Streptokinase proteins, corresponding genes, corresponding plasmid recombinants, corresponding transformants and processes for preparing same

Abstract: This invention relates to a novel chemically synthesized gene including a base sequence coding for the primary amino acid sequence of natural-type streptokinase, a corresponding plasmid recombinant, corresponding transformant and process for preparing streptokinase by the incubation of the transformant, the invention further relating to novel streptokinase derivative proteins having streptokinase activity and a modified primary amino acid sequence corresponding to the primary amino acid sequence of natural-type streptokinase which is deficient in the amino acid residues at the 373-position to the C-terminal, and wherein at least one of the amino acid residue may be deficient, replaced or inserted, chemically synthesized gene including a base sequence coding for the derivative protein, plasmid recombinant containing the gene, transformant transformed by the recombinant, and process for preparing the streptokinase derivative protein by the incubation of the transformant.

Description: The present invention relates to a novel chemically synthesized gene including a base sequence coding for the primary amino acid sequence of streptokinase, a corresponding plasmid recombinant, corresponding transformant and process for preparing streptokinase by the incubation of the transformant, the invention further relating to novel streptokinase derivative proteins having a modified primary amino acid sequence, chemically synthesized gene including a base sequence coding for the protein, plasmid recombinant containing the gene, transformant transformed by the recombinant, and process for preparing the streptokinase derivative protein by the incubation of the transformant. Streptokinase is a protein produced or secreted by hemolytic streptococci and having thrombolytic activity and a molecular weight of about 47000. The primary amino acid sequence of the protein is represented by the formula (1) given below. The total base sequence coding for the protein has also been determined from the DNA of Streptococcus equisimilis H46A (Group C) Malke et al., 1985, Gene 34 , pp. 357-362 which produces the protein. Streptokinase-coding recombinant vectors are known in the art, EP-A-151337 discloses a DNA sequence encoding natural streptokinase in a recombinant vector. Streptokinase is already in clinical use as a thrombolytic agent in U.S. and European countries, is widely used for treating patients with lung thrombus and acute myocardial infarction and has been proved fully effective. However, when streptokinase is prepared by incubating original bacteria, the resulting culture contains some extracellular secretions, many of which are toxic or likely to be toxic to humans, so that the preparation of streptokinase from the culture for clinical use requires many purification steps which must be executed with utmost care. Further it has been reported that the streptokinase obtained by the incubation of original bacteria cause various symptoms of shock due to the antigenicity thereof. It has therefore been desired in the art to develop techniques for reducing the antigenicity or to provide novel streptokinase derivatives with reduced antigenicity. For clinical use, it is also desired to improve streptokinase in its stability in blood and its specificity of thrombolytic activity. An object of the present invention is to provide a process for preparing a large quantity of streprokinase with ease and high purity by resorting to gene recombination techniques, chemically synthesized gene of streptokinase wherein nucleotide codons are used to practice the process easily with use of E . coli , corresponding plasmid recombinant (streptokinase expression vector) having the gene introduced therein, host cell (transformant) transformed with the recombinant, and techniques for incubating the cell. Another object of the invention is to provide a novel streptokinase derivative protein which has activities inherent in streptokinase, especially thrombolytic activity, and which is reduced in the binding activity to a streptokinase-specific antibody and in the productivity of the specific antibody (antigenicity) when the protein is administered. Another object of the invention is to provide the derivative protein which is improved in its stability in blood and in thrombolytic specificity. Another object of the invention is to provide a process for preparing the novel streptokinase derivative protein by utilizing gene recombination techniques capable of producing the protein in a large quantity with ease and high purify, chemically synthesized gene wherein nucleotide codons are used to practice the process easily with use of E . coli , corresponding plasmid recombinant (expression vector) having the gene inserted therein, host cell (transformant) transformed with the reccmbinant, and techniques for preparing the protein by incubating the cell. The present invention provides a chemically synthesized gene having the base sequence listed in claim 1 and coding for streptokinase of the natural-type having a primary amino acid sequence represented by the following formula (1), a corresponding plasmid recombinant having the gene introduced therein (natural-type streptokinase expression vector), a corresponding transformant obtained from the recombinant by transformation, and a process for preparing natural-type streptokinase by incubating the transformant. The present invention further provides a streptokinase derivative protein which has a modified primary amino acid sequence corresponding to the amino acid sequence respresented by the formula (1) of natural-type streptokinase which is deficient in the amino acid residues at the 373-position to the C-terminal and wherein at least one amino acid residue may optionally be deficient, replaced or inserted. The symbols as used in the formula (1) and hereinafter to represent amino acid sequences and amino acid residues, as well as base sequences, nucleic acid bases, etc., are according to the nomenclature of IUPAC-IUB and to the practice in the art, as exemplified below. Ala: alanine Asn: asparagine Cys: cysteine Glu: glutamic acid His: histidine Leu: leucine Met: methionine Pro: proline Thr: threonine Tyr: tyrosine A: adenine G: guanine Arg: arginine Asp: aspartic acid Gln: glutamine Gly: glycine Ile: isoleucine Lys: lysine Phe: phenylalanine Ser: serine Try: tryptophan Val: valine T: thymine C: cytosine The positions of the amino acid residues in amino acid sequences are all expressed according to the amino acid sequence of the formula (1) even when an amino acid or amino acids are deficient or inserted. Given below are preferred examples of streptokinase derivative proteins of the invention having modified primary amino acid sequences. (1) Polypeptide having the amino acid sequence (represented by the formula (1)) of natural-type streptokinase wherein the amino acid residues at the 373-position to the C-terminal are deficient. (2) Polypeptide having the amino acid sequence of natural-type streptokinase wherein the amino acid residues at the 373-position to the C-terminal are deficient, and further the amino acid sequence of at least from Arg at the 45-position to Gly at the 68-position is deficient. (3) Polypeptide having the amino acid sequence of natural-type streptokinase wherein the amino acid residues at the 373-position to the C-terminal are deficient, and further at least Phe at the 118-position is deficient or replaced by other amino acid residue. (4) Polypeptide having the amino acid sequence of natural-type streptokinase wherein the amino acid residues at the 373-position to the C-terminal are deficient, and further at least Lys at the 256-position and Lys at the 257-position are deficient or replaced by other amino acid residues. (5) Polypeptide having the amino acid sequence of natural-type streptokinase wherein the amino acid residues at the 373-position to the C-terminal are deficient, and further other amino acid residue is inserted in the position next to at least each of Lys at the 256-position and Lys at the 257-position. The other amino acid residue which can be substituted or inserted in the above modified amino acid sequences can be any of those constituting the protein of streptokinase and is preferably selected from among the α-amino acids constituting the proteins of the human body. Examples of such amino acids are Pro, Gln, Thr, Ser, His and the like. More specifically, examples of preferred amino acid residues to be substituted in the 256-position are Gln, Thr, His and the like. Those to be substituted in the 257-position are Pro, Gln, Ser and His. Those to be inserted in the 256- and 257-positions include Pro. The present invention further provides a chemically synthesized gene including a base sequence coding for the streptokinase derivative protein having the modified amino acid sequence, a corresponding plasmid recombinant having the gene inserted therein (streptokinase derivative protein expression vector), a host cell transformed with the recombinant, and a process for preparing the streptokinase derivative protein by incubating the transformant. A detail description will be given of the streptokinase derivative protein of the invention, the process for preparing the same and the process for preparing natural-type streptokinase of the invention as well as the chemically synthesized gene, the expression vector and the transformant which are used in these processes. In accordance with the primary amino acid sequence of the formula (1), various base sequences can be determined for the chemically synthesized gene coding for the primary amino acid sequence of natural-type streptokinase of the invention. It is desirable to employ the following standards for this purpose. 1) Select triplet codons which are frequently used in the host cell, e.g., in E . coli . 2) Give specific restriction enzyme recognition sites within the base sequence to be determined and at the opposite ends thereof to ensure facilitated ligation to the base sequence of other gene or the like and facilitated insertion into the plasmid vector utilizing these sites as desired. 3) In joining or ligating chemically synthesized genes (DNA fragments), avoid or minimize ligations other than the desired ligation. 4) Eliminate objectionable sequences, such as terminator, from the base sequence to be designed. 