alpha 2-3 Sialyltransferase

Abstract: There are provided a novel α2→3 sialyltransferase expressed by a cloned gene from human cells, a cDNA encoding the α2→3 sialyltransferase, a method for detecting or suppressing the expression of an α2→3 sialyltransferase by use of said cDNA, a recombinant vector containing said cDNA, a cell containing said vector, and their production processes.

Description: The present invention relates to a novel α2 → 3 sialyltransferase utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as a substrate, DNA encoding the α2 → 3 sialyltransferase, recombinant vector in which the DNA is incorporated and a cell containing the recombinant vector as well as a process for producing them. Further, the present invention relates to a process for producing a sugar chain using the α2 →3 sialyltransferase and a process for producing a sugar chain by expressing the α2 →3 sialyltransferase in a transformed cell. Further, the present invention relates to a method for detecting or inhibiting the expression of α2 →3 sialyltransferase using a cDNA encoding the α2 →3 sialyltransferase of the present invention. The α2 →3 sialyltransferase of the present invention is useful for producing a sugar chain or modification thereof having useful physiological activity and improving a sugar chain which is attached to a useful physiologically active protein. While proteins produced by a procaryote such as Escherichia coli have no sugar chain, the proteins and lipids produced by an eucaryote such as yeast, fungi, plant cells, animal cells and the like have an attached sugar chain in many cases. As a sugar chain in animal cells, N-linked sugar chain (also called as N-glycan) which binds to asparagine (Asn) residue in proteins, and O-linked sugar chain (also called as O-glycan) which binds to serine (Ser) or threonine (Thr) residue are known to be added to glycoproteins. There has been recently revealed that a certain lipids containing a sugar chain are bound covalently to a number of proteins and the proteins are attached to the cell membrane via those lipids. Those lipids containing a sugar chain are called as glycosyl phosphatidylinositol anchor. The other example of a sugar chain in animal cells is glycosaminoglycan. A compound wherein a protein and a glycosaminoglycan are covalently bound is called as proteoglycan. Although glycosaminoglycan which is a component of sugar chain of proteoglycan has similar structure to that of O-glycan which is glycoprotein sugar chain, glycosaminoglycan has chemical properties different from those of O-glycan. Glycosaminoglycan has the characteristic structure composed of disaccharide unit repeats containing glucosamine or galactosamine and uronic acid (except that keratan sulfate has no uronic acid), wherein the sulfate groups are covalently bound thereto (except that hyaluronic acid has no sulfate groups). Furthermore, as a sugar chain in animal cells, there is a sugar chain contained in glycolipid. As glycolipid in animal cells, there are known sphingoglycolipid in which sugar, long chain fatty acid and sphingosine which is long chain base are covalently bound, and glyceroglycolipid in which sugar chain is covalently bound to glycerol. Recently, elucidation on the function of a sugar chain has been rapidly advanced together with advance in molecular biology and cell biology, and a variety of functions of a sugar chain have been revealed. Firstly, a sugar chain play an important role on clearance of glycoprotein in blood. Erythropoietin obtained by transferring a gene in Escherichia coli manifests in vitro its activity, but is known to be rapidly in vivo clearanced [Dordal et al.: Endocrinology, 116 , 2293 (1985) and Browne et al.: Cold Spr. Harb. Symp. Quant. Biol., 51 , 693, 1986]. Human granulocyte-macrophage colony stimulating factor (hGM-CSF) has naturally two N-linked sugar chains, but it is known that, as the number of sugar chains is decreased, the clearance rate in rat plasma is raised proportionally thereto [Donahue et al.: Cold Spr. Harb. Symp. Quant. Biol., 51 , 685 (1986)]. The clearance rate and clearanced places vary depending upon the structure of a sugar chain. It is known that, while hGM-CSF to which sialic acid is added is clearanced in kidney, hGM-CSF from which sialic acid is removed is raised in the clearance rate and is clearanced in liver. Additionally, the clearance rates in rat plasma and rat perfusion liquid were studied with respect to α1-acid glycoproteins having different sugar chain which were biosynthesized by rat liver primary culture in the presence of various N-linked sugar chain biosynthesis inhibitors. In both cases, the clearance rates were slower in descending order of high mannose type, sugar chain deficient type, hybrid type and complex type (natural type). It is also known that the clearance in blood of tissue-type plasminogen activator (t-PA) used as a fibrinolytic agent is significantly influenced by the structure of a sugar chain. It is also known that a sugar chain endows a protein with the protease resistance. For example, when formation of sugar chain of fibronectin is inhibited by zunicamycin, the resulting sugar chain deficient fibronectin is promoted in the intracellular degrading rate. It is also known that addition of a sugar chain increases the heat stability and anti-freezing properties. It is also known that a sugar chain makes a contribution to increase the solubility of protein. A sugar chain also makes a contribution to holding the correct steric structure of proteins. It is known that, although the removal of two N-linked sugar chains naturally present in membrane-bound glycoprotein of vesicular stomatitis virus inhibits the transport of proteins to cell surface, new addition of a sugar chain to said proteins recovers the transport. In this case, it has been revealed that the removal of a sugar chain induces the association between protein molecules via disulfide bond and, as the result, the transport of proteins is inhibited. Since the correct steric structure is retained due to inhibition of this association by new addition of sugar chain, the transport of proteins becomes possible. It is shown that the position to which new sugar chain is added is considerably flexible. To the contrary, it has been revealed that the transport of proteins having natural sugar chains is completely inhibited, in some cases, depending upon the introduction site of the additional sugar chain. There is known the case where a sugar chain masks antigen site on polypeptide. From the experiments using polyclonal antibody or monoclonal antibody reacting with a particular region on polypeptide in hGM-CSF, prolactin, interferon - γ, Rauscher leukemia virus gp70 and influenza hemagglutinin, it is considered that a sugar chain of the above proteins inhibits a reaction with antibody. There is also known the case where a sugar chain itself has direct relationship with manifestation of the activity of glycoprotein. For example, a sugar chain is considered to participate in manifestation of the activity of glycoprotein hormones such as luteinizing hormone, follicule stimulating hormone or chorionic gonadotropin. In addition, EP-A 0370205 discloses that granulocyte colony-stimulating factor (G-CSF) and prourokinase (pro-UK) can be improved in the properties by artificially and intentionally introducing a sugar chain into the proteins using recombinant DNA techniques. Furthermore, as an important function of a sugar chain, there is participation in recognition phenomena between cells, between proteins or between cells and proteins. For example, it is known that the place where a sugar chain is clearanced in the living body is different depending upon the difference in the structure of a sugar chain. In addition, it has been found that a ligand of ELAM-1 which is inflammatory response-specifically expressed on blood vessel endothelial cell and promotes adhesion to neutrophil is a sugar chain called as Sialyl-Le X [NeuAc α2 →3Gal β1 →4 (Fuc α1 →3)GlcNAc:NeuAc, sialic acid; Gal, galactose; Fuc, fucose; GlcNAc, N-acetylglucosamine]. As the result, there has risen the possibility that a sugar chain itself or modification thereof is used in pharmaceuticals and the like [Phillips et al.: Science 250 , 1130 (1990), Goelz et al.: Trends in Glycoscience and Glycotechnology 4 , 14 (1992)]. Further, it is suggested that L-selectin which is expressed in a part of T lymphocytes and neutrophil and GMP-140 (also called as P-selectin) which is expressed in the membrane surface of platelet and blood vessel endothelial cell by inflammatory stimulation participate in inflammatory response as same as ELAM-1, and ligands thereof are also sugar chains similar to the Sialyl-Le X sugar chain which is a ligand of ELAM-1 [Rosen et al.: Trends in Glycoscience and Glycotechnology 4 , 1 (1992), Larsen et al.: Trends in Glycoscience and Glycotechnology) 4 , 25 (1992), Aruffo et al.: Trends in Glycoscience and Glycotechnology 4 , 146 (1992)]. Also in metastasis of cancer as in the inflammatory response, it is suggested that ELAM-1 and GMP-140 promote metastasis of cancer by causing adhesion of cancer cells to inner wall of blood vessel or aggregation between cancer cells and platelets [Goelz et al.: Trends in Glycoscience and Glycotechnology) 4 , 14 (1992), Larsen et al.: Trends in Glycoscience and Glycotechnology) 4 , 25 (1992)]. These suggestions are consistent with the findings that expression of the Sialyl-Le X sugar chain is high in cancer cells having high metastasis ability [Irimura et al.: Experimental Medicine 6 , 33 (1988)]. From these findings, it is expected that the Sialyl-Le X sugar chain or derivatives thereof manifest the excellent anti-inflammatory effects and anti-metastatic effects by binding to ELAM-1, L-selectin or GMP-140. Additionally, in view of the mechanism of the above-described inflammatory response and metastasis of cancer, inflammatory response could be inhibited and metastasis of cancer could be prevented by inhibiting the expression of glycosyltransferase which controls synthesis of ligand sugar chain recognized by ELAM-1, L-selectin or GMP-140. Antisense RNA/antisense DNA techniques [Tokuhisa: Bioscience and Industry 50 , 322 (1992), Murakami: Chemistry 46 , 681 (1991)] or Triple helix techniques [Chubb and Hogan : Trends in Biotechnology 10 , 132 (1992)] are useful for inhibiting the expression of some particular genes. Since information on those gene or nucleotide sequence of those gene is necessary in order to inhibit the expression of desired glycosyltransferase using this antisense RNA/DNA techniques, the cloning of a gene of desired glycosyltransferase and analysis of the information on the nucleotide sequence are important. Further, diagnosis of malignancy in inflammatory diseases or cancer can be also performed by investigating the expression of the particular glycosyltransferase in inflammatory lymphocyte and cancer cells. For investigating the expression of desired glycosyltransferase, Northern hybridization method which uses as a probe the relevant gene labelled with radioactivity and the like [Sambrook, Fritsch, Maniatis, Molecular Cloning, A laboratory manual, 2nd edition, Cold Spring Harbor Laboratory Press, 1989] and Polymerase Chain Reaction method (abbreviated as PCR method hereinafter) [Innis et al.: PCR Protocols, Academic Press, 1990] are useful. For applying these methods, the information on desired glycosyltransferase gene or nucleotide sequence thereof is necessary. Also from this respect, the cloning of a desired glycosyltransferase gene and analysis of information of that nucleotide sequence are important. As described above, alteration in the structure of glycoprotein and mass production of the particular sugar chain or modification thereof are industrially extremely important themes. The means by which the structure of a sugar chain is altered have recently advanced remarkably. In particular, the structure of a sugar chain can be altered by a high specific enzyme (exoglycosidase) which successively dissociates a sugar chain or glycopeptidase and endo-type glycosidase which cleavages a bonding between peptide chain and sugar chain without changing both peptide chain and sugar chain. As the result, the biological role of a sugar chain can be studied in detail. Further, endoglycoceramidase which cleavages between a sugar chain of glycolipid and ceramide has been recently found [Ito and Yamagata: J. Biol. Chem. 261 , 14278 (1986)]. This finding has not only facilitated the preparation of a sugar chain of glycolipid but also advanced the study on the function of cell surface glycolipid. In addition, new addition of sugar chain has been possible by using glycosyltransferase. For example, sialic acid can be newly added to the end of sugar chain by using sialyltransferase [Sabesan and Paulson: J. Am. Chem. Soc. 108 , 2068 (1986)]. A sugar chain to be added can be varied by using other various glycosyltransferases or glycosidase inhibitors [Alan et al.: Annu. Rev. Biochem. 56 , 497 (1097)]. However, mass production of glycosyltransferase used in synthesis of a sugar chain is extremely difficult. For that reason, it is desired that glycosyltransferase is produced in a large amount by cloning glycosyltransferase using recombinant DNA techniques and effectively expressing glycosyltransferase in a host cell. As a method for cloning glycosyltransferase, there is known a method of purifying a protein, producing an antibody reacting with it and performing immunoscreening using the antibody [Weinstein et al.: J. Biol. Chem. 262, 17735 (1987)], and a method of purifying a protein, determining amino acid sequence thereof, producing synthetic DNA which corresponds thereto and performing hybridization using the DNA as a probe [Narimatsu et al.: Proc. Natl. Acad. Sci., USA, 83, 4720 (1986)]. A method is also known where hybridization is performed using cloned glycosyltransferase gene as a probe and thereby glycosyltransferase gene having homology with the glycosyltransferase is cloned [John. B. Lowe et al.: J. Biol. Chem. 266 , 17467 (1991)]. In addition, there is known a cloning method by direct expression cloning using panning method as screening method, in which antibody or lectin reacting with a sugar chain is employed [John. B. Lowe et al.: Proc. Natl. Acad. Sci.,USA, 86 , 8227 (1989), John. B. lowe et al.: Genes Develop., 4 , 1288 (1990)]. WO91/12340 describes the cloning of genes encoding glycosyltransferases; namely fucosyltransferases and a galactosyltransferase, by using a gene transfer approach. There is no case where a glycosyltransferase can be cloned using lectin-resistance as an index. From the studies on various lectin-resistant mutants of CHO cell, it has been revealed that there are cases where a new glycosyltransferase is expressed, where the activity of a certain glycosyltransferase disappears, and where synthesis of sugar nucleotide or its transfer to Golgi body is inhibited [Pamela Stanley et al.: Methods in Enzymology, 96 , 157]. Therefore, it is considered that cloning of a glycosyltransferase can be performed using lectin-resistance as an index by introducing a gene derived from a cell expressing glycosyltransferase to be cloned into CHO cell or lectin-resistant mutants of CHO cell [Ravindra Kumar et al.: Mol. Cell. BioI., 9 , 5713 (1989)]. James Ripka et al. have tried to clone N-acetylglucosaminyltransferase I by introducing human genomic DNA derived from A431 cell into lectin-resistant mutants of CHO cell (Lecl) using resistance to lectin concanavalin A as an index. However, they could not clone glycosyltransferase by the screening method using lectin-resistance as an index [James Ripka et al.: Biochem. Biophys. Res. Commun., 159, 554 (1989)]. Heffernan et al. have cloned mouse sialic acid hydroxylase using resistance to lectin WGA (wheat germ agglutinin) as an index by introducing cDNA library into CHO cell [Michael Heffernan et al.: Nucleic Acids Res., 19, 85 (1991)] which was made to produce large T antigen of polyoma [Michael Heffernan et al.: Glycoconjugate J., 8 , 154 (1991)]. However, there is no report in which a glycosyltransferase could be cloned in a screening system using the lectin-resistance as an index. In addition, with respect to hosts, Stanley, Ripka, Heffernan et al. all used CHO cell or lectin-resistant mutants of CHO cell as a host. With respect