Novel plasminogen activator

Abstract: A novel plasminogen activator which is identical in peptide sequence to naturally occurring human prourokinase except that the 155th amino acid counting from the N-terminal amino acid (serine)] is other than the 155th amino acid (proline) of naturally occurring human prourokinase, optionally with the addition of methionine at the N terminus.

Description: FIELD OF THE INVENTION This invention relates to a novel plasminogen activator constructed by using recombinant DNA technology, a deoxyribonucleic acid (DNA) coding for the activator, a recombinant plasmid containing the DNA, a microbial or animal cell transformed with the recombinant plasmid, and a method of producing the plasminogen activator using said microbial or animal cell. The novel plasminogen activator according to the invention is stable against the protease (thrombin) which cleaves the urokinase type plasminogen activator into an inactive double-chain structure. Therefore, it can be readily recovered and purified to a pure form. This novel plasminogen activator has an increased specific activity, is stable and has good thrombolytic activity. Accordingly, the novel plasminogen activator according to the invention can be used, as expected, as a therapeutic agent for the treatment and/or prevention of cerebral thrombosis and myocardial infarction, among others. BACKGROUND OF THE INVENTION Urokinase (UK), streptokinase (SK) and tissue plasminogen activator (t-PA) are currently used as thrombolytic agents. The primary structure of human urokinase has been described by Heynecker et al. (JP-A-59-51300, the term "JP-A" used herein means "an unexamined published Japanese patent application") and the folded structure thereof has been described and shown in Fig. 1. This folded structure, which has been proposed based on homology to other proteins, can be divided into the following three domains. The first domain on the N-terminal side is a domain homologous to epidermal growth factor. Hereinafter, this domain is referred to as "growth factor domain". The second domain is the so-called "kringle domain". The third domain at the C-terminal side is the so-called serine protease domain. Hereinafter, the third domain is referred to as "protease domain". Human urokinase occurs in two forms, a single-chain form and a double-chain form. The single chain form of UK is called prourokinase (hereinafter referred to as "pro-UK") and, in such a structure, it has no thrombolytic activity. Only upon cleavage by plasmin into a double-chain structure, it can exhibit thrombolytic activity. However, when pro-UK is cleaved by thrombin, which cleaves pro-UK at a site two amino acids upstream from the plasmin cleavage site, the resulting double-chain structure has no thrombolytic activity any longer [Ichinose et al., J. Biol. Chem., 261 , 3486 (1986)]. Thus, after administration into the blood, pro-UK is cleaved by thrombin which is present in the blood into an inactive double-chain structure. Furthermore, if in the purification process for pro-UK there is a contamination by a thrombin-like protease, the protease will possibly cleave pro-UK into an inactive double-chain structure. To prevent pro-UK from being cleaved by a thrombin-like protease, a pro-UK derivative resulting from the introduction of a substitute amino acid into the thrombin cleavage site at position P₁ or P₁', has been proposed (EP-A-0200451 and JP-A-62-143686). However, since the thrombin cleavage site is separated by only two amino acids from the plasmin cleavage site, the substitute amino acid introduced in the vicinity of the plasmin cleavage site, namely at P₃ to P₁', simultaneously makes it difficult for plasmin cleavage to occur, hence the conversion of the pro-UK derivative obtained into an active form becomes difficult and a decreased specific activity results. In fact, with the pro-UK derivative mentioned above, in which the position of amino acid substitution is P₃ or P₂ relative to the plasmin cleavage site, a decrease in susceptibility to plasmin, hence a decrease in specific activity, has been observed. As regards the numbering of the amino acid residues of the substrate peptide to be cleaved, the peptide bond to be cleaved is represented by -P₁-P₁'-and the amino acids on the amino side to the peptide bond to be cleaved are given position codes P₁, P₂, P₃, P₄ etc. from the amino acid constituting the amino side of said peptide bond toward the amino terminus. The amino acids on the carboxyl side of this bond are given position codes P₁', P₁', P₃', P₄' etc. from the amino acid constituting the carboxyl side of said peptide bond toward the carboxyl terminus. It is an object of the invention to develop a novel pro-UK derivative which is stable against any thrombin-like protease capable of inactivating pro-UK and shows no substantial decrease in specific activity. Heretofore, reports have been published on the construction of pro-UK derivatives, which are more or less resistant to cleavage by thrombin, by introducing a substitute amino acid in position P₁ with respect to the thrombin cleavage site (position P₃ with respect to the plasmin cleavage site; 156th amino acid of mature pro-UK) or position P₁' with respect to the thrombin cleavage site (P₂ with respect to the plasmin cleavage site; 157th amino acid of mature pro-UK). However, since the position of amino acid substitution in these derivatives is only two or three amino acids away from the plasmin cleavage site, the pro-UK derivatives may possibly have a decreased specific activity as a result of a decrease in susceptibility to plasmin. In fact, the present inventors have confirmed that amino acid substitution in mature pro-UK at position 157 (P₂ with respect to the plasmin cleavage site) results in a decrease in specific activity. Furthermore, in the case of amino acid substitution at position 157, it has been found that there is another problem in addition to specific activity decrease. Namely, the resistance of the pro-UK derivatives to thrombin is still unsatisfactory. SUMMARY OF THE INVENTION In an attempt to solve such problems, the present inventors made a novel amino acid substitution in a unique site, that is in position 155 [proline (Pro)] of mature pro-UK. As a result, it was surprisingly found that the pro-UK derivatives obtained by amino acid substitution in position 155 are completely resistant to thrombin and, in addition, have an increased specific activity as compared with naturally occurring pro-UK. The present invention provides a novel plasminogen activator of the human prourokinase type which is characterized in that the 155th [from the N terminal serine of mature human prourokinase] amino acid residue proline has been replaced by another amino acid residue. The invention further provides a DNA coding for the peptide constituting this activator, a recombinant plasmid with this DNA inserted in it, a microbial or animal cell transformed with this plasmid, and a method of producing this plasminogen activator which uses this microbial or animal cell. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates the folded structure of human pro-UK. Fig. 2 illustrates the construction scheme for the plasmid pUKT6. Fig. 3 illustrates the construction scheme for the plasmid pUKT4 and pUKS3. Fig. 4 illustrates the construction scheme for the plasmid pSEUKT6. Fig. 5 illustrates the construction scheme for the plasmid pSEUKT4. Fig. 6 illustrates the construction scheme for the plasmid pSEUKS3. Fig. 7 (1) and (2) illustrate the levels of resistance of various pro-UK derivatives to thrombin in comparison with that of naturally occurring pro-UK to thrombin, the mark  being for naturally occurring pro-UK, ○ for the pro-UK derivative UK-T6, ▵ for the pro-UK derivative UK-T4, ▴ for the pro-UK derivative UK-S3 and □ for the pro-UK derivative UK-T. In (1) and (2), the thrombin concentrations are 5.0 IU/mℓ and 100 IU/mℓ, respectively. Fig. 7 (3) illustrates the results of SDS-polyacrylamide gel electrophoresis of naturally occurring pro-UK and various pro-UK derivatives after treatment with 5.0 IU/mℓ of thrombin, comparatively showing the digestion of each single-chain polypeptide during the course of time. Fig. 8 illustrates the scheme for the synthesis of cDNA by the Okayama-Berg method and the construction of a recombinant plasmid DNA containing said cDNA. Fig. 9 illustrates the construction scheme for the plasmid DNA pCCK1. Fig. 10 illustrates the construction scheme for the plasmid pCCK2. Fig. 11 illustrates the construction scheme for the plasmid pUK11 carrying human pro-UK cDNA. Fig. 12 illustrates the construction scheme for the plasmid pTrS20. Fig. 13 illustrates the construction scheme for the plasmid pTrS33. Fig. 14 illustrates the construction scheme for the plasmid pTerm2. Fig. 15 illustrates the construction scheme for the plasmid pTSF10. Fig. 16 illustrates the construction scheme for the plasmid pTA4. Fig. 17 illustrates the construction scheme for the plasmid pTkSD217. Fig. 18 illustrates the construction scheme for the plasmid pTkSL11. Fig. 19 illustrates the construction scheme for the plasmid pTkSS4. Fig. 20 illustrates the construction scheme for the plasmid pTkSJ1. Fig. 21 illustrates the construction scheme for the plasmid pTkSR18. Fig. 22 illustrates the construction scheme for the plasmid pUKA2. Fig. 23 illustrates the construction scheme for the plasmid pUKB101. Fig. 24 illustrates the construction scheme for the plasmid pTG3. Fig. 25 illustrates the construction scheme for the plasmid phPA2. Fig. 26 illustrates the construction scheme for the single-stranded pUKmpS1. Fig. 27 illustrates the construction scheme for the plasmid pUKS1. Fig. 28 illustrates the construction scheme for the plasmid pAGE105M. Fig. 29 illustrates the construction scheme for the plasmid pAGE106. Fig. 30 illustrates the construction scheme for the plasmid pSE1PA1-5. Fig. 31 illustrates the construction scheme for the plasmid pSE1PA1-9. Fig. 32 illustrates the construction scheme for the plasmid pUC19H. Fig. 33 illustrates the construction scheme for the plasmid pSE1PA1-9A. Fig. 34 illustrates the construction scheme for the plasmid pUKF2. Fig. 35 illustrates the construction scheme for the plasmid pUKFpro. Fig. 36 illustrates the construction scheme for the plasmid pSEUK1-1A. Fig. 37 illustrates the construction scheme for the plasmid pSPAS1-9A Fig. 38 illustrates the construction scheme for the plasmid pUKT1. Fig. 39 illustrates the construction scheme for the plasmid pSEUKT. In the figures mentioned above, the following abbreviations are used in the human t-PA cDNA region: F for finger domain, G for growth factor domain, K1 for kringle 1, K2 for kringle 2, K2' for kringle resulting from partial replacement of kringle 2 by kringle 1, and P for protease domain. In the human pro-UK cDNA region (dotted region), the following abbreviations are used: G for growth factor domain, K for kringle, and P for protease domain. DETAILED DESCRIPTION OF THE INVENTION The novel plasminogen activator according to the invention is a human prourokinase-type plasminogen activator resulting from substitution of an amino acid residue other than a proline residue for the 155th [from the N terminus, serine, of mature human prourokinase] amino acid residue, proline, of mature human prourokinase. This amino acid substitution results in an increase in plasminogen activator activity and in acquisition of resistance to thrombin-like proteases. While substitution of a threonine residue for the 155th amino acid residue, proline, is preferable, any amino acid residue selected from asparagine, aspartic acid, alanine, arginine, isoleucine, glycine, glutamine, glutamic acid, threonine, serine, cysteine, tyrosine, tryptophan, valine, histidine, phenylalanine, methionine, lysine and leucine can be used as a substitute provided that said residue can afford resistance to thrombin-like proteases. It is also possible to increase the resistance to said proteases by replacing the 153rd amino acid residue (leucine) by some other amino acid residue simultaneously with the amino acid substitution in position 155. In this case, too, the plasminogen activator activity is not lowered. A novel plasminogen activator resulting from substitution of an asparagine residue for the 153rd amino acid, leucine, residue and of a threonine residue for the 155th amino acid residue, proline, has a newly created site for N-glycosylation which can stabilize the protein. It is expected that when this plasminogen activator is expressed in animal cells, N-glycosylation might occur on the 153rd amino acid residue, asparagine. Hereinafter, the novel plasminogen activator according to the invention is referred to as "pro-UK derivative" for short. The recombinant plasmid according to the invention is produced by insertion of a DNA fragment coding for the above-mentioned pro-UK derivative into an appropriate plasmid capable of DNA expression in host cells. The DNA fragment coding for the pro-UK derivative of this invention can be prepared by introducing a base substitution which causes amino acid substitution into a human UK DNA. As the human UK DNA, a cDNA obtained by reverse transcription of a messenger RNA coding for human UK and a human UK DNA obtained from chromosomal DNA are used. The human UK cDNA may be any cDNA coding for human UK. Specifically, the human UK cDNA contained in the plasmid pUK1 or pUK11 can be used. pUK1 and pUK11 are plasmids produced by the present inventors and the procedures for their preparation are described in Reference Examples 1, 2 and 3. The human UK cDNA in pUK1 codes for a pro-UK modification lacking part of the N-terminal region of pro-UK while that in pUK11 codes for a pro-UK modification lacking part of the C-terminal region of pro-UK. However, the base sequences of the respective cDNAs agree in part with the base sequence shown in Table 4. An Escherichia coli strain harboring pUK1 and an Escherichia coli strain harboring pUK11 have been deposited, since September 3, 1987, with the Fermentation Research Institute, Agency of Industrial Science and Technology of 1-3, Higashi 1-chome, Tsukubashi, Ibaraki 305, Japan under the names (and deposit numbers) of Escherichia coli EUK1 (FERM BP-1463) and Escherichia coli EUK11 (FERM BP-1464), respectively. The plasmid into which the DNA coding for a proUK derivative is to be inserted may be any plasmid allowing expression of said DNA in microbial or animal cells. Escherichia coli is preferably used as the microbial cells. For causing expression of a pro-UK derivative in Escherichia coli , a plasmid which allows insertion of a foreign DNA thereinto at a site downstream from an appropriate promoter, for example trp or lac promoter and in which the distance between the Shine-Dalgarno sequence (hereinafter referred to as "SD sequence") and the initiation codon (ATG) is appropriate, for example 6 to 18 bases long, can be used. Specifically, pKYP10 (U.S. Patent