5) Provide a restriction enzyme recognition site at a suitable position so as to facilitate the subsequent modification of the gene. The formula (2) given below represents a base sequence thus determined of chemically synthesized gene of natural-type streptokinase. The corresponding amino acid sequence is also given. The base sequence represented by the formula (2) is in the form of a double-stranded DNA sequence coding for the primary amino acid sequence (formula (1)) of natural-type streptokinase and comprising single-stranded DNA sequences which are complementary to each other. These complementary DNA sequences are also included in the present invention. In view of the use of E . coli as the host cell, codons which are frequently used for E . coli are preferentially selected for designing the base sequence of the formula (2), whereas the codons to be used for the present gene are not limited to such codons. Furthermore, the host cells to be used in the present invention are not limited to those of E . coli as will be described later. Accordingly, codons which are frequently used in the host cell can be selected for use in combination. The base sequence of the formula (2) can be altered or modified by local change, deletion or addition of some nucleic acid bases. Such alteration or modification of the base sequence includes use of genetic codons coding for the same primary amino acid sequence as the one coded for by the base sequence of the formula (2), and provision of various restriction enzyme recognition sites for effecting ligation with regulators, such as promoter, which are required in actually inserting the resulting base sequence into a suitable vector for expression in a microorganism. In constructing the desired expression vector by gene engineering techniques utilizing the gene of the present invention, it is necessary to suitably attach various regulators such as promoter, Shine-Dalgarno sequence (SD sequence) and like ribosomal binding sites, protein synthesis initiation codon and termination codon, etc. Since the ligation and cleavage of base sequences are effected with use of restriction enzymes, there is a need to provide suitable restriction enzyme recognition site at the upstream and downstream ends thereof. The formula (3) given below represents an example of gene wherein such suitable restriction enzyme recognition sites are attached. The gene comprises the base sequence of the formula (2) and specific restriction enzyme recognition sites provided at the upstream and downstream ends thereof for attaching promoter and like regulators which are required for the expression of the desired protein encoded by the base sequence. The gene is desirable for the subsequent construction of expression vector. The restriction enzyme recognition sites present in the base sequence of the formula (3) are shown by the formula (4). Of course, the formula (4) shows only examples of recognition sites; those to be present in the gene of the invention can be suitably modified or selected in accordance with the kind of expression vector to be constructed. The chemically synthesized gene coding for the streptokinase derivative protein of the invention having a modified primary amino acid sequence can be designed based on the gene coding for natural-type streptokinase, using a DNA sequence (codons) coding for amino acid residues corresponding to the modification of the primary amino acid sequence. A specific portion of the DNA sequence can be modified by various methods known in the art. The modification procedure includes cleavage or removal of a specified region with use of restriction enzymes, or replacement of a specific region by a chemically synthesized oligonucleotide including a modified portion which is separately prepared. Preferably, the chemically synthesized gene of the present invention is prepared by chemically synthesizing some oligonucleotide fragments according to the base sequence designed and ligating these fragments. The oligonucleotide fragments can be chemically synthesized with ease by usual methods using a commercial DNA synthesizer or the like. These methods include, for example, the solid-phase phosphite triester method [Nature, 310 , 105(1984)] and the solid-phase phosphoamidide method (S.L. Beaucage and M.H. Caruthers, Tetrahedron Letters, 22 , 1859(1981)). The oligonucleotide fragments prepared can be isolated and purified by a usual method such as high performance liquid chromatography. The purified base sequences can be checked, for example, by the Maxam-Gilbert method [A.M. Maxam and W. Gilbert, Proc. Natl. Acad. Sci., U.S.A., 74 , 560 (1977); A.M. Maxam and W. Gilbert, Methods in Enzymol., 65 , 499, Acad. Press (1980)]. The desired gene of the invention can be constructed by phosphorylating the resulting oligonucleotides at the hydroxyl groups of their 5′ ends with T4 polynucleotidekinase, followed by annealing, ligating the oligonucleotides into blocks with use of T4 DNA ligase, similarly ligating the blocks into some subunits, and ligating the subunits. These subunits and the desired gene can be incorporated into suitable vectors, e.g. pBR322, for preservation or amplification. It is desirable to isolate the subunits or the gene from the vector for use in the subsequent procedure. The above gene preparation procedure is described specifically in examples given later. The plasmid vector can be constructed by employing usual procedures or methods of gene engineering techniques. These include cleavage of DNA with restriction enzymes, S1 nuclease and the like, ligation of DNA fragments with T4 DNA ligase or the like, isolation and purification of DNA by agarose gel electrophoresis, polyacrylamide gel electrophoresis or the like, collection and purification of DNA by the phenol extraction method, etc. The host cells can be checked for the presence of the plasmid vector by the alkali SDS extraction method [H.C. Birnboim and J. Doly, Nucleic Acids Res., 7 , 1513 (1979)], i.e., by treating the plasmid DNA collected therefrom with restriction enzymes and checking the DNA for the presence of corresponding restriction enzyme recognition sites or checking the length of DNA fragment produced. The presence of the vector can be checked also by analyzing the base sequence of the gene by the direct dideoxy method [F. Sanger et al., Proc., Natl. Acad. Sci., U.S.A., 74 , 5463 (1977)] or like method. Suitable vectors for use in constructing the plasmid recombinant incorporating the gene of the invention are pBR322 and various plasmid vectors derived therefrom. However, useful vectors are not limited to these but include various known ones such as bacteriophage, virus vectors inclusive of animal and vegetable viruses, plasmids, cosmids and the like. For the transformant obtained by incorporating the vector to express the desired streptokinase or the protein of a derivative thereof, the vector needs to have, in addition to the gene of the invention, various regulators such as promoter, terminator, poly-A tail adding signal (in the case where the host cells are eucaryotic cells) and the like for transcription, and ribosomal binding site and the like for translation. Various promoters are known in different host cells. Examples of promoters in E . coli are trp promoter, lac promoter, tac promoter, λP L promoter, lpp promoter and the like, those in Bacillus subtilis are SP01 promoter, SP02 promoter, pen promoter and the like, those in yeast and other eucaryotic cells are PH05 promoter, PGK promoter, SV40 -derived promoter and the like. The desired vector of the invention can be obtained by selecting a plasmid containing such regulators as a vector, or isolating such regulators from a plasmid by the usual method or chemically synthesizing the regulators and incorporating the regulators into a suitable vector. The vector (recombinant) obtained for expressing streptokinase or derivative protein thereof has the gene of the invention, promoter and ribosomal binding site which are bound to the upstream of the gene, and a terminator binding to the downstream thereof, i.e., information for expressing streptokinase (natural type or derivative) protein. The vector is introduced into suitable host cells, whereby the cells were caused to express (produce or accumulate) the protein of contemplated streptokinase or a derivative thereof. Especially in the case where E . coli is used as the host cell, the expression vector includes a system for directly expressing the desired protein in the cell, and a system for causing the periplasm to secrete the protein for expression. In the secreting expression system, it is necessary to construct a vector having the chemically synthesized gene of the invention and a gene coding for a signal peptide bound to the upstream end of the gene. The secreting expression vector will be described below in detail. The term "signal peptide" refers an amino acid sequence of more than ten to several tens of hydrophobic amino acid residues which is present at the amino ends of various secretory proteins. The signal peptide acts to withdraw the protein from the cytoplasm and is excised from the secretory protein by signal peptidase. The use of such signal peptides in gene recombination methods and the resulting advantages are known as described in detail, for example, in Unexamined Japanese Patent Publication SHO 61-149089. The signal peptide to be used in the present invention may be the same as, or different from, those disclosed in the publication. Examples of useful signal peptides are E . coli β-lactamase ( bla ), alkaline phosphatase ( pho S ), E . coli outer membrane protein ( Omp A, Omp F , Lpp ) and the like. The base sequence coding for such a signal peptide can be chemically synthesized, or a natural one can be utilized. pKTN mentioned in examples given later is an example of vector containing such a base sequence. pKTN is a vector containing a base sequence coding for E . coli bla signal peptide, more specifically, a vector having gene information comprising tac promoter, SD sequence, and base sequence coding for bla signal peptide, as arranged in the same direction. E . coli , the strain JM103 harboring pKTN has been deposited in Fermentation Research Institute, Agency of Industrial Science and Technology, MITI, with the designation " Escherichia coli , JM-103, pKTN-2-2'' and the deposition number FERM BP-1398 A preferred example of secreting expression system is the plasmid pSKXT constructed with use of pKTN as will be described later in an example. The secreting expression vector pSKXT has a base sequence which comprises the bla signal peptide encoded base sequence and a first codon coding for the amino acid residue at the first position of the streptokinase and directly attached to the downstream end of the base sequence, i.e., a base sequence coding for the fusion protein of the signal peptide and streptokinase. With this vector there is no likelihood of displacement of the reading frame, permitting E . coli cell to express the fusion protein by virtue of the action of tac promoter, such that the desired protein only is passed through the inner membrane and secreted and accumulated in the periplasm. E . coli , the strain JM109 harboring the plasmid pSKXT has been deposited in Fermentation Research Institute, Agency of Industrial Science and Technology, MITI with the designation " Escherichia coli , JM-109, pSKT" and the deposition number FERM BP-2464. The desired expression vector thus obtained is introduced into suitable host cells (transformation), whereby the cells are given ability to produce streptokinase or protein of derivative thereof. The host cell to be used is not limited specifically but can be any of those known, such as cells of E . coli and like gram-negative bacteria, Bacillus subtilis and like gram-positive bacteria, actinomycetes and yeast, and animal or vegetable cells. Among these, E . coli is preferable, which are more preferably the strain HB101 [H. W. Boyer and D. Roulland-Dussoix., J. Mol. Biol., 41 , 459 (1969)] and the strain JM109 [J. Messing et al., Nucleic Acids Res., 9 , 309 (1981)] derived from the strain K12. The vector of the invention can be introduced into the host cell for transformation by a usual method, for example, by treating the host cell in an aqueous solution containing calcium chloride at a low temperature and adding the vector to the solution [E. Lederberg and S. Cohen, J. Bacteriol., 119 , 1072 (1974)]. The present invention also provides the host cell thus transformed (transformant having introduced therein the vector for expressing natural-type streptokinase of the invention or streptokinase