to a sialyltransferase, a cDNA encoding an enzyme having β galactoside α2 →6 sialyltransferase activity has been isolated and the nucleotide sequence thereof has been revealed [Weinstein et al.: J. Biol. Chem., 262, 17735 (1987)]. With respect to an enzyme having β galactoside α2 →3 sialyltransferase activity, Gillespie et al. have reported cloning of a gene encoding an enzyme which adds sialic acid to galactose in O-linked sugar chain of glycoprotein (sugar chain which is added to serine or threonine residue), but base sequence thereof has not been revealed [Gillespie et al.: Glycoconjugate J., 7 , 469 (1990)]. Gillespie et al. describe the cloning and sequencing of a cDNA encoding β galactoside α2,3 sialyltransferase with a 62% identity to a portion of an α2,6 sialyltransferase [Gillespie et al., FASEB J. 4(7) abstract No. 2173 (1990)]. In addition, Weinstein et al. have been reported a method for purifying an enzyme having β galactoside α2 →3 sialyltransferase activity from rat liver [Weistein et al.: J. Biol. Chem., 257, 13835 (1982)]. And, Joziasse et al. have reported the method for purification of an enzyme having a β galactoside α2-3 sialyltransferase activity from human placenta which utilizes the oligosaccharides with the terminal Galβ1-3GalNAc structure efficiently [Joziasse, D. H. et al.: J. Biol. Chem. 260, 4941-4951 (1985)]. However, desired enzyme can be obtained in an extremely small amount. Hitherto, there have been no reports in which sialic acid is added in α2 →3 linkage to a desired position on a sugar chain of a glycoprotein, glycolipid or oligosaccharide using recombinant DNA techniques. The main object of the present invention is to provide a novel α2 →3 sialyltransferase utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as a substrate which effectively alters a sugar chain of protein and produces a particular sugar chain, a cDNA encoding said α2 →3 sialyltransferase and a vector containing said cDNA. These objects as well as other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings. The present inventors constructed a cDNA library by incorporating cDNA which was synthesized using mRNA extracted from animal cells as a template into expression cloning vector, introduced said cDNA library into a cell, the resultant cell was cultured in the presence of lectin having the activity which inhibits growth of said cell, and growing cells were isolated to obtain a cloned gene which was introduced in a host cell to express. As the result, we found that a novel α2 →3 sialyltransferase utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as a substrate was expressed, resulting in completion of the present invention. According to the present invention, there is provided a novel α2 →3 sialyltransferase utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as a substrate and having the amino acid sequence shown by SEQUENCE IDENTIFIER NUMBER (Seq.ID NO:2 or 7, cDNA encoding said α2 →3 sialyltransferase, and a The recombinant vector containing said DNA. The α2 →3 Sialyltransferase of the present invention is a glycosyltransferase having the βgalactoside α2 →3 sialyltransferase activity, and has the activity which adds sialic acid to the end of a substrate sugar chain in α2 →3 linkage. Fig. 1 is a flow sheet showing construction of plasmid pAGEL106. Fig. 2 is a flow sheet showing construction of plasmid pASLB3-3-1. Fig. 3 is a flow sheet showing construction of plasmid pASLB3-3. Fig. 4 is a flow sheet showing construction of plasmid pSALBE3-3. Fig. 5 is a flow sheet showing construction of plasmid pASLBC. Fig. 6 is a flow sheet showing construction of plasmid pASLBEC. Fig. 7 is a flow sheet showing construction of plasmid pASLBEC2. Fig. 8 is a flow sheet showing construction of plasmid pAMoEC2. Fig. 9 is a flow sheet showing construction of plasmid pAMoEC3. Fig. 10 is a flow sheet showing construction of plasmid pAMoERC3. Fig. 11 is a flow sheet showing construction of plasmid pAGE207. Fig. 12 is a flow sheet showing construction of plasmid pAGE207ScN. Fig. 13 is a flow sheet showing construction of plasmid pAMoC3Sc. Fig. 14 is a flow sheet showing construction of plasmid pAMoERC3Sc. Fig. 15 is s flow sheet showing construction of cDNA. library. Fig. 16 is a flow sheet showing construction of plasmid pUC119-LEC. Fig. 17 is a view showing the results of the sialyltransferase activity measured by HPLC. Panels a. and b. show HPLC pattern on KJM-1 strain in which pAMoERL is introduced and Panels c. and d. show HPLC pattern on KJM-1 strain in which the vector pAMoERC3Sc is introduced, respectively. Panels a. and c. show HPLC pattern where an assay solution containing CMP-sialic acid as sugar donor is used and Panels b. and d. show HPLC pattern where an assay solution not containing CMP-sialic acid is used, respectively. Fig. 18 is a view showing the results of analysis by HPLC after peak 1 and peak 2 are treated with sialidase. Panels a. and c. show HPLC pattern on peak 1 and peak 2 without sialidase treatment, respectively. Panels b. and d. show HPLC pattern on peak 1 and peak 2 after sialidase treatment, respectively. Peak 3 shows the product obtained by treating peak 1 and peak 2 with sialidase. Fig. 19 is a view showing the results of analysis by a flow cell sorter FCS-1 (manufactured by Nihonbunko) after indirect fluorescent antibody staining. Panel a. shows the results of indirect fluorescent antibody staining using KM93 or serum of normal mouse on KJM-1 in which pAMoERC3Sc is introduced. Dotted line shows the pattern when normal mouse serum is used, and solid line shows the pattern when KM93 is used. Panel b.shows the results of indirect fluorescent antibody staining using KM93 on KJM-1 strain in which pA-MoERL is introduced. Dotted line is the same one shown in Panel a., and solid line shows the pattern when KM93 is used. Fig. 20 is a flow sheet showing construction of plasmid pAGE147. Fig. 21 is a flow sheet showing construction of plasmid pAGE247. Fig. 22 is a flow sheet showing construction of plasmid pAMN6hyg. Fig. 23 is a flow sheet showing construction of plasmid pAMoERSA. Fig. 24 is a flow sheet showing construction of plasmid pAMoPRC3Sc. Fig. 25 is a flow sheet showing construction of plasmid pAMoPRSA. Fig. 26 is a flow sheet showing construction of plasmid pAMoPRSAL-35F. Fig. 27 is a flow sheet showing construction of plasmid pUC119-WM17. Fig. 28 is a view showing the results of analysis by EPICS Elite Flow Cytometer (manufactured by COULTER) after indirect fluorescent antibody staining. As a control, the results of indirect fluorescent antibody staining using normal mouse serum on KJM-1 strain in which pAMoPRC3Sc (control plasmid) is introduced are also shown. In addition, the results of indirect fluorescent antibody staining using KM93 on KJM-1 strain in which pAMoPRC3Sc (control plasmid) or pAMoPRWM17 (α2 →3 sialyltransferase expression plasmid) is introduced are also shown as pAMoPRC3Sc or pAMoPRWM17, rescpectively. Fig. 29 is a flow sheet showing construction of plasmid pAMoPRSAW17-31F. The abbreviations used herein have the following meanings. dhfr: dihydrofolate reductase gene hG-CSF: human granulocyte colony stimulating factor gene bp: base pairs kb: kilobase pairs G418/Km: G418 and kanamycin resistance gene derived from transposon 5 (Tn5) hyg: hygromycin resistance gene Ap: ampicillin resistance gene derived from pBR322 Tc: tetracycline resistance gene derived from pBR322 P1: P1 promoter derived from pBR322 Pkt: Herpes simplex virus (HSV) thymidine kinase (tk) gene promoter Sp. βG: rabbit βglobin gene splicing signal A. βG: rabbit βglobin gene poly A addition signal A. SE: simian virus 40 (SV40) early gene poly A addition signal Atk: Herpes simplex virus (HSV) thymidine kinase (tk) gene poly A addition signal Pse: simian virus 40 (SV40) early gene promoter pmo: Moloney murine leukemia virus long terminal repeat (LTR) promoter HTLV-1: human T-cell leukemia virus type-1 (HTLV-1) gene EBNA-1: Epstein-Barr virus EBNA-1 gene oriP: Epstein-Barr virus replication gene ori: pUC119 replication gene lac'Z: a part of Escherichia coli β-galactosidase gene IG: intergenic region of M13 phage DNA A cDNA encoding the α2 → 3 sialyltransferase of the present invention includes (a) DNA having nucleotide sequence shown in Seq ID NO: 1 or 6 or a part thereof, (b) DNA the complementary strand of which hybridizes to the DNA of (a) or part thereof, (c) DNA which is degenerate with respect to the DNA of (a) or (b) or parts thereof, which cDNA encodes a polypeptide having the biological activity of an α2 → 3 sialyltransferase and utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as substrate. In this context, the term "hybridization" refers to conventional hybridization conditions, preferably to stringent hybridization conditions. The α2 → 3 sialyltransferase of the present invention includes all of the α2 → 3 sialyltransferases encoded by a DNA defined in the above (a), (b) or (c). A process for producing a cDNA which encodes a α2 → 3 sialyltransferase of the present invention is explained below by exemplifying a process for producing cDNA defined in the above (a). A cDNA library is constructed by incorporating cDNA synthesized using mRNA extracted from animal cells as a template into expression cloning vector. This cDNA library is introduced in animal or insect cells, and the cells are cultured in the presence of lectin which inhibits the growth of the cells. A cell transfected with cDNA encoding appropriate glycosyltransferases which change the structure of sugar chain recognized by lectin grows in the presence of lectin. This cell is isolated, and cDNA encoding desired α2 →3 sialyltransferase is obtained from the cell. As an animal cell used in the above method, any animal cell can be used so long as cDNA encoding α2 →3 sialyltransferase of the present invention can be expressed in the animal cell. For example, human histiocytic leukemia cell line TYH [Haranaka et al.: Int. J. Cancer, 36 , 313 (1985)], human melanoma cell line WM266-4 (ATCC CRL1676) and the like are used. As a vector in which cDNA synthesized using mRNA extracted from these cells as a template is incorporated, any vectors can be used in which said cDNA can be incorporated and expressed. For example, pAMoERC3Sc and the like are used. As an animal or insect cell in which cDNA library constructed using said vector is introduced, any cells can be used in which said cDNA library can be introduced and expressed. For example, human Namalwa cell [Hosoi et al.: Cytotechnology, 1 , 151 (1988)] and the like can be used. As lectin to be used in the present invention, any lectins can be used which can inhibit the growth of cell in which cDNA is introduced. For example, Ricinus communis 120 lectin is used. After resistance of the host cell to lectin is determined, the lectin is used in such the concentration as can inhibit the growth of host cell. A plasmid having a cDNA encoding the α2 → 3 sialyltransferase of the present invention or a DNA fragment containing the cDNA part is recovered from cells which grow in the presence of lectin by the known method, for example, Hirt method [Robert F. Margolskee et al.: Mol. Cell. Biol., 8 , 2837 (1988)]. As a plasmid having a cDNA encoding the enzyme of the present invention, there is, for example, pUC119-LEC or pUC119-WM17. Escherichia coli HB101/pUC119-LEC containing pUC119-LEC and Escherichia coli HB101/pUC119-WM17 have been deposited with Fermentation Research Institute, Agency of Industrial Science and Technology, 1-3, Higashi 1 chome Tsukuba-shi Ibaraki-ken 305, Japan on October 29, 1991 and on September 22, 1992 under the Budapest Treaty, and have been assigned the accession number FERM BP-3625 and FERM BP-4013, respectively. DNA defined by the above (b) and (c) can be prepared by well known recombinant DNA techniques such as hybridization method, and a method introducing mutations in DNA based on a DNA encoding an α2 →3 sialyltransferase obtained by the above process. Alternatively, a cDNA encoding the α2 →3 sialyltransferase of the present invention can be prepared using chemosynthetic method. The DNA encoding the α2 →3 sialyltransferase of the present invention obtained in the above process is inserted downstream of appropriate promoter to construct recombinant vector which is introduced in a host cell, and the resulting cell is cultured to obtain α2 →3 sialyltransferase of the present invention. As a host cell, any host cells can be used to which recombinant DNA techniques have been applied such as procaryotic cells, animal cells, yeasts, fungi or insect cells. For example, there are Escherichia coli cell as procaryotic cell, CHO cell which is Chinese hamster ovary cell, COS cell which is a monkey cell, Namalwa cell which is a human cell as animal cell. In particular, direct expression system using Namalwa cell as a host cell is suitably used due to such the advantages that efficiency which introduces a cDNA library in Namalwa cell as a host cell is extremely high, the introduced plasmid (cDNA library) can exist extra-chromosomally, and the plasmid is easily recovered from the resultant lectin resistant strain. As a vector for introducing DNA encoding the present α2 →3 sialyltransferase therein, there can be used any vectors in which DNA encoding the α2 →3 sialyltransferase can be incorporated and which can be expressed in a host cell. For example, there are pAGE107 [JP-A 3-22979, Miyaji et al.: Cytotechnology, 3 , 133 (1990)], pAS3-3 [EP-A 0370205], pAMoERC3Sc, and CDM8 [Brian Seed et al.: Nature, 329, 840 (1987)]. For expressing the cDNA encoding the enzyme of the present invention in Escherichia coli , it is preferred that there is used a plasmid wherein foreign DNA can be inserted downstream of promoter having the strong transcription activity such as trp promoter and the like and the distance between Shine-Dalgarno sequence (abbreviated as SD sequence hereinafter) and initiation codon is appropriately adjusted (for example, said distance is 6 to 18 bases). More particularly, there are pKYP10 [EP-A 0083069], pLSAl [Miyaji et al: Agric. Biol. Chem., 53, 277 (1989)], and pGEL1 [Sekine et al.: Proc. Natl. Acad. Sci., USA, 82 , 4306 (1985)]. As general procedures of recombinant DNA techniques used in the present invention, there can be used those described in EP-A 0370205 or those described by Sambrook, Fritsch, Maniatis et al. [Molecular Cloning, A laboratory manual, 2nd edition, Cold Spring arbor Laboratory Press, 1989]. Isolation of mRNA and synthesis of cDNA library can be effected using the above-described methods and many kits on the market. For introducing DNA in an animal cell, there can be used any currently known methods. For example, the electroporation method [Miyaji et al.: Cytotechnology, 3 , 133 (1990)], calcium phosphate method [EP-A 0370205] or lipofection method [Philip L. Felgner et al.: Proc. Natl. Acad. Sci., USA, 84 , 7413 (1987)] can be used. Obtaining of transfected cells and culturing them can be performed according to the method described in EP-A 0370205 or JP-A 2-257891. As a process for producing cloned the α2 →3 sialyltransferase, there are a method for expressing it in a host cell, a method for secreting it from a host cell, and a method for expressing it on cell surface of a host cell. The place where expression is effected varies depending upon the kind of host cell to be used and upon the form of glycosyltransferase to be produced. When glycosyltransferase is produced as it is, using an animal cell as a host cell, it is generally produced in a host cell or on cell surface of a host cell, and a part of it is extracellularly excreted upon cleavage by protease. To secrete glycosyltransferase intentionally, it is produced in a form where a signal peptide is added to a part containing an active site of glycosyltransferase using recombination DNA techniques according to a method by Paulson et al. [C. Paulson et al.: J. Biol. Chem., 264 , 17619 (1989)] and a method by Lowe et al [John B. Lowe et al.: Proc. Natl. Acad. Sci., USA, 86 , 8227 (1989), John B. Lowe et al.: Genes Develop., 4 , 1288 (1990)]. Alternatively, the production yield can be increased using gene amplification system using for example the dihydrofolate reductase gene according to a method described in EP-A 0370205. The α2 →3 Sialyltransferase of the present invention thus produced can be purified by a conventional purifying method for