No. 4,686,191) and pTrS33 (Reference Example 4), both constructed by the present inventors, are preferred examples of such a plasmid. The plasmid to be used in causing expression of the DNA coding for a pro-UK derivative in animal cells may be any plasmid provided that it allows expression of said DNA in animal cells. Preferred is a plasmid which allows insertion of a foreign DNA at a site downstream from an appropriate promotor, for example the SV40 early promoter or SV40 late promoter, and which contains a poly(A) signal, splicing signal and so forth. Preferred plasmids are pAGE103 [Mizukami et al., J. Biochem., 101 , 1307-1310 (1987)] and pSE1PA1SE1dhfr1-9A (hereinafter referred to as "pSPAS1-9A" for short) (Reference Example 14), both constructed by the present inventors. An Escherichia coli strain harboring pAGE103 has been deposited, since March 23, 1987, with the Fermentation Research Institute under the designation (and deposit number) Escherichia coli EAGE103 (FERM BP-1312). As a plasmid having the dihydrofolate reductase (hereinafter referred to as "dhfr") gene as a selective marker, there may be mentioned pSV2-dhfr [S. Subramani et al, Mol. Cell. Biol., 1 , 854 (1981)], among others. Recombination between the DNA coding for a pro-UK derivative and a vector DNA can be carried out by using available techniques that are generally used in the recombinant DNA technology, namely by digesting both DNAs with one or more restriction enzymes, followed by ligation using T4 DNA ligase. Prior to ligation, the termini of the DNA fragments obtained by digestion with the restriction enzyme or enzymes may be filled in using a DNA polymerase Klenow fragment or may be filled in or processed for paring off using T4 DNA polymerase. DNA linkers may also be used. We now describe the construction of recombinant plasmids using pUK1 as the pro-UK cDNA-containing plasmid and incorporating respective DNAs coding for various pro-UK derivatives, it necessary by the intermediary of one or more chemically synthesized DNA fragments, and the construction of recombinant plasmids by incorporation of respective DNAs coding for various pro-UK derivatives into the vector pSPAS1-9A (Reference Example 14) for expression in animal cells as examples and for illustration purposes only. In the first example that follows, the recombinant plasmid pUKT6 coding for a pro-UK derivative (UK-T6) is constructed. As shown in Fig. 2, phPA2 (Reference Example 10) constructed from pUK1 is cleaved with Eco RI and Hin dIII and a DNA fragment of about 3.4 kb is purified. Separately, pUKS1 (Reference Example 11) is cleaved with Hin dIII and Cfr I and a DNA fragment of about 0.75 kb is purified. The two DNA fragments thus obtained, together with the two 5'-phosphorylated synthetic DNAs shown in Fig. 2, are ligated together in the presence of T4 DNA ligase to give pUKT6 coding for a pro-UK derivative (UK-T6) differing from pro-UK in that the 155th (from the N terminus) amino acid residue Pro of pro-UK has been replaced by Thr. In the next example that follows, the recombinant plasmid pUKT4 coding for a pro-UK derivative (UK-T4) and the recombinant plasmid pUKS3 coding for another pro-UK derivative (UK-S3) are constructed. As shown in Fig. 3, phPA2 (Reference Example 10) is cleaved with Eco RI and Hin dIII and a DNA fragment of about 3.4 kb is purified. Separately, pUKS1 (Reference Example 11) is cleaved with Hin dIII and Cfr I and a DNA fragment of about 0.75 kb is purified. The two DNA fragments thus obtained, together with the two 5'-phosphorylated synthetic DNAs shown in Fig. 3, are ligated together in the presence of T4 DNA ligase to give pUKT4 coding for a pro-UK derivative (UK-T4) differing from pro-UK in that the 153rd (from the N terminus) amino acid residue Leu and 155th amino acid residue Pro of pro-UK have been replaced by Ser and Thr, respectively. pUKS3 coding for a pro-UK derivative (UK-S3) differing from pro-UK in that the 153rd (from the N terminus) amino acid residue Leu and 155th amino acid residue Pro of pro-UK have been replaced by Asn and Thr, respectively, is also obtained. Plasmid DNAs capable of expressing the respective pro-UK derivatives (UK-T6, UK-T4, UK-S3 and UK-T) and natural-type pro-UK in animal cells can be prepared, for example, in the following manner. As shown in Fig. 4, pSPAS1-9A (Reference Example 14) is cleaved with Xho I and Kpn I and a DNA fragment of about 8.6 kb is purified. Separately, psE1UKpro1-1A (hereinafter referred to as pSEUK1-1A for short) (Reference Example 13) is cleaved with Xho I and Bgl II and a DNA fragment of about 0.75 kb is purified. Further, separately, pUKT6 is cleaved with Bgl II and Kpn I and a DNA fragment of about 1.15 kb is purified. The three DNA fragments thus obtained are ligated together using T4 DNA ligase to give the plasmid pSE1UKT6SEd1-3 (hereinafter referred to as pSEUKT6 for short) capable of expressing a pro-UK derivative (UK-T6). Then, as shown in Fig. 5, pSPAS1-9A (Reference Example 14) is cleaved with Xho I and Kpn I and a DNA fragment of about 8.6 kb is purified. Separately, pSEUK1-1A (Reference Example 13) is cleaved with Xho I and Bgl II and a DNA fragment of about 0.75 kb is purified. Further, separately, pUKT4 is cleaved with Bgl II and Kpn I and a DNA fragment of about 1.15 kb is purified. The three DNA fragments thus obtained are ligated together using T4 DNA ligase to give the plasmid pSE1UKT4SEd1-3 (hereinafter referred to as pSEUKT4 for short) capable of expressing a pro-UK derivative (UK-T4). Then, as shown in Fig. 6, pSPAS1-9A (Reference Example 14) is cleaved with Xho I and Kpn I and a DNA fragment of about 8.6 kb is purified. Separately, pSEUK1-1A (Reference Example 13) is cleaved with Xho I and Bgl II and a DNA fragment of about 0.75 kb is purified. Further, separately, pUKS3 is cleaved with Bgl II and Kpn I and a DNA fragment of about 1.15 kb is purified. The three DNA fragments thus obtained are ligated together in the presence of T4 DNA ligase to give the plasmid pSE1UK3SEd1-3 (hereinafter referred to as pSEUKS3 for short) capable of expressing a pro-UK derivative (UK-S3). The DNA sequences for the pro-UK derivatives UK-T6, UK-T3 and UK-S3 and amino acid sequences corresponding thereto are shown in Tables 1, 2 and 3, respectively. The amino acid sequence of naturally occurring pro-UK and the corresponding DNA sequence are shown in Table 4. The reaction conditions generally employed in the above recombination techniques are summarized as follows. The digestion of DNA with a restriction enzyme or enzymes is generally carried out in a reaction mixture containing 0.1 to 20 µg of DNA, 2 to 200 mM (preferably 10 to 40 mM) Tris-HCl (pH 6.0 to 9.5, preferably pH 7.0 to 8.0), 0 to 200 mM NaCl and 2 to 20 mM (preferably 5 to 10 mM) MgCl₂, using 0.1 to 100 units (preferably 1 to 3 units per microgram of DNA) of each restriction enzyme, at 20 to 70°C (the optimal temperature may vary depending on the restriction enzyme or enzymes) for 15 minutes to 24 hours. The reaction is generally terminated by heating at 55 to 75°C for 5 to 30 minutes. The reaction may also be terminated by inactivating the restriction enzyme or enzymes using an appropriate reagent such as phenol or diethyl pyrocarbonate. The DNA fragment resulting from restriction enzyme digestion can be purified by low temperature gelation agarose gel electrophoresis (hereinafter referred to as "LGT method") [L. Wieslander, Analytical Biochemistry, 98 , 305 (1979)] or agarose gel freezing and thawing (hereinafter referred to as "AFT method"). This AFT method comprises admixing a DNA fragment-containing agarose gel (0.7 to 1.5%) slice with an equal volume of TE buffer [10 mM Tris-HCl (pH 7.5), 1 mM EDTA] and 2 volumes of phenol (saturated with TE buffer), repeating two cycles of freezing (-70°C) and thawing (65°C), centrifuging, separating the resulting upper aqueous layer and recovering the DNA fragment by precipitation with ethanol. Alternatively, the DNA fragment may also be electrophoretically eluted and purified from an agarose or polyacrylamide gel using a Maxfield model AE-3241 DNA fragment collector (Atto), for instance. (Hereinafter, the latter method is referred to as "electrophoretic elution method"). The ligation of DNA fragments is generally carried out in a reaction mixture containing 2 to 200 mM (preferably 10 to 40 mM) Tris-HCl (pH 6.1 to 9.5, preferably pH 7.0 to 8.0), 2 to 20 mM (preferably 5 to 10 mM) MgCl₂, 0.1 to 10 mM (preferably 0.5 to 2.0 mM) ATP and 1 to 50 mM (preferably 5 to 10 mM) dithiothreitol (hereinafter referred to as "DTT") at 1 to 37°C (preferably 3 to 20°C) for 15 minutes to 72 hours (preferably 2 to 20 hours), using 1 to 1,000 units of T4 DNA ligase. The recombinant plasmid DNA resulting from the ligation reaction is introduced into Escherichia coli , if necessary using the transformation method of Cohen et al. [S.N. Cohen et al., Proc. Natl. Acad. Sci. USA, 69 , 2110 (1972)] or the transformation method of Hanahan [J. Mol. Biol., 166 , 557 (1983)]. The recombinant M13 phage RF DNA produced by the ligation reaction is introduced into Escherichia coli , strain JM105 [J. Messing et al., Gene, 33 , 103 (1985)], for instance, if necessary using the known method of transfection [Y. Kuchino et al., Tampakushitsu, Kakusan, Koso (Protein, Nucleic Acid and Enzyme), 29 , 294 (1984)]. The recombinant plasmid DNA or recombinant M13 phage RF DNA can be isolated from the Escherichia coli , strain harboring the same by the method of Birnboim et al. [H.C. Birnboim et al., Nucleic Acids Res., 7 , 1513 (1979)], for instance. The isolation of the single-stranded DNA from the recombinant M13 phage can be performed by the known method [Y. Kuchino et al., Tampakushitsu, Kakusan, Koso, 29 , 294 (1984)]. The deoxyoligonucleotides to be used in the practice of the invention can be synthesized by solid-phase synthesis by the phosphoamidite method [S.L. Beaucage et al., Tetrahedron Lett., 22 , 1859 (1981) and L. J. BcBrie et al., ibid. 24 , 245 (1983)] using an Applied Biosystems model 380A DNA synthesizer (Applied Biosystems Inc., Foster City, CA 94404). For joining a deoxyoligonucleotide thus synthesized to another or other DNA fragments, about 20 picomoles of the deoxyoligonucleotide is 5'-phosphorylated in 20 µℓ of T4 kinase buffer [50 mM Tris-HCl (pH 7.6), 10 mM MgCl₂, 5 mM DTT, 0.1 mM EDTA, 0.5 mM ATP], with addition of 5 units of T4 DNA kinase. For use as a probe for hybridization, the deoxyoligonucleotide is radioactively labeled at the 5' end thereof using 20 to 50 µCi of 5'-[γ-³²P]ATP (3,000 Ci/mmol, Amersham, Arlington Heights, IL) in lieu of 0.5 mM ATP in the T4 kinase buffer mentioned above. For structural analysis of plasmid DNAs, each plasmid DNA is subjected to digestion with one to ten restriction enzymes, followed by agarose gel or polyacrylamide gel electrophoresis for checking the cleavage sites. Furthermore, if necessary, the base sequence of a DNA can be determined by the dideoxy sequence method using M13 phage. The pro-UK derivative polypeptides according to the invention can be produced by using Escherichia coli or animal cells as the host, for example in the following manner. The pro-UK derivative polypeptides are produced using animal cells as the host as follows. The host for use in causing expression of the pro-UK derivative polypeptides may be any animal cell strain or line provided that it allows expression of said polypeptides. Preferred specific animal cells include dhfr-deficient CHO cells [G. Urlaub & L.A. Chasin, Proc. Natl. Acad. Sci. USA, 77 , 4216 (1980)], among others. In the following, an example of the pro-UK derivative polypeptide production is described in which pSEUKT6 is used as a plasmid for expression of the pro-UK derivative UK-T6 and dhfr-deficient CHO cells are used as the host. The plasmid pSEUKT6 is introduced into dhfr-deficient CHO cells by, for example, the calcium phosphate method [Graham & Van der Eb, Virology, 52 , 546 (1978)]. Transformant strains harboring pSEUKT6 can be selected using, for example, MEM ALPHA medium (ribonucleic acid-free and deoxyribonucleic acid-free; Gibco-Oriental) supplemented with G418 and dialyzed fetal calf serum. Furthermore, a transformant strain with the gene for the physiologically active polypeptide being amplified can be selected from the transformant resistant to methotrexate. The transformant strain thus obtained is cultured in a medium, whereby the pro-UK derivative polypeptide can be produced in the culture. Suitable media are HAM F10 medium, HAM F12 medium (both available from Flow Laboratories), Dulbecco's MEM medium, RPMI-1640 medium (latter two available from Nissui Pharmaceutical), MEM ALPHA medium, and mixed media derived from these, each supplemented with any of various sera (e.g. fetal calf serum). If necessary, 0.5 to 5 mM glutamine, antibiotics (penicillin (25 U/mℓ), streptomycin (25 µg/mℓ), G418 (0.3 mg/mℓ), etc.], sodium bicarbonate (0.01%) and so forth may be added to the medium each in an appropriate amount. For cultivation, various types of culture bottles, dishes, roller bottles, spinner flasks, jar fermenters and the like can be used. The cultivation is generally carried out at a seed cell density of 5×10⁴ to 1×10⁶ cells/mℓ at 30 to 40°C for 2 to 10 days, whereupon the substance according to the invention is secreted mainly extracellularly in an amount depending on the cell density. Cells are removed from the culture by centrifugation and the pro-UK derivative polypeptide is isolated and purified from the supernatant after centrifugation. The plasminogen activating activity of the pro-UK derivative polypeptide obtained can be assayed by the fibrin plate assay method [Granelli-Piperno & Reich, J. Exp. Med., 148 , 223 (1978)]. While the production of UK-T6, one the novel pro-UK derivatives, has been described hereinabove in detail, other pro-UK derivatives can be produced substantially in the same manner following the descriptions and instructions given in this specification. The following examples are further illustrative of the present invention, but are not intended to limit the scope of the invention. EXAMPLE 1 Construction of the recombinant plasmid pUKT6 coding for the pro-UK derivative UK-T6 (cf. Fig. 2) : About 2 µg of the pUKS1 plasmid DNA obtained in Reference Example 11 were dissolved in 30 µℓ of Y-100 buffer [solution containing 10 mM Tris-HCl (pH 7.5), 100 mM NaCl, 7 mM MgCl₂, 6 mM 2-mercaptoethanol], 16 units of Cfr I (Takara Shuzo; hereinafter, unless otherwise specified, the restriction enzymes used were obtained from Takara Shuzo) and 10 units of Hin dIII were added, and the digestion reaction was performed at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Separately, about 2 µg of the phPA2 plasmid DNA obtained in Reference Example 10 were dissolved in 30 µℓ of Y-100 buffer, 10 units of Hin dIII and 1 unit of Eco RI were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.4 kb was purified using the AFT method. The following