derivative protein of the invention). The transformant can be incubated using usual media, which include, for example, L-broth medium, E medium, M-9 medium, etc. These media are usable with addition of various carbon sources, nitrogen sources, inorganic salts, vitamins, natural extracts, physiologically active substances and the like which are generally known. The incubation can be carried out by various methods under conditions which are suitable to the growth of the host cell in respect of pH, temperature, aeration, stirring, etc. For example, it is desirable to incubate E . coli at a pH of about 5 to about 8, more preferably 7, at a temperature of about 20 to about 43 °C with aeration and stirring. The scale of incubation is not limited specifically. The composition of the medium and incubation conditions can be altered suitably in order to produce an increased amount of expressed protein or promote or inhibit the secretion of the desired protein. For example, when E . coli is thus incubated which is transformed with a vector for secreting and expressing the streptokinase of the invention or protein of a derivative thereof, the desired protein is secreted and accumulated in the periplasm, and the protein can be separated off, collected and purified by usual methods. For this purpose, for example, the periplasm prepared by the osmotic shock method can be subjected to gel filtration, adsorption chromatography, ion-exchange chromatography, high performance liquid chromatography or the like method. Such methods can be employed in a suitable combination. The protein secreted in the periplasm or obtained in the culture supernatant is advantageous in that it is easy to separate off and purify by the above procedure. In this way, streptokinase and protein of derivative thereof can be prepared by gene engineering techniques according to the present invention. The protein obtained can be readily identified with reference to the fact that it exhibits a single peak when subjected to high performance liquid chromatography or a single band when subjected to polyacrylamide gel electrophoresis. The desired streptokinase or derivative protein thereof, as purified to a higher degree, can be identified by the same methods as usually used for analyzing the structure of polypeptides or proteins, for example, by analyzing the molecular weight by SDS-PAGE, measuring isoelectric point by isoelectric focusing, determining the amino acid composition by amino acid analyzer, analyzing the amino acid sequence by protein sequencer, etc. More advantageously, the streptokinase derivative protein having a modified primary amino acid sequence of the invention can be produced by preparing a natural-type streptokinase expression vector by the foregoing method, cleaving the vector with suitable restriction enzyme to remove a specific region, repairing the gene with a chemically synthesized oligonucleotide fragment having the desired base sequence, incorporating the gene into a suitable vector again similarly to construct the desired derivative protein expression vector, introducing the vector into host cells and incubating the cells. With this method, the amino acid residue at an optional position in the natural-type streptokinase of the formula (1) can be removed or replaced or a contemplated amino acid residue can be inserted in a desired position by procedures already described or to be described in detail in the examples to follow. In this way, natural-type streptokinase and derivative protein thereof can be produced in large quantities with high purity and ease by gene engineering techniques. Especially, the streptokinase derivative protein obtained according to the invention is lower in antigenicity, more stable in blood and higher in the selectivity of thrombolytic activity and specificity thereof (thrombus selectivity) than natural-type streptokinase. The protein is therefore advantageously usable for medical applications wherein natural-type streptokinase is used. EXAMPLES Given below for a better understanding of the present invention are Examples 1 to 4 wherein natural-type streptokinase was prepared, and Examples 5 to 12 wherein streptokinase derivative proteins were prepared according to the invention. The following drawings be referred to in the examples. Fig. 1 is a diagram showing procedures in Example 1 for constructing block 1 to block 8 from synthetic oligonucleotides and for constructing streptokinase gene subunits SKA, SKB, SKC and SKD from the blocks; Fig. 2 is a diagram showing procedures in Example 1 for preparing plasmids pSKA, pSKB, pSKC and pSKD having the respective subunits and for preparing streptokinase cloning vector pSKX from these plasmids via plasmid vectors pSKAB and pSKABX; Fig. 3 is a diagram showing a procedure for preparing streptokinase secreting expression vector pSKXT from vector pSKX and plasmid pKTN2-2 in Example 2; Fig. 4 is a diagram showing a procedure for preparing streptokinase derivative protein (1-372) expression vector in Example 5; Fig. 5 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 118 deficient) in Example 6; Fig. 6 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 45-68 deficient) in Example 7; Fig. 7 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 256Gln, 257Gln replaced) in Example 8; Fig. 8 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) in Example 9; Fig. 9 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) in Example 10; Fig. 10 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) in Example 11; and Fig. 11 is a diagram showing a procedure for preparing a vector for expressing streptokinase derivative protein (1-372, 256, 257Lys-Pro-Lys-Pro altered) in Example 12. The methods and procedures employed in Examples are as follows unless otherwise stated. 1. Cleavage of DNA with restriction enzymes The restriction enzymes used were those produced by Takara Shuzo Co., Ltd., Toyobo Co., Ltd. and Nippon Gene Co., Ltd. The mixtures to be reacted were those of compositions specified by the respective companies. For reaction, each mixture was allowed to stand in a water bath at 37°C for 3 hours. As a standard, the restriction enzyme was used in an amount of one unit per microgram of DNA, such that the amount of reaction mixture eventually obtained was 100 µl. A sterilized 1.5-ml Eppendorf tube was used as the reactor. 2. Phenol extraction This extraction method was practiced after the completion of the enzymatic reaction to inactivate the enzyme and terminate the reaction. To the reaction mixture was added TE-saturated phenol (phenol saturated with 10 mM tris-HCl (pH 8.0) containing 1 mM EDTA) in one-half the amount of the reaction mixture, followed by full stirring by shaking and then by centrifugation (12000 r.p.m., 5 minutes) to obtain an aqueous layer containing DNA. To the aqueous layer was added the same amount of ether, and the mixture was similarly stirred and centrifuged to collect an aqueous layer. This procedure was repeated two to three times. To the resulting aqueous layer were added 3 M sodium acetate buffer (pH 5.0) in 0.1 times the amount of the layer and cold ethanol in 2.5 times the amount of the layer. The mixture was stirred by shaking, then allowed to stand at -80°C for at least 30 minutes and thereafter centrifuged (12000 r.p.m., 5 minutes), whereby the DNA was collected as a sediment. 3. Ligation of DNA fragments with T4 DNA ligase The DNA was dissolved in an aqueous solution containing 67 mM tris-HCl (pH 7.6), 6.7 mM magnesium chloride, 10 mM dithiothreitol and 1 mM ATP, and T4 DNA ligase (product of Takara Shuzo Co., Ltd.) was added to the solution in an amount of one unit per microgram of DNA. The mixture was reacted at 12°C for at least 5 hours or at 4°C overnight to ligate the DNA fragments. The amount of reaction mixture finally obtained was 100 µl. After the reaction, the DNA was collected by the phenol extraction method. 4. Method of transformation E . coli , the strain HB-101 or JM-109 was used as the host cell. The strain was incubated with shaking in L-broth medium (1% bacto trypton, 0.5% bacto yeast extract and 0.5% sodium chloride) at 37°C until the absorbance at 600 nm reached 0.25. The culture (10 ml) was centrifuged (7000 r.p.m., 5 minutes) to collect the cells, which were then ice-cooled. The cells were suspended with addition of 5 ml of 0.1 M magnesium chloride and washed, followed by centrifugation (7000 r.p.m., 1 minute) to collect the cells. A 5 ml quantity of ice-cooled mixture of 0.1 M calcium chloride and 0.05 M magnesium chloride was added to the cells to obtained a suspension, which was then allowed to stand in ice for at least 30 minutes and thereafter centrifuged (7000 r.p.m., 1 minute). The cells collected were suspended in 0.5 ml of the same solution again. To 0.2 ml of the suspension was added 20 µl of the reaction mixture of DNA ligated with T4 DNA ligase, and the mixture was ice-cooled for 30 minutes. Subsequently, the mixture was heated in a water bath at 42.5 °C for 30 seconds. With addition of 1.0 ml of L-broth medium, the mixture was allowed to stand in a water bath at 37°C for 1 hour. The transformant to be obtained was selected by the following procedure utilizing antibiotic resistance for identification, i.e., by spreading 0.2 ml portions of the reaction mixture over a plate medium prepared by adding 50 µg/ml of ampicillin to L-broth medium containing 1.5% agar, and allowing the mixture to stand overnight at 37°C. The active colonies were isolated. 5. Isolation and purification of plasmid The strain harboring plasmids was incubated with shaking at 37°C for 12 to 16 hours in 400 ml of L-broth medium containing 50 µg/ml of ampicillin. The culture was centrifuged (6000 r.p.m., 10 minutes) to collect the cells, to which was added 14 ml of solution I (50 mM glucose, 10 mM EDTA, 25 mM tris-HCl (pH 8.0) and 2 mg/ml of lysozyme, as sterilized) to obtained a suspension. The suspension was allowed to stand in ice for 30 minutes. With addition of 28 ml of solution II (0.2N sodium hydroxide and 1% sodium dodecylsulfate), the suspension was stirred and allowed to stand in ice for 5 minutes. A 21 ml quantity of solution III (3 M sodium acetate (pH 4.8), as sterilized) was then added to the suspension, and the mixture was allowed to stand in ice for at least 60 minutes and thereafter centrifuged (8000 r.p.m., 10 minutes) to obtain a sediment. The sediment was dissolved in 11 ml of solution IV (0.1 M sodium acetate and 0.05 M tris-HCl (pH 8.0)), cold ethanol was added to the resulting solution in 2.5 times the amount thereof, and the mixture was allowed to stand at -80°C for 30 minutes. The mixture was centrifuged again (12000 r.p.m., 15 minutes) to collect a sediment. The sediment was dissolved in 4 ml of TE buffer (solution of 10 mM tris-HCl (pH 7.5) and 1 mM EDTA), 4.62 g of cesium chloride was dissolved in the solution with stirring, and 0.42 ml of ethidium bromide solution (5 mg/ml) was added. The combined solution was centrifuged (3000 r.p.m., 10 minutes) to remove the suspended matter, and the resulting solution was ultracentrifuged (50000 r.p.m., for 15 hours). The resulting product was irradiated with ultraviolet rays to collect a plasmid DNA portion emitting fluorescence. This portion was subjected to extraction five to six times using isopropanol saturated with 5 M sodium chloride solution to remove the ethidium bromide. Finally, the portion was subjected to Biogel A-50 (product of Bio Rad Laboratories) column chromatography (column size: 2.5 cm x 15-20 cm, eluent: TE buffer + 0.5 M sodium chloride solution, detection at UV 254 nm) to remove the cesium chloride and RNA and the like present. Plasmid DNA was collected by the phenol extraction method. The amount of purified plasmid DNA was determined from OD 260 nm measurement, with OD₂₆₀=0.022 calculated as 1 µg/ml of DNA. 6. Synthesis of oligonucleotide DNA was chemically synthesized using a DNA synthesizer, Model 381A type, product of Applied Biochemicals (solid-phase β-cyanoethyl phosphoamidide method, with use of 0.2 µM column). The procedure followed was according to the manufacturer's manual. The oligonucleotide obtained by the synthesizer has the column carrier resin attached to the 3′ end, protective groups to the active groups, and dimethoxytrityl group to the 5′ end, so that the resin and these groups need to be removed. The removal procedure was also according to the manual. Next, HPLC was conducted until a single peak was obtained for fractionation and purification to separate the by-product, free protective groups and protective group removing agents from the desired oligonucleotide. The column used was YMCA-PACK AM-303 ODS (4.6 x 250 mm, Waters type, product of Yamamura Chemical Laboratories). Gradient elution was conducted using as eluents 5% to 40% acetonitrile/0.1 M triethyl ammonium acetate aqueous solutions (pH 7.2). The system used had a pump CCPM, UV visible detector UV-8000 and controller CCP which were products of Tosoh Corporation. 