glycosyltransferase [J. Evan. Sadler et al. : Methods of Enzymology, 83 , 458]. When the enzyme is produced in Escherichia coli , it can be effectively purified in combination of the above method and the method described in EP-A 0272703. Alternatively, purification can be carried out by producing the enzyme of the present invention as a fused protein with other protein and subjecting it to affinity chromatography using substance having the affinity with the fused protein. For example, the enzyme of the present invention can be produced as a fused protein with protein A and purified by affinity chromatography using immunoglobulin G according to the method by Lowe et al. [John. B. Lowe et al.: Proc. Natl. Acad. Sci., USA, 86 , 8227 (1989), John. B. Lowe et al.: Genes Develop., 4 , 1288 (1990)]. Alternatively, the enzyme can be purified by affinity chromatography using an antibody reacting with the enzyme itself. The activity of the sialyltransferase is determined by the known method [J. Evan. Sadler et al.: Methods in Enzymology, 83 , 458, Naoyuki Taniguti et al.: Methods in Enzymology, 179 , 397]. A sugar chain can be synthesized in vitro using the α2 →3 sialyltransferase of the present invention. For example, sialic acid can be added in α2 →3 linkage to non-reducing end-group of lactosamine structure (Gal β1 →4GlcNAc structure) contained in glycoprotein, glycolipid or oligosaccharide. The structure of sugar chain at non-reducing terminus can also be converted to Sialyl-Le X structure by acting the α2 →3 sialyltransferase of the present invention to glycoprotein, glycolipid or oligosaccharide as a substrate. An oligosaccharide having Sialyl-Le X and modification thereof at non-reducing terminus can be synthesized using known α1 →3 fucosyltransferase [Lowe et al.: Genes Develop., 4 , 1288 (1990), Goelz et al.: Cell, 63 , 1349 (1990)] after acting the α2 →3 sialyltransferase of the present invention to an oligosaccharide having the lactosamine structure at non-reducing terminus. DNA encoding the α2 →3 sialyltransferase of the present invention can be used to be expressed together with DNAs encoding glycoprotein, glycolipid or oligosaccharide having the useful physiological activity in an animal cell or insect cell producing a sugar chain substrate to the α2 →3 sialyltransferase, and the produced α2 →3 sialyltransferase can be acted on glycoprotein, glycolipid or oligosaccharide to obtain glycoprotein, glycolipid or oligosaccharide having the altered sugar chain structure. Further, a part of an oligosaccharide can be excised from the resulting glycoprotein, glycolipid or oligosaccharide having the altered sugar chain structure by using the known enzymatic or chemical method. DNA encoding the α2 →3 sialyltransferase of the present invention can be used not only in modification of sugar chain of glycoproteins, glycolipids or oligosaccharides and efficient production of the particular sugar chain but also in therapy for diseases such as inflammation or cancer metastasis using antisense RNA/DNA techniques and diagnosis on those diseases using Northern hybridization method or PCR method. Expression of the activity of the α2 →3 sialyltransferase of the present invention can be inhibited using, for example, DNA encoding the α2 →3 sialyltransferase by antisense RNA/DNA techniques [Tokuhisa: Bioscience and Industry, 50 , 322 (1992), Murakami: Kagaku, 46 , 681 (1991), Miller: Biotechnology, 9 , 358 (1992), Cohen: Trends in Biotechnology, 10 , 87 (1992), Agrawal: Trends in Biotechnology, 10 , 152 (1992)] or triple helix technique [Chubb and Hogan: Trends in Biotechnology, 10 , 132 (1992)]. More particularly, expression of the α2 →3 sialyltransferase can be inhibited by administering in the living body an oligonucleotide which is designed and prepared based on nucleotide sequence of a part of the α2 →3 sialyltransferase gene, preferably, 10-50 bases sequence in the initiation region. As nucleotide sequence of a synthetic oligonucleotide, there can be used the completely same sequence as that of a part of antisense strand disclosed herein or a sequence which is modified so that the α2 →3 sialyltransferase expression inhibiting activiy is not lost. When triple helix technique is used, nucleotide sequence of synthetic oligonucleotide is designed based on the information about nucleotide sequence of both sense strand and antisense strand. In addition, expression of a cDNA encoding the α2 →3 sialyltransferase of the present invention can be detected by hybridizaiton method or PCR as follows. For detecting expression of the cDNA encoding the α2 →3 sialyltransferase of the present invention using the Northern hybridization method or the PCR method, probe DNA or synthetic oligonucleotide is prepared based on a cDNA encoding the α2 →3 sialyltransferase of the present invention or nucleotide sequence thereof. Northern hybridization method and PCR method are performed according to Molecular Cloning, A laboratory manual, 2nd edition [Cold Spring Harbor Laboratory Press, 1989] and PCR Protocols [Academic Press, 1990]. The following Examples further illustrate the present invention in detail but are not be construed to limit the scope thereof. In the following Examples, T4 polynucleotide kinase and T4 DNA ligase used were those manufactured by Takarashuzo K.K. Example 1 1. Construction of the direct expression cloning vector pAMoERC3Sc (1) Construction of pAGEL106 (see Fig 1) According to the method described below, plasmid pAGEL106 having a promoter in which the simian virus 40 (SV40) early gene promoter and a part of the R region and the U5 region of the long terminal repeat of human T-cell leukemia virus type-1 (HTLV-1) are fused was constructed. That is, a DNA fragment containing a part of the R region and the U5 region [ Ban II- Sau 3AI fragment (0.27 kb)] was isolated from pATK03 and inserted between Bgl I site and Bam HI site of pAGE106 via synthetic linker. pAGE106 (1 µg) obtained by the method described in EP-A 0370205 was dissolved in a buffer (abbreviated as Y-100 buffer hereinafter) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 100 mM sodium chloride and 6 mM 2-mercaptoethanol, and digested with 10 units of Bgl I (manufactured by Takarashuzo, restriction enzymes were used those manufactured by Takarashuzo hereinafter unless others indicated) and 10 units of Bam HI at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to obtain an about 4.9 kb DNA fragment. Separately, 1 µg of pATK03 [Shimizu at al.: Proc. Natl. Acad. Sci., USA, 80 , 3618 (1983)] was dissolved in 30 µl of Y-100 buffer, and 10 units of Ban II was added thereto. The mixture was subjected to digestion reaction at 37°C for two hours, and subjected to agarose gel electrophoresis to give an about 0.4 kb DNA fragment. resulting DNA fragment was dissolved in Y-100 buffer (30 µl) and 10 units of Sau 3AI was added thereto. The mixture was subjected to digestion reaction at 37°C for two hours, and subjected to agarose gel elctrophoresis to give an about 0.27 kb DNA fragment. Separately, for linking Bgl I cleavage site and Ban II cleavage site, the following DNA linker was synthesized. The 5 mer and 6 mer single-stranded DNAs of the DNA linker were synthesized using a DNA synthesizer model 380A (Applied Biosystems). Each 0.2 µg of the synthesized DNAs was dissolved in 40 µl of a buffer (abbreviated as T4 kinase buffer hereinafter) containing 50 mM Tris-HCl (pH 7.5), 10 mM magnesium chloride, 5 mM dithiothreitol (abbreviated as DTT hereinafter), 0.1 nM EDTA and 1 mM adenosine triphosphate (abbreviated as ATP hereinafter) and phosphorylated with 30 units of T4 polynucleotide kinase at 37°C for two hours. The DNA fragments thus obtained, i.e., 0.2 µg of the Bgl I- Bam HI fragment (4.9 kb) derived from pAGE106 and 0.01 µg of the Ban II- Sau 3AI fragment (0.27 kb) derived from pATK03 were dissolved in 30 µl of buffer (abbreviated as T4 ligase buffer hereinafter) containing 66 mM Tris-HCl (pH 7.5), 6.6 mM magnesium chloride, 10 mM DTT and 0.1 mM ATP. 0.01 µg of the above DNA linker was added to the solution and both the DNA fragments and DNA linker were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain [Bolivar et al.:Gene, 2 , 75 (1977)] was transformed using the above reaction mixture according to the method by Cohen et al. [S. N. Cohen et al.: Proc. Natl. Acad. Sci., USA, 69 , 2110 (1972)] to obtain kanamycin resistant strains. From these transformants, a plasmid was isolated according to the known method [H.C.Birnboim et al.: Nucleic Acids Res., 7 , 1513 (1979)]. This plasmid was designated as pAGEL106, and its structure was confirmed by restriction enzyme digestion. (2) Construction of pASLB3-3-1 (see Fig. 2) According to the method described below, expression plasmid pASLB3-3-1 of human granulocyte colony stimulating factor (hG-CSF) having a promoter in which the SV40 early gene promoter and a part of the R region and the U5 region of the long terminal repeat (LTR) of HTLV-1 are fused was constructed. 0.5 µg of pAGEL106 obtained in Sec. 1(1) of this Example was dissolved in 30 µl of a buffer (abbreviated as K-20 buffer hereinafter) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 20 mM potassium chloride and 6 mM 2-mercaptoethanol, and digested with 10 units of Sma I at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of T4 ligase buffer, and the digested DNA and 0.01 µl of Sal I linker (5'pGGTCGACC3' : manufactured by Takarashuzo) were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of a buffer (abbreviated as Y-175 buffer hereinafter) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 175 mM sodium chloride and 6 mM 2-mercaptoethanol, and digested with 10 units of Sal I and 10 units of Mlu I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 1.7 kb DNA fragment. On the other hand, 1 µg of pAS3-3 obtained by the method described in EP-A 0370205 was dissolved in 30 µl of Y-175 buffer, and digested with 10 units of Sal I and 10 units of Mlu I at 37°C for 2 hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 6.7 kb DNA fragment. DNA fragments thus obtained, i.e., 0.1 µg of the Mlu I- Sal I fragment (1.7 kb) derived from pAGEL106 and 0.2 µg of the Mlu I- Sal I fragment (6.7 kb) derived from pAS3-3 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12 °C for 16 hours. Escherichia coli HB101 strain was transformed using the above reaction solution according to the method by Cohen et al. to obtain a kanamycin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLB3-3-1, and its structure was confirmed by restriction enzyme digestion. (3) Construction of pASLB3-3 (see Fig. 3) According to the method described below, in order to construct plasmid pASLB3-3 wherein ampicillin resistance gene is introduced in pASLB3-3-1, DNA fragment [ Xho I- Mlu I fragment (2.58 kb)] containing ampicillin resistance gene of pAS3-3 was introduced between Xho I site and Mlu I site of pASLB3-3-1. 1 µg of pASLB3-3-1 obtained in Sec. 1(2) of this Example was dissolved in 30 µl of a buffer (abbreviated as Y-150 buffer) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 150 mM sodium chloride and 6 mM 2-mercaptoethanol and digested with 10 units of Xho I and 10 units of Mlu I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 7.26 kb DNA fragment. Separately, 1 µg of pAS3-3 was dissolved in Y-150 buffer (30 µl) and digested with 10 units of Xho I and 10 units of Mlu I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 2.58 kb DNA fragment. DNA fragments thus obtained, i.e., 0.2 µg of the Xho I- Mlu I fragment (7.26 kb) derived from pASLB3-3-1 and 0.1 µg of the Xho I- Mlu I fragment (2.58 kb) derived from pAS3-3 were dissolved in T4 ligase buffer (30 µl) and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the above reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLB3-3, and its structure was confirmed by restriction enzyme digestion. (4) Construction of pASLBE3-3 (see Fig. 4) According to the method described below, the dihydrofolate reductase (dhfr) expression unit was removed from pASLB3-3 to obtain plasmid pASLBE3-3 wherein the replication origin (oriP) of the Epstein-Barr virus and the EBNA-1 gene are introduced. EBNA-1 gene encodes a factor which causes replication by trans-acting to oriP. For that use, oriP and EBNA-1 gene were isolated from plasmid p220.2 wherein Sma I- Hae III fragment containing multicloning sites derived from pUC12 [Messing et al.: Methods in Enzymology, 101 , 20 (1983)] is incorporated at Nar I site of p201 [Bill Sugden et al., Nature, 313 , 812 (1985)] 1 µg of Plasmid p220.2 was dissolved in Y-100 buffer (30 µl) and digested with 20 units of Eco RI at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of DNA polymerase I buffer containing 50 mM Tris-HCl (pH 7.5), 10 mM magnesium chloride, 0.1 mM dATP (deoxyadenosine triphosphate), 0.1 mM dCTP (deoxycytidine triphosphate), 0.1 mM dGTP (deoxyguanosine triphosphate), and 0.1 mM dTTP (deoxythymidine triphosphate), and 6 units of Escherichia coli DNA polymerase I Klenow fragment was added to react at 37°C for 60 minutes, which resulted in conversion of 5' protruding cohesive end produced by Eco RI digestion into blunt end. The reaction was stopped by extraction with phenol, extracted with chloroform, and precipitated with ethanol. The precipitate was dissolved in 20 µl of T4 ligase buffer, and Xho I linker (5'pCCTCGAGG3', manufactured by Takarashuzo) (0.05 µg) and 175 units of T4 DNA ligase were added to react at 12°C for 16 hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of Y-100 buffer, and digested with 10 units of Bam HI at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of DNA polymerase I buffer, and 6 units of Escherichia coli DNA polymerase I Klenow fragment was added to react at 37°C for 60 minutes, which resulted in conversion of 5' protruding cohensive end into blunt end. The reaction was stopped by extraction with phenol, extracted with chloroform, and precipitated with ethanol. The precipitate was dissolved in 30 µl of Y-100 buffer, and digested with 10 units of Xho I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 4.9 kb DNA fragment. Separately, 1 µg of pASLB3-3 was dissolved in 30 µl of Y-100 buffer, and 20 units of Xho I was added to perform digestion reaction at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of DNA polymerase I buffer, and 6 units of Eschericchia coli DNA polymerase I Klenow fragment was added to react 37°C for 60 minutes, which resulted in conversion of 5' protruding cohesive end produced by XhoI digestion into blunt end. The reaction was stopped by extraction with phenol, extracted with chloroform, precipitated with ethanol, the precipitate was dissolved in a buffer (abbreviated as Y-0 buffer hereinafter) containing 10 mM Tris-ECl (pH 7.5), 6 mM magnesium chloride and 6 mM 2-mercaptoethanol, and 20 units of Kpn I was added to perform digestion reaction at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 1.3 kb DNA fragment. Separately, 1 µl of pAGE107 [JP-A 3-22979, Miyaji et al.: Cytotechnology, 3 , 133 (1990)] was dissolved in 30 µl of Y-0 buffer, and 20 units of Kpn I was added to perform digestion reaction at 37°C for two hours. Thereafter, sodium chloride was added to give an NaCl concentration of 100 mM, and 20 units of Xho I was added to perform digestion reaction at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 6.0 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Xho I- Bam HI (blunt end) fragment (4.9 kb) derived from p220.2, 0.1 µg of the Xho I (blunt end)- Kpn I fragment (1.3 kb) derived from pASLB3-3 and 0.2 µg of the Kpn I- Xho I fragment (6.0 kb) derived from pAGE107 were dissolved in 30 µl of T4 ligase buffer and ligated with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLBE3-3, and its structure was confirmed by restriction enzyme digestion. (5) Construction of pASLBC (see Fig. 5) According to the method described below, plasmid pASLBC was constructed wherein hG-CSF cDNA was removed from pASLB3-3 and multicloning sites were introduced. Multicloning sites were produced using synthetic DNA. 1 µg of pASLB3-3 obtained in Sec. 1(4) of this Example was dissolved in 30 µl of Y-175 buffer and digested with 20 units of Sal I and 20 units of Mlu I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 3.1 kb DNA fragment. Separately, 1 µg of pASLB3-3 was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for two hours. Then, sodium chloride was added to this reaction mixture to give NaCl concentration of 150 mM, and this plasmid was digested with 20 units of MluI at 37°C for another two hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 6.0 kb DNA fragment. Separately, as a linker for linking Sal I cleavage site and Kpn I cleavage site, the following DNA linker was synthesized (Seq. ID NO:3, Seq. ID NO:4). Restriction enzyme cleavage sites Hind III, Eco RV, Sfi I, Stu I and Not I are incorporated in the linker. The 52 mer and 44 mer single-stranded DNAs of the DNA linker were synthesized using a DNA synthesizer model 380A (Applied Biosystems). Each 0.2 µg of the synthesized DNAs was dissolved in 20 µl of T4 kinase buffer and phosphorylated with 30 units of T4 polynucleotide kinase at 37°C for two hours. The DNA fragments thus obtained, i.e., 0.1 µg of the Sal I- Mlu I fragment (3.1 kb) and 0.2 µg of the Kpn I- Mlu I fragment (6.0 kb) derived from pASLB3-3 were dissolved in T4 ligase buffer (30 µl). 