHI SATOMI, TAKANO MASAYUKI, YAJIMA KAZUYOSHI, YAMADA YUKIO, IKENAKA, YASUHIRO, NANBA, HIROKAZU, TAKAHASHI, SATOMI, TAKANO, MASAYUKI, YAJIMA, KAZUYOSHI, YAMADA, YUKIO, IKENAKA, YASUHIRO 2-13-1037, KAWASAKI-CHO, NANBA, HIROKAZU KOUNRYO 2-63, OKIHAMA-CHO, TAKAHASHI, SATOMI 1-13-13, SHINWADAI, TAKANO, MASAYUKI 299-3-106, HIGASHIFUTAMI, YAJIMA, KAZUYOSHI 1-413-305, NAKAOCHIAI 1-CHOME, YAMADA, YUKIO 34-6, MINORI
Applicants: KANEGAFUCHI CHEMICAL IND, KANEGAFUCHI KAGAKU KOGYO KABUSHIKI KAISHA
Application year: 1993
Main classification: C12N15
IPC codes: C12N15, C12N9, C12P13