7. Agarose gel electrophoresis Agarose gel was used at a concentration of 0.9 or 1.6%. The agarose gel was prepared by weighing out an amount of Agarose I (product of Dojin Chemical Lab.) and dissolving the agarose in TBE buffer (containing 0.089 M tris-boric acid and 0.02 M EDTA) to the specified concentration with heating. Electrophoresis was performed using a mini gel electrophoresis system Mupid-2 (product of Cosmo-Bio Co., Ltd.) and TBE buffer as an electrophoretic buffer. The resulting gel was dipped in a solution of 0.5 µg/ml of ethidium bromide and observed under ultraviolet light to detect DNA fragments emitting fluorescence. The DNA was eluted from the gel by removing the desired band portions with a knife, placing the portions into a dialysis tube (3500 MW cut), filling the tube with TE buffer and electrophoresing the portions for 30 minutes using the same system as above. The DNA obtained was concentrated to dryness, a small amount of distilled water was added thereto, and the mixture was treated by the phenol extraction method to collect the DNA. 8. Analysis of base sequence The base sequences of streptokinase and the protein of derivative thereof were analyzed by the M-13 dideoxy method [J. Messing., Methods in Enzymology, 101 , 20 (1983)] using M-13 sequencing kit (with use of (α-32P) dCTP, product of Amersham International Ltd.) manufactured by Toyobo Co., Ltd. according to the manufacturer's manual. The sequencing gel used was FUJI GENSOR GEL SHEET (SO802, concentration 8%) manufactured by Fuji Photo Film Co., Ltd. Example 1 Preparation of streptokinase expression vector pSKXT (1) Construction of pSKX A detailed description will be given of the construction of plasmid pSKX which has incorporated therein a chemically synthesized gene of natural-type streptokinase by cloning. 1) Synthesis of oligonucleotides In constructing the whole base sequence represented by the formula (3) and including the structural gene of streptokinase, the base sequence was first divided into 52 oligonucleotide fragments (A-1 to A-14, B-1 to B-12, C-1 to C-12 and D-1 to D-14) having 43 to 56 bases and listed in Tables 1 to 4 below, and the individual fragments were chemically synthesized by the solid-phase β-cyanoethyl phosphoamidide method. 2) Ligation of synthesized oligonucleotides The whole base sequence of the formula (3) was constructed as divided into subunits SKA, SKB, SKC and SKD. Subunit SKA comprises, as shown by the following formula (5), a base sequence starting with EcoR I restriction enzyme recognition site of the whole base sequence, including Apa I and Nhe I restriction enzyme recognition sites and ending with Sal I restriction enzyme recognition site. Subunit SKB comprises, as shown by the following formula (6), a base sequence starting with EcoR I restriction enzyme recognition site, including Nhe I and Sac I restriction enzyme recognition sites and ending with Sal I restriction enzyme recognition site. Subunit SKC comprises, as shown by the following formula (7), a base sequence starting with EcoR I restriction enzyme recognition site, including Sca I and AatI I restriction enzyme recognition sites and ending with Sal I restriction enzyme recognition site. Subunit SKD comprises, as shown by the following formula (8), a base sequence starting with EcoR I restriction enzyme recognition site, including Aat II restriction enzyme recognition site and ending with Sal I restriction enzyme recognition site. The subunits were constructed as divided into blocks 1 to 8, which were constructed in the manner to be described below, as schematically shown in Fig. 1. With reference to Fig. 1, the solid dot at one end of the line representing a chemically synthesized oligonucleotide represents introduction of a phosphate group at the 5′ end. Of the 52 chemically synthesized oligonucleotides A-1 to D-14, the 44 oligonucleotides other than A-1, A-8, B-1, B-7, C-1, C-7, D-1 and D-8 were treated in the following manner to introduce the phosphate group into each oligonucleotide at the 5′ end. A 20 µl quantity of 1 to 3 µg/ml solution of each oligonucleotide was prepared, 5 units of T4 DNA polynucleotidekinase (product of Takara Shuzo Co., Ltd.), 5 µl of reaction buffer specified by the company and 1 µl of 100 mM ATP were added to the solution, and the amount of the mixture was adjusted to 50 µl with addition of water. The mixture was then reacted at 37°C for 1 hour. Next, to DNA solutions (2 µl each) of oligonucleotides serving as the starting materials for each of the blocks to be constructed (e.g. oligonucleotides A-1, A-2, A-3, A-12, A-13 and A-14 in the case of block 1) were added 1 µl of 100 mM ATP and 10 µl of ligase reaction buffer (solution of 660 mM tris-HCl (pH 7.6), 66 mM magnesium chloride and 100 mM dithiothreithol), and the amount of mixture to be reacted was adjusted to 100 µl. The mixture was heated in a water bath at 100°C for 2 minutes and thereafter cooled spontaneously. Subsequently, with addition 2.5 units of T4 DNA ligase (product of Takara Shuzo Co., Ltd.), the mixture was reacted at 4°C overnight to ligate the nucleotides. The ligated reaction product was subjected to phenol extraction and then to 10% polyacrylamide gel electrophoresis. A double-stranded portion which was the block of the desired size was removed from the gel and subjected to elution. Blocks 1 to 8 were constructed by the same procedure as above. 3) Preparation of pSKA, pSKB, pSKC and pSKD Subunit SKA comprising block 1 and block 2 constructed by the procedure 2) was incorporated into plasmid pBR322 by the following procedure to obtain vector pSKA. Similarly, pSKB, pSKC and pSKD were prepared. Fig. 2 schematically shows the procedures. With reference to the diagram, Ap r stands for ampicillin resistance, Tc r for tetracycline resistance, and ori for a replication origin. The restriction enzyme recognition sites of base sequences are to be represented by the following symbols, the same as the diagrams to follow. A ... Aat II E ... EcoR I Na .. Nae I Sc .. Sac I Ap .. Apa I EV .. EcoR V P ... Pst I B ... BamH I N ... Nhe I S ... Sal I 3)-1 Preparation of pSKA First, pBR322 was treated with restriction enzymes EcoR I and Sal I and then electrophoresced on 0.9% agarose gel to obtain a DNA fragment of about 3.7 kb. The DNA fragment was mixed with block 1 and block 2 prepared above, T4 DNA ligase, ATP and ligase reaction buffer, followed by reaction at 4°C overnight to ligate DNA. E . coli HB-101 was transformed by the calcium method using the reaction mixture. DNA was collected from the colonies obtained by the simplified boiling method, followed by screening to select the colonies containing the desired pSKA. The alkali-SDS extraction method, CsCl equilibrium density ultracentrifugation method and Biogel A-50 column chromatography were further practiced to collect purified DNA, and a restriction enzyme cleavage map was prepared using restriction enzymes ( EcoR I, Bgl II, Apa I, Nco I, Sal I, Xho II, Hinf I, etc.) Consequently, it was confirmed that the colonies selected contained the desired pSKA. Further the SKA introduction portion of pSKA was checked for base sequence by the M-13 dideoxy method. More specifically, EcoR I- Sal I fragment was subcloned in M-13 mp18 or mp19 to analyze the base sequence. As a result, the desired sequence was identified. 3)-2 Preparation of pSKB, pSKC and pSKD By the same procedure as the above 3)-1, E . coli HB-101 was transformed with subunit SKB comprising block 3 and block 4, followed by colony screening, and collection and purification of plasmid DNA, to prepare pSKB, which was similarly identified. Similarly, subunit SKC comprising block 5 and block 6 was introduced in E . coli HB-101 for transformation, followed by screening of colonies, and collection and purification of plasmid DNA to prepare pSKC, which was identified similarly. E . coli HB-101 was likewise transformed with subunit SKD comprising block 7 and block 8, followed by screening of colonies, and collection and purification of plasmid DNA. 4) Preparation of pSKAB This plasmid is an intermediate vector for constructing pSKX contemplated. The procedure for preparing the vector, which is schematically illustrated in Fig. 2, is as follows. pSKA obtained by the procedure 3) was treated with restriction enzyme Pst I and Nhe I and electrophoresced on agarose gel to isolate and purify a 1072 bp DNA fragment. Similarly, pSKB obtained by the procedure 3) was treated with the same restriction enzymes as above to isolate and purify a 3250 bp DNA fragment. The two DNA fragments were ligated with T4 DNA ligase, and E . coli HB-101 was transformed with the resulting reaction mixture to obtain colonies containing the desired pSKAB. Plasmid DNA was collected from the colonies and purified. pSKAB of 4323 bp was confirmed by restriction enzyme cleavage mapping. 5) Preparation of pSKABX Construction of the streptokinase secreting expression vector requires a plasmid vector with a base sequence not having the codon for Met attached to the front of the first-position amino acid of streptokinase. Accordingly pSKABX was prepared by altering the N-terminal of pSKAB through the following procedure. Fig. 2 shows the procedure schematically. pSKAB was treated with restriction enzymes EcoR I and Apa I and ligated with oligonucleotides E-1 and E-2 freshly chemically synthesized, in the presence of T4 DNA ligase. E . coli was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme cleavage map was prepared to obtain the desired pSKABX. Given below are the base sequences of oligonucleotides E-1 and E-2 and of a linker obtained with use of these nucleotides for constructing the secreting expression vector. Linker Number of of bases Base sequence E-1 18 AATTCGATATCGCGGGCC E-2 10 CGCGATATCG Linker for constructing secreting expression vector The plasmid vector thus obtained can be used for constructing a system for secreting and expressing streptokinase. The E-1 and E-2 introducing portion of pSKABx was found to have the above base sequence by the M-13 dideoxy method [J. Messing, Methods in Enzymology, 101 , 20 (1983)]. 