0.01 µg of the above DNA linker and both DNA fragments were ligated with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLBC, and its structure was confirmed by restriction enzyme digestion. (6) Construction of pASLBEC (see Fig. 6) According to the method described below, plasmid pASLBEC was constructed by removing the dihydrofolate reductase (dhfr) expression unit from pASLBC and introducing oriP and EBNA-1 genes therein. 1 µg of pASLBE3-3 obtained in Sec. 1(4) of this Example was dissolved in 30 µl of Y-150 buffer and digested with 20 units of Mlu I and 20 units of Xho I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 1.3 kb DNA fragment. Separately, 1 µg of pASLBE3-3 was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give NaCl concentration of 150 mM and this plasmid was partially digested with 5 units of Mlu I at 37°C for 20 minutes. The reaction solution was subjected to agarose gel electrophoresis to give an about 9.6 kb DNA fragment. Then, 1 µg of pASLBC obtained in Sec. 1(5) of this Example was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give a NaCl concentration of 100 mM and digested with 20 units of XhoI at 37°C for another two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 0.6 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of Mlu I- Xho I fragment (1.3 kb) and 0.2 µg of the Kpn I- Mlu I fragment (9.6 kb) derived from pASLBE3-3 and 0.05 µg of the Kpn I- Xho I fragment (0.6 kb) derived from pASLBC were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLBEC, and its structure was confirmed by restriction enzyme digestion. (7) Construction of pASLBEC2 (see Fig. 7) According to the method described below, Bam HI linker was introduced in Stu I site in multicloning site of pASLBEC to obtain plasmid pASLBEC2. In pASLBEC2, Stu I site in multicloning sites is lost. 1 µg of pASLBEC obtained in Sec. 1(6) of this Example was dissolved in 30 µl of Y-100 buffer and partially digested with 5 units of Stu I at 37°C for 20 minutes. The reaction solution was subjected to agarose gel electrophoresis to give an about 11.5 kb DNA fragment. The resulting DNA fragment was dissolved in 30 µl of T4 ligase buffer. The DNA fragment and 0.01 µg of the Bam HI linker (5'pCCGGATCCGG3': manufacture by Takarashuzo) were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Bam HI at 37°C for two ours. The reaction solution was subjected to agarose gel electrophoresis to give an about 11.5 kb DNA fragment. The resulting DNA fragment was dissolved in T4 ligase buffer (20 µl) and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From, this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pASLBEC2, and its structure was confirmed by restriction enzyme digestion. (8) Construction of pAMoEC2 (see Fig. 8) According to the method described below, plasmid pAMoEC2 was constructed wherein the promoter in pASLBEC2 [promoter in which the SV40 early gene promoter and a part of the R region and the U5 region of the long terminal repeat (LTR) of HTLV-1 are fused] was replaced by the promoter of the long terminal repeat (LTR) of Moloney murine leukemia virus. For that use, Moloney murine leukemia virus LTR promoter was isolated from plasmid Molp-1 [Akinori Ishimoto et al.: Virology, 141, 30 (1985)]. 1 µg of pASLBEC2 obtained in Sec. 1(7) of this Example was dissolved in a buffer (abbreviated as K-50 buffer hereinafter) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 50 mM potassium chloride and 6 mM 2-mercaptoethanol and digested with 20 units of Hind III and 20 units of Aat II (manufactured by Toyoboseki) at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 4.8 kb DNA fragment. Separately, 1 µg of pASLBEC2 was dissolved in 30 µl of K-50 buffer and digested with 20 units of Aat II at 37°C for two hours. Thereafter, this plasmid was partially digested with 5 units of XhoI at 37°C for 20 minutes. The reaction solution was subjected to agarose gel electrophoresis to give an about 6.1 kb DNA fragment. Then, as a linker for linking Xho I cleavage site and ClaI cleavage site, the following linker was synthesized. The 9 mer and 7 mer single-stranded DNAs of the above DNA linker were synthesized using a DNA synthesizer model 380A (Applied Biosystems). Each 0.2 µg of the synthesized DNAs was dissolved in 40 µl of T4 kinase buffer and phosphorylated with 30 units of T4 polynucleotide kinase at 37°C for two hours. Separately, 1 µl of Molp-1 [Akinori Ishimoto et al.: Virology, 141 , 30 (1985)] was dissolved in 30 µl of K-20 buffer and digested with 20 units of Sma I at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give an NaCl concentration of 50 mM and digested with 20 units of Cla I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 0.6 kb DNA fragment. The resulting DNA fragment was dissolved in 30 µl of T4 ligase buffer. The DNA fragment, 0.01 µg of the above DNA linker and 0.03 µg of Hind III linker (5'pCAAGCTTG3': manufactured by Takarashuzo) were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of a buffer (abbreviated as Y-50 buffer hereinafter) containing 10 mM Tris-HCl (pH 7.5), 6 mM magnesium chloride, 50 mM sodium chloride and 6 mM 2-mercaptoethanol and digested with 10 units of Hind III at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give NaCl concentration of 100 mM and digested with 10 units of Xho I at 37°C for another two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 0.6 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Hind III- Aat II fragment (4.8 kb) and 0.2 µg of the Aat II- Xho I fragment (6.1 kb) derived from pASLBEC2 and 0.05 µg of the Hind III- Xho I fragment (0.6 kb) derived from Molp-1 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoEC2, and its structure was confirmed by restriction enzyme digestion. (9) Construction of pAMoEC3 (see Fig. 9) According to the method described below, plasmid pAMoEC3 was constructed by inserting, as stuffer DNA, a DNA fragment [ Dra I- Pvu II fragment (2.5 kb)] containing the tetracycline resistant gene of pBR322 into Bam HI site in multicloning sites of pAMoEC2. 1 µg of pAMoEC2 obtained in Sec. 1(8) of this Example was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Bam HI at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of DNA polymerase I buffer, and 6 units of Escherichia coli DNA polymerase I Klenow fragment was added to react at 37°C for 60 minutes, which resulted in conversion of 5' protruding cohensive end produced by Bam HI into blunt end. The reaction solution was subjected to agarose gel electrophoresis to give an about 11.5 kb DNA fragment. Separately, 1 µg of pBR322 [Bolivar et al.: Gene, 2 , 95 (1977)] was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Dra I and 20 units of Pvu II at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 2.5 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.1 µg of the Bam HI (blunt end) fragment (11.5 kb) derived from pAMoEC2 and 0.2 µg of the Dra I- Pvu II fragment (2.5 kb) derived from pBR322 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12 °C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin and tetracycline resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoEC3, and its structure was confirmed by restriction enzyme digestion. (10) Construction of pAMoERC3 (see Fig. 10) According to the method described below, plasmid pAMoERC3 was constructed by inverting the orientation of oriP and EBNA-1 gene in plasmid pAMoEC3. 1 µg of pAMoEC3 obtained in Sec. 1(9) of this Example was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Xho I at 37 °C for two hours. Thereafter, this plasmid was added to 30 µl of 1M Tris-HCl (pH 8.0) and dephosphorylated with one unit of Escherichia coli alkaline phosphatase (manufactured by Takarashuzo) at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 30 µl of a buffer (abbreviated as TE buffer hereinafter) containing 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA, and the mixture was subjected to agarose gel electrophoresis to give an about 9.1 kb DNA fragment. Separately, 1 µg of pAMoEC3 was dissolved in Y-100 buffer (30 µl) and digested with 20 units of Xho I at 37°C for two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 4.9 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.1 µg of the XhoI fragment (9.1 kb) and 0.2 µg of the Xho I fragment (4.9 kb) derived from pAMoEC3 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoERC3, and its structure was confirmed by restriction enzyme digestion. (11) Construction of pAGE207 (see Fig. 11) According to the method described below, plasmid pAGE207 wherein the G418 resistance gene in pAGE107 is replaced by hygromycin (hyg) resistance gene was constructed. For that use, the hyg resistance gene was isolated from p201 [Bill Sugden et al., Nature, 313, 812 (1985)]. 1 µg of pAGE107 obtained by the method described in JP-A 3-22979 was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Cla I at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give NaCl concentration of 150 mM and digested with 20 units of MluI at 37°C for another two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 4.6 kb DNA fragment. Separately, 0.5 µg of p201 [Bill Sudgen et al.: Nature, 313 ,812 (1985)] was dissolved in 30 µl of Y-50 buffer and digested with 20 units of NarI [manufactured by New England Biolab] at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in DNA polymerase I buffer (30 µl), and 6 units of Escherichia coli DNA polymerase I Klenow fragment was added to react at 37°C for 60 minutes, which resulted in conversion of 5' protruding cohensive end produced by NarI digestion into blunt end. The reaction was stopped by extraction with phenol, extracted with chloroform, precipitated with ethanol, the precipitate was dissolved in 20 µl of T4 ligase buffer. The DNA and 0.05 µg of Cla I linker (5'pCATCGATG3': manufactured by Takarashuzo) were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. After precipitaiton with ethanol, the precipitate was dissolved in 30 µl of Y-50 buffer and digested with 10 units of ClaI at 37°C for two hours. Thereafter, sodium chloride was added to this reaction mixture to give a NaCl concentration of 150 mM and digested with 10 units of MluI at 37°C for another two hours. The reaction solution was subjected to agarose gel electrophoresis to give an about 1.6 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Cla I- Mlu I fragment (4.6 kb) derived from pAGE107 and 0.1 µg of the Cla I- Mlu I fragment (1.6 kg) derived from p201 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAGE207, and its structure was confirmed by restriction enzyme digestion. (12) Construction of pAGE207ScN (see Fig. 12) According to the method described below, in order to remove the similar sequence with Sfi I site present in the rabbit βglobin gene, plasmid pAGE207ScN in which Sca I linker is inserted at Bal I site of pAGE207 was constructed. In pAGE207ScN, the number of inserted Sca I linkers is indefinite. 0.5 µg of pAGE207 obtained in Sec. 1(11) of this Example was dissolved in 30 µl of Y-0 buffer and digested with 10 units of Bal I at 37°C for two hours. After precipitation with ethanol, the precipitate was dissolved in 20 µl of T4 ligase buffer. The DNA and 0.01 µg of Sca I linker (5'pAAGTACTT3': manufactured by Takarashuzo) were ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method by Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAGE207ScN, and its structure was confirmed by restriction enzyme digestion. (13) Construction of Plasmid pAMoC3Sc (see Fig. 13) According to the method as described below, plasmid pAMoERC3Sc was constructed, in which case for the purpose of removing a similar sequence with Sfi I site of the rabbit βglobin gene in the plasmid pAMoERC3, the rabbit βglobin gene in the plasmid pAMoERC3 was replaced with the rabbit βglobin gene in the plasmid pAGE207ScN from which that similar sequence had already been removed. For convenience, plasmid pAMoC3Sc was first constructed, and then the plasmid pAMoERC3Sc was constructed. The number of Sca I linkers inserted into the plasmid pAGE207ScN to remove the similar sequence with Sfi I site is unknown. In the case of pAMoERC3Sc, however, from the fact that the pAGE207ScN was once digested with Sca I at the time of its construction, it is deduced that the number of Sca I sites inserted thereinto is only one. First, 1 µg of the pAGE207ScN obtained in Sec. 1(12) of this Example was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for 2 hours. Then, sodium chloride was added to give a NaCl concentration of 100 mM, and this plasmid was further digested with 20 units of Sca I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.7 kb DNA fragment. Also, 1 µg of the pAGE207ScN was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Sca I and 20 units of Cla I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.9 kb DNA fragment. Separately, 1 µg of the pAMoERC3 obtained in Sec. 1(10) of this Example was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for 2 hours. Then, sodium chloride was added to give a NaCl concentration of 100 mM, and this plasmid was further digested with 20 units of Xho I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 3.2 kb DNA fragment. Next, 1 µg of the pAGE107 obtained by the method described in the EP-A 0370205 was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Xho I and 20 units of Cla I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 4.3 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.1 µg of the Kpn I- Sca I fragment (0.7 kb) derived from the pAGE207ScN, 0.1 µg of the Sca I- Cla I fragment (0.9 kb) derived from the same plasmid, 0.3 µg of the Kpn I- Xho I fragment (3.2 kb) derived from the pAMoERC3, and 0.3 µg of the Xho I- Cla I fragment (4.3 kb) derived from the pAGE107 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using the reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoC3Sc, and its structure was confirmed by restriction enzyme digestion. (14) Construction of Plasmid pAMoERC3Sc (see Fig. 14) First, 1 µg of the pAMoERC3 obtained in Sec. 1(10) of this Example was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for 2 hours. Then, sodium chlorite was added to give an Nacl concentration of 150 mM, and this plasmid was further digested with 20 units of Mlu I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 6.8 kb DNA fragment. Also, 1 µg of the pAMoERC3 was dissolved in 30 µl of Y-150 buffer and digested with 20 units of XhoI and 20 units of Mlu I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 1.3 Kb DNA fragment. Separately, 1 µg of the pAMoC3Sc was dissloved in 30 µl of Y-0 buffer and digested with 20 units of Kpn I at 37°C for 2 hours. Then, sodium chloride was added to this reaction mixture to give an NaCl concentration of 100 mM, and this plasmid was digested with 20 units of Xho I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 5.9 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Kpn I- Mlu I fragment (6.8 kb) derived from the pAMoERC3, 0.05 µg of the Xho I- Mlu I fragment (1.3 kb) derived from the same plasmid, and 0.2 µg of the Kpn I- Xho I fragment (5.9 kb) derived from the pAMoC3Sc were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoERC3Sc, and its structure was confirmed by restriction enzyme digestion. The plasmid pAMoERC3Sc has a long terminal repeat of Moloney murine leukemia virus as a promoter for expression of heterogeneous genes. For the purpose of attaining high efficiency in the expression of heterogeneous genes, this plasmid is designed to have a rabbit βglobin gene splicing signal, a rabbit βglobin gene poly A addition signal, and an SV40 early gene poly A addition signal after the position of a heterogeneous genes to be inserted. Moreover, this plasmid has a G418 resistance gene as a drug resistance marker for animal cells and a kanamycin resistance gene (the same as the G418 resistance gene) and an ampicillin resistance gene as drug resistant markers for Escherichia coli cells, respectively. Further, this plasmid has a replication origin (oriP) of Epstein-Barr virus and an EBNA-1 gene which is a trans-acting factor on the oriP to cause replication, so that it can be present in many kinds of cells, including Namalwa cells other than rodent cells, in the form of a plasmid without being incorporated into their chromosomes. 