two synthetic DNAs (each having 43 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer: The thus-obtained synthetic DNAs (25 picomoles each) were then phosphorylated at the 5' end by treating in 10 µℓ of a solution (hereinafter referred to as "T4 kinase buffer") containing 50 mM Tris-HCl (pH 7.6), 10 mM MgCl₂, 5 mM DTT, 0.1 mM EDTA and 0.5 mM ATP, with 5 units of T4 DNA kinase (Takara Shuzo) added, at 37°C for 30 minutes. The thus-obtained pUKS1-derived DNA fragment (about 7.5 kb, about 0.1 µg), phPA2-derived DNA fragment (about 3.4 kb, about 0.1 µg) and two 5'-phosphorylated synthetic DNAs (1-picomole each) were dissolved in 20 µℓ of a buffer (hereinafter referred to as "T4 ligase buffer") containing 20 mM Tris-HCl (pH 7.6), 10 mM MgCl₂, 10 mM DTT and 1 mM ATP, 300 units of T4 ligase (Takara Shuzo) were added, and the ligation reaction was carried out at 4°C for 18 hours. The resulting reaction mixture was used to transform Escherichia coli MM294 [F⁻ hsdR⁻ hsdM⁺ endoI⁻ thi] [ATCC31446; the same strain as Escherichia coli 294 so referred to in the report of K. Backman et al., Proc. Natl. Acad. Sci. USA, 73 , 4174 (1976)] to give ampicillin (hereinafter, "Ap")-resistant transformants. Plasmid DNAs were isolated from these transformants and subjected to structural analysis by digestion with restriction enzymes and to DNA sequence determination by the M13 dideoxy sequencing method. A plasmid DNA thus found to have the desired structure was named pUKT6 (cf. Fig. 2). EXAMPLE 2 Construction of the recombinant plasmid pUKT4 and pUKS3 coding for the pro-UK derivative UK-T4 and UK-S3, respectively (cf. Fig. 3) : About 2 µg of the pUKS1 plasmid DNA obtained in Reference Example 11 were dissolved in 30 µℓ of Y-100 buffer, 16 units of Cfr I and 10 units of Hin dIII were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Separately, about 2 µg of the phPA2 plasmid DNA obtained in Reference Example 10 were dissolved in 30 µℓ of Y-100 buffer, 10 units of Hin dIII and 1 unit of Eco RI were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.4 kb was purified using the AFT method. The following two synthetic DNAs (each having 43 bases) were synthesized and 5'-phosphorylated as described in Example 1. The thus-obtained pUKS1-derived DNA fragment (about 0.75 kb, about 0.1 µg), phPA2-derived DNA fragment (about 3.4 kb, about 0.1 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 ligase were added, and the ligation reaction was carried out at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. Plasmid DNAs were isolated from these transformants and subjected to structural analysis by digestion with restriction enzymes and to DNA sequence determination by the M13 dideoxy sequence method. A plasmid DNA having the desired structure and coding for an amino acid sequence modified in that the 153rd (from the N terminus) amino acid Leu and 155th amino acid Pro have been replaced by Ser and Thr, respectively, was named pUKT4 and another plasmid DNA also desirable in structure and coding for an amino acid sequence modified in that the 153rd (from the N terminus) amino acid Leu and 155th amino acid Pro have been replaced by Asn and Thr, respectively, was named pUKS3 (cf. Fig. 3) EXAMPLE 3 Construction of the UK-T6 expression plasmid DNA pSEUKT6 (cf. Fig. 4) : About 2 µg of the pSPAS1-9A plasmid DNA obtained in Reference Example 14 were dissolved in 30 µℓ of Y-0 buffer [solution containing 10 mM Tris-HCl (pH 7.5), 7 mM MgCl₂ and 6 mM 2-mercaptoethanol], 10 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Xho I were added and the digestion reaction was further carried out at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 8.6 kb was purified using the AFT method. Separately, about 3 µg of the pSEUK1-1A plasmid DNA were dissolved in 30 µℓ of Y-100 buffer, 12 units of Bgl II and 12 units of Xho I were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Further, separately, about 3 µg of the pUKT6 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 15 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 12 units of Bgl II were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified using the AFT method. The thus-obtained pSPAS1-9A-derived DNA fragment (about 8.6 kb, about 0.1 µg), psEUK1-1A-derived DNA fragment (about 0.75 kb, about 0.02 µg) and pUKT6-derived DNA fragment (about 1.15 kb, about 0.02 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was carried out at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSEUKT6 isolated from one of said transformants was subjected to structural analysis by digestion with restriction enzymes and found to have the desired structure (cf. Fig. 4). EXAMPLE 4 Construction of the UK-T4 expression plasmid DNA pSEUKT4 (cf. Fig. 5) : About 2 µg of the pSPAS1-9A plasmid DNA obtained in Reference Example 14 were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Xho I were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 8.6 kb was purified using the AFT method. Separately, about 3 µg of the pSEUK1-1A plasmid DNA were dissolved in 30 µℓ of Y-100 buffer, 12 units of Bgl II and 12 units of Xho I were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Further, separately, about 3 µg of the pUKT4 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 15 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 12 units of Bgl II were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified using the AFT method. The thus-obtained pSPAS1-9A-derived DNA fragment (about 8.6 kb, about 0.1 µg), pSEUK1-1A-derived DNA fragment (about 0.75 kb, about 0.02 µg) and pUKT4-derived DNA fragment (about 1.15 kb, about 0.02 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was carried out at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSEUKT4 isolated from one of these transformants was found to have the desired structure upon structural analysis by digestion with restriction enzymes (cf. Fig. 5). EXAMPLE 5 Construction of the UK-S3 expression plasmid pSEUKS3 (cf. Fig. 6) : About 2 µg of the pSPAS1-9A plasmid DNA obtained in Reference Example 14 were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Xho I were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 8.6 kb was purified using the AFT method. Separately, about 3 µg of the pSEUK1-1A plasmid DNA were dissolved in 30 µℓ of Y-100 buffer, 12 units of Bgl II and 12 units of Xho I were added, and the digestion reaction was carried out at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Further, separately, about 3 µg of the pUKS3 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 15 units of Kpn I were added, and the digestion reaction was carried out at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 12 units of Bgl II were added, and the digestion reaction was further carried out at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified using the AFT method. The thus-obtained pSPAS1-9A-derived DNA fragment (about 8.6 kb, about 0.1 µg), pSEUK1-1A-derived DNA fragment (about 0.75 kb, about 0.02 µg) and pUKS3-derived DNA fragment (about 1.15 kb, about 0.02 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was carried out at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSEUKS3 isolated from one of these transformants was found to have the desired structure upon structural analysis by digestion with restriction enzymes (cf. Fig. 6). EXAMPLE 6 Production of various pro-UK derivatives and natural pro-UK in animal cells : (1) Production of the UK-T6 polypeptide in CHO cells carrying pSEUKT6: pSEUKT6 (obtained in Example 3) was introduced into dhfr-deficient CHO cells essentially by the calcium phosphate method. Thus, 5 mℓ of MEM α (non-selective medium) supplemented with 1/10 volume of PCS (fetal calf serum) and 1/50 volume of 7.5% NaHCO₃ (Flow Laboratories) were inoculated with CHO cells to a cell concentration of 1×10⁵ cells/mℓ [LUX dishes 6 cm in diameter were used for culturing (LUX dishes were used also in the subsequent examples)] and the cells were cultured in a CO₂ incubator at 37°C for 1 day. Separately, 10 µg of the pSEUKT6 DNA were dissolved in 450 µℓ of 10 mM Tris-HCl (pH 7.5), and 500 µℓ of a solution containing 280 mM NaCl, 1.5 mM Na₂HPO₄ and 50 mM HEPES (N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid) (pH 7.1) were added to the solution. After mixing, 50 µℓ of 2.5 M CaCl₂ were further added and, after mixing, the resulting mixture was allowed to stand at room temperature for 5 minutes. The whole quantity of this DNA solution was added to a dhfr-deficient CHO cell culture prepared by discarding the medium, adding 10 mℓ of fresh MEM α (nonselective medium) and culturing for 1 hour. The resulting mixture was incubated for 8 hours. Then, cells were washed with PBS (phosphate buffer-saline; 8 g/liter NaCl, 0.2 g/liter KCl, 1.15 g/liter Na₂HPO₄ (anhydrous), 0.2 g/liter KH₂PO₄) and, after addition of 5 mℓ of MEM α (nonselective medium), cultured for 16 hours. Cells were washed with PBS, 3 mℓ of a solution containing 0.05% trypsin and 0.2% EDTA (ethylenediaminetetraacetic acid) were added thereto, the excess solution was removed, and incubation was conducted at 37°C for 5 minutes (trypsin treatment). MEM α (selective medium) supplemented with 10% dialyzed FCS (Gibco-Oriental), 1/50 volume of 7.5% NaHCO₃, 1/100 volume of 100 × nonessential amino acid solution and 0.3 mg/mℓ G418 (Gibco-Oriental) was added to the cells for thoroughly suspending the cells therein. Incubation was performed in a CO₂ incubator at 37°C for 5 days using a dish having a diameter of 10 cm. Cells were washed with PBS and, after addition of MEM α (selective medium), cultured for 5 days. The same procedure as above was followed and incubation was further carried out for 5 days. Cells were washed with PBS, subjected to trypsin treatment, suspended in 10 mℓ of MEM α (selective medium) and cultured in a dish having a diameter of 6 cm in a CO₂ incubator at 37°C for 3 to 7 days. Colonies that had appeared were subjected to trypsin treatment and then inoculated into a dish having a diameter of 10 cm to a cell concentration of 5×10⁴ cells/mℓ using 10 mℓ of MEM α (selective medium) containing 50 mM MTX. Medium exchange was performed three times at 5 day intervals using the same medium mentioned above. MTX-resistant colonies that had appeared were respectively isolated and each colony was cultured in a dish having a diameter of 6 cm until confluence. Then, the medium was replaced with 5 mℓ of MEM α (selective medium) containing 50 nM MTX and, after 1 day, the culture supernatant was examined for the activity of UK-T6 by the fibrin plate assay method [Granelli-Piperno & Reich, J. Exp. Med., 148 , 233 (1978)]. With the clone No. 24, the highest activity was detected and the production of UK-T6 amounted to 5 µg/10⁶ cells/day. This clone was cultured in 100 mℓ of MEM α (selective medium) containing 50 nM MTX in a Falcon model 3027 roller bottle. After confluence, culturing was conducted for 3 days in a medium identical in composition to the above-mentioned one except that it was FCS-free and contained 10 KIU/mℓ of aprotinin (Boehringer-Mannheim). The culture supernatant (100 mℓ per run) thus obtained was used in Example 7. (2) Production of the UK-T4 polypeptide in animal cells carrying pSEUKT4: Using the recombinant plasmid pSEUKT4 obtained in Example 4 and dhfr-deficient CHO cells and following the same procedure as described above, there were obtained UK-T4-producing cell strains. Among them, the clone No. 8 was found to be the highest in activity, the UK-T4 production amounting to 3 µg/10⁶ cells/day. This clone was cultured in 100 mℓ of MEM α (selective medium) containing 50 nM MTX in a Falcon model 3027 roller bottle. After confluence, culturing was conducted for 3 days in a medium identical in composition to the above-mentioned one except that it was FCS-free and contained 10 KIU/mℓ of aprotinin (Boehringer-Mannheim). The culture supernatant (100 mℓ per run) thus obtained was used in Example 7. (3) Production of the UK-S3 polypeptide in animal cells carrying pSEUKS3: Using the recombinant plasmid pSEUKS3 obtained in Example 5 and dhfr-deficient CHO cells and following the same procedure as described above, there were obtained UK-S3-producing cell strains. Among them, the clone No. 13 was found to be the highest in activity, the UK-S3 production amounting to 3 µg/10⁶ cells/day. This clone was cultured in 100 mℓ of MEM α (selective medium) containing 50 nM MTX in a Falcon model 3027 roller bottle. After confluence, culturing was conducted for 3 days in a medium identical in composition to the above-mentioned one except that it was FCS-free and contained 10 KIU/mℓ of aprotinin (Boehringer-Mannheim). The culture supernatant (100 mℓ per run) thus obtained was used in Example 7. (4) Production of the pro-UK polypeptide in animal cells carrying pSE1UKT1-1d (hereinafter referred to as "pSEUKT"): Using the recombinant plasmid pSEUKT obtained in Reference Example 16 and dhfr-deficient CHO cells and following the same procedure as described above, there were obtained UK-T-producing cell strains. Among them, the clone No. 21 was found to be the highest in activity, the UK-T4 production amounting to 3 µg/10⁶ cells/day. This clone was cultured in 100 mℓ of MEM α (selective medium) containing 50 nM MTX in a Falcon model 3027 roller bottle. After confluence, culturing was conducted for 3 days in a medium identical in composition to the above-mentioned one except that it was FCS-free and contained 10 KIU/mℓ of aprotinin (Boehringer-Mannheim). The culture supernatant (100 mℓ per run) thus obtained was used in Example 7. (5) Production of the pro-UK polypeptide in animal cells carrying pSEUK1-1A: Using the recombinant plasmid pSEUK1-1A obtained in Reference Example 13 and pSV2-dhfr and dhfr-deficient CHO cells and following the same procedure as described above, there were obtained pro-UK-producing cell strains. Among them, the clone No. 5 was found to be the highest in activity, the pro-UK production amounting to 3 µg/10⁶ cells/day. This clone was cultured in 100 mℓ of MEM α (selective medium) containing 50 nM MTX in a Falcon model 3027 roller bottle. After confluence, culturing was conducted for 3 days in a medium identical in composition to the above-mentioned one except that it was FCS-free and contained 10 KIU/mℓ of aprotinin (Boehringer-Mannheim). The culture supernatant (100 mℓ per run) thus obtained was used in Example 7. EXAMPLE 7 Purification of natural type pro-UK and various pro-UK derivatives from animal cell culture supernatants : The serum-free culture supernatants (200 mℓ each) obtained in Example 6 and containing natural type pro-UK or a pro-UK derivative were respectively applied to an anti-UK monoclonal antibody column (4 mℓ) [prepared by binding an anti-UK monoclonal antibody (prepared by the method of Milstein et al. [C. Milstein et al., Nature, 256 , 495-497 (1975)] using low-molecular urokinase (Nippon Chemical Research) as an antigen)] to CNBr4-activated Sepharose 4B (Pharmacia