6) Preparation of pSKX Fig. 2 schematically shows a procedure for preparing pSKX. pSKABX was treated with restriction enzymes Sac I and Pst I to obtain a DNA fragment having 1352 bp, which was ligated with a DNA fragment of 309 bp obtained by treating pSKC with restriction enzymes Sac I and Aat II and a DNA fragment of 3305 pb obtained by treating pSKD with restriction enzymes Aat II and Pst I. E . coli was transformed with the reaction mixture, followed by the same procedures as above, i.e. collection and purification of vector DNA and restriction enzyme mapping. The procedure revealed that the desired pSKX (4966 pb) was obtained. Example 2 Construction of streptokinase secreting expression vector A vector was constructed for secreting and expressing streptokinase in the periplasm of E . coli . The chemically synthesized streptokinase gene was ligated to tac promoter and bla signal peptide so that the codon frame of the streptokinase structural gene would not be displaced from the codon stream of amino acid sequence of bla signal peptide, whereby the desired streptokinase secreting expression vector was constructed. Fig. 3 shows the construction procedure. Same symbols as above were used for indicating restriction enzyme recognition sites. Spbla stands for bla signal peptide, SD for ribosomal binding site, and Ptac for tac promoter. tac promoter is represented by the blank area, bla signal peptide by the hatched area, and the base sequence coding for the streptokinase gene by the solid black area. 1) Preparation of pSKXT pSKX was treated with restriction enzymes EcoR V and Sal I to obtain a DNA fragment with 1251 bp. Similarly, pSKX was treated with restriction enzymes EcoR I and Sal I to obtain a DNA fragment with 3711 bp. pKTN2-2 having tac promoter and bla signal peptide portions was treated with restriction enzymes EcoR I and Nae I to obtain a DNA fragment with 460 bp. ( E . coli JM-103 harboring pKTN2-2 has been deposited as FERM BP-1398). These fragments were collected by agarose gel or polyacrylamide gel electrophoresis and reacted for ligation. E . coli JM-109 was then transformed with the resulting reaction mixture. Vector DNA was collected from the colonies obtained, purified, followed by restriction enzyme cleavage mapping to obtain the desired vector pSKXT. E . coli JM-109 harboring pSKXT produces within the cell the fused protein of bla signal peptide and streptokinase through the action of tac promoter, the fused protein is then transferred by the action of bla signal peptide to the inner membrane where the signal peptide is excised by protease present in the membrane, and streptokinase only is secreted in the periplasm between the inner membrane and the outer membrane. Example 3 Expression and identification of streptokinase 1) Incubation of E . coli JM-109 harboring streptokinase secreting expression vector pSKXT E . coli JM-109 harboring vector pSKXT and obtained in Example 2 was incubated with shaking in the following manner using M-9 casamino acid liquid medium of the composition listed in Table 5 below. Ingredient Amount Disodium phosphate 5.8 g Potassium dihydrogen phosphate 3.0 g Sodium chloride 5.0 g Ammonium chloride 1.0 g 1 M calcium chloride* 0.1 ml 1 M magnesium chloride* 1.0 ml Glucose* 5.0 g Casamino acid (product of Difco) 5.0 g L-proline 50 mg Vitamin B₁ 1 mg Water amount needed to make 1 liter of medium Each asterisked ingredient was separately sterilized by autoclaving (at 121°C for 15 minutes). When required, ampicillin sterilized by filtration was added to a final concentration of 50 µg/ml. The culture (1 ml) prepared above was placed into a Sakaguchi flask containing 100 ml of the medium and incubated at 37°C with reciprocating shaking. About 3 hours (OD₆₀₀=about 0.3) after the start of incubation, IPTG (isopropyl-β-D-thiogalactoside, product of Sigma) was added to the culture to a final concentration of 0.25 mg/ml, followed by further incubation. 2) Extraction of desired product from cells Incubation conducted under the above conditions was discontinued about 4 hours after the addition of IPTG, and fractions were obtained by the following steps. First, the culture was centrifuged (5000 r.p.m., 10 minutes) to separate cells and culture supernatant. The supernatant thus obtained will be referred to as the "medium fraction". The cells obtained were suspended in 30 mM tris-HCl (pH 8.0)-20% sucrose buffer in the same amount as the culture, EDTA aqueous solution was added to the suspension to a final concentration of 0.01 M, and the mixture was stirred by a rotary shaker at 24°C at 180 r.p.m. for 10 minutes and then centrifuged (6000 r.p.m., 10 minutes). The resulting supernatant will be referred to as the "sucrose buffer fraction". The sediment resulting from the centrifugation was suspended again in ice-cooled water in the same amount as the culture, and the suspension was allowed to stand in ice for 15 minutes with occasional stirring and thereafter centrifuged (10000 r.p.m., 5 minutes). The resulting supernatant will be referred to as the "periplasm fraction." 3) Determination of streptokinase activity The plasminogen activator activity of the streptokinase was determined by measuring the activity of plasmin produced by the plasminogen activation of the streptokinase by the method of Jackson et al. [K.W. Jackson et al, Methods in Enzymology, 80 , 387 (1981)]. The activity of plasmin was measured using a synthetic substrate, i.e., S-2251 (D-Val-Leu-Lys-p-nitroaniline, product of Kabi Vitrum), that is, by determining p-nitroaniline released by plasmin in terms of an increase in the absorbance at 405 nm, as will be described in detail below. The specimen was diluted with 0.025% aqueous solution of Triton X-100, a 25 µl portion of the dilution and 25 µl of 50 mM tris-HCl buffer (pH 7.5) were placed into a test tube, and the mixture was incubated at 37°C for 5 minutes. Human plasminogen (product of Sigma) was diluted with 50 mM tris-HCl buffer (pH 7.5) containing 1 mg/ml of bovine serum albumin (BSA) to a concentration of 80µ g/ml. The diluted human plasminogen (25 µl) was placed into the test tube, and the mixture was incubated at 37°C. Fifteen minutes thereafter, 25 µl of S-2251 diluted to 2.5 mg/ml with 50 mM tris-HCl buffer (pH 7.5) containing 1.6 M sodium chloride was added to the culture, followed by incubation at 37°C for 10 minutes. The reaction was then terminated by addition of 1.5 ml of 0.2 M acetic acid aqueous solution, and the absorbance of the culture was measured at 405 nm. The activity unit of the specimen was determined with reference to a standard curve for a standard substance, i.e., streptokinase derived from streptococcus equisimilis H46A (Group C) and having specific activity of 100 international units/µg, the curve being obtained by using the standard substance in the same manner as above. 4) Western blotting The streptokinase present in the cells obtained by the procedure 2) above was detected by the Western blotting method [H. Towbin, T. Staehelin and J. Gordon, Proc. Natl. Acad. Sci., U.S.A., 76 , 4350 (1979)] using streptokinase-specific antibody as will be described below in detail. Streptokinase-specific anti-serum was prepared by immunizing rabbits with purified streptokinase derived from Streptococcus equisimilis H46A (Group C) as an antigen, i.e., by dissolving 1 mg of freeze-dried Streptokinase in 0.5 ml of physiological saline, adding 0.5 ml of Freunds complete adjuvant to the solution to emulsify the streptokinase, subcutaneously giving the emulsion to the back of three rabbits, similarly giving the emulsified streptokinase at the same dose every two weeks to thus immunize the rabbits four times in total, collecting the total blood 10 days after the final immunization and separating off the serum. The anti-serum thus obtained was used for detecting streptokinase. A specimen for polyacrylamide gel electrophoresis to be performed in the presence of sodium dodecylsulfate (hereinafter referred to as "SDS-PAGE") was prepared, for example, by suspending cells obtained from 1 ml of culture in 1 ml of 10 mM tris-HCl buffer (pH 8.0) containing 2% SDS and 2% dithiothreithol, heating the suspension at 100°C for 5 minutes and the centrifuging the suspension (12000 r.p.m., 10 minutes) to obtain a supernatant. SDS-PAGE was performed by electrophrescing the specimen thus prepared at 2 mA/cm on 15% polyacrylamide containing 0.1% of SDS according to the Laemmli method [U.K. Laemmli, Nature, 227 , 680 (1970)]. The streptokinase was detected by electrophoretically transferring to a nitrocellulose film (product of Bio Rad) the protein in the resulting gel and specifically staining the streptokinase in the protein with use of specific antibody. For the staining, the nitrocellulose film having the protein transferred thereto was shaken at room temperature for 1 hour in a 10% solution of bovine serum albumin (BSA) in 20 mM tris-HCl buffer (pH 7.5, hereinafter referred to briefly as "TBS") containing 0.15 M sodium chloride and then reacted, at room temperature for 3 hours, with the streptokinase anti-serum as diluted to 1000-fold with 0.1% BSA-TBS. The resulting reaction mixture was washed with TBS four times and treated for 60 minutes with peroxidase-bound anti-rabbit IgG (product of Cappel) as diluted to 2000-fold with 0.1% BSA-TBS. The resulting mixture was washed again and thereafter treated with 0.02 M citric acid buffer (pH 6.5) containing 0.03% hydrogen peroxide and 0.5 mg/ml 4 chloro-1-naphthol for color development. 5) Determination of amount of streptokinase expressed E . coli JM-109 harboring secreting expression vector pSKXT was incubated and fractionated by the procedures 1) and 2) above and checked by the procedure 3) to determine the amount of streptokinase expressed. Consequently, no activity was detected from the medium supernatant fractions but 200 international units/ml of activity was detected from the cell fraction, i.e., almost entirely from the periplasm fraction. Further when analyzed by the procedure 4) above, the immunoactivity appeared at the position of about 47000 in molecular weight the same as in the case of natural type. These results indicate that the streptokinase detected by the invention is the same as natural-type streptokinase and was produced by translation within the cell, then passed through the inner membrane by the action of the signal peptide, further processed for the removal of the signal peptide and secreted. Example 4 Preparation and identification of recombinant streptokinase 1) Extraction from cells A 2.4-liter quantity of culture obtained by the incubation method of Example 3, 1) was centrifuged (8000 r.p.m., 10 minutes) to collect the cells, which were treated by the osmotic shock method described in Example 3, 2) to obtain a periplasm fraction. 