2. Resistance of Namalwa cells to Ricinus communis 120 lectin Namalwa cells conditioned for serum-free media (KJM-1 strain) (Hosoi et al., Cytotechnology, 1 , 151 (1988)) were cultured in the presence of Ricinus communis 120 lectin at various concentrations, and the resistance of the KJM-1 strain to Ricinus communis 120 lectin was examined. The KJM-1 strain was suspended in RPMI1640·ITPSGF medium (RPMI1640 medium (Nissui Seiyaku) containing a 1/40 volume of 7.5% NaHCO 3 , 200 mM L-glutamine solution (GIBCO) of 3% in volume, penicillin-streptomycin solution (GIBCO; 5000 units/ml penicillin and 5000 µg/ml streptomycin) of 0.5% in volume, 10 mM HEPES, 3 µg/ml insulin, 5 µg/ml transferrin, 5 mM sodium pyruvate, 125 nM sodium selenate, 1 mg/ml galactose, and 0.1% (w/v) prulonic F68) to give a concentration of 5 × 10 4 cells/ml, and the suspension was distributed in 200-µl portions into wells of a 96-well microtiter plate. Various concentrations of Ricinus communis 120 lectin (Seikagaku Kogyo) were added thereto in 1/100 volumes, and the plate was incubated in a CO 2 incubator at 37°C for 1 to 2 weeks. As the result, it was found that the minimum concentration of Ricinus communis 120 lectin to cause complete inhibition of the KJM-1 strain growth was 50 ng/ml. Four million cells of the KJM-1 strain were examined, and the natural occurrence of Ricinus communis 120 lectin resistant strain was not observed. 3. Cloning of an α2→3 sialyltransferase cDNA (LEC) from human histiocytic leukemia cell line TYH (1) Isolation of mRNA from TYH cells From 1 × 10 8 TYH cells (Haranaka et al., Int. J. Cancer, 36 , 313 (1985)), about 40 µg of mRNA was isolated using the mRNA extraction kit "Fast Track" (Invitrogen; trade No. K1593-02). (2) Preparation of a cDNA library (see Fig. 15) From 8 µg of mRNA obtained above, double-stranded cDNA was prepared using the cDNA synthesis kit "The Librarian I" (Invitrogen) with a random primer as a primer. Then, each of the SfiI linkers (Seq. ID NO:5) as shown below was provided, instead of Bst XI linkers, at either terminus of the cDNA. The cDNA was fractioned in size by agarose gel electrophoresis and cDNA fragments larger than about 1.2 kb were isolated. Sfi I Linkers: The 11 mer and 8 mer single-stranded DNAs of Sfi I linkers were synthesized using a DNA synthesizer model 380A (Applied Biosystems). Then, 50 µg of each of the synthesized DNAs was dissolved in 50 µl of T4 kinase buffer and phosphorylated with 30 units of T4 polynucleotide kinase (Takara Shuzo) at 37°C for 16 hours. The specific reagents and procedures were as described in the manufacturer's instructions accompanying the kit used, except that the above Sfi I linkers were used in place of Bst XI linkers. Also, 24 µg of pAMoERC3Sc, which is a direct expression cloning vector, was dissolved in 590 µl of Y-50 buffer and digested with 80 units of Sfi I at 37°C for 16 hours. Then, 5 µl of this solution was subjected to agarose gel electrophoresis, and after the completion of digestion was confirmed, for the purpose of decreasing the amount of clones having no cDNA insert incorporated at the time of cDNA library construction, this plasmid was further digested with 40 units of Bam HI at 37°C for 2 hours. The reaction mixture was subjected to electrophoresis to give an about 11.5 kb DNA fragment. Next, 2 µg of the Sfi I fragment (11.5 kb) derived from the pAMoERC3Sc, together with the cDNA obtained above, was dissolved in 250 µl of T4 ligase buffer and ligated together with 2000 units of T4 DNA ligase at 12°C for 16 hours. Then, 5 µg of transfer RNA (tRNA) was added thereto, followed by ethanol precipitation, and the resulting precipitate was dissolved in 20 µl of TE buffer. With this reaction mixture, Escherichia coli strain LE392 (Maniatis et al., ed., Molecular Cloning, 2.58, Cold Spring Harbor, 1989) was tranformed by electroporation (William J. Dower et al., Nucleic Acids Res., 16, 6127 (1988)) to give about 200,000 ampicillin resistant transformants. (3) Cloning of the α2→3 sialyltransferase cDNA (LEC) About 200,000 ampicillin-resistant transformants (cDNA library) obtained in Sec. 3(2) of this Example were mixed, after which plasmids were prepared using >plasmid<maxi kit (trade No. 41031; Qiagen) which is a plasmid preparation kit. The obtained plasmids were ethanol precipitated, and the resulting precipitate was dissolved in TE buffer to give a concentration of 1 µg/µl. The above plasmids were introduced into the KJM-l strain by electroporation (Miyaji et al., Cytotechnology, 3 , 133 (1990)) at a proportion of 4 µg per 1.6 × 10 6 cells. After the introduction of the plasmids, these cells were suspended in 8 ml of RPMI1640·ITPSGF medium, and the cells were incubated in a CO 2 incubator at 37°C for 24 hours. Then, the cells were supplemented with G418 (GIBCO) to give a concentration of 0.5 mg/ml and further cultured for 5 to 7 days to give transformants. The obtained transformants were suspended in RPMI1640·ITPSGF medium containing Ricinus communis 120 lectin (50 ng/ml) to give a concentration of 5 × 10 4 cells/ml, and the cells were distributed in 200-µl portions into wells of a 96-well microtiter plate. The cells were cultured in a CO 2 incubator at 37°C for 2 to 3 weeks, and 7 strains were obtained which had become resistant to a Ricinus communis 120 lectin. From this resistant strain, a plasmid was isolated according to the Hirt method (Robert F. Margolskee et al., Mol. Cell. Biol., 8 , 2837 (1988)), and Escherichia coli strain LE392 was transformed with this plasmid by electroporation (Willium J. Dower et al., Nucleic Acids Res., 16 , 6127 (1988)). From this transformant, a plasmid was prepared using >plasmid< maxi kit (trade No. 41031; Qiagen), and its structure was examined by restriction enzyme digestion to find that it contained about 1.9 kb cDNA. The cDNA containing plasmid was designated as pAMoERL. When this plamid was also introduced into the KJM-1 strain by the above method, this strain became resistant to Ricinus communis 120 lectin; it was therefore found that this cDNA is a gene responsible for lectin resistance. The KJM-1 strain containing the plasmid pAMoERL was able to grow even in the presence of 200 ng/ml of Ricinus communis 120 lectin. 4. Sequencing of an α2→3 sialyltransferase cDNA (LEC) (1) Introduction of the α2→3 sialyltransferase cDNA (LEC) into pUC119 (see Fig. 16) First, 1 µg of pAMoERL obtained in Sec. 3(3) of this Example was dissloved in 30 µl of Y-100 buffer and digested with 20 units of Eco RV and 20 units of Asp 718 (Boehringer Mannheim) at 37°C for 2 hours. The reaction mixture was subjected to electrophoresis to give an about 1.97 kb DNA fragment. Separately, 1 µg of pUC119 (Messing et al., Methods in Enzymology, 153 , 3 (1987)) was dissolved in 30 µl of K-20 buffer and digested with 20 units of Sma I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 100 mM, and this plasmid was further digested with 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to electrophoresis to give an about 3.16 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Eco RV- Asp 718 fragment (1.97 kb) derived from the pAMoERL and 0.1 µg of the Sma I- Asp 718 fragment (3.16 kb) derived from the pUC119 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pUC119-LEC, and its structure was confirmed by restriction enzyme digestion. (2) Construction of deletion plasmids for sequencing First, 2 µg of pUC119-LEC obtained in Sec. 4(1) of this Example was dissloved in 30 µl of Y-150 buffer and digested with 20 units of Bam HI and 20 units of Sph I at 37°C for 2 hours. After ethanol precipitation, the resulting precipitate was dissolved in 100 µl of ExoIII buffer (deletion kit for kilo-sequence; Takara Shuzo). Also, 2 µg of pUC119-LEC was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Sac I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 150 mM, and this plasmid was further digested with 20 units of NotI at 37°C for 2 hours. After ethanol precipitation, the resulting precipitate was dissolved in 100 µl of Exo III buffer. From the Bam HI- Sph I fragment and the Sac I- Not I fragment thus obtained from the pUC119-LEC, several tens of deletion plasmids were prepared, respectively, using the deletion kit for kilo-sequence (Takara Shuzo). The nucleotide sequence of the deletion plasmid obtained above was determined using the Taq DyeDeoxy terminator cycle sequencing kit (trade No. 401113; Applied Biosystems). The determined nucleotide sequence is shown in the Sequence Listing (Seq. ID NO:1). Also, it was found from the corresponding amino acid sequence (Seq. ID NO:2) that this protein has a common structure to glycosyltransferase (hereinafter abbreviated to GT). That is, this protein seems to have a structure where 8 amino acids in the N-terminal portion are put out at the cytoplasm side, a highly hydrophobic region consisting of the subsequent 18 amino acids is used for binding to the membrane, and most of the remaining C-terminal portion including the catalytic site is exposed to the internal cavity of the Golgi's apparatus. The comparison of amino acid sequence between this protein and other GTs made clear that this protein has a certain homology with the rat α2→6 sialyltransferase. For these reasons, it is considered that Ricinus communis 120 lectin resistance gene encodes GT. 5. Measurement of α2→3 sialyltrasferase activity of the KJM-1 strain having expression plasmid for Ricinus communis 120 lectin resistance gene The KJM-1 strain having the plasmid pAMoERL obtained in Sec. 3 of this Example was suspended in 30 ml of RPMI1640·ITPSGF medium containing 0.5 mg/ml of G418 to give a concentration of 5 × 10 5 cells/ml, and the cells were cultured in a CO 2 incubator at 37°C for 3 days. After the culturing, the cells were collected by centrifugation at 160 × g for 10 minutes and washed with 10 ml of PBS (3 g/l NaCl, 0.2 g/l KCl, 1.15 g/l anhydrous sodium monohydrogenphosphate, 0.2 g/l potassium dihydrogenphosphate), followed by further centrifugation to collect the cells. About 1.4 × 10 7 cells obtained above were suspended in 100 µl of homogenization buffer (250 mM saccharose, 10 mM Tris-HCl (pH 7.4)), and lysed by sonication. The lysate was centrifuged at 550 × g for 10 minutes to obtain a supernatant. Also, as a control, the KJM-1 strain having the vector plasmid pAMoERC3Sc was prepared, and the above procedures were followed to obtain a supernatant. Then, 20 µl of each of the two supernatants obtained above was allowed to react in an assay solution (0.1 M cacodylic acid-HCl (pH 6.5), 0.01 M MnCl 2 , 0.45% Triton X-100, 0.08 mM substrate, 5 mM CMP-sialic acid (added or not added)) having the final volume of 50 µl at 37°C for 2 hours, and the products were identifed by high performance liquid chromatography (HPLC) to determine the α2→3 sialyltransferase activity in the respective supernatants. The activity determinations were carried out using 200 µg of proteins in the supernatant, and protein quantitation was achieved using BCA protein assay reagent (PIERCE). As the substrate, a sugar chain fluorescence-labeled with aminopyridine (Galβ1→4GlcNAcβ1→3Galβ1→4Glc-aminopyridine) was used. The fluorescence labeling of the substrate was carried out using lacto-N-neotetraose (BioCarb Chemicals) by the conventional method (Akimoto Kondo, et al., Agric. Biol. Chem., 54 , 2169 (1990)). Each of the supernatants was allowed to react with an assay solution containing or not containing CMP-sialic acid as a sugar donor. The reaction mixture was separated by HPLC, and the peaks appearing only with the assay solution containing CMP-sialic acid were considered as the products. After completion of the reaction, the assay solution was treated at 100°C for 5 minutes, and centrifuged at 10,000 × g for 10 minutes. The resulting supernatant was subjected to HPLC which was carried out on the TSKgel ODS-80T M column (4.6 mm × 30 cm; Tosoh) eluting with 0.02 M ammonium acetate buffer (pH 4.0) at a temperature of 50°C at a rate of 1 ml/min. The products were detected using the fluorescence HPLC monitor model RF-535T (Shimazu Seisakusho) with an excitation wavelength of 320 nm and an emission wavelength of 400 nm. As the result, peaks 1 and 2 were detected as the products, as shown in Fig. 17. From the facts that the elution time was identical with that of the standard and that the substrate was regenerated by sialidase treatment of the products, it was found that the peak 1 corresponds to NeuAcα2→6Galβ1→ 4GlcNAcβ1→3Galβ1→4Glc-aminopyridine and the peak 2 corresponds to NeuAcα2→3Galβ1→4GlcNAcβ1→3Galβ1→4Glc-aminopyridine. The sialidase treatment of the products was carried out as follows. First, 10 µl of the supernatant was subjected to HPLC, and the peaks 1 and 2 were fractioned and freeze-dried, independently, followed by dissolution in 50 µl of buffer containing 20 mM Tris-maleic acid (pH 6.0) and 1 mM calcium citrate. Then, 20 µl of the solution was treated with 2 µl of 400 mU/ml sialidase (neuraminidase; Sigma, N-2133) at 37°C for 16 hours. Also, as a control, the same reaction was conducted using 2 µl of water in place of sialidase. After completion of the reaction, the solution was treated at 100°C for 5 minutes, and centrifuged at 10,000 × g for 10 minutes. Then, 10 µl of the supernatant was subjected to the above HPLC. The results are shown in Fig. 18. Both from the peak 1 and from the peak 2, peak 3 was detected as the product by sialidase treatment. In view of its elution time, the peak 3 is considered to be Galβ1→4GlcNAcβ1→3Galβ1→4Glc-aminopyridine as the substrate. The comparison of HPLC pattern between the KJM-1 strain having the plasmid pAMoERL and the KJM-1 strain having the plasmid pAMoERC3Sc made clear that both the strains gave approximately the same peak 1 but the strain having the pAMoERL strain gave a significantly higher peak 2 than that of the strain having the pMoERC3Sc. The ratio of peak 2 to peak 1 for the KJM-1 strain having the pAMoERL was 6 to 7 times greater than that for the KJM-1 strain having the pAMoERC3sc as the vector (see Fig. 17). From these results, it was shown that this Ricinus communis 120 lectin resistance gene is an α2→3 sialyltransferase gene and that oligosaccharides with sialic acid added can be produced using α2→3 sialyltransferase encoded in the said gene. Example 2 Synthesis of sialyl-Le x in the strain KJM-1 having the expression plasmid for α2→3 sialyltransferase: The KJM-1 strain having the plasmid pAMoERL obtained in Sec. 3(3) of Example 1 and the strain KJM-1 having the direct expression cloning vector pAMoERC3Sc obtained in Sec. 1(14) of Example 1 were independently cultured in the RPMI1640·ITPSGF medium containing 0.5 mg/ml of G418. Then, about 1 × 10 6 cells of each strain were taken in a microtube (1.5 ml; Eppendorf) and centrifugated at 550 × g for 7 minutes. The collected cells were washed with 1 ml of PBS containing 0.1% sodium azide (hereinafter abbreviated as A-PBS), and the expression of sialyl-Le x in the cells of each strain was examined by indirect fluorescent antibody staining with KM93 (Shitara et al., Anticancer Res., 9 , 999 (1989)) which is a monoclonal antibody reacting with sialyl-Le x , as described below. The cells of each strain were suspended in 50 µl of A-PBS solution containing 10 µg/ml KM93 and allowed to react at 4°C for 1 hour. After washing three times with A-PBS, these cells were suspended in 20 µl of A-PBS containing the anti-mouse antibodies IgG and IgM (Cappel) diluted 20-fold with A-PBS which had been fluorescence-labeled with fluorescein isothiocyanate (FITC), and