Fine Chemicals)] equilibrated with 50 mM phosphate buffer (pH 7.5) containing 0.01% Tween 80, 0.05% NaN₃ and 100 mM NaCl. Then, 3 bed volumes of the same buffer as used for column equilibration were applied to the column. Elution was then performed using 16 mℓ of phosphate-citrate buffer (pH 4.0) containing 0.01% Tween 80, 0.05% NaN₃, 100 mM NaCl and 200 mM arginine and the eluate was fractionated in 1 mℓ portions. The active eluate fractions were immediately adjusted to pH 7.5 with phosphoric acid and then dialyzed against phosphate buffer (pH 7.5) containing 0.01% Tween 80, 0.05% NaN₃, 100 mM NaCl and 200 mM arginine (hereinafter referred to as "UK buffer") at 4°C for 24 hours. The dialyzate was used as a final purified sample in the subsequent experiments. SDS-polyacrylamide gel electrophoresis revealed that the final purified sample contained a single-chain product with a purity of not less than 95%. The SDS-polyacrylamide gel electrophoresis suggested carbohydrate chain addition to the pro-UK derivative UK-S3, which has an N-glycosylation site, namely -Asn-X-Thr-. An increase in molecular weight was confirmed as well. EXAMPLE 8 Comparison in specific activity between the natural type pro-UK and pro-UK derivatives : The final purified samples respectively containing the natural type pro-UK and pro-UK derivatives as obtained in Example 7 were each applied to a reversed-phase HPLC column (TSK gel ODS-120T, Tosoh). The peak area was compared with the peak area for natural type pro-UK for which the concentration had been determined by the Lowry method [Lowry et al., J. Biol. Chem., 193 , 265 (1951)] to determine the concentration of each protein. The activity of each sample was determined by the fibrin plate assay method. The fibrin plate assay method was carried out in the following manner. Bovine thrombin (Sigma) (1,000 U) was dissolved in 5 mℓ of 50 mM phosphate buffer (pH 7.0). The solution was filter sterilized through a 0.45 µm membrane filter to give a thrombin solution. Separately, a fibrinogen solution was prepared by dissolving bovine fibrinogen (Nakalai-Tesque) in 100 mℓ of sterilized 50 mM phosphate buffer (pH 7.0) with 30 minutes of stirring and then removing the insoluble matter using sterilized glass wool to give a fibrinogen solution. Furthermore, a 2% agar solution was prepared by adding 2 g of agar (Sigma) to 100 mℓ of 50 mM phosphate buffer (pH 7.0), sterilizing at 1.5 atmospheres and 120°C for 20 minutes and then maintaining the solution at 60°C. In each dish, there were placed 0.25 mℓ of the thrombin solution, 5 mℓ of the fibrinogen solution and 5 mℓ of the agar solution. The mixture was stirred and then allowed to stand for solidification of the agar. A fibrin plate was thus obtained. Human urokinase (double-chain form, urine-derived, Nippon Chemical Research) was dissolved in UK buffer and the concentration was adjusted to 10, 100, 1,000 and 10,000 IU/mℓ. These solutions were used as standards. Each sample was diluted to 0.5 µg/mℓ with UK buffer. Holes having a diameter of 2 mm were made on each fibrin plate, the number of the holes being the same as the number of the standards and samples. The holes were filled with 5 µℓ each of the standards or samples prepared as described above. After 24 hours of incubation at 37°C, the diameter of each resultant halo was measured. The activity of each sample was determined using a calibration curve (activity: halo diameter) prepared by using the standards. The specific activity data for the natural type pro-UK and respective pro-UK derivatives are shown herein below in Table 5. While the specific activities of UK-T6, UK-T4 and UK-S3 were higher (1,2- to 1,8-fold) than that of natural type pro-UK, the specific activity of the pro-UK derivative UK-T (with an amino acid substitution in position P₁' relative to the thrombin cleavage site) was only 60% of that of natural type pro-UK. This decrease in specific activity was presumably due to that introduction of amino acid substitution in the neighborhood of the plasmin cleavage site (namely in position P₂ relative to the plasmin cleavage site) which made it difficult for the cleavage with plasmin to take place. Therefore, the pro-UK derivatives (UK-T6, UK-T4 and UK-S3) with amino acid substitution in position P₂ are advantageous from the specific activity viewpoint as well. EXAMPLE 9 Comparison in susceptibility to thrombin between the natural type pro-UK and pro-UK derivatives : The final purified samples respectively containing the natural type pro-UK and pro-UK derivatives as obtained in Example 7 were each applied to a reversed-phase HPLC column (TSK gel ODS-120, Tosoh). Each sample was diluted with the diffusate solution (obtained in the dialysis in Example 7) to 30 µg/mℓ on the basis of the peak area obtained. To 100 µℓ of this dilution were added 20 µℓ of 30 IU/mℓ or 600 IU/mℓ human thrombin, and the mixture was incubated at 37°C. The human thrombin was obtained from Sigma. The human thrombin was used after 1.5 hours of treatment with 10 IU/1,000 IU thrombin of aprotinin at 37°C. Fifteen (15), 30, 60, 120 and 240 minutes after addition of thrombin, sampling was made in 24 µℓ portions, followed by addition of 4 µℓ of 84 µM thrombin inhibitor (THROMSTOP, American Diagnostica) for termination of the reaction. A sample was also prepared by adding the thrombin inhibitor immediately after addition of thrombin. This sample was used as the 0 minute-sample. Furthermore, in a control group, samples without addition of thrombin were incubated at 37°C. A comparison with respect to the susceptibility to thrombin was performed by determining the residual activities of the natural type pro-UK and pro-UK derivatives in the reaction mixtures obtained as described above by the fibrin plate assay method (cf. Example 8). As a result, it was revealed that the pro-UK derivatives had apparently decreased susceptibility to thrombin as compared with natural type pro-UK (cf. Fig. 7-(1)). In particular, it was also shown that UK-T6, UK-T4 and UK-S3 were still lower in susceptibility to thrombin than UK-T (cf. Fig. 7-(2)). Sample Specific activity (×10⁴ IU/mg) Natural type pro-UK 12.8 ± 0.92 100 UK-T6 15.4 ± 0.87 120 UK-T4 16.2 ± 1.26 127 UK-S3 22.7 ± 0.90 177 UK-T 7.8 ± 0.74 61 For further confirmation of the above results, 10 µℓ each of the above reaction mixtures were subjected to SDS-polyacrylamide gel electrophoresis [Laemmli, Nature, 227 , 680 (1970)]. While natural type pro-UK was cleaved with thrombin by degrees, the pro-UK derivatives were hardly cleaved with thrombin (cf. Fig. 7-(3)). These results indicated, in good agreement with the results of fibrin plate assay, that the pro-UK derivatives had decreased susceptibility to thrombin. Strains of Escherichia coli harboring the plasmids pSEUKS3, pSEUKT4 and pSEUKT6 have been deposited, since June 15, 1989, with the Fermentation Research Institute under the Budapest Treaty under the designations (and accession numbers) Escherichia coli ESEUKS3 (FERM BP-2478), Escherichia coli ESEUKT4 (FERM BP-2479) and Escherichia coli ESEUKT6 (FERM BP-2480), respectively. The reference examples referred to in the above examples are given below. REFERENCE EXAMPLE 1 Construction of the plasmid ptPA7 carrying human t-PA cDNA : (1) Preparation of poly(A) RNA from Detroit 562 cells: RNA was prepared from Detroit 562 human pharyngeal carcinoma cells by the guanidine thiocyanate-lithium chloride method [Cathala et al., DNA, 2 , 329 (1983)] as follows. Detroit 562 human pharyngeal carcinoma cells [W. D. Peterson, Jr, et al., Proc. Soc. Exp. Biol. Med., 136 , 1187 (1971)] were grown in 50 mℓ MEM medium (Nissui Pharmaceutical) containing 10% FCS, 1/100 volume of 100 × nonessential amino acid solution (Flow Laboratories), 1 mM sodium pyruvate and 0.1% lactoalbumin hydrolyzate (Gibco-Oriental) in a tissue culture flask (Corning; 150 cm²). After cultivation at 37°C until confluence, cells were washed with PBS, 100 ng/mℓ of phorbol myristate acetate (PMA) were added and, after addition of 30 mℓ of the same medium as mentioned above except that it was FCS-free, incubation was performed at 37°C for 24 hours. Then, cells were treated with 10 mℓ of a solution containing 0.05% trypsin and 0.02% EDTA to give a cell suspension. Six tissue culture flasks of the above type were used to give a total of 1×10⁸ cells. Cells were collected from the pooled cell suspension by centrifugation (1,100 × g, 4°C, 10 minutes), washed with 80 mℓ of phosphate buffer, and solubilized in 10 mℓ of a solution containing 5 M guanidine thiocyanate, 10 mM EDTA, 50 mM Tris-HCl (pH 7) and 8% (v/v) 2-mercaptoethanol using a vortex mixer. The solubilization product was transferred to a centrifuge tube, 80 mℓ of 4 M LiCl were added and, after stirring, the resulting mixture was allowed to stand at 4°C for 20 hours. Centrifugation using a Hitachi RPR10 rotor (9,000 rpm, 90 minutes) resulted in recovery of RNA as a precipitate. The RNA precipitate was suspended in 50 mℓ of a solution comprising 4 M urea and 2 M lithium chloride, and RNA was recovered again as a precipitate by centrifugation using a Hitachi RPR10 rotor (9,000 rpm, 60 minutes). The RNA precipitate was dissolved in 10 mℓ of a solution containing 0.1% sodium lauryl sulfate, 1 mM EDTA and 10 mM Tris-HCl (pH 7.5) and, after extraction with phenol-chloroform, recovered by precipitation with ethanol. About 2.5 mg of the RNA obtained were dissolved in 1 mℓ of a solution containing 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA. After 5 minutes of incubation at 65°C, 0.1 mℓ of 5 M NaCl were added. The resulting mixture was subjected to oligo(dT)-cellulose column [P-L Biochemicals) chromatography (column volume: 0.5 mℓ). The poly(A)-containing mRNA adsorbed was eluted with a solution containing 10 mM Tris-HCl (pH 7.5) and 1 mM EDTA. About 90 µg of poly(A)-containing mRNA were thus obtained. (2) Synthesis of cDNA and insertion of said DNA into a vector: Synthesis of cDNA and construction of a recombinant plasmid with the cDNA inserted therein were performed by the Okayama-Berg method [Mol. Cell. Biol., 2 , 161 (1982)]. The processes are schematically illustrated in Fig. 8. 400 µg of pCDV1 [Okayama & Berg, Mol. Cell. Biol., 3 , 280 (1983)] were added to 300 µℓ of a solution containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl₂ and 10 mM NaCl, 500 units of Kpn I were added, and the reaction was carried out at 37°C for 6 hours for the cleavage of the plasmid at the Kpn I site occurring therein. After extraction with phenol-chloroform, DNA was recovered by precipitation with ethanol. About 200 µg of the Kpn I-cleaved DNA were added to 200 µℓ of a buffer containing 40 mM sodium cacodylate, 30 mM Tris-HCl (pH 6.8), 1 mM CaCl₂ and 0.1 mM dithiothreitol (DTT) (hereinafter referred to as "TdT buffer") with dTTP added to a concentration of 0.25 mM. Furthermore, 81 units of terminal deoxynucleotidyl transferase (hereinafter referred to as "TdT") (P-L Biochemicals) were added, and the reaction was carried out at 37°C for 11 minutes, whereupon a poly(dT) chain comprising about 67 dT residues was added to the 3' terminus of the Kpn I cleavage site of pCDV1. After phenol-chloroform extraction, about 100 µg of the poly(dT) chain-containing pCDV1 DNA were recovered from the solution by precipitation with ethanol. The DNA thus obtained was added to 150 µℓ of a buffer containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl₂ and 100 mM NaCl, 360 units of Eco RI were further added, and the reaction was carried out at 37°C for 2 hours. The reaction mixture was treated by the LGT method and a DNA fragment of about 3.1 kb was recovered. About 60 µg of poly(dT) chain-added pCDV1 were obtained. The resulting DNA was dissolved in 500 µℓ of a solution containing 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA, the solution was incubated at 65°C for 5 minutes and then cooled on ice, and 50 µℓ of 5 M NaCl were added to the solution. The resultant mixture was subjected to oligo(dA)-cellulose column (Collaborative Research) chromatography. Molecules having a sufficiently long poly(dT) chain were adsorbed on the column. They were eluted with a solution containing 10 mM Tris-HCl (pH 8.0) and 1 mM EDTA to give 27 µg of poly(dT) chain-added pCDV1 (hereinafter referred to as "vector primer"). A linker DNA was prepared in the following manner. About 14 µg of pL1 [Okayama & Berg, Mol. Cell. Biol., 3 , 280 (1983)] were added to 200 µℓ of a buffer containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl₂ and 50 mM NaCl, 50 units of Pst I were added, and the reaction was carried out at 37°C for 4 hours for the cleavage of the pL1 DNA at the Pst I site occurring therein. The reaction mixture was subjected to phenol-chloroform extraction and then about 13 µg of the Pst I-cleaved pL1 DNA were recovered by precipitation with ethanol. About 13 µg of the thus-obtained DNA were added to 50 µℓ of a solution prepared by adding dGTP to TdT buffer to a final concentration of 0.25 mM, 54 units of TdT (P-L Biochemicals) were further added, and the mixture was incubated at 37°C for 13 minutes, whereby a (dG) chain comprising about 14 dG residues was added to the 3' end of the Pst I cleavage site of pL1. After phenol-chloroform extraction, DNA was recovered by precipitation with ethanol. The resulting DNA was added to 100 µℓ of a buffer containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl₂ and 60 mM NaCl, 80 units of Hin dIII were added, and the mixture was incubated at 37°C for 3 hours, whereby the pL1 DNA was cleaved at the Hin dIII site. The reaction mixture was fractionated by agarose gel electrophoresis, and a DNA fragment of about 0.5 kb was recovered by the DEAE paper method [Dretzen et al., Anal. Biochem., 112 , 295 (1981)]. An oligo(dG) chain-containing linker DNA (hereinafter referred to as "linker DNA" for short) was thus obtained. About 4 µg of the poly(A) RNA and about 1.4 µg of the vector primer, each prepared as described above, were dissolved in 22.3 µℓ of a solution containing 50 mM Tris-HCl (pH 8.3), 8 mM MgCl₂, 30 mM KCl, 0.3 mM DTT, 2 mM dNTP (dATP, dTTP, dGTP and dCTP) and 10 units of ribonuclease inhibitor (P-L Biochemicals), 10 units of reverse transcriptase (Seikagaku Kogyo) were added, and incubation was performed at 41°C for 90 minutes for the synthesis of a DNA complementary to the mRNA. The reaction mixture was subjected to phenol-chloroform extraction, followed by precipitation with ethanol, whereby the vector primer DNA with an RNA-DNA double strand added thereto was recovered. The resulting DNA was dissolved in 20 µℓ of TdT buffer containing 66 µM dCTP and 0.2 µg of poly(A), 14 units of TdT (P-L Biochemicals) were added, and the mixture was incubated at 37°C for 2 minutes for the addition of a (dC) chain comprising 20 dC residues to the 3' end of cDNA. The reaction mixture was subjected to phenol-chloroform extraction and then a (dC) chain-added cDNA-vector primer DNA was recovered therefrom by precipitation