2) Purification of streptokinase Streptokinase can be collected from the extract of the fraction by a combination of separating methods such as salting-out with ammonium sulfate, isoelectric focusing, gel filtration, ion-exchange chromatography and hydrophobic chromatography, whereby a single purified product can be obtained. As an example, the combination of hydrophobic chromatography and anion-exchange chromatography will be described below. First, ammonium sulfate was added to 600 ml of the periplasm fraction to a concentration of 0.6 M, and the mixture was passed through a Butyl Toyo pearl column (3 x 20 cm, product of Tosoh Corporation) equilibrated with 0.6 M ammonium sulfate. After washing the column with 500 ml of 0.3 M ammonium sulfate, 50 mM tris-HCl buffer (pH 8.5) was passed through the column to elute a fraction having streptokinase activity. The fraction was then dialyzed against 50 mM tris-HCl buffer (pH 8.5), and the dialyzate was separated by a fractionating HPLC system (Model HLC-837, product of Tosoh Corporation) including DEAE-5PW (2.15 x 15 cm, product of Tosoh Corporation) at a flow rate of 4 ml/min using 50 mM tris-HCl buffer (pH 8.5) as an eluent at a straight concentration gradient of 0.1 M to 0.3 M sodium chloride. The Butyl Toyo pearl eluate fraction after dialyzation was injected into the HPLC system, and the main protein peak portion was pooled. The streptokinase activity fraction was eluted at a peak in match with the protein peak. The active eluate fraction pooled was further desalted by Sephadex G-25 column (2.5 x 25 cm, product of Pharmacia) and then separated again by a DEAE-5PW column under the same condition as above. Finally, the streptokinase active fraction was desalted by the same Sephadex G-25 column and lyophilized to obtain a final purified product. 3) Identification of final purified product 3)-1 Measurement of molecular weight by SDS-PAGE A sample of the purified product obtained by the procedure 2) was analyzed by SDS-PAGE in the presence of and also in the absence of a reducing agent. Consequently, the sample exhibited a single band at the position of 47300 in molecular weight the same as the streptokinase derived from Streptococcus equisimilis H46A (Group C). 3)-2 Amino acid analysis With addition of 6N hydrochloric acid, a sample of the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using an automatic amino acid analyzer, Model Hitachi 835. Table 6 below shows the result. For comparison, Table 6 also shows the result obtained by similarly analyzing the streptokinase derived from Streptococcus equisimilis H46A (Group G) (designated as "Natural" in Table 6. Amino acid Natural Recombinant Calculated Asp 65.5 63.8 65 Thr 29.4 29.2 30 Ser 23.6 23.0 25 Glu 46.1 45.4 44 Pro 21.2 22.6 21 Gly 20.8 20.4 20 Ala 21.9 21.6 21 Val 21.5 21.1 23 1/2 Cys Not detected Not detected 0 Met 3.6 3.6 4 Ile 20.8 21.0 23 Leu 38.2 38.6 39 Tyr 22.1 21.3 22 Phe 15.2 15.1 15 Trp Not detected Not detected 1 Lys 32.4 32.2 32 His 8.8 9.0 9 Arg 19.8 19.9 20 3)-3 Analysis of amino acid sequence The sequence of 20 amino acid residues at the amino end of a sample of the purified product was analyzed by the method of Heiwick et al. [R.M. Heiwick et al., J. Biol. Chem., 256 , 7990 (1981)]. Consequently, the sequence of 20 amino acid residues at the amino end of the sample was found identical with that of the streptokinase (natural type) derived from Streptococcus equisimilis H46A (Group C). It was possible for the above analysis procedure to identify Trp which was the sixth amino acid residue from the N-terminal and which was not detectable by the amino acid analysis procedure 3)-2 above. The amount of recombinant streptokinase collected by the foregoing method was found to be 2.5 mg by the amino acid analysis of the final purified product sample. The specific plasminogen activator activity was 101.5 international units/µg, which was comparable to that (100 international units/µg) of the natural type. These results revealed that the streptokinase obtained was identical with the streptokinase (of the natural type) derived from Streptococcus equisimilis H46A (Group C). Example 5 Preparation of streptokinase derivative protein expression vector pSKXT (1-372) and expression of the protein A streptokinase derivative protein (1-372) expression vector was prepared by the following procedure using streptokinase expression vector pSKXT obtained in Example 2. Fig. 4 schematically shows the procedure. 1) Chemical synthesis of DNA fragments For the preparation of a streptokinase derivative protein corresponding to streptokinase wherein amino acid residues toward the C-terminal are deficient, the following freshly chemically synthesized oligonucleotide (DNA) fragments F-1 and F-2 were used. These DNA fragments were prepared by the same method as already described. The vector for expressing the derivative protein was prepared by removing fragments having restriction enzyme recognition site Acc I (present at the 367-position of streptokinase and at three other positions) and restriction enzyme recognition site Sal I (present downstream from the termination codon of streptokinase and at another position) from pSKXT, and inserting the fragments F-1 and F-2 into the plasmid. 2) Construction of pSKXT (1-372) pSKXT was treated with restriction enzymes Pst I and Mlu I to obtain a DNA fragment of 1635 bp, with restriction enzymes Pst I and Sal I to produce a DNA fragment of 2962 bp, and further with restriction enzymes Acc I and Mlu I to produce a DNA fragment of 677 bp. These fragments were collected by agarose gel electrophoresis and reacted with the chemically synthesized DNA fragment obtained by the procedure 1) above for ligation. E . coli JM109 was transformed with the reaction mixture, vector DNA collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372). 3) Expression and recognition of derivative protein (1-372) E . coli JM-109 harboring vector pSKXT (1-372) obtained by the procedure 2) was incubated with shaking in the same manner as in Example 3, 1) using M-9 casamino acid liquid medium of the composition listed in Table 5. About 4 hours after the addition of IPTG, incubation was discontinued, and the culture was separated into cells and a culture supernatant in the same manner as in Example 3, 2), and a sucrose buffer fraction and periplasm fraction were obtained. In the same manner as in Example 3, 3) and 4), the fractions were checked for streptokinase activity and subjected to Western blotting, and the amount of expression of protein was determined. Consequently, no activity was detected from the medium supernatant but expression of about 1000 international units/ml was detected from the cell fraction, i.e. almost entirely from the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 42300. These results indicate that the streptokinase derivative protein (1-372) was produced by translation within the cell, then passed through the inner membrane by the action of the signal peptide, further processed for the removal of the signal peptide and secreted. 4) Preparation and identification of recombinant streptokinase derivative protein (1-372) In the same manner as in the procedure 3), 2.4 liters of culture was centrifuged (8000 r.p.m., 10 minutes) to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. Streptokinase derivative protein (1-372) was collected from the extract and purified in the same manner as in Example 4, 2) and finally lyophilized to obtain a purified product. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 42300. With addition of 6N hydrochloric acid, the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using the automatic amino acid analyzer, Model Hitachi 835. Table 7 below shows the result. Amino acid Purified product Calculated Asp 56.6 58 Thr 25.9 27 Ser 20.8 23 Glu 39.5 38 Pro 18.5 17 Gly 18.4 18 Ala 19.4 19 Val 20.3 22 1/2 Cys Not detected 0 Met 3.6 4 Ile 19.9 22 Leu 36.3 37 Tyr 15.8 16 Phe 15.0 15 Trp Not detected 1 Lys 30.2 30 His 7.7 8 Arg 16.7 17 The sequence of 20 amino acid residues from the amino end of the purified product was analyzed by the method of Heiwick et al., and the sequence of 5 residues from the carboxyl terminal thereof was analyzed with use of carboxypeptidases A and B. As a result, the product was identified as the desired streptokinase derivative protein (1-372). The amount of recombinant streptokinase derivative protein (1-372) obtained by the above method was found to be 9 mg by the amino acid analysis of a sample of the purified product. The specific plasminogen activator activity was 107 in mole ratio based on that of the natural-type streptokinase which was taken as 100 and was comparable to that of the natural type. Example 6 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 118 deficient) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, 118 deficient) was prepared by the following procedure using streptokinase derivative expression vector pSKXT (1-372) obtained in Example 5. Fig. 5 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 118 def)" in the figure. 1) Chemical synthesis of DNA fragments The desired derivative protein expression vector was prepared by replacing the sequence of pSKXT (1-372) between restriction enzyme recognition sites Nhe I and Hinf I by the following freshly chemically synthesized oligonucleotide (DNA) fragments a-1 to a-4. These DNA fragments were chemically prepared individually in the same manner as already stated. For the analysis to be performed later, restriction enzyme recognition site BamH I was provided which was not present in pSKXT (1-372). 2) Construction of pSKXT (1-372, 118 deficient) pSKXT (1-372) was treated with restriction enzymes Pst I and Nhe I to form a DNA fragment of 1522 bp, with restriction enzymes Pst I and Sac I to form a DNA fragment of 3488 bp and further with restriction enzymes Sac I and Hinf I to form a DNA fragment of 212 bp. These fragments were collected by agarose gel electrophoresis and reacted with the four DNA fragments chemically synthesized by the procedure 1) above for ligation. (Oligomers a-2 and a-4 were used as phosphorylated at their 5 ends). E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372, 118 deficient). 3) Expression and recognition of derivative protein (1-372, 118 deficient) E . coli JM109 harboring vector pSKXT (1-372, 118 deficient) obtained by the procedure 2) was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. As a result, no activity was detected from the medium supernatant but expression of about 250 international units/ml was detected from the cell fraction, i.e., almost entirely from the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 42200. 4) Preparation and identification of recombinant streptokinase derivative protein (1-372, 118 deficient) In the same manner as in Example 5, 4), 1.6 liters of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 118 deficient) was collected from the extract and purified. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 42200. With addition of 6N hydrochloric acid, the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using the automatic amino acid analyzer, Model Hitachi 835. Table 8 below shows the result. Amino acid Purified product Calculated Asp 57.7 58 Thr 26.5 27 Ser 21.7 23 Glu 39.0 38 Pro 17.4 17 Gly 19.1 18 Ala 19.7 19 Val 19.8 22 1/2 Cys Not detected 0 Met 3.3 4 Ile 20.1 22 Leu 37.5 37 Tyr 15.4 16 Phe 14.0 14 Trp Not detected 1 Lys 30.0 30 His 7.9 8 Arg 16.5 17 The amount of recombinant streptokinase derivative protein (1-372, 118 deficient) obtained by the above method was found to be 2.4 mg by the amino acid analysis of a sample of the purified product. The specific plasminogen activator activity was 100 in mole ratio based on that of the natural-type streptokinase which was taken as 100 and was equivalent to that of the natural type. Example 7 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 45-68 deficient) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, 45-68 deficient) was prepared by the following procedure using streptokinase derivative expression vector pSKXT (1-372) obtained in Example 5. Fig. 6 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 45-68 def)" in the figure. 