allowed to react at 4°C for 30 minutes. After washing three times with A-PBS, these cells were again suspended in A-PBS, and analysis was carried out with the flow cell sorter FCS-1 (Nihon Bunko). As a control, the same analysis was carried out using the normal mouse serum diluted 500-fold with A-PBS in place of KM93. The results are shown in Fig. 19. The fluorescence intensity of the cells stained with KM93 for the KJM-1 strain having the direct expression cloning vector pAMoERC3Sc was stronger than that of the control (see Fig. 19(a)). This indicates that sialyl-Le x is also expressed in the original KJM-1 strain. The fluorescence intensity of the cells stained with KM93 for the KJM-1 strain having the plasmid pAMoERL capable of expressing cDNA encoding α2→3 sialyltrasferase of the present invention was further stronger than that of the strain KJM-1 containing the pAMoERC3Sc (see Fig. 19(b)). This indicates that sialyl-Le x is synthesized in cells by the α2→3 sialyltransferase of the present invention. Example 3 Production by animal cells of α2→3 sialyltransferase derived from TYH cells: 1. Construction of plasmid pAMoPRSAL-35F for expression of a cDNA encoding α2→3 sialyltransferase (1) Construction of pAGE147 (see Fig. 20) According to the method as described below, the plasmid pAGE147 was constructed by replacing the SV40 early gene promoter of the plasmid pAGE107 with a long terminal repeat (LTR) of Moloney murine leukemia virus as a promoter. First, 2 µg of the plasmid pPMOL1 obtained by the method as described in JP-A 1-63394 was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Sma I at 30°C for 3 hours. Then, sodium chloride was added to give an NaCl concentration of 50 mM, and this plasmid was further digested with 20 units of Cla I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.6 kb DNA fragment containing the LTR promoter of Moloney murine leukemia virus. Next, 25 pmoles of each of the two synthetic DNA fragments as shown below, which had been synthesized in Sec. 1(8) of Example 1, were dissolved in 10 µl of T4 kinase buffer and phosphorlylated at their 5'-termini with 5 units T4 DNA kinase at 37°C for 30 minutes. The DNA fragments thus obtained, i.e., 0.05 µg of the Cla I- Sma I fragment (0.6 kb) derived from the pPMOL1, two 5'-phosphorlylated synthetic DNA fragments (each 1 pmol), and Hind III linker (5'-pCAAGCTTG-3'; Takara Shuzo) (1 pmol) were dissolved in 30 µl of T4 ligase buffer and ligated together with 200 units of T4 DNA ligase at 12°C for 16 hours. After recovery by ethanol precipitation, the DNA fragments were dissolved in Y-100 buffer and digested with 10 units of Hind III and 10 units of Xho I at 37°C for 2 hours. The reaction was stopped by phenol/chloroform extraction and the DNA fragments were recovered by ethanol precipitation. Independently, 1 µg of pAGE107 (JP-A 3-22979; Miyaji et al., Cytotechnology, 3 , 133 (1990)) was dissloved in 30 µl of Y-100 buffer and digested with 10 units of Hind III and 10 units of Xho I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 6.0 kb DNA fragment containing the G418 resistance gene and ampicillin resistance gene. The DNA fragments thus obtained, i.e., 0.3 µg of the Hind III- Xho I fragment (6.0 kb) derived from the pAGE107 and 0.01 µg of the Hind III- Xho I fragment (0.6 kb) derived from the pPMOL1 were dissolved in 20 µl of T4 ligase buffer and ligated together with 200 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAGE147, and its structure was confirmed by restriction enzyme digestion. (2) Construction of pAGE247 (see Fig. 21) According to the method as described below, the plasmid pAGE247 was constructed by replacing the SV40 early gene promoter of the plasmid pAGE207 with a long terminal repeat (LTR) of Moloney murine leukemia virus as a promoter. First, 2 µg of the plasmid pAGE147 obtained in Sec. 1(1) of this Example was dissolved in 30 µl of Y-100 buffer and digested with 10 units of Hind III and 10 units of Xho I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.63 kb DNA fragment containing the LTR promoter of Moloney murine leukemia virus. Independently, 2 µg of the plasmid pAGE207 obtained in Sec. 1(11) of Example 1 was dissloved in 30 µl of Y-100 buffer and digested with 10 units of Hind III and 10 units of Xho I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 5.84 kb DNA fragment containing the hyg resistance gene and the ampicillin resistance gene. The DNA fragments thus obtained, i.e., 0.05 µg of the Hind III- Xho I fragment (0.63 kb) derived from the pAGE147 and 0.1 µg of the Hind III- Xho I fragment (5.84 kb) derived from the pAGE207 were dissolved in 30 µl of T4 ligase buffer and ligated together with 100 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAGE247, and its structure was confirmed by restriction enzyme digestion. (3) Construction of pAMN6hyg (see Fig. 22) According to the method as described below, the plasmid pAMN6hyg for expression of human granulocyte colony stimulating factor derivative was constructed which contains the LTR of Moloney murine leukemia virus as a promoter and the hyg resistance gene as a marker. First, 2 µg of the plasmid pAGE247 obtained in Sec. 1(2) of this Example was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Cla I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 175 mM, and this plasmid was further digested with 20 units of Sal I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 4.8 kb DNA fragment containing the LTR promotor of Moloney murine leukemia virus, the ampicillin resistance gene and the hyg resistance gene. Separately, 2 µg of the plasmid pASN6 obtained by the method as described in EP-A 0370205 was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Cla I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 175 mM, and this plasmid was further digested with 20 units of Sal I and 20 units of Mlu I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 5.0 kb DNA fragment containing the human granulocyte colony stimulating factor derivative gene. The DNA fragments thus obtained, i.e., 0.1 µg of the Cla I- Sal I fragment (4.8 kb) derived from the pAGE247 and 0.1 µg of the Cal I- Sal I fragment (5.0 kb) derived from the pASN6 were dissolved in 20 µl of T4 ligase buffer and ligated together with 200 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMN6hyg and its structure was confirmed by restriction enzyme digestion. (4) Construction of secretory expression vector pAMoERSA (see Fig. 23) According to the methods as described below, the secretory expression vector pAMoERSA for secretory expression of an arbitrary protein as a fused protein with the binding region of Staphylococcus aureus protein A to immunoglobulin G (IgG) was constructed. First, 2 µg of the plasmid pAMN6hyg obtained in Sec. 1(3) of this Example was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Sna BI at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 100 mM, and this plasmid was further digested with 20 units of Xba I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.33 kb DNA fragment containing the signal sequence of human granulocyte colony stimulating factor. Also, 2 µg of the pPrAS1 (Saito et al., Protein Engineering, 2 , 481 (1989)) was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Cla I at 37°C for 2 hours. After ethanol precipitation, the precipitate was dissolved in 30 µl of DNA polymerase I buffer, and the 5'-cohesive end produced by the Cla I digestion was converted into a blunt end with 6 units of the Klenow fragment of Escherichia coli DNA polymerase I at 37°C for 60 minutes. The reaction was stopped by phenol extraction, and after chloroform extraction and ethanol precipitation, the resulting precipitate was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Bam HI at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.21 kb DNA fragment containing the IgG-binding region of Staphylococcus aureus protein A. Separately, 2 µg of the plasmid pAMoERC3Sc obtained in 1(14) of Example 1 was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Xba I and 20 units of Bam HI at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 12.1 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.05 µg of the Sna BI- Xba I fragment (0.33 kb) derived from the pAMN6hyg, 0.05 µg of the Cla I(blunt)- Bam HI fragment (0.21 kb) derived from the pPrAS1, and 0.1 µg of the Xba I- Bam HI fragment (12.1 kb) derived from the pAMoERC3Sc were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known methods. This plasmid was designated as pAMoERSA, and its structure was confirmed by restriction enzyme digestion. (5) Construction of pAMoPRC3Sc (see Fig. 24) In cases where certain cells capable of expressing EBNA-1, such as Namalwa cells, are used as a host, the plasmid introduced into the host can be present as it is, without being integrated into the chromosome, even if it has no EBNA-1 gene in the plasmid pAMoERC3Sc. According to the methods as described below, therefore, the plasmid pAMoPRC3Sc was constructed by removing the EBNA-1 gene in the plasmid pAMoERC3Sc. The plasmid pAMoPRC3Sc can be used as a direct expression cloning vector similarily to the case of the plasmid pAMoERC3Sc. First, 2 µg of the plasmid pAMoERC3Sc obtained in Sec. 1(14) of Example 1 was dissolved in 30 µl of Y-50 buffer and digested with 20 units of Nsi I (New England Biolabs) at 37°C for 2 hours. After ethanol precipitation, the resulting precipitate was dissolved in 30 µl of DNA polymerase I buffer, and the 3'-cohesive end produced by the Nsi I digestion was converted into a blunt end with 6 units of the Klenow fragment of Escherichia coli DNA polymerase I at 37°C for 60 minutes. The reaction was stopped by phenol extraction, and after chloroform extraction and ethanol precipitation, the resulting precipitate was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Not I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 8.1 kb DNA fragment. Separately, 2 µg of the same plasmid as above was dissloved in 30 µl of Y-100 buffer and digested with 20 units of Xho I at 37°C for 2 hours. After ethanol precipitation, the resulting precipitate was dissolved in 30 µl of DNA polymerase I buffer, and the 5'-cohesive end produced by the Xho I digestion was converted into a blunt end with 6 units of the Klenow fragment of Escherichia coli DNA polymerase I at 37°C for 60 minutes. The reaction was stopped by phenol extraction, and after chloroform extraction and ethanol precipitation, the resulting precipitate was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Not I at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 3.2 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.1 µg of the Nsi I(blunt)- Not I fragment (8.1 kb) derived from the plasmid pAMoERC3Sc and 0.1 µg of the Xho I(blunt)- Not I fragment (3.2 kb) derived from the same plasmid were dissloved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoPRC3Sc and its structure was confirmed by restriction enzyme digestion. (6) Construction of pAMoPRSA (see Fig. 25) According to the methods as described below, the plsamid pAMoPRSA was constructed by removing the EBNA-1 gene in the pAMoERSA. The plasmid pAMoPRSA can be used as a secretory expression vector similarily to the case of the plasmid pAMoERSA. First, 2 µg of the plasmid pAMoERSA obtained in Sec. 1(4) of this Example was dissolved in 30 µl of a buffer containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl 2 , 80 mM NaCl, and 6 mM 2-mercaptoethanol (hereinafter abbreviated as Y-80 buffer) and digested with 20 units of Xba I and 20 units of Asp 718 (Boehringer Mannheim) at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 1.3 kb DNA fragment. Separately, 2 µg of the plasmid pAMoPRC3Sc was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Xba I and 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 8.5 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.05 µg of the Xba I- Asp 718 fragment (1.3 kb) derived from the pAMoERSA and 0.1 µg of the Xba I- Asp 718 fragment (8.5 kb) derived from the pAMoPRC3Sc were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoPRSA, and its structure was confirmed by restriction enzyme digestion. (7) Construction of pAMoPRSAL-35F (see Fig. 26) The cloned α2→3 sialyltransferase seems to have a structure where 8 amino acids at the N-terminal portion are put out at the cytoplasm side, the highly hydrophobic region consisting of the subsequent 18 amino acids is used for binding to the membrane, and most of the remaining C-terminal portion including the catalytic site is exposed to the internal cavity of Golgi's apparatus. According to the methods as described below, therefore, the secretory expression of α2→3 sialyltransferase was achieved by removing the membrane-binding region from α2→3 sialyltransferase and adding instead the signal sequence of granulocyte colony stimulating factor and the IgG-binding region of Staphylococcus aureus protein A. The cDNA portion encoding a certain region on and after the membrane-binding region of α2→3 sialyltransferase (from 35th Phe to 333rd Phe) was prepared by the polymerase chain reaction (PCR) method and inserted into the secretory expression vector pAMoPRSA obtained in Sec. 1(6) of this Example. As a set of primers using PCR, the following two synthetic DNA fragments, i.e., L-A(35F) (44 mer) and L-3NN (36 mer), were synthesized by DNA synthesizer model 380A (Applied Biosystems). The primers using PCR L-A(35F) and L-3NN are designed to have an Eco RV site and an Asp 718 site, respectively, so that DNA fragments amplified by PCR can be incorporated between the Stu I site and the Asp 718 site of the plasmid pAMoPRSA after digestion with Eco RV and Asp 718. The PCR was carried out using a GeneAmp™ DNA amplification reagent kit with AmpliTaq™ recombinant Taq DNA polymerase (Takara Shuzo). The reaction mixture was prepared accodring to the methods as described in the kit, and Perkin Elmer Cetus DNA thermal cycler (Takara Shuzo) was used for incubation. Thirty cycles of amplification were carried out according to the following scheme: 94°C for 1 minute, 55°C for 1 minute and 72°C for 3 minutes. Then, further incubation was carried out at 72°C for 7 minutes. As a template, 1 ng of the plasmid pUC119-LEC obtained in Example 1 was used. After completion of the reaction, chloroform extraction and ethanol precipitation were successively carried out. The resulting precipitate was then dissolved in 30 µl of Y-100 buffer and digested with 20 units of Eco RV and 20 units of Asp 718 at 37°C for 2 hours. The reaction micture was subjected to agarose gel electrophoresis to give an about 0.9 kb DNA fragment. Separately, 2 µg of the plasmid pAMoPRSA was dissloved in 30 µl of Y-100 buffer and digested with 20 units of Stu I and 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 9.06 kb DNA fragment. The DNA fragments thus obtained. i.e., 0.1 µg of the Eco RV- Asp 718 fragment (0.9 kb) derived from the DNA fragments amplified by PCR and 0.1 µg of the Stu I- Asp 718 fragment (9.06 kb) derived from the pAMoPRSA were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoPRSAL-35F, and its structure was confirmed by restriction enzyme digestion. 