with ethanol. The resulting DNA was dissolved in 400 µℓ of a solution containing 10 mM Tris-HCl (pH 7.5), 6 mM MgCl₂ and 60 mM NaCl, 20 units of Hin dIII were added, and the cleavage at the Hin dIII site was effected by performing incubation at 37°C for 2 hours. The reaction mixture was subjected to phenol-chloroform extraction and then to precipitation with ethanol, which gave 0.5 picomole of a (dC) chain-added cDNA-vector primer DNA. A 0.2-picomole portion of the DNA and 0.4 picomole of the linker DNA mentioned above were dissolved in 100 µℓ of a solution containing 10 mM Tris-HCl (pH 7.5), 0.1 M NaCl and 1 mM EDTA. Incubation was performed at 65°C for 10 minutes, then at 42°C for 25 minutes further at 0°C for 30 minutes. The reaction mixture was modified to give a total volume of 1,000 µℓ of a reaction mixture containing 20 mM Tris-HCl (pH 7.5), 4 mM MgCl₂, 10 mM (NH₄)₂SO₄, 0.1 M KCl and 0.1 mM β-NAD. To this reaction mixture were added 25 units of Escherichia coli -derived DNA ligase (New England Biolabs) and incubation was performed at 11°C for 18 hours. The reaction mixture was supplemented with dNTP (each to 40 µM) and β-NAD (to 0.15 mM), 10 units of Escherichia coli -derived DNA ligase, 20 units of Escherichia coli -derived polymerase I (P-L Biochemicals) and 10 units of Escherichia coli -derived ribonuclease H (P-L Biochemicals) were added, and incubation was performed at 12°C for 1 hour and then at 25°C for 1 hour. In the reactions mentioned above, the cDNA-containing recombinant DNA was circularized and the RNA portion off the RNA-DNA double strand was replaced by DNA, whereby a recombinant plasmid in the form of a complete, double-stranded DNA was formed. (3) Selection of a human t-PA cDNA-containing recombinant DNA: The t-PA cDNA was selected out, by colony hydridization, as a clone capable of associating with a ³²p-labeled synthetic DNA probe having the DNA sequence 5'-ATGGATGCAATGAAGAGAGGGCTCTGCTGT-3' in agreement with a part of the base sequence of the t-PA signal peptide region of the human t-PA cDNA [Pennica et al., Nature, 301 , 214 (1983)], as follows. First, Escherichia coli C600SF8 [Cameron, Proc. Natl. Acad. Sci. USA, 72 , 3416 (1975)] was transformed with the recombinant plasmid obtained above in section (2) by the method of Hanahan [J. Mol. Biol., 166 , 557 (1983)]. About 10,000 colonies obtained were immobilized on a nitrocellulose filter by the method of Hanahan and Meselson [Methods in Enzymology, 100 , 333 (1983)]. Then, filter prehydridization was performed in a solution containing 6 × NET [1 × NET = 150 mM NaCl, 15 mM Tris-HCl (pH 7.5), 1 mM EDTA], 10 × Denhardt solution and 100 µg/mℓ of fragmented Escherichia coli chromosome DNA at 65°C for at least 4 hours. The above-mentioned ³²P-labeled probe was added to this prehydridization solution and allowed to associate with DNA on the filter (65°C, at least 16 hours). Then, the filter was washed twice with 6 × SSC (1 × SSC = 150 mM NaCl, 15 mM sodium citrate) (room temperature, 5 minutes/wash) and then washed with a solution containing 2 × SSC and 0.1% SDS at 65°C for 30 minutes. After further washing with a solution containing 2 × SSC and 0.1% SDS at 65°C for 15 minutes, the filter was washed twice with 6 × SSC at room temperature (5 minutes per wash). The filter was air-dried and one positive clone was identified by autoradiography. The DNA sequence of the cDNA of the plasmid ptPA7 carried by this positive clone was determined by the dideoxy sequencing method using M13 phage. It was found that the cDNA of ptPA7 codes for t-PA completely identical in amino acid sequence to that t-PA reported by Pennica et al. [Nature, 301 , 214 (1983)]. It was revealed, however, that the codons for the 95th amino acid (aspartic acid) and 512th amino acid (threonine) of mature t-PA were GAT and ACC instead of GAC and ACA, respectively. This bacterial strain has been deposited with the Fermentation Research Institute under the designation Escherichia coli EtPA7 (FERM BP-1467) under the Budapest Treaty. REFERENCE EXAMPLE 2 Construction of the human pro-UK cDNA-carrying plasmid pUK1 : The Detroit 562 cell-derived cDNA library prepared in Reference Example 1 was screened by the technique of colony hydridization and a human pro-UK cDNA clone was isolated. Thus, the recombinant plasmid obtained in Reference Example 1 was used to transform Escherichia coli C600SF8 [Cameron, Proc. Natl. Acad. Sci. USA, 72 , 3416 (1975)] by the method of Hanahan [J. Mol. Biol., 166 , 557 (1983)]. About 30,000 colonies obtained were immobilized on a nitrocellulose filter by the method of Hanahan and Meselson [Methods in Enzymology, 100 , 333 (1983)]. Then, filter prehybridization was effected in a solution containing 6 × NET, 10 × Denhardt solution and 100 µg/mℓ of fragmented Escherichia coli chromosome DNA at 65°C for at least 4 hours. Then, a ³²P-labeled synthetic 41-base DNA probe identical in DNA sequence to a part of the kringle domain of human pro-UK cDNA [Holmes et al., Bio/Technology, 3 , 923 (1985)] and thus having the DNA sequence 5'-GGGAATGGTCACTTTTACCGAGGAAAGGCCAGCACTGACAC-3' (for the human pro-UK cDNA isolated by the present inventors, this corresponds to the underlined part of the base sequence shown in Table 4) was added to the above-mentioned prehybridization solution and allowed to associate with DNA on the filter (65°C, at least 16 hours). The filter was then washed twice with 6 × SSC (room temperature, 5 minutes per wash) and then with a solution containing 1 × SSC and 0.1% SDS at 57°C for 30 minutes. The filter was further washed with a solution containing 1 × SSC and 0.1% SDS at 57°C for 15 minutes and then twice with 6 × SSC (room temperature, 5 minutes per wash). The filter was air-dried and one positive clone was identified by autoradiography. The DNA sequence of the cDNA of the plasmid pUK1 carried by this positive clone was determined by the dideoxy method using M13 phage. It was found that the cDNA of pUK1 codes for the translational region for that portion of pro-UK downstream from the 41st amino acid residue (Cys) of pro-UK (according to the numbering of amino acid residues of pro-UK as used in Table 4) and for the 3'-nontranslational region. The amino acid sequence of pro-UK encoded by the cDNA of pUK1 was in agreement with that reported by Holmes et al. [Bio/Technology, 3 , 923 (1985)]. However, the codons for the following amino acids were different in the third base from those reported by Holmes et al.: 254th amino acid Asn: AAC → AAT, 340th amino acid Leu: CTA → CTG, 345th amino acid Pro: CCC → CCA, 346th amino acid Gln: CAA → CAG. The above bacterial strain has been deposited with the Fermentation Research Institute under the designation Escherichia coli EUK1 (FERM BP-1463) under the Budapest Treaty. REFERENCE EXAMPLE 3 Construction of the human pro-UK cDNA-carrying plasmid pUK11 : Since the pro-UK cDNA encoded by the plasmid pUK1 obtained in Reference Example 2 does not contain the signal region and growth factor domain region of pro-UK, a cDNA containing these regions was cloned as described below. First, a vector, pCCK2, for cDNA cloning was constructed as follows. (1) Construction of the recombinant plasmid pCCK1: Escherichia coli HB101 was transformed with the plasmid pRC19 constructed by Kuwano et al. [J. Biochem., 96 , 923-926 (1984)] and carrying a cDNA for rat brain cholecystokinin (CCK) precursor, was cultured and the pRC19 DNA was prepared from cultured cells in the conventional manner. About 3 µg of the pRC19 DNA obtained were dissolved in 30 µℓ of Y-50 buffer [solution containing 10 mM Tris-HCl (pH 7.5), 50 mM NaCl, 7 mM MgCl₂ and 7 mM 2-mercaptoethanol], 1 unit of Pvu II was added, and the digestion reaction was carried out at 37°C for 1 hour, whereby the DNA was partially digested by Pvu II. After 10 minutes of heat treatment at 65°C, a DNA fragment about 530 bp was purified using the AFT method. Separately, about 1 µg of the plasmid DNA pUC19 constructed by Norrander et al. [Norrander, J. et al., Gene, 26 , 101 (1983); available from Takara Shuzo] was dissolved in 30 µℓ of Y-0 buffer containing 20 mM KCl, 16 units of Sma I were added, and the digestion reaction was carried out at 30°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.7 kb was purified using the AFT method. The thus-obtained pRC19-derived DNA fragment (about 530 bp, about 0.01 µg) and the pUC19-derived DNA fragment (about 2.7 kb, about 0.05 µg) were dissolved in 20 µℓ of T4 ligase buffer, 200 units of T4 DNA ligase were added, and the ligation reaction was carried out at 4°C for 18 hours. The recombinant plasmid mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pCCK1 isolated from one of these transformants was subjected to structural analysis with restriction enzymes and found to have the desired structure (cf. Fig. 9). (2) Construction of the recombinant plasmid pCCK2: About 2 µg of the pCCK1 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of Sac I were added, and the digestion reaction was performed at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Bam HI were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.55 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 plasmid DNA obtained in Reference Example 4 to be given later herein were treated in the same manner as mentioned above and a Sac I- Bam HI fragment of about 2.85 kb was purified using the AFT method. The thus-obtained pCCK1-derived DNA fragment (about 0.55 kb, about 0.02 µg) and the pTrS33-derived DNA fragment (about 2.85 kb, about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The recombinant plasmid mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pCCK2 isolated from one of these transformants was subjected to structural analysis using restriction enzymes and found to have the desired structure (cf. Fig. 10). (3) Isolation of the human pro-UK cDNA-carrying plasmid pUK11: About 8 µg of the Detroit 562 cell-derived poly(A) RNA (mRNA) [dissolved in 7 µℓ of 10 mM Tris-HCl (pH 7.5)-0.5 mM EDTA] were heated at 65°C for 10 minutes and then quenched in ice. The composition of this solution was adjusted so that the resultant solution contained, in a total volume of 80 µℓ, 50 mM Tris-HCl (pH 8.3), 8 mM MgCl₂, 30 mM KCl, 5 mM DTT, 1 mM dNTP (dATP, dTTP, dGTP and dCTP), 10 units of ribonuclease inhibitor (P-L Biochemicals) and 5 µg/mℓ of oligo(dT)₁₂₋₁₈ (Collaborative Research). The solution was maintained at 41°C for 15 minutes. Then, 20 units of reverse transcriptase (Seikagaku Kogyo) were added and the mixture was maintained at 41°C for 90 minutes for the synthesis of a cDNA complementary to the mRNA. The reaction mixture was subjected to phenol-chloroform extraction. Addition of ethanol gave a precipitate, which was dissolved in 40 µℓ of 0.3 M NaOH. The solution was allowed to stand at 37°C for 15 hours for effecting the hydrolysis of mRNA. Then, the solution was neutralized by addition of 10 µℓ of 1 M Tris-HCl (pH 7.5) and 40 µℓ of 0.3 N HCl, and a single-stranded cDNA was recovered by precipitation with ethanol and dissolved in 28.5 µℓ of H₂O. The composition of this solution was adjusted so that the resultant solution contained, in a total volume of 40 µℓ, 50 mM Tris-HCl (pH 8.3), 8 mM MgCl₂, 30 mM KCl, 5 mM DTT, 1 mM dNTP (dATP, dTTP, dGTP and dCTP) and 2.5 µg/mℓ of the synthetic DNA primer CATGAGAGCCCTGCTGG (idential to a part of the base sequence for the signal peptide region of human pro-UK). The solution was maintained at 65°C for 10 minutes and then at 41°C for 30 minutes. Then, 10 units of reverse transcriptase were added, and the mixture was maintained at 41°C for 60 minutes for conversion of the single-standed cDNA to a double-stranded cDNA. The reaction mixture was subjected to phenol-chloroform extraction. Addition of ethanol gave a precipitate, which was dissolved in 30 µℓ of Y-0 buffer containing 25 mM NaCl, 25 units of Bss HII (New England Biolabs) were added to the solution, and the digestion reaction was conducted at 50°C for 2 hours. Furthermore, 1.25 µℓ of 2 M NaCl and 12 units of Bam HI were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a cDNA fragment of about 1.1 to 1.4 kb was purified using the AFT method. Separately, about 2 µg of the pCCK2 plasmid DNA obtained as described above were cleaved with Bss HII and Bam HI in the same manner as above, and a Bss HII- Bam HI fragment of about 2.9 kb was purified using the AFT method. The thus-obtained cDNA fragment (about 1.1 to 1.4 kb, about 0.02 µg) and pCCK2-derived DNA fragment (about 2.9 kb, about 0.05 µg) were dissolved in 20 µℓ of T4 ligase buffer, 200 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli C600SF8 to give about 25,000 Ap-resistant transformants. Among these Ap-resistant strains, about 1,000 positive clones were obtained by the colony hybridization method mentioned in Reference Example 2, which were capable of associating with the same probe as used for isolating the pro-UK cDNA in Reference Example 2. The hybridization conditions and filter washing conditions were the same as employed in Reference Example 2. The plasmid pUKll (cf. Fig. 11) isolated from one of the positive clones thus obtained was subjected to base sequence determination for the pro-UK signal peptide, growth factor domain and kringle domain regions by the dideoxy method using Ml3 phage. The DNA sequence thus found was in agreement with those reported by Holmes [Bio/Technology, 3 , 923 (1985)]. REFERENCE EXAMPLE 4 Construction of the recombinant plasmid pTrS33 : (1) Construction of the ATG vector pTrS20: According to the scheme shown in Fig. 12, the ATG vector pTrS20 in which the SD sequence is 14 bases apart from the initiation codon ATG and which has a Sac I site just behind the ATG codon was constructed. First, 3 µg of pKYP10 prepared as described in U.S. Patent No. 4,686,191 were dissolved in 30 µℓ of Y-100 buffer, 6 units of the restriction enzyme Ban III and 6 units of the restriction enzyme Nru I (New England Biolabs) were added, and the cleavage reaction was conducted at 37°C for 3 hours. From the reaction mixture, there were recovered about 0.5 µg of a Ptrp-containing DNA fragment ( Ban III- Nru I fragment) of about 3.8 kb by the LGT method. Separately, the following DNA linker was synthesized by the phosphotriester method for providing the initiation codon ATG downstream from Ptrp. The synthetic 19-mer and 17-mer DNAs (10 picomoles each) were dissolved in a total volume of 20 µℓ of a solution containing 50 mM Tris-HCl (pH 7.5), 10 mM MgCl₂, 5 mM dithiothreitol, 0.1 mM EDTA and 1 mM ATP, 3 units of T4 polynucleotlde kinase (Takara Shuzo) were added, and the phosphorylatlon reaction was conducted at 37°C for 60 minutes. Then, 0.1 µℓ of the pKYP10-derived Ban III- Nru I fragment (about 3.8 kb) obtained as described above and about 0.5 picomole of the DNA linker mentioned above were dissolved in 20 µℓ of T4 ligase buffer, 2 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The recombinant plasmid mixture obtained was used to transform Escherichia coli