1) Chemical synthesis of DNA fragments The desired derivative protein expression vector was prepared by replacing the sequence of pSKXT (1-372) at the region between restriction enzyme sites Nsp(7524) V and Xho I by the following freshly chemically synthesized oligonucleotide (DNA) fragments (b-1 and b-2). These fragments correspond to a region of streptokinase wherein the 45- to 68-positions are deficient and were individually chemically synthesized in the same manner as above. 2) Construction of pSKXT (1-372, 45-68 deficient) pSKXT (1-372) was treated with restriction enzymes Pst I and Nsp(7524) V to form a DNA fragment of 1325 bp and with restriction enzymes Pst I and Xho I to form a DNA fragment of 3864 bp. These fragments were collected by agarose gel electrophoresis and reacted with the two DNA fragments chemically synthesized by the procedure 1) above for ligation. E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372, 45-48 deficient). 3) Expression and recognition of derivative protein (1-372, 45-68 deficient) E . coli JM109 harboring vector pSKXT (1-372, 45-68 deficient) obtained by the procedure 2) was incubated with shaking similarly using the same M-9 casamino acid liquid medium, and the cells were collected and checked for streptokinase activity (plasminogen activator activity). Consequently, an amount of expression of about 1200 international units/ml was detected from the cell fraction and was found almost entirely in the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 39600. 4) Preparation and identification of recombinant streptokinase derivative protein (1-372, 45-68 deficient) In the same manner as in Example 5, 4), cells were collected from 800 ml of culture, and a periplasm fraction was extracted from the cells by the osmotic shock method. The desired derivative protein (1-372, 45-68 deficient) was collected from the extract, purified and lyophilized to obtain a purified product. Table 9 below shows the result of amino acid analysis of the product. Amino acid Purified product Calculated Asp 54.3 57 Thr 24.9 25 Ser 19.2 20 Glu 36.7 36 Pro 13.9 14 Gly 13.9 14 Ala 17.0 17 Val 22.0 22 1/2 Cys Not detected 0 Met 3.8 4 Ile 20.0 22 Leu 36.6 36 Tyr 14.7 16 Phe 13.9 14 Trp Not detected 1 Lys 27.2 27 His 6.8 7 Arg 15.1 16 The amount of recombinant streptokinase derivative protein (1-372, 45-68 deficient) obtained by the above method was found to be 2 mg by the amino acid analysis of a sample of the purified product. The specific plasminogen activator activity was 112 in mole ratio based on that of the natural-type streptokinase which was taken as 100 and was comparable to that of the natural type. Example 8 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 256Gln, 257Gln replaced) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, replaced by Gln at each of the 256- and 257-positions) was prepared by the following procedure using streptokinase derivative expression vector pSKXT (1-372) obtained in Example 5. Fig. 7 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 256Gln, 257Gln sub)" in the figure. 1) Chemical synthesis of DNA fragments The desired derivative protein expression vector was prepared by replacing the sequence of pSKXT (1-372) at the region between restriction enzyme sites Hind III and SanB I by the following freshly chemically synthesized oligonucleotide (DNA) fragments (c-1 to c-4). These fragments include a region of streptokinase wherein Gln is substituted at each of the 256- and 257-positions. The four fragments c-1 to c-4 were individually chemically synthesized in the same manner as above. For the analysis to be practiced later, restriction enzyme recognition site EcoR V was further provided to the sequence. 2) Construction of pSKXT (1-372, 256Gln, 257Gln replaced) pSKXT (1-372) was treated with restriction enzymes Hind IIII and Nco I to form a DNA fragment of 608 bp and with restriction enzymes Nco I and SnaB I to form a DNA fragment of 4612 bp. These fragments were collected by agarose gel electrophoresis and reacted with the four DNA fragments chemically synthesized by the procedure 1) above for ligation. (Oligomers c-2 and c-4 were used as phosphorylated at their 5′ ends). E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector. 3) Expression and recognition of derivative protein (1-372, 256Gln, 257Gln replaced) E . coli JM109 harboring the vector obtained by the procedure 2) was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. As a result, no activity was detected from the medium supernatant but expression of about 70 international units/ml was detected from the cell fraction, i.e., almost entirely from the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 42300. 4) Preparation and identification of recombinant streptokinase derivative protein (1-372, 256Gln, 257Gln replaced) In the same manner as in Example 5, 4), 2.4 liters of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 256Gln, 257Gln replaced) was collected from the extract, purified and lyophilized to obtain a purified product. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 42300. Table 10 below shows the result obtained by subjecting the purified product to amino acid analysis. Amino acid Purified product Calculated Asp 56.6 58 Thr 25.8 27 Ser 21.0 23 Glu 40.7 40 Pro 17.5 17 Gly 18.3 18 Ala 19.3 19 Val 19.9 22 1/2 Cys Not detected 0 Met 3.6 4 Ile 19.5 22 Leu 35.9 37 Tyr 15.5 16 Phe 15.1 15 Trp Not detected 1 Lys 27.9 28 His 7.5 8 Arg 16.8 17 The amount of recombinant streptokinase derivative protein (1-372, 256Gln, 257Gln replaced) obtained by the above method was found to be 4 mg by the amino acid analysis of the purified product. The specific plasminogen activator activity was 7 in mole ratio based on that of the natural-type streptokinase which was taken as 100. Example 9 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 118 deficient, 256Gln, 257Gln replaced) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) was prepared by the following procedure using streptokinase derivative expression vector pSKXT (1-372, 118 deficient) obtained in Example 6 and streptokinase derivative protein expression vector pSKXT (1-372, 256Gln, 257Gln replaced) obtained in Example 8. Fig. 8 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 118 def, 256Gln, 257Gln sub)" in the figure. 1) Construction of pSKXT (1-372, 118 deficient, 256Gln, 257Gln replaced) pSKXT (1-372, 118 deficient) was treated with restriction enzymes Bgl II and Hind III to form a DNA fragment of 515 bp. pSKXT (1-372, 256Gln, 257Gln replaced) was treated with restriction enzymes Pst I and Bgl II to form a DNA fragment of 1443 bp and with restriction enzymes Pst I and Hind III to form a DNA fragment of 3335 bp. These DNA fragments were collected by agarose gel electrophoresis and ligated together by reaction. E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372, 118 deficient, 256Gln, 257Gln replaced). 2) Expression and recognition of derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) E . coli JM109 harboring the vector obtained by the procedure 1) was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. As a result, no activity was detected from the medium supernatant but expression of about 160 international units/ml was detected from the cell fraction, i.e., almost entirely from the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 42200. 3) Preparation and identification of recombinant streptokinase derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) In the same manner as in Example 5, 4), 1.2 liters of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) was collected from the extract and purified. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 42200. With addition of 6N hydrochloric acid, the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using the automatic amino acid analyzer, Model Hitachi 835. Table 11 shows the result. Amino acid Purified product Calculated Asp 58.8 58 Thr 26.4 27 Ser 21.4 23 Glu 40.0 40 Pro 17.2 17 Gly 18.4 18 Ala 19.3 19 Val 19.9 22 1/2 Cys Not detected 0 Met 3.7 4 Ile 19.5 22 Leu 37.6 37 Tyr 15.7 16 Phe 13.9 14 Trp Not detected 1 Lys 27.7 28 His 7.7 8 Arg 16.7 17 The amount of recombinant streptokinase derivative protein (1-372, 118 deficient, 256Gln, 257Gln replaced) obtained by the above method was found to be 5.4 mg by the amino acid analysis of the purified product. The specific plasminogen activator activity was 9.98 in mole ratio based on that of the natural-type streptokinase which was taken as 100. Example 10 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 45-68 deficient, 256Gln, 257Gln replaced) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) was prepared by the following procedure using streptokinase derivative expression vectors pSKXT (1-372, 45-68 decicient) and pSKXT (1-372, 256Gln, 257Gln replaced) obtained in Examples 7 and 8, respectively. Fig. 9 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 45-68 def, 256Gln, 257Gln sub)" in the figure. 1) Construction of pSKXT (1-372, 45-68 deficient, 256Gln, 257Gln replaced) pSKXT (1-372, 45-68 deficient) was treated with restriction enzymes Pst I and Mlu I to form a DNA fragment of 1563 bp. pSKXT (1-372, 256Gln, 257Gln replaced) was treated with restriction enzymes Pst I and Mlu I to form a DNA fragment of 3661 bp. These DNA fragments were collected by agarose gel electrophoresis and ligated together by reaction. E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372, 45-68 deficient, 256Gln, 257Gln replaced). 2) Expression and recognition of derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) E . coli JM109 harboring the vector obtained by the procedure 1) was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. As a result, no activity was detected from the medium supernatant but expression of about 40 international units/ml was detected from the cell fraction, i.e., almost entirely from the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 39800. 3) Preparation and identification of recombinant streptokinase derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) In the same manner as in Example 5, 4), 0.8 liter of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) was collected from the extract and purified. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 39800. With addition of 6N hydrochloric acid, the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using the automatic amino acid analyzer, Model Hitachi 835. Table 12 shows the result. Amino acid Purified product Calculated Asp 54.4 57 Thr 24.9 25 Ser 18.9 20 Glu 39.0 38 Pro 14.6 14 Gly 14.1 14 Ala 16.9 17 Val 21.5 22 1/2 Cys Not detected 0 Met 3.8 4 Ile 20.1 22 Leu 36.7 36 Tyr 15.2 16 Phe 13.9 14 Trp Not detected 1 Lys 25.0 25 His 6.5 7 Arg 15.3 16 The amount of recombinant streptokinase derivative protein (1-372, 45-68 deficient, 256Gln, 257Gln replaced) obtained by the above method was found to be 0.56 mg by the amino acid analysis of the purified product. The specific plasminogen activator activity was 6.30 in mole ratio based on that of the natural-type streptokinase which was taken as 100. Example 11 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) was prepared by the following procedure using streptokinase derivative expression vectors pSKXT (1-372, 118 deficient) and pSKXT (1-372, 45-68 deficient, 256Gln, 257Gln replaced) obtained in Example 6 and 10, respectively. Fig. 10 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 45-68 def, 118 def, 256Gln, 257Gln sub)" in the figure. 