2. Secretory expression of the cDNA encoding the α2→3 sialyltransferase using Namalwa KJM-1 cell as host The plasmid pAMoPRSA (secretory expression vector; control) obtained in Sec. 1(6) of this Example and the plasmid pAMoPRSAL-35F (plasmid for secretory expression of α2→3 sialyltransferase) obtained in Sec. 1(7) of this Example were prepared using the plasmid preparation kit >plasmid<maxi kit (trade No. 41031; Qiagen). Each of the plasmids thus obtained was ethanol precipitated and dissolved in TE buffer to give a concentration of 1 µg/µl. Then, both plasmids were independently introduced into the Namalwa KJM-1 cells at a proportion of 4 µg per 1 ×10 6 cells by electroporation (Miyaji et al., Cytotechnology, 3 , 133 (1990)). The cells were suspended in 8 ml of RPMI1640·ITPSGF medium, and the cells were cultured in a CO 2 incubator at 37°C for 24 hours. Then, the cells were supplemented with G418 (GIBCO) to give a concentration of 0.5 mg/ml and further cultured for 7 to 14 days to obtain transformants. Each of the transformants was suspended in 30 ml of RPMI1640·ITPSGF medium containing 0.5 mg/ml of G418 at 1 × 10 5 cells/ml, and the cells were cultured in a CO 2 incubator at 37°C for 8 days. Then, centrifugation at 160 × g for 10 minutes gave the separation of a supernatant from the cells, and the supernatant was further centrifuged at 1500 × g for 10 minutes. The culture supernatant thus obtained was stored at -80°C until it was used. The α2→3 Sialyltransferase encoded in the plasmid pAMoPRSAL-35F can readily be purified using IgG Sepharose because of its secretory expression as a fused protein with the IgG-binding region of Staphylococcus aureus protein A. To the culture supernatant obtained above, sodium azide was added to give a final concentraion of 0.1%. Then, 100 µl of IgG Sepharose (Pharmacia) which had been pre-treated according to the accompanying instructions was added, and the mixture was gently stirred at 4°C overnight. The IgG Sepharose was recovered by centrifugation at 160 × g for 10 minutes, and washed three times with 1 ml of RPMI1640·ITPSGF medium. The sialyltranseferase activity was measured by directly using 5 µl of this IgG Sepharose. The activity measurement was carried out by reacting in 30 µl of the assay solution (0.1 M cacodylate buffer (pH 6.5), 0.01 M MnCl 2 , 0.45% Triton X-100, 0.1 mM substrate, the above IgG Sepharose (5 µl), 5 mM CMP-sialic acid (added or not added)) at 37°C for 2 hours, and then identifying the products by high performance liquid chromatography (HPLC). As the substrate, various sugar chains (lacto-N-neotetraose (LNnT), lacto-N-tetraose (LNT), lacto-N-fucopentaose III (LNFP-III) and lacto-N-fucopentaose V (LNFP-V); all are available from Oxford GlycoSystems, and the respective structures are shown in Fig. 1) fluorescence-labeled with aminopyridine were used. The fluorescence labeling of the substrate was carried out according to the conventional method (Akihiro Kondo et al., Agric. Biol. Chem., 54 , 2169 (1990)). The IgG Sepharose was allowed to react with an assay solution containing or not contaning CMP-sialic acid as a sugar donor. The reaction mixture was separated by HPLC, and the peaks appearing only with the assay solution containing CMP-sialic acid were considered as the products. After completion of the reaction, the assay solution was treated with at 100°C for for 5 minutes, and centrifuged at 10,000 × g for 10 minutes. The resulting supernatant was subjected to HPLC which was carried out on the TSKgel ODS-80T M column (4.6 mm × 30 cm; Tosoh) eluting with 0.02M ammonium acetate buffer (pH 4.0) at a temperature of 50°C at a rate of 1 ml/min. The products were detected using the fluorescence HPLC monitor model RF-535T (Shimazu Seisakusho) with an excitation wavelength of 320 nm and an emission wavelength of 400 nm. The products was identified from the facts that the elution time was coincided with that of the standard and that the substrate was regenerated by sialidase treatment of the products. The quantitative analysis of the products was carried out with the use of pyridylaminated lactose as the standard for comparison of fluorescence intensity. The results of this assay are shown in Table 1. The activity to various sugar chain substrates is also shown in terms of a relative activity when the activity to the substrate LNnT is taken as 100. In cases where the IgG Sepharose derived from the culture supernatant of Namalwa cells having the plasmid pAMoPRSAL-35F was used, α2→3 sialyltransferase activity was detected when any of the sugar chains was used as the substrate. On the other hand, in cases where the IgG Sepharose derived from the culture supernatant of Namalwa cells containing the vector pAMoPRSA, no activity was detected when any of the sugar chains was used as the substrate. These results indicates that the α2→3 sialyltransferase undergoes secretory production into the culture supernatant as a fused protein with the IgG-binding region of Staphylococcus aureus protein A and that it can readily be recovered and purified using IgG Sepharose. Also shown in Table 1 are the relative activities of a known α2→3 sialyltransferase, for which purification has been reported so far (Wienstein et al., J. Biol. Chem., 257 , 13845 (1982)), to various sugar chain substrates when its activity to the substrate LNT is taken as 100. The known α2→3 sialyltransferase exhibits a higher specificity to LNT than LNnT, whereas the α2→3 sialyltransferase of the present invention exhibits a higher specificity to LNnT than LNT. This indicates that the α2→3 sialyltransferase of the present invention is a novel enzyme having a different substrate specificity from that of the known enzyme. It was also found that the α2→3 sialyltransferase of the present invention can take LNFP-III as the substrate. No report has been made so far on such α2→3 sialyltransferase having an enzymatic activity to LNFP-III. This indicates that the use of this enzyme can make possible the direct in vitro synthesis of sialyl-Le x sugar chains from Le x sugar chains. Because the α2→3 sialyltransferase of the present invention can take not only LNnT but also LNT as the substrate, it is possible to synthesize sialyl-Le x sugar chains, as well as sialyl-Le a sugar chains. That is, with the use of the α2→3 sialyltransferase of the present invention, the terminal structure of a sugar chain can be converted into NeuAcα2-3Galβ1-4GlcNAc or NeuAcα2-3Galβ1-3GlcNAc, which are then made into a sialyl-Le x sugar chain or a sialyl-Le a sugar chain, using α1→3 fucosyltransferase or α1→4 fucosyltransferase, respectively. Moreover, the α2→3 sialyltransferase of the present invention exhibits a higher substrate specificity to LNnT than that attained by the known α2→3 sialyltransferase, and is therefore superior to the known α2→3 sialyltransferase with respect to the capability for synthesis of sialyl-Le x sugar chains. Example 4 Production by animal cells of α2→3 sialyltransferase derived from human melanoma cell strain WM266-4: 1. Cloning of an α2→3 sialyltransferase cDNA (WM17) derived from human melanoma cell line WM266-4 (1) Isolation of mRNA from human melanoma WM266-4 cell line From 1 × 10 8 WM266-4 cells (ATCC CRL1676), about 30 µg of mRNA was isolated using the mRNA extraction kit "Fast Track" (trade No. K1593-02; Invitrogen), according to the manufacturer's instructions accompanying the kit used. (2) Preparation of a cDNA library From 8 µg of mRNA obatined above, double-stranded cDNA was prepared using the cDNA synthesis kit "The Librarian I" (Invitrogen) with a random primer as a primer. Then, each of the following Sfi I linkers (Seq. ID: 5) as prepared in Example 1 was added, instead of Bst XI linker, at either terminus of the cDNA. The cDNA was fractioned in size by agarose gel electrophoresis and cDNA fragments larger than about 1.2 kb were isolated. Before use, these Sfi I linkers (11 mer and 8 mer) had been independently dissloved at 100 nM in 50 µl of T4 kinase buffer and phosphorylated with 30 units of T4 polynucleotide kinase (Takara Shuzo) at 37°C for 16 hour. The specific reagents and procedures were as described in the manufacturer's instructions accompanying the kit used, except that the above Sfi I linkers were used in place of Bst XI linkers. As the direct expression cloning vector, the vector pAMoPRC3Sc obtained in Sec. 1(5) of Example 3 was used. First, 24 µg of the vector pAMoPRC3Sc was dissolved in 590 µl of Y-50 buffer and digested with 80 units of Sfi I at 37°C for 16 hours. Then, the reaction mixture was taken at a volume of 5 µl for agarose gel electrophoresis, and after the completion of digestion was confirmed, this vector was further digested with 40 units of Bam HI at 37°C for 2 hours. The digestion with Bam HI was done for the purpose of decreasing the amount of background (i.e., clones containing no cDNA insert) at the time of cDNA library construction. The reaction mixture was subjected to agarose gel electrophoresis to give an about 8.8 kb DNA fragment. Then, 2 µg of the Sfi I fragment (8.8 kb) derived from the pAMoPRC3Sc, together with the cDNA obtained above, was dissolved in 250 µl of T4 ligase buffer and ligated together with 2000 units of T4 DNA ligase at 12°C for 16 hours. Thereafter, 5 µg of transfer RNA (tRNA) was added, and after ethanol precipitation, the resulting precipitate was dissolved in 20 µl of TE buffer. The reaction mixture was used to transform Escherichia coli LE392 strain (Maniatis et al., Molecular Cloning, 2.58 , Cold Spring Harbor, 1989) by electroporation (William J. Dower et al., Nucleic Acids Res., 16 , 6127 (1988)) to give about 200,000 ampicillin resistant transformants. (3) Cloning of an α2→3 sialyltransferase cDNA (WM17) About 200,000 ampicillin resistant transformants (cDNA library) obtained in Sec. 1(2) of this Example were mixed, after which plasmids were prepared using >plasmid<maxi kit (trade No. 41031; Qiagen) which is a plasmid preparation kit. The obtained plasmids were ethanol precipitated and the resulting precipitate was dissloved in TE buffer to give a concentration of 1 µg/µl. The above plasmids were introduced into the KJM-1 strain by electroporation (Miyaji et al., Cytotechnology, 3 , 133 (1990)) at a proportion of 4 µg per 1.6 × 10 6 cells. After the introduction of plasmids, these cells were suspended in 8 ml of RPMI1640·ITPSGF medium, and the cells were cultured in a CO 2 incubator at 37°C for 24 hours. Then, the cells were supplemented with G418 (GIBCO) to give a concentration of 0.5 mg/ml and further cultured for 5 to 7 days to obtain transformants. The obtained transformant was suspended in RPMI1640·ITPSGF medium containing Ricinus communis 120 lectin (50 ng/ml) to give a concentration of 5 x 10 4 cells/ml, and the suspension was distributed in 200-µl portions into wells of a 96-well microtiter plate. The cells were cultured in a CO 2 incubator at 37°C for 4 weeks, and a certain strain was obtained which had become resistant to Ricinus communis 120 lectin. After culturing of this resistant cell, a plasmid was isolated from about 5 × 10 6 cells according to the Hirt method (Robert F Margolskee et al., Mol. Cell. Biol., 8 , 2837 (1988)). The isolated plasmid was introduced into Escherichia coli strain LE392 by electroporation (William J. Dower et al., Nucleic Acids Res., 16 , 6127 (1988)) to give an ampicillin resistant transformant. From this transformant, a plasmid was prepared using >plasmid< maxi kit (Qiagen), and its structure was examined by restriction enzyme digestion to find that it contained about 1.9 kb cDNA. The cDNA containing plasmid was designated as pAMoPRWM17. When this plasmid was also introduced into the KJM-1 strain by the above method, the transformant became resistant to Ricinus communis 120 lectin; it was therefore found that this cDNA is a gene responsible for lectin resistance. The KJM-1 strain containing the plasmid pAMoPRWM17 was able to grow even in the presence of 200 ng/ml Ricinus communis 120 lectin. 2. Sequencing of an α2→3 sialyltransferase cDNA (WM17) (1) Incorporation of the α2→3 sialyltransferase cDNA (WM17) into plasmid pUC119 (see Fig. 27) First, 1 µg of the plasmid pAMoPRWM17 was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Eco RV and 20 units of Asp 718 (Boehringer Mannheim) at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 1.9 kb DNA fragment. Separately, 1 µg of the plasmid pUC119 (Messing et al., Methods in Enzymology, 153 , 3 (1987)) was dissloved in 30 µl of K-20 buffer and digested with 20 units of Sma I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 100 mM, and this plasmid was further digested with 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 3.16 kb DNA fragment. The DNA fragments thus obtained, i.e., 0.2 µg of the Eco Rv- Asp 718 fragment (1.9 kb) derived from the pAMoPRWM17 and 0.1 µg of the Sma I- Asp 718 fragment (3.16 kb) derived from the pUC119 were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pUC119-WM17, and its structure was confirmed by restriction enzyme digestion. (2) Construction of deletion plasmids for sequencing First, 2 µg of the plasmid PUC119-WM17 was dissloved in 30 µl of Y-150 buffer and digested with 20 units of Bam HI and 20 units of Sph I at 37°C for 2 hours. After ethanol precipitation, the resuling precipitate was dissolved in 100 µl of Exo III buffer (accompanying the deletion kit for kilo-sequence; Takara Shuzo). Independently, 2 µg of the same plasmid was dissolved in 30 µl of Y-0 buffer and digested with 20 units of Sac I at 37°C for 2 hours. Then, sodium chloride was added to give an NaCl concentration of 150 mM and digested with 20 units of Not I at 37°C for 2 hours. After ethanol precipitation, the resulting precipitate was dissolved in 100 µl of Exo III buffer. From the Bam HI- Sph I fragment derived from the plasmid pUC119-WM17 and the Sac I- Not I fragment derived from the same plasmid, several tens of deletion plasmids were respectively prepared using the deletion kit for kilo-sequence (Takara Shuzo). Specific reagents and procedures were as described in the manufacturer's instructions accompanying the kit used. The nucleotide sequence of the deletion plasmid obtained above was determined using the Taq DyeDeoxy terminator cycle sequencing kit (trade No. 401113; Applied Biosystems). The determined nucleotide sequence is shown in the sequence listing (Seq. ID NO:6). A poly A tail follows 1742 base pairs (bp) nucleotide. From this nucleotide sequence, it was found that this gene encodes a protein consisting of 329 amino acids. The amino acid sequence of this protein is also shown in the sequence listing (Seq. ID NO:7). It was also found that the amino acid sequence has about 91% homology with that encoded by the α2 →3 sialyltransferase cDNA cloned from TYH cells in Example 4. From these results, it is considered that the gene (WM17) isolated in terms of resistance to Ricinus communis 120 lectin encodes the α2→3 sialyltransferase. 3. Measurement of α2→3 sialyltransferase activity of KJM-1 strain containing the WM17 expression plasmid In the same manner as described in Sec. 5 of Example 5, comparison of the α2→3 sialyltransferase activity was made between the KJM-1 strain containing the WM17 expression plasmid (pAMoPRWM17) and the KJM-1 strain containing the plasmid pAMoPRC3Sc as a control. As the result, the α2→3 sialyltransferase activity of the KJM-1 strain containing the plasmid pAMoPRWM17 was 6 to 7 times higher than that of the KJM-1 strain containing the plasmid pAMoPRC3Sc. 4. Synthesis of an sialyl-Le x sugar chain in KJM-1 strain having an expression plasmid for the α2→3 sialyltransferase The KJM-1 strain containing the pAMoPRWM17 (expression plasmid for α2→3 sialyltransferase) obtained in Sec. 1 of Example 4 and the KJM-1 strain containing the pAMoPRC3Sc (control plasmid) were independently cultured in the RPMI1640·ITPSGF medium containing 0.5 mg/ml of G418, after which about 1 × 10 6 cells of each strain were taken in a microtube (1.5 ml; Eppendorf) and collected by centrifugation at 550 × g for 7 minutes. Then, these cells were washed with 1 ml of A-PBS and subjected to indirect fluorescent antibody staining with KM93 (Shitara et al., Anticancer Res., 9 , 999 (1989)) which is an antibody reacting with the sialyl-Le x sugar chain, thereby examining the production of sialyl-Le x sugar chain in these cells. The collected cells of each strain were suspended in 50 µl (10 µg/ml) of KM93 and allowed to react at 4°C for 1 hour. Then, these cells were washed three times with A-PBS and suspended in 20 µl of fluorescence-labeled anti-mouse IgG and IgM antibodies (Cappel, used after 20-fold dilution with A-PBS), followed by reaction at 4°C for 30 minutes. After washing three times with A-PBS, these cells were again suspended in A-PBS and subjected to anlysis using EPICS elite flow cytometer (Coulter). As a control, the same analysis was carried out using the normal mouse serum diluted 500-fold with A-PBS in place of KM93. The results are shown in Fig. 28. It is found that for the KJM-1 strain containing the direct expression cloning vector pAMoPRC3Sc (control plasmid), the cells stained with KM93 exhibit a higher fluorescence intensity than that of the control. This indicates that the KJM-1 strain is originally able to express the sialyl-Le x sugar chains. Also found is that the fluorescence intensity obtained when the cells of the KJM-1 strain containing the pAMoPRWM17 (α2→3 sialyltransferase expression plasmid) were stained with KM93 is further higher than that obtained when the cells of the KJM-1 strain having the pAMoPRC3Sc (control plasmid) were stained with KM93. This indicates that the α2→3 sialyltransferase encoded in the gene WM17 can synthesize sialyl-Le x sugar chains in cells. 