HB101 [Bolivar et al., Gene, 2 , 75 (1977)] to give Ap-resistant colonies. Each colony was cultured and a plasmid DNA was recovered from cultured cells and examined for its structure by cleavage with the restriction enzymes Eco RI, Ban III, Hin dIII, Sac I and Nru I, followed by agarose gel electrophoresis. A plasmid found to have the desired structure was named pTrS20 (cf. Fig. 12). It was confirmed by the dideoxy sequencing method using M13 phage that pTrS20 had the following DNA sequence in the neighborhood of the Ban III and Hin dIIII sites (2) Construction of pTrS33: About 3 µg of the pTrS20 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of Sac I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Pst I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified by the AFT method. Separately, 2 µg of pKYP26 prepared as described in EP-A-0214555 [an Escherichia coli strain harboring pKYP26 has been deposited with the Fermentation Research Institute under the designation Escherichia coli IKYP26 (FERM BP-863) under the Budapest Treaty] were dissolved in 30 µℓ of Y-100 buffer, 8 units of Pst I and 10 units of Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.7 kb was purified by the AFT method. Further, separately, 2 µg of the M13mp18 RF DNA [Norrander, J. et al., Gene, 26 , 101 (1983); obtained from Takara Shuzo] were dissolved in 30 µℓ of Y-0 buffer, 10 units of Sac I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 1 M NaCl and 10 units of Cla I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.65 kb was purified by the AFT method. Further, separately, the following two synthetic DNAs (43 bases and 45 bases) (also shown in Fig. 13) were synthesized using an Applied Biosystems model 380A DNA synthesizer: They were separately 5'-phosphorylated in the same manner as described hereinbefore. The thus-obtained pTrS20-derived DNA fragment (about 1.15 kb, about 0.1 µg), pKYP26-derived DNA fragment (about 1.7 kb, about 0.1 µg), M13mp18-derived DNA fragment (about 0.65 kb, about 0.05 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The recombinant plasmid mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid pTrS33 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequencing by the dideoxy method using M13 phage (cf. Fig. 13). REFERENCE EXAMPLE 5 Construction of the recombinant plasmid pTerm2 : About 2 µg of the pKYP26 plasmid DNA (EP-A-0214555) were dissolved in 30 µℓ of a solution containing 10 mM Tris-HCl (pH 8.0), 75 mM NaCl, 7 mM MgCl₂ and 6 mM 2-mercaptoethanol, 16 units of Asp 718 (Boehringer-Mannheim) and 10 units of Pst I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.7 kb was purified using the AFT method. Separately, about 2 µg of the pTrS20 plasmid DNA obtained in Reference Example 4-(1) were dissolved in 30 µℓ of Y-100 buffer, 8 units of Pst I and 10 units of Nru I (Boehringer-Mannheim) were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.5 kb was purified using the AFT method. Further, separately, the two synthetic DNAs (19 bases and 23 bases) shown in Fig. 14 were synthesized using an Applied Biosystems model 380A DNA synthesizer and respectively 5'-phosphorylated by the same method as mentioned hereinabove. The thus-obtained pKYP26-derived DNA fragment (about 1.7 kb, about 0.1 µg), pTrS20-derived DNA fragment (about 1.15 kb) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTerm2 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequencing by the dideoxy method using M13 phage (cf. Fig. 14). REFERENCE EXAMPLE 6 Construction of the recombinant plasmid pTkSS4 : (1) Construction of the recombinant plasmid pTSF10: Escherichia coli C600SF8 transformed with the human t-PA cDNA-containing plasmid ptPA7 obtained in Reference Example 1 was cultured and the ptPA7 DNA was prepared from cultured cells by a conventional method. About 2 µg of the ptPA7 DNA obtained were dissolved in 30 µℓ of Y-100 buffer, 8 units of Bgl II were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.0 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 DNA (Reference Example 4) were dissolved in 30 µℓ of Y-100 buffer, 10 units of the restriction enzyme Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.8 kb was purified using the AFT method. The thus-obtained ptPA7-derived DNA fragment (about 2.0 kb, about 0.1 µg) and pTrS33-derived DNA fragment (about 2.8 kb, about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTSF10 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 15). (2) Construction of the recombinant plasmid pTA4: About 3 µg of the pTSF10 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 3 M NaCl and 12 units of Bst EII (New England Biolabs) were added, and the digestion reaction was further conducted at 60°C for 2 hours. Then, a DNA fragment of about 0.3 kb was purified using the AFT method. Separately, Escherichia coli IGHA2 (deposited with the Fermentation Research Institute under the deposit number FERM BP-400) was cultured and the pGHA2 plasmid DNA (U.S. Patent No. 4,868,125) was prepared from cultured cells by a conventional method. About 2 µg of the pGHA2 DNA obtained were dissolved in 30 µℓ of Y-100 buffer, 8 units of Pst I and 8 units of Bgl II were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Further, separately, about 3 µg of the ptPA7 DNA (Reference Example 1) were dissolved in 30 µℓ of a solution containing 20 mM Tris-HCl (pH 7.6), 150 mM NaCl, 7 mM MgCl₂ and 6 mM 2-mercaptoethanol (hereinafter referred to as "Y-150 buffer"), 10 units of Bgl II were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 12 units of Bst EII were added and the digestion reaction was further conducted at 60°C for 2 hours. A DNA fragment of about 1.55 kb was purified from the reaction mixture using the AFT method. Further, separately, about 2 µg of the pTerm2 DNA obtained in Reference Example 5 were digested in 30 µℓ of Y-0 buffer in the presence of 12 units of Kpn I at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 8 units of Pst I were added, and the digestion reaction was further conducted at 37°C for 2 hours. Thereafter, a DNA fragment of about 1.7 kb was purified using the AFT method. The thus-obtained four DNA fragments (0.03 µg of the pTSF10-derived fragment, 0.05 µg of the pGHA2-derived fragment, 0.1 µg of the ptPA7-derived fragment and 0.1 µg of the pTerm-2-derived fragment) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTA4 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 16). (3) Construction of the recombinant plasmid pTkSD217: About 10 µg of the pTA4 plasmid DNA obtained as described above were dissolved in 100 µl of Y-100 buffer, 30 units of Xho I were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, the product DNA fragment was recovered by precipitation with ethanol and dissolved in 50 µℓ of TE buffer [10 mM Tris-HCl (pH 7.5), 0.5 mM EDTA]. To 10 µℓ of this DNA solution were added 10 µℓ of 5 times concentrated BAL31 buffer [100 mM Tris-HCl (pH 8.0), 3 M NaCl, 60 mM CaCl₂, 60 mM MgCl₂, 5 mM EDTA], 30 µℓ of water and 0.5 units of the exonuclease BAL31 (Takara Shuzo), and the reaction was conducted at 30°C for 5 minutes. These are the conditions under which DNA can be pared off by about 0.5 kb from the Xho I terminus. The reaction was terminated by extraction with phenol. After further extraction with chloroform, the product DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 30 µℓ of Y-100 buffer, 10 units of Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.5 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 plasmid DNA (Reference Example 4) were digested in 30 µℓ of Y-0 buffer in the presence of about 12 units of Sac I at 37°C for 2 hours. After phenol extraction and chloroform extraction, the product DNA fragment was recovered by precipitation with ethanol and dissolved in a total volume of 40 µℓ of a buffer containing 50 mM Tris-HCl (pH 7.8), 7 mM MgCl₂, 6 mM 2-mercaptoethanol, 0.25 mM dATP, 0.25 mM dCTP, 0.25 mM dGTP and 0.25 mM dTTP (hereinafter referred to as "polymerase buffer"), 6 units of Klenow fragment (Klenow Pol I) (Takara Shuzo) were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Sac I protruding ends to blunt ends. The reaction was terminated by extraction with phenol. After chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 30 µℓ of Y-100 buffer, 10 units of Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.8 kb was purified using the AFT method. The thus-obtained pTA4-derived DNA fragment (about 1.5 kb, about 0.2 µg) and pTrS33-derived DNA fragment (about 2.8 kb, about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTkSD217 isolated from one of these transformants was subjected to structural analysis comprising digestion with restriction enzymes and its DNA sequence downstream from the Escherichia coli tryptophan promoter (Ptrp) was determined by the dideoxy sequence method using M13 phage [J. Messing et al., Gene , 19 , 269 (1985)]. As a result, it was confirmed that pTkSD217 had the desired structure and that the DNA sequence was as follows (cf. Fig. 17). (4) Construction of the recombinant plasmid pTkSL11: About 3 µg of were pTkSD217 plasmid DNA obtained as described above was dissolved in 30 µℓ of Y-100 buffer, 10 units of Hin dIII and 15 units of Sca I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.23 kb was purified using the AFT method. Separately, about 2 µg of the pTerm2 plasmid DNA obtained in Reference Example 5 were dissolved in 30 µℓ of Y-100 buffer, 10 units of Hin dIII and 10 units of Nsi I (New England Biolabs) were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.8 kb was purified using the AFT method. Furthermore, the following two synthetic DNAs (35 bases and 31 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer and respectively 5'-phosphorylated by the method mentioned hereinbefore. The thus-obtained pTkSD217-derived DNA fragment (about 0.23 kb, about 0.01 µg), pTerm2-derived DNA fragment (about 2.8 kb, about 0.1 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTkSL11 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the M13 dideoxy sequence method (cf. Fig. 18). (5) Construction of the recombinant plasmid pTkSS4: About 2 µg of the ptPA7 plasmid DNA obtained in Reference Example 1 were dissolved in 30 µℓ of Y-100 buffer, 12 units of the restriction enzyme Sca I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.0 kb was purified using the AFT method. Separately, about 2 µg of the pTkSL11 plasmid DNA obtained as described above subjected to the same reaction as above and, after 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.0 kb was purified using the AFT method. The thus-obtained ptPA7-derived DNA fragment (about 2.0 kb, about 0.1 µg) and pTkSL11-derived DNA fragment (about 2.0 kb, about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTkSS4 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 19). REFERENCE EXAMPLE 7 Construction of the recombinant plasmid pTkSR18 : (1) Construction of the recombinant plasmid pTkSJ1: About 2 µg of the pTA4 plasmid obtained in Reference Example 6-(2) were dissolved in 30 µℓ of Y-0 buffer, 10 units of Eco RI and 30 units of Bbe I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a Bbe I- Eco RI fragment of about 2.8 kb was purified using the AFT method. Separately, about 3 µg of the pTA4 DNA were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 1 unit of Eco RI were added and the digestion reaction was further conducted at 37°C for 1 hour. Under these reaction conditions, the digestion of the DNA with Kpn I was complete and that with Eco RI was partial. After 10 minutes of heat treatment at 65°C, an Eco RI- Kpn I fragment of about 1.4 kb was purified using the AFT method. Separately, the following two synthetic DNAs (16 bases and 24 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer and respectively 5'-phosphorylated in the same manner as mentioned hereinabove. The thus-obtained pTA4-derived Bbe I- Eco RI fragment (about 2.8 kb, about 0.1 µg), pTA4-derived Eco RI- Kpn I fragment (about 1.4 kb, about 0.05 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTkSJ1 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequencing by the dideoxy method using M13 phage (cf. Fig. 20). (2) Construction of the recombinant plasmid pTkSR18: About 3 µg of the pTkSJ1 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-100 buffer, 10 units of Xho I and 15 units of Sca I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.5 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 plasmid DNA obtained in Reference Example 4 were dissolved in Y-0 buffer containing 150 mM KCl, 8 units of Pvu I and 15 units of Sal I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes heat treatment at 65°C, a DNA fragment of about 1.0 kb was purified using the AFT method. Further, separately, about 2 µg of the pTerm2 plasmid DNA obtained in Reference Example 5 were dissolved in 30 µℓ of Y-150 buffer, 8 units of Pvu I and 8 units of Nsi I (New England Biolabs) were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.85 kb was purified using the AFT method. Furthermore, the following two synthetic DNAs (35 bases and 31 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer and respectively 5'-phosphorylated by the same method as mentioned above. The thus-obtained pTkSJ1-derived Xho I- Sca I fragment (about 0.5 kb, about 0.05 µg), and pTrS33-derived Pvu I- Sal I fragment (about 1.0 kb, about 0.1 µg), pTerm2-derived NsiI-PvuI fragment (about 1.85 kg, about 0.1 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA·pTkSR18 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequencing by the dideoxy method using M13 phage (cf. Fig. 21). REFERENCE EXAMPLE 8 Construction of the recombinant plasmid pUKB101: (1) Construction of the recombinant plasmid pUKA2: The pUK1 DNA was prepared from an Escherichia coli C600SF8 transformant strain harboring the human pro-UK cDNA-containing plasmid pUK1 obtained in Reference Example 2. About 2 µg of the pUK1 DNA obtained were dissolved in 30 µℓ of Y-100 buffer, 8 units of the restriction enzyme Nco I and 8 units of the restriction enzyme Stu I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.2 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 plasmid DNA obtained in Reference Example 4 were dissolved in 30 µℓ of a solution containing 10 mM Tris-HCl (pH 7.5), 25 mM KCl, 7 mM MgCl₂ and 6 mM 2-mercaptoethanol (hereinafter referred to as "K-25 buffer"), 16 units of the restriction enzyme Sma I were added, and the digestion reaction was conducted at 30°C for 2 hours. Then, 1.5 µℓ of 1 M NaCl and 10 units of Nco I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.85 kb was purified using the AFT method. The thus-obtained pUK1-derived DNA fragment (about 1.2 kb, about 0.05 µg) and pTrS33-derived DNA fragment (about 2.85 kb, about 0.1 µg) were dissolved in 20 µℓ of a buffer containing 20 mM Tris-HCl (pH 7.6), 10 mM MgCl₂, 10 mM dithiothreitol (DTT) and 1 mM ATP, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give ampicillin Ap-resistant