1) Construction of pSKXT (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) pSKXT (1-372, 118 deficient) was treated with restriction enzymes Bgl II and Hind III to form a DNA fragment of 515 bp. pSKXT (1-372, 45-68 deficient, 256Gln, 257Gln replaced) was treated with restriction enzymes Pst I and Bgl II to form a DNA fragment of 1371 bp and with restriction enzymes Pst I and Hind III to form a DNA fragment of 3335 bp. These DNA fragments were collected by agarose gel electrophoresis and ligated together by reaction. E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector pSKXT (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced). E . coli JM109 harboring the above vector has been deposited in Fermentation Research Institute, Agency of Industrial Science and Technology, MITI with the designation " Escherichia coli , JM-109, pSKXT (1-372, Δ45-68, Δ118, QQ") and the deposition number FERM BP-2828. 2) Expression and recognition of derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) E . coli JM109 (FERM BP-2828) harboring the vector obtained by the procedure 1) was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. Consequently, no activity was detected from the medium supernatant but an amount of expression of about 170 international units/ml was detected from the cell fraction and was almost entirely found in the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 39700. 3) Preparation and identification of recombinant streptokinase derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) In the same manner as in Example 5, 4), 1.2 liters of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) was collected from the extract and purified. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 39700. With addition of 6N hydrochloric acid, the purified product was hydrolyzed at 110°C for 24 hours and subjected to amino acid analysis by the ninhydrin method using the automatic amino acid analyzer, Model Hitachi 835. Table 13 shows the result. Amino acid Purified product Calculated Asp 57.8 57 Thr 24.6 25 Ser 18.7 20 Glu 37.9 38 Pro 14.4 14 Gly 14.1 14 Ala 17.0 17 Val 19.6 22 1/2 Cys Not detected 0 Met 3.6 4 Ile 19.5 22 Leu 36.2 36 Tyr 15.7 16 Phe 12.9 13 Trp Not detected 1 Lys 24.5 25 His 7.1 7 Arg 16.1 16 The amount of recombinant streptokinase derivative protein (1-372, 45-68 deficient, 118 deficient, 256Gln, 257Gln replaced) obtained by the above method was found to be 2.4 mg by the amino acid analysis of the purified product. The specific plasminogen activator activity was 7.74 in mole ratio based on that of the natural-type streptokinase which was taken as 100. Example 12 Preparation of streptokinase derivative protein expression vector pSKXT (1-372, 256, 257Lys-Pro-Lys-Pro altered) and expression of the protein A vector for expressing a streptokinase derivative protein (1-372, altered to -Lys-Pro-Lys-Pro- at the 256- and 257-positions) was prepared by the following procedure using streptokinase derivative expression vector pSKXT (1-372) obtained in Example 5. Fig. 11 schematically shows the procedure. The desired vector is shown as "pSKXT (1-372, 256, 257LysProLysPro rep)" in the figure. 1) Chemical synthesis of DNA fragments The desired derivative protein expression vector was prepared by replacing the sequence of pSKXT (1-372) at the region thereof between restriction enzyme recognition sites Hind III and SnaB I by the new DNA fragments d-1 to d-4 chemically synthesized and represented below. These fragments include Lys-Pro-Lys-Pro substituted for Lys-Lys at the 256- and 257-positions of streptokinase and are used for an alteration by replacing some amino acid residues by other amino acid residues according to the invention. The four oligonucleotides d-1 to d-4 were individually synthesized chemically. For the analysis to be performed later, new restriction enzyme recognition site EcoR V was provided. 2) Construction of pSKXT (1-372, 256, 257Lys-Pro-Lys-Pro altered) pSKXT (1-372) was treated with restriction enzymes Hind III and Nco I to form a DNA fragment of 608 bp and with restriction enzymes Nco I and SnaB I to form a DNA fragment of 4612 bp. These fragments were collected by agarose gel electrophoresis and reacted with the four chemically synthesized DNA fragments (with the oligomers d-2 and d-4 phosphorylated at the 5′ ends). E . coli JM109 was transformed with the reaction mixture, vector DNA was collected from the resulting colonies and purified, and a restriction enzyme map was prepared to obtain the desired vector. 3) Expression and recognition of derivative protein (1-372, 256, 257Lys-Pro-Lys-Pro altered) E . coli JM109 harboring the vector obtained by the procedure 2) above was incubated with shaking in the same manner as already stated using the same M-9 casamino acid liquid medium as above. After the addition of IPTG, the cells were collected and checked for streptokinase activity (plasminogen activator activity) similarly. Consequently, no activity was detected from the medium supernatant but an amount of expression of about 7 international units/ml was detected from the cell fraction and was almost entirely found in the periplasm fraction. Further analysis by Western blotting revealed the immunoactivity at the position of calculated molecular weight of about 42500. 4) Preparation and identification of recombinant streptokinase derivative protein (1-372, 256, 257Lys-Pro-Lys-Pro altered) In the same manner as in Example 5, 4), 2.4 liters of culture was centrifuged to collect the cells, from which a periplasm fraction was extracted by the osmotic shock method. The desired derivative protein (1-372, 256, 257Lys-Pro-Lys-Pro altered) was collected from the extract and purified to obtain a lyophilized purified product. When analyzed by SDS-PAGE in the presence of a reducing agent, the purified product exhibited a single band at the position of contemplated molecular weight of about 42500. Table 14 below shows the result obtained by subjecting the purified product to amino acid analysis. Amino acid Purified product Calculated Asp 60.6 58 Thr 27.2 27 Ser 22.4 23 Glu 38.7 38 Pro 18.1 19 Gly 18.5 18 Ala 19.9 19 Val 20.7 22 1/2 Cys Not detected 0 Met 3.4 4 Ile 20.2 22 Leu 38.3 37 Tyr 15.8 16 Phe 14.8 15 Trp Not detected 1 Lys 30.1 30 His 8.1 8 Arg 16.1 17 The amount of recombinant streptokinase derivative protein (1-372, 256, 257Lys-Pro-Lys-Pro altered) thus obtained was found to be 0.46 mg by the amino acid analysis of the purified product. The specific plasminogen activator activity was 17.7 in mole ratio based on that of the natural-type streptokinase which was taken as 100.

Claims: Claims for the following Contracting States : AT, CH, DE, DK, FR, GB, IT, LI, NL, SE A chemically synthesized streptokinase gene having the base sequence: or the base sequence: A plasmid recombinant comprising a plasmid vector having introduced therein the chemically synthesized gene defined in claim 1. A transformant transformed by the plasmid recombinant defined in claim 2. A streptokinase derivative protein having streptokinase activity and a modified primary amino acid sequence corresponding to the primary amino acid sequence represented by the following formula (1) of streptokinase which is deficient in the amino acid residues at the 373-position to the C-terminal, at least one of the amino acid residues of the sequence of the formula (1) being or not being deficient or replaced, or at least one amino acid residue being or not being inserted in the sequence of the formula (1). A streptokinase derivative protein as defined in claim 4 which has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Arg at the 45-position to Gly at the 68-position are further deficient. A streptokinase derivative protein as defined in claim 4 which has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Phe at the 118-position is further deficient or replaced by other amino acid residue. A streptokinase derivative protein as defined in claim 4 which has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Lys at the 256-position and Lys at the 257-position are each deficient or replaced by other amino acid residue. A streptokinase derivative protein as defined in claim 4 which has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein other amino acid residue is further inserted in the position next to each of Lys at the 256-position and Lys at the 257-position. A streptokinase derivative protein as defined in claim 4 which has a modified primary amino acid sequence including the combination of at least two of the modifications defined in claims 5 to 8. A chemically synthesized gene coding for the streptokinase derivative protein defined in any one of claims 4 to 9. A plasmid recombinant comprising a plasmid vector having introduced therein the chemically synthesized gene defined in claim 10. A transformant transformed by the recombinant defined in claim 11. A transformant as defined in claim 12 wherein the host cell is E. coli. A process for preparing a streptokinase derivative protein characterized by incubating the transformant defined in claims 12 or 13 and collecting and purifying the streptokinase derivative protein expressed. Claims for the following Contracting State : ES A process for preparing a plasmid recombinant comprising a plasmid vector, characterized by introducing therein a chemical synthesized streptokinase gene having the base sequence or the base sequence: A process for preparing a transformant comprising the transformation of a host cell by the plasmid recombinant defined in claim 1. A process for preparing a plasmid recombinant comprising a plasmid vector, characterized by introducing therein a chemically synthesized gene coding for a streptokinase derivative protein having streptokinase activity and a modified primary amino acid sequence corresponding to the primary amino acid sequence represented by the following formula (1) of streptokinase which is deficient in the amino acid residues at the 373-position to the C-terminal, at least one of the amino acid residues of the sequence of the formula (1) being or not being deficient or replaced, or at least one amino acid residue being or not being inserted in the sequence of formula (1). A process according to claim 3, characterized in that the streptokinase derivative protein has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Arg at the 45-position to Gly at the 68-position are further deficient. A process according to claim 3, characterized in that the streptokinase derivative protein has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Phe at the 118-position is further deficient or replaced by other amino acid residue. A process according to claim 3, characterized in that the streptokinase derivative protein has a modified primary amino acid sequence corresponding to the sequence of the formula (1) wherein Lys at the 256-position and Lys at the 257-position are each deficient or replaced by other amino acid residue. A process according to claim 3, characterized in that the streptokinase derivative protein has a modified primary amino acid sequence corresponding to the sequence of the fomula (1) wherein other amino acid residue is further inserted in the position next to each of Lys at the 256-position and Lys at the 257-position. A process according to claim 3, characterized in that the streptokinase derivative protein has a modified primary amino acid sequence including the combination of at least two of the modifications defined in claims 4 to 7. A process for preparing a transformant comprising the transformation of a host cell by the plasmid recombinant defined in any of claims 3 to 8. A process according to claim 9, wherein the host cell is E. coli. A process for preparing a streptokinase derivative protein, characterized by incubating a transformant as defined in any of claims 9 and 10 