5. Secretory production by animal cells of the α2→3 sialyltransferase derived from the WM266-4 cell line (1) Construction of plasmid pAMoPRSAW17-31F for secretory expression of a cDNA encoding the α2→3 sialyltransferase (see Fig. 29) In view of the amino acid sequence, α2→3 sialyltransferase encoded in the cloned gene WM17 seems to have a structure where 8 amino acids in the N-terminal portion are put out at the cytoplasma side, the highly hydrophobic region consisting of the subsequent 18 amino acids is used for binding the membrane, and most of the remaining C-terminal portion (including the catalytic site) is exposed to the internal cavity of Golgi's apparatus. According to the method as described below, therefore, the secretory production of the α2→3 sialyltransferase was achieved by removing the membrane-binding region from the α2→3 sialyltransferase and adding instead the signal sequence of the granulocyte colony stimulating factor and the IgG-binding region of Staphylococcus aureus protein A. The gene portion encoding a certain region following the membrane-binding region of the α2→3 sialyltransferase (from 31st Phe to 329th Phe) was prepared by PCR method and inserted into the secretory expression vector pAMoPRSA obtained in Sec. 1(6) of Example 3. As a set of primers using PCR, the following two synthetic DNA fragments, i.e., W17-A(31F) (44 mer) and W17-C (36 mer), were synthesized by DNA synthesizer model 380A (Applied Biosystems). The primers using PCR W17-A (31F) and W17-C are designed to have an Eco RV site and an Asp 718 site, respectively, so that DNA fragments amplified by PCR can be incorporated between the Stu I site and the Asp 718 site of the plasmid pAMoPRSA after digestion with Eco RV and Asp 718. The PCR was carried out using a GeneAmp™ DNA amplification reagent kit with AmpliTaq™ recombinant Taq DNA polymerase (Takara Shuzo). The reaction mixture was prepared according to the method as described in the kit, and Perkin Elmer Cetus DNA thermal cycler (Takara Shuzo) was used for incubation. Thirty cycles of amplification were carried out according to the following scheme: 94°C for 1 minute, 55°C for 1 minute and 72°C for 3 minutes. Then, further incubation was carried out at 72°C for 7 minutes. As a templete, 1 ng of the plasmid pUC119-WM17 obtained in Sec. 2(1) of this Example was used. After completion of the reaction, chloroform extraction and ethanol precipitation were successively carried out. The resulting precipitate was then dissolved in 30 µl of Y-100 buffer and digested with 20 units of Eco RV and 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 0.91 kb DNA fragment. Separately, 2 µg of the plasmid pAMoPRSA was dissolved in 30 µl of Y-100 buffer and digested with 20 units of Stu I and 20 units of Asp 718 at 37°C for 2 hours. The reaction mixture was subjected to agarose gel electrophoresis to give an about 9.06 kb DNA fragment. Then, 0.1 µg of the Eco RV- Asp 718 fragment (0.91 kb) derived from the amplified DNA by PCR and 0.1 µg of the Stu I- Asp 718 fragment (9.06 kb) derived from the plasmid pAMoPRSA were dissolved in 30 µl of T4 ligase buffer and ligated together with 175 units of T4 DNA ligase at 12°C for 16 hours. Escherichia coli HB101 strain was transformed using this reaction mixture according to the method of Cohen et al. to obtain an ampicillin resistant strain. From this transformant, a plasmid was isolated according to the known method. This plasmid was designated as pAMoPRSAW17-31F, and its structure was confirmed by restriction enzyme digestion. (2) Secretory expression of a cDNA encoding the α2→3 sialyltransferase with Namalwa KJM-1 cells used as a host The plasmid pAMoPRSA (secretory expression vector; control) obtained in Sec. 1(6) of Example 3 and the plasmid pAMoPRSAW17-31F (secretory expression plasmid for α2→3 sialyltransferase) obtained in Sec. 5(1) of this Example were prepared using the plasmid preparation kit, >plasmid<maxi kit (trade No. 41031; Qiagen). Each of the plasmids thus obtained was precipitated by ethanol and dissolved in TE buffer to give a concentration of 1 µg/µl. Then, both plasmids were independently introduced into the Namalwa KJM-1 cells at a proportion of 4 µg per 1.6 × 10 6 cells by electroporation (Miyaji et al., Cytotechnology, 3 , 133 (1990)). The cells were suspended in 8 ml of RPMI1640·ITPSGF medium, and the cells were cultured in a CO 2 incubator at 37°C for 24 hours. Then, the cells were supplemented with G418 (GIBCO) at a concentration of 0.5 mg/ml and further cultured for 7 to 14 days, resulting in transformants. Each of the transformants was suspended in 30 ml of RPMI1640·ITPSGF medium containing 0.5 mg/ml of G418 to give a concentration of 1 × 10 5 cells/ml, and the cells were cultured in a CO 2 incubator at 37°C for 8 days. Then, centrifugation at 160 × g for 10 minutes gave the separation of a supernatant from the cells, and the supernatant was further centrifuged at 1500 × g for 10 minutes. The culture supernatant thus obtained was stored at -80°C until it was used. The α2→3 sialyltransferase encoded in the plasmid pAMoPRSAW17-31F can readily be purified using IgG Sepharose because of its secretory production as a fused protein with the IgG-binding region of Staphylococcus aureus protein A. To the culture supernatant obtained above, sodium azide was added to give a final concentration of 0.1%. Then, 100 µl of IgG Sepharose (Pharmacia) which had been pre-treated according to the accompanying instructions was added, and the mixture was gently stirred at 4°C overnight. The IgG Sepharose was recovered by centrifugation at 160 × g for 10 minutes, and washed three times with 1 ml of RPMI1640·ITPSGF medium. The sialyltransferase activity was measured by directly using 5 µl of this IgG Sepharose. The activity measurement was carried out by reacting in 30 µl of the assay solution (0.1 M cacodylate buffer (pH 6.5), 0.01 M MnCl 2 , 0.45% Triton X-100, 0.1 mM substrate, the above IgG Sepharose (5 µl), 5 mM CMP-sialic acid (added or not added)) at 37°C for 2 hours, and then identifying the products by high performance liquid chromatography (HPLC). As the substrate, various sugar chains (lacto-N-neotetraose (LNnT), lacto-N-tetraose (LNT), lacto-N-fucopentaose III (LNFP-III) and lacto-N-fucopentaose V (LNFP-V); all are available from Oxford GlycoSystems, and the respective structures are shown in Fig. 1) fluorescence-labeled with aminopyridine were used. The fluorescence labeling of the substrate was carried out according to the conventional method (Akihiro Kondo et al., Agric. Biol. Chem., 54 , 2169 (1990)). The IgG Sepharose was allowed to react with an assay solution containing or not containing CMP-sialic acid as a sugar donor. The reaction mixture was analysed by HPLC, and the peaks appearing only with the assay solution containing CMP-sialic acid were considered as the products. After completion of the reaction, the assay solution was treated at 100°C for 5 minutes, and centrifuged at 10,000 × g for 10 minutes. Then, 10 µl of the resulting supernatant was subjected to HPLC which was carried out on the TSKgel ODS-80T M column (4.6 mm × 30 cm; Tosoh) eluting with 0.02 M ammonium acetate buffer (pH 4.0) at a temperature of 50°C at a rate of 1 ml/min. The products were detected using the fluorescence HPLC monitor model RF-535T (Shimazu Seisakusho) with an excitation wavelength of 320 nm and an emission wavelength of 400 nm. The products were identified from the facts that the elution time was coincided with that of the standard and that the substrate was regenerated by sialidase treatment of the products. The quantitative analysis of the products was carried out with the use of pyridylaminated lactose as the standard for comparison of fluorescence intensity. The results of this assay are shown in Table 2. The activity to various sugar chain substrates is also shown in terms of a relative activity when the activity to the substrate LNnT is taken as 100. In cases where the IgG Sepharose derived from the culture supernatant of Namalwa cells having the plasmid pAMoPRSAW17-31F was used, α2→3 sialyltransferase activity was detected when any of the sugar chains was used as the substrate. On the other hand, in cases where the IgG Sepharose derived from the culture supernatant of Namalwa cells having the vector pAMoPRSA, no activity was detected when any of the sugar chains was used as the substrate. These results indicate that the α2→3 sialyltransferase undergoes secretory production into the culture supernatant as a fused protein with the IgG-binding region of Staphylococcus aureus protein A and that it can readily be recovered and purified using IgG Sepharose. Also shown in Table 2 are the relative activities of a known α2→3 sialyltransferase, for which purification has been reported so far (Wienstein et al., J. Biol. Chem., 257, 13845 (1982)), to various sugar chain substrates when its activity to the substrate LNT is taken as 100. The known α2→3 sialyltransferase exhibits a higher specificity to LNT than LNnT, whereas the α2→3 sialyltransferase of the present invention exhibits a higher specificity to LNnT than LNT. This indicates that the α2→3 sialyltransferase of the present invention is a novel enzyme having a different susbtrate specificity from that of the known enzyme. It was also shown that the α2→3 sialyltransferase of the present invention can take LNFP-III as the substrate. No report has been made so far on such α2→3 sialyltransferase having an enzymatic activity to LNFP-III. This indicates that the use of this enzyme can make possible the direct in vitro synthesis of sialyl-Le x sugar chains from Le x sugar chains. Because the α2→3 sialyltransferase of the present invention can take not only LNnT but also LNT as the substrate, it is possible to synthesize sialyl-Le a sugar chains, as well as sialyl-Le x sugar chains. That is, with the use of the α2→3 sialyltransferase of the present invention, the terminal structure of a sugar chain can be converted into or NeuAcα2-3Galβ1-3GlcNAc, NeuAcα2-3Galβ1-4GlcNAc which are then made into a sialyl-Le x sugar chain or a sialyl-Le a sugar chain, using an α1→3 fucosyltransferase or α1→4 fucosyltransferase, respectively. Moreover, the α2→3 sialyltransferase of the present invention exhibits a higher substrate specificity to LNnT than that attained by the known α2→3 sialyltransferase, and is therefore superior to the known α2→3 sialyltransferase with respect to the capability for synthesis of sialyl-Le x sugar chains. "Sequence Listing" SEQ ID NO: 1 SEQUENCE LENGTH: 1919 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double MOLECULE TYPE: cDNA to mRNA ORIGINAL SOURCE ORGANISM: human CELL LINE: TYH cell CELL TYPE: histiocytoma cell SEQ ID NO: 2 SEQUENCE LENGTH: 333 SEQUENCE TYPE: amino acid TOPOLOGY: linear MOLECULE TYPE: protein ORIGINAL SOURCE ORGANISM: human CELL LINE: TYH cell CELL TYPE: histiocytoma cell SEQ ID NO :3 SEQUENCE LENGTH: 52 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Other nucleic acid synthetic DNA SEQ ID NO: 4 SEQUENCE LENGTH: 44 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Other nucleic acid synthetic DNA SEQ ID NO: 5 SEQUENCE LENGTH: 11 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double TOPOLOGY: linear MOLECULE TYPE: Other nucleic acid synthetic DNA SEQ ID NO: 6 SEQUENCE LENGTH: 1766 SEQUENCE TYPE: nucleic acid STRANDEDNESS: double MOLECULE TYPE: cDNA to mRNA ORIGINAL SOURCE ORGANISM: human CELL LINE: WM266-4 cell CELL TYPE: melanoma SEQ ID NO: 7 SEQUENCE LENGTH: 329 SEQUENCE TYPE: amino acid TOPOLOGY: linear MOLECULE TYPE: protein ORIGINAL SOURCE ORGANISM: human CELL LINE: WM266-4 cell CELL TYPE: melanoma

Claims: A DNA which encodes a polypeptide having the biological activity of an α2→3 sialyltransferase and utilizing more preferentially lacto-N-neotetraose than lacto-N-tetraose as a substrate and which is selected from the group consisting of: (a) a cDNA having a nucleotide sequence as designated by Seq. ID NO:1 or 6 or a part thereof; (b) a DNA the complementary strand of which hybridizes to the DNA of (a) or a part thereof; and (c) a DNA which is degenerate with respect to the sequence of (a) or (b) or parts thereof. A polypeptide encoded by the DNA sequence according to claim 1. The polypeptide according to claim 2 which has the amino acid sequence as designated by Seq. ID NO:2 or 7. A recombinant vector containing a DNA sequence according to claim 1. A recombinant vector containing a cDNA which encodes an amino acid sequence as designated by Seq. ID NO:2 or 7. A process of preparing a cDNA according to claim 1 which comprises the steps of: constructing a cDNA library by incorporating into a vector, a cDNA synthesized using as an template an mRNA extracted from an animal cell; introducing said cDNA library into a cell and preparing a culture of the obtained cell in the presence of a lectin having an activity to inhibit the growth of said cell; and separating the grown cell from said culture and isolating from said cell, a DNA which encodes an α2→3 sialyltransferase. A process of preparing a recombinant vector according to claim 4 or 5, which comprises the steps of: constructing a cDNA library by incorporating into a vector, a cDNA synthesized using as an template an mRNA extracted from an animal cell; introducing said cDNA library into a cell and preparing a culture of the obtained cell in the presence of a lectin having an activity to inhibit the growth of said cell; separating the grown cell from said culture and isolating from said cell, a DNA which encodes an α2→3 sialyltransferase; and introducing said DNA into a vector downstream from the position of a promoter therein. A process for production of an α2→3 sialyltransferase according to claim 2 or 3, which comprises the steps of: preparing a culture of a cell containing a recombinant vector according to claim 4 or 5 in a medium and allowing the α2→3 sialyltransferase produced by said cell to accumulate in said culture; and isolating the α2→3 sialyltransferase from said culture. A process according to claim 6 or 7, wherein said animal cell is a TYH cell or a human melanoma WM266-4 cell. A process according to claim 6 or 7, wherein said lectin is Ricinus communis 120 lectin. Plasmid pUC119-LEC which is contained in E. coli HB101/pU119-LEC (FERM BP-3625). Plasmid pUC119-WM17 which is contained in E. coli HB101/pUC119-WM17 (FERM BP-4013). A cell containing a recombinant vector according to claim 4 or 5. A method for adding sialic acid at non-reducing terminus in the lactosamine structure of a glycoprotein or a glycolipid in α2→3 linkage by use of a cell according to claim 13. A method for converting non-reducing terminus of sugar chains on glycoproteins or glycolipids into sialyl-Le x structure by use of a cell according to claim 13. A method for adding sialic acid at a non-reducing terminus in the lactosamine structure of glycoproteins or glycolipids in α2→3 linkage by use of an α2→3 sialyltransferase according to claim 2 or 3. A method for converting non-reducing terminus of glycoproteins or glycolipids into sialyl-Le x structure by use of an α2→3 sialyltransferase according to claim 2 or 3. A method for detecting the expression of an α2→3 sialytransferase according to claim 2 or 3 by hybridization using a cDNA according to claim 1. A method for detecting the expression of an α2→3 sialyltransferase according to claim 2 or 3 by the polymerase chain reaction using synthesized oligonucleotides as primers that are parts of the nucleotide sequence of a cDNA according to claim 1. A method for suppressing the expression of an α2→3 sialyltransferase according to claim 2 or 3 using an oligonucleotide which contains part or all of the nucleotide sequence of a cDNA according to claim 1. An Escherichia coli strain containing a recombinant vector according to claim 4 or 5. Escherichia coli strain HB101/pUC119-LEC (FERM BP-3625). Escherichia coli strain HB101/pUC119-WM17 (FERM BP-4013).

Inventors: HANAI NOBUO, HASEGAWA MAMORU, NISHI TATSUNARI, SASAKI KATSUTOSHI, SEKINE SUSUMU, WATANABE ETSUYO, HANAI, NOBUO, HASEGAWA, MAMORU, NISHI, TATSUNARI, SASAKI, KATSUTOSHI, SEKINE, SUSUMU, WATANABE, ETSUYO
Applicants: KYOWA HAKKO KOGYO KK, KYOWA HAKKO KOGYO CO., LTD.
Application year: 1992
Main classification: C12N15
IPC codes: C12N15, C12N1, C12N9