transformants. The plasmid DNA pUKA2 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 22). (2) Construction of the recombinant plasmid pUKB101: About 2 µg of the pUKA2 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of the restriction enzyme Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Nco I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.2 kb was purified using the AFT method. Separately, about 2 µg of the pTrS33 plasmid DNA obtained in Reference Example 4 were dissolved in 30 µℓ of K-25 buffer, 16 units of Sma I were added, and the digestion reaction was conducted at 30°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Pst I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified using the AFT method. Separately, about 2 µg of the pTerm2 plasmid DNA obtained in Reference Example 5 were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Pst I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.7 kb was purified using the AFT method. Furthermore, the following two synthetic DNAs (41 bases and 45 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer. These synthetic DNAs (20 picomoles each) were individually phosphorylated at the 5' end by conducting the reaction in 20 µℓ of T4 kinase buffer in the presence of 5 units of T4 DNA kinase at 37°C for 30 minutes. The thus-obtained pUKA2-derived DNA fragment (about 1.2 kb, about 0.05 µg), pTrS33-derived DNA fragment (about 1.15 kb, about 0.05 µg), pTerm2-derived DNA fragment (about 1.7 kb, about 0.05 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pUKB101 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the M13 dideoxy sequence method (cf. Fig. 22). REFERENCE EXAMPLE 9 Construction of the recombinant plasmid pTG3 : About 2 µg of the pTkSS4 plasmid DNA obtained in Reference Example 6 were dissolved in 30 µℓ of Y-0 buffer, 10 units of the restriction enzyme Nar I (New England Biolabs) were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.0 µℓ of 2 M NaCl and 12 units of Bam HI were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.3 kb was purified using the AFT method. Separately, about 3 µg of the pTkSR18 plasmid DNA obtained in Reference Example 7 were subjected to the same reaction as above and, after 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.2 kb was purified using the AFT method. The thus-obtained pTkSS4-derived DNA fragment (about 3.3 kb, about 0.1 µg) and pTkSR18-derived DNA fragment (about 0.2 kb, about 0.01 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pTG3 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 24). REFERENCE EXAMPLE 10 Construction of the recombinant plasmid phPA2 : About 2 µg of the pTG3 plasmid DNA obtained in Reference Example 9 were dissolved in 30 µℓ of Y-100 buffer, 10 units each of Eco RI and Pvu I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.7 kb was purified using the AFT method. Separately, about 2 µg of the pUKB101 plasmid DNA obtained in Reference Example 8 were dissolved in 30 µℓ of Y-100 buffer, 10 units each of Nco I and Pvu I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.0 kb was purified using the AFT method. Further, separately, about 2 µg of the pTkSR18 plasmid DNA obtained in Reference Example 7 were dissolved in 30 µℓ of Y-100 buffer, 10 units each of Hin dIII and Aat II were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.55 kb was purified using the AFT method. Furthermore, the following four synthetic DNAs (37 bases and 41 bases as well as 41 bases and 45 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer. These synthetic DNAs (20 picomoles each) were individually phosphorylated at the 5' end by conducting the reaction in 20 µℓ of T4 kinase buffer in the presence of 5 units of T4 DNA kinase at 37°C for 30 minutes. The thus-obtained pTG3-derived DNA fragment (about 1.7 kb, about 0.05 µg), pUK101-derived DNA fragment (about 3.0 kb, about 0.05 µg), pTkSR18-derived DNA fragment (about 0.55 kb, about 0.05 µg) and four 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA phPA2 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the Ml3 dideoxy sequencing method (cf. Fig. 25). REFERENCE EXAMPLE 11 Construction of the recombinant plasmid pUKS1 coding for the pro-UK derivative UK-S1 : (1) Construction of a single-stranded template DNA (single-stranded pUKmpS1): About 3 µg of the pUK1 obtained in Reference Example 2 were dissolved in 30 µℓ of Y-100 buffer, 10 units each of the restriction enzyme Pst I and Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a 890-bp Pst I- Bam HI DNA fragment was purified using the AFT method [Bio/Technology, 2 , 66-67 (1984)]. Separately, about 1 µg of the M13 phage vector M13mp18 RF DNA (Takara Shuzo) was dissolved in a total of 30 µℓ of Y-100 buffer, 10 units each of the restriction enzymes Pst I and Bam HI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 7.2 kb was purified using the AFT method. The thus-obtained pUK1-derived DNA fragment (890 bp) and M13mp18RF-derived DNA fragment (about 7.2 kb) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The above reaction mixture was used to transfect Escherichia coli JM105 by a known method [Messing et al., Methods in Enzymology, 101 , 20 (1983)] to give a recombinant phage. Then, Escherichia coli JM105 was infected with this recombinant phage by the Messing et al. method. A single-stranded phage DNA was recovered from the culture supernatant, while a double-stranded phage DNA was recovered from cultured cells in the same manner as in recovering plasmid DNAs. The structure of this double-stranded phage DNA (pUKmpS1) was confirmed by digestion with restriction enzymes (cf. Fig. 26). (2) Mutagenesis in the UK cDNA using an oligonucleotide: (a) Preparation of a synthetic DNA for mutagenesis and phosphorylation thereof: For producing a UK derivative in which the 164th amino acid residue is Asn in lieu of Phe in UK and which has a carbohydrate chain added thereto (hereinafter this derivative is referred to as "UK-S1"), a 17-base synthetic DNA, 5'-GGGGAGAAAACACCACC-3', was synthesized using an Applied Biosystems model 380A DNA synthesizer. Then, 25 picomoles of the thus-obtained synthetic DNA were phosphorylated at the 5' end by conducting the reaction in 10 µℓ of T4 kinase buffer in the presence of 5 units of T4 DNA kinase (Takara Shuzo) at 37°C for 30 minutes. (b) Site-directed mutagenesis using two oligonucleotide primers: A solution prepared by mixing 6.5 µℓ of a solution containing the single-stranded recombinant phage DNA (about 2 µg) obtained as described above, 1 µℓ of 10-fold concentrated polymerase buffer [containing 500 mM Tris-HCl (pH 7.8), 70 mM MgCl₂, 60 mM 2-mercaptoethanol, 0.25 mM dATP, 0.25 mM dCTP, 0.25 mM dGTP and 0.25 mM dTTP] and 2 µℓ of a solution containing the synthetic DNA (2.5 picomoles) for mutagenesis obtained as described above were allowed to stand at 65°C for 5 minutes, at 55°C for 5 minutes, at 37°C for 10 minutes and then at 25°C for 10 minutes. Then, 3 units of Escherichia coli DNA polymerase I Klenow fragment (Takara Shuzo) (hereinafter referred to as "Klenow fragment" for short) were added, and the reaction was conducted at 25°C for 30 minutes. Then to the reaction mixture were added 1 µℓ of 10-fold concentrated polymerase buffer, 6 µℓ of 0.5 picomole/µℓ M13 primer M4 (Takara Shuzo) and 3 units of Klenow fragment. After conducting the reaction at 37°C for 10 minutes and then at 25°C for 40 minutes, 2 µℓ of 10 mM ATP and 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 11°C for 18 hours. The reaction mixture was subjected to phenol extraction and chloroform extraction, and a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 30 µℓ of Y-100 buffer, 12 units of Eco RI and 12 units of Pst I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a Pst I- Eco RI fragment (about 600 bp) was purified using the AFT method. (c) Insertion of the mutant DNA fragment into a vector: About 3 µg of the pUK11 plasmid DNA obtained in Reference Example 3 were dissolved in 30 µℓ of Y-50 buffer, 10 units of Aat II (Toyobo) and 8 units of Pst I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, an Aat II- Pst I fragment of about 1.0 kb was purified using the AFT method. Separately, about 3 µg of the pUKB101 plasmid DNA obtained in Reference Example 8 were dissolved in 30 µℓ of Y-50 buffer, 10 units of Aat II and 10 units of Eco RI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, an Aat II- Eco RI fragment of about 2.9 kb was purified using the AFT method. The thus-obtained pUK11-derived Aat II- Pst I fragment (about 0.05 µg) and pUKB101-derived Aat II- Eco RI fragment (about 0.1 µg) were dissolved, together with the mutated Pst I- Eco RI fragment (about 600 bp), in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli C600SF8 [Proc. Natl. Acad. Sci. USA, 72 , 3416 (1975)] to give Ap resistant transformants. A recombinant plasmid, pUKS1, capable of hybridizing with a probe prepared by radiolabeling the above-mentioned synthetic DNA for mutagenesis with ³²P at the 5' end was isolated from one of the transformants using the technique of colony hybridization. Structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the dideoxy method using M13 phage confirmed that pUKS1 had the desired structure (cf. Fig. 27). REFERENCE EXAMPLE 12 Construction of the t-PA expression plasmid pSE1PA1-9A : (1) Construction of the recombinant plasmid pAGE105M: An Escherichia coli HB101 transformant harboring the plasmid pAGE28 constructed by the present inventors [Mizukami et al., J. Biochem., 101 , 1307-1310 (1987)] was cultured and the pAGE28 DNA was prepared from cultured cells by a conventional method. About 2 µg of the pAGE28 DNA obtained were dissolved in 30 µℓ of Y-100 buffer, 8 units of Xho I and 12 units of Sca I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.8 kb was purified using the AFT method. Separately, an Escherichia coli HB101 transformant harboring the plasmid pAGE103 constructed by the present inventors (Mizukami et al., J. Biochem., 101 , 1307-1310 (1987)] was cultured and the pAGE103 DNA was prepared from cultured cells in a conventional manner. About 3 µg of the pAGE103 DNA obtained were dissolved in 30 µℓ of Y-100 buffer, 10 units of Eco RI were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Eco RI protruding ends to blunt ends. The reaction was terminated by phenol extraction and, after chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 30 µℓ of Y-100 buffer, 12 units of Xho I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.4 kb was purified using the AFT method. Separately, an Escherichia coli transformant harboring the plasmid pKCR constructed by O'Hara et al. [Proc. Natl. Acad. Sci. USA, 78 , 1527 (1981)] was cultured and the pKCR DNA was prepared from cultured cells by the conventional method. About 2 µg of the pKCR DNA obtained were dissolved in 30 µℓ of Y-150 buffer, 12 units of Bam HI and 16 units of Sal I were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour for the conversion of the Bam HI and Sal I protruding ends to blunt ends. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.85 kb was purified using the AFT method. The thus-obtained pAGE28-derived DNA fragment (about 2.8 kb, about 0.05 µg), pAGE103-derived DNA fragment (about 0.4 kb, about 0.03 µg) and pKCR-derived DNA fragment (about 1.85 kb, about 0.2 µg) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The thus-obtained reaction mixture was used to transform Escherichia coli MM294 to give kanamycin (hereinafter referred to as "Km")-resistant transformants. The plasmid pAGE105M isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 28). (2) Construction of the recombinant plasmid pAGE106: About 2 µg of the pAGE105M DNA obtained as described above were dissolved in 30 µℓ of Y-100 buffer, 12 units of Sca I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 5.0 kb was purified using the AFT method. Separately, 20 picomoles of the Eco RI linker GGAATTCC (Collaborative Research) was 5'-phosphorylated in the same manner as mentioned hereinbefore. The thus-obtained pAGE105M-derived DNA fragment (about 5.0 kb, about 0.1 µg), 5'-phosphorylated Eco RI linker (1 picomole) were dissolved in 20 µℓ T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The thus-obtained reaction mixture was used to transform Escherichia coli MM294 to give Km-resistant transformants. The plasmid DNA pAGE106 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 29). (3) Construction of the t-PA expression plasmid pSE1PA1-5: About 2 µg of the pAGE106 DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Bam HI were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 5.0 kb was purified using the AFT method. Separately, about 3 µg of the ptPA7 plasmid DNA obtained in Reference Example 1 were dissolved in 30 µℓ of Y-0 buffer containing 75 mM NaCl, 12 units of Fok I and 12 units of Eco RI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.7 kb was purified using the AFT method. Further, separately, about 3 µg of the pTA4 DNA were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 1 unit of Eco RI were added, and the digestion reaction was further conducted at 37°C for 1 hour. Under the above conditions, the digestion of the DNA with Kpn I was complete and that with Eco RI was partial. After 10 minutes of heat treatment at 65°C, and Eco RI- Kpn I fragment of about 1.4 kb was purified using the AFT method. Furthermore, the following two synthetic DNAs (each having 21 bases) were synthesized using an Applied Biosystems model 380A DNA synthesizer and individually 5'-phosphorylated by the same method as mentioned before. The thus-obtained pAGE106-derived DNA fragment (about 5.0 kb, about 0.1 µg), ptPA7-derived DNA fragment (about0.7 kb, about 0.1 µg), pTA4-derived Eco RI- Kpn I fragment (about 1.4 kb, about 0.05 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The thus-obtained reaction mixture was used to transform Escherichia coli MM294 to give Km-resistant transformants. The plasmid DNA pSE1PA1-5 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the dideoxy method using M13 phage (cf. Fig. 30). (4) Construction of the t-PA expression plasmid pSE1PA1-9: About 2 µg of the pSE1PA1-5 DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 8 units of Hin dIII were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 5.0 kb was purified using the AFT method. Separately, about 2 µg of the pSE1PA1-5 DNA were dissolved in 30 µℓ of Y-0 buffer, 12 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.0 µℓ of 2 M NaCl and 10 units of Nco I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 4.9 kb was purified using the AFT method. Furthermore, the following four synthetic DNAs (47 bases, 49 bases, 49 bases and 47 bases; these synthetic DNAs constituting a part of the 5'-nontranslational region of the t-PA cDNA) were synthesized using an Applied Biosystems model 380A DNA synthesizer and individually 5'-phosphorylated in the same manner as mentioned above. The thus-obtained pSE1PA1-5-derived 5.0 kb DNA fragment (about 0.1 µg), pSE1PA1-5-derived 4.9 kb DNA fragment (about 0.1 µg), and four 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The thus-obtained reaction mixture was used to transform Escherichia coli MM294 to give Km-resistant transformants. The plasmid DNA pSE1PA1-9 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the dideoxy method using M13 phage (cf. Fig. 31). (5) Construction of the recombinant plasmid pUC19H (for rendering the Ap resistance gene portable): About 2 µg of the plasmid DNA pUC19 constructed by Norrander et al. [Norrander, J. et al., Gene, 26 , 101 (1983); available from Takara Shuzo] were dissolved in 30 µℓ of Y-50 buffer, 10 units of Hin dIII and 1 unit of Dra I were added, and the digestion reaction was conducted at 37°C for 1 hour. Under these conditions, the digestion of the DNA with Hin dIII was complete while that with Dra I was partial. After 10 minutes of heat treatment at 65°C, two DNA fragments, namely a Hin dIII- Dra I fragment of about 1.55 kb and a Dra I- Hin dIII fragment of about 1.1 kb, were purified using the AFT method. Separately, 20 picomoles of the Hin dIII linker CAAGCTTG (Collaborative Research) was 5'-phosphorylated by the same method as mentioned hereinabove. The thus-obtained pUC19-derived DNA fragments (about 1.55 kb and about 1.1 kb), and 5'-phosphorylated Hin dIII linker (2 picomoles) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pUC19H isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 32). (6) Construction of the recombinant plasmid pSE1PA1-9A (Insertion of Ap-resistance gene into pSE1PA1-9): About 2 µg of the pUC19H plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-50 buffer, 8 units of Hin dIII were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour for converting the Hin dIII protruding ends to blunt ends. The reaction was terminated by phenol extraction and, after chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 30 µℓ of Y-50 buffer, 8 units of Pvu II were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.4 kb was purified using the AFT method. Separately, about 2 µg of the t-PA expression plasmid pSE1PA1-9 obtained as described above were dissolved in 30 µℓ of Y-150 buffer, 8 units of Xho I and 8 units of Eco RV were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 5.9 kb was purified using the AFT method. Furthermore, about 3 µg of the pAGE28 plasmid DNA prepared as described hereinbefore were dissolved in 30 µℓ of Y-l50 buffer, 10 units of Xho I and 10 units of Eco RV were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.85 kb was purified using the AFT method. The thus-obtained pUC19H-derived DNA fragment (about 1.4 kb, about 0.1 µg), pSE1PA1-9-derived DNA fragment (about 5.9 kb, about 0.1 µg) and pAGE128-derived DNA fragment (about 0.85 kb, about 0.05 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give transformants resistant to both Ap and Km. The plasmid DNA pSE1PA1-9A isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 33). REFERENCE EXAMPLE 13 Construction of the human pro-UK expression plasmid pSEUK1-1A ; (1) Construction of the recombinant plasmid pUKF2: About 3 µg of the pUK11 plasmid DNA obtained in Reference Example 3 were dissolved in 30 µℓ of Y-100 buffer, 12 units of Nco I and 12 units of Hin dIII were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.45 kb was purified using the AFT method. Separately, about 2 µg of the pUKA2 plasmid DNA obtained in Reference Example 8-(1) were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Nco I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.2 kb was purified using the AFT method. Furthermore, about 2 µg of the pTerm2 plasmid DNA obtained in Reference Example 5 were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 8 units of Hin dIII were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.85 kb was purified using the AFT method. The thus-obtained pUK11-derived DNA fragment (about 0.45 kb, about 0.02 µg), pUKA2-derived DNA fragment (about 1.2 kb, about 0.05 µg) and pTerm2-derived DNA fragment (about 2.85 kb, about 0.05 µg) were dissolved in 20 µℓ of T4 ligase buffer, 50 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pUKF2 isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 34). (2) Construction of the recombinant plasmid pUKFpro: About 2 µg of the pUKF2 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer containing 25 mM NaCl, 10 units of Bss HII (New England Biolabs) were added, and the digestion reaction was conducted at 50°C for 2 hours. Then, 1.0 µℓ of 2 M NaCl and 10 units of Hin dIII were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 4.3 kb was purified using the AFT method. Separately, the following six synthetic DNAs (39 bases, 41 bases, 41 bases, 39 bases, 17 bases and 17 bases) were synthesized and respectively 5'-phosphorylated following the procedure mentioned above. The thus-obtained pUKF2-derived DNA fragment (about 4.3 kb, about 0.1 µg) and six 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pUKFpro isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes and DNA sequence determination by the M13 dideoxy method (cf. Fig. 35). (3) Construction of the recombinant plasmid pSEUK1-1A: About 2 µg of the pSE1PA1-9A plasmid DNA obtained in Reference Example 12 were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Hin dIII were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 6.3 kb was purified using the AFT method. Separately, about 3 µg of the pUKFpro plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 15 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Hin dIII were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.55 kb was purified using the AFT method. The thus-obtained pSE1PA1-9A-derived DNA fragment (about 6.3 kb, about 0.1 µg) and pUKFpro-derived DNA fragment (about 1.55 kb, about 0.05 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSEUK1-1A isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 36). REFERENCE EXAMPLE 14 Construction of the t-PA expression plasmid pSPAS1-9A ; (1) Construction of the recombinant plasmid psE1dhfr1A: About 2 µg of the pAGE106 plasmid DNA obtained in Reference Example 12-(2) were dissolved in 30 µℓ of Y-50 buffer, 12 units of Asp 718 (Boehringer-Mannheim) were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Asp 718 protruding ends to blunt ends. Then, after phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol and dissolved in a total of 30 µℓ of Y-150 buffer, 10 units of Mlu I was added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.3 kb was purified using the AFT method. Separately, about 3 µg of the dhfr gene-containing pSV2dhfr plasmid DNA [Subramani et al., Mol. Cell. Biol. 1 , 854 (1981)] were dissolved in 30 µℓ of Y-100 buffer, 12 units of Bgl II were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in a total of 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Bgl II protruding ends to blunt ends. Then, after phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol and dissolved in a total of 30 µℓ of Y-100 buffer, 12 units of Hin dIII were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Furthermore, about 3 µg of the pSE1PA1-9A plasmid DNA obtained in reference Example 12 were dissolved in 30 µℓ of Y-100 buffer, 12 units of Hin dIII were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 1 M NaCl and 12 units of Mlu I were added, and the digestion reaction was further conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 2.9 kb was purified using the AFT method. The thus-obtained pAGE106-derived DNA fragment (about 0.1 µg), pSV2dhfr-derived DNA fragment (about 0.03 µg) and pSE1PA1-9A-derived DNA fragment (about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSE1dhfr1A isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 37). (2) Construction of the recombinant plasmid pSPAS1-9A: About 3 µg of the pSE1dhfr1A plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-100 buffer, 12 units of Xho I were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Xho I protruding ends to blunt ends. Then, after phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol and dissolved in 30 µℓ of Y-150 buffer, 12 units of Mlu I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 4.4 kb was purified using the AFT method. Separately, about 3 µg of the pSE1PA1-9A plasmid DNA obtained in reference Example 12 were dissolved in 30 µℓ of Y-50 buffer, 12 units of Cla I were added, and the digestion reaction was conducted at 37°C for 2 hours. After phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol. This DNA fragment was dissolved in 40 µℓ of polymerase buffer, 6 units of Klenow fragment were added, and the reaction was conducted at 15°C for 1 hour to thereby convert the Cla I protruding ends to blunt ends. Then, after phenol extraction and chloroform extraction, a DNA fragment was recovered by precipitation with ethanol and dissolved in 30 µℓ of Y-150 buffer, 12 units of Mlu I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 6.75 kb was purified using the AFT method. The thus-obtained pSE1dhfr1A-derived DNA fragment (about 0.1 µg) and pSE1PA1-9A-derived DNA fragment (about 0.1 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSPAS1-9A isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 37). REFERENCE EXAMPLE 15 Construction of the recombinant plasmid pUKT1 coding for the pro-UK derivative UK-T : About 2 µg of the pUK11 plasmid DNA obtained in Reference Example 3-(3) were dissolved in 30 µℓ of Y-100 buffer, 16 units of Eco O109 and 10 units of Hin dIII were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Separately, about 2 µg of the phPA2 plasmid DNA obtained in Reference Example 10 were dissolved in 30 µℓ of Y-100 buffer, 10 units each of Hin dIII and Eco RI were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 3.4 kb was purified using the AFT method. Furthermore, the following two synthetic DNAs (each having 43 bases) were synthesized and 5'-phosphorylated following the procedure already mentioned hereinbefore. The thus-obtained pUK11-derived DNA fragment (about 0.75 kb, about 0.1 µg), phPA2-derived DNA fragment (about 3.4 kb, about 0.1 µg) and two 5'-phosphorylated synthetic DNAs (1 picomole each) were dissolved in 20 µℓ of T4 ligase buffer, 300 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. Plasmid DNAs were isolated from these transformants and subjected to structural analysis comprising digestion with restriction enzymes and to DNA sequence determination by the M13 dideoxy sequence method. As a result, it was confirmed that pUKTl had the desired structure (cf. Fig. 38). REFERENCE EXAMPLE 16 Construction of the UK-T expression plasmid DNA pSEUKT : About 2 µg of the pSPAS1-9A plasmid DNA obtained in Reference Example 14 were dissolved in 30 µℓ of Y-0 buffer, 10 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 10 units of Xho I were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 8.6 kb was purified using the AFT method. Separately, about 3 µg of the pSEUK1-1A plasmid DNA were dissolved in 30 µℓ of Y-100 buffer, 12 units of Bgl II and 12 units of Xho I were added, and the digestion reaction was conducted at 37°C for 2 hours. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 0.75 kb was purified using the AFT method. Further, separately, about 3 µg of the pUKT1 plasmid DNA obtained as described above were dissolved in 30 µℓ of Y-0 buffer, 15 units of Kpn I were added, and the digestion reaction was conducted at 37°C for 2 hours. Then, 1.5 µℓ of 2 M NaCl and 12 units of Bgl II were added, and the digestion reaction was further conducted at 37°C for 1 hour. After 10 minutes of heat treatment at 65°C, a DNA fragment of about 1.15 kb was purified using the AFT method. The thus-obtained pSPAS1-9A-derived DNA fragment (about 8.6 kb, about 0.1 µg), pSEUK1-1A-derived DNA fragment (about 0.75 kb, about 0.02 µg) and pUKS1-derived DNA fragment (about 1.15 kb, about 0.02 µg) were dissolved in 20 µℓ of T4 ligase buffer, 100 units of T4 DNA ligase were added, and the ligation reaction was conducted at 4°C for 18 hours. The reaction mixture thus obtained was used to transform Escherichia coli MM294 to give Ap-resistant transformants. The plasmid DNA pSEUKT isolated from one of these transformants was found to have the desired structure upon structural analysis comprising digestion with restriction enzymes (cf. Fig. 39). As described hereinabove in detail, the present invention makes it possible to supply polypeptides usable as thrombolytic agents on an industrial scale utilizing the recombinant DNA technology and the use of these polypeptides for therapeutic benefit. While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims: A plasminogen activator which is identical in peptide sequence to naturally occurring human prourokinase except that the 155th amino acid from serine, the N-terminal amino acid, is other than proline, the 155th amino acid of naturally occurring human prourokinase, optionally with the addition of methionine at the N terminus. The plasminogen activator of Claim 1, wherein the 153rd amino acid from serine, the N-terminal amino acid, is other than leucine, the 153 rd amino acid of naturally occurring human prourokinase. The plasminogen activator of Claim 1 or 2, wherein the 155th amino acid is selected from the group consisting of asparagine, aspartic acid, alanine, arginine, isoleucine, glycine, glutamine, glutamic acid, threonine, serine, cysteine, tyrosine, tryptophan, valine, histidine, phenylalanine, methionine, lysine and leucine. The plasminogen activator of Claim 1 or 2, wherein the 155th amino acid is threonine. The plasminogen activator of Claim 2, wherein the 153rd amino acid is selected from the group consisting of asparagine and serine. The plasminogen activator of Claim 2, wherein the 153rd amino acid is asparagine and the 155th amino acid is threonine. The plasminogen activator of Claim 2, wherein the 153rd amino acid is serine and the 155th amino acid is threonine. The plasminogen activator of Claim 1 or 2 which has the peptide sequence shown in Table 1, Table 2 or Table 3. A deoxyribonucleic acid (DNA) which codes for the plasminogen activator of Claim 1 or Claim 2. A recombinant DNA derived from the DNA of Claim 9 by insertion thereof into a vector DNA. A microbial or animal cell containing the recombinant DNA of Claim 10. Use of the plasminogen activator of claim 1 or 2 in the preparation of a medicament for treatment or prevention of cerebral thrombosis or myocardial infarction. A pharmaceutical composition for the prevention or treatment of cerebral thrombosis or myocardial infarction comprising an effective amount of the plasminogen activator of Claim 1 or Claim 2 together with a pharmaceutically acceptable carrier or diluent.

Inventors: ITO SEIGA, NISHI TATSUNARI, YASAMURA SHIGEYOSHI, ITO, SEIGA, NISHI, TATSUNARI, YASAMURA, SHIGEYOSHI
Applicants: KYOWA HAKKO KOGYO KK, KYOWA HAKKO KOGYO CO., LTD.
Application year: 1990
Main classification: C12N15
IPC codes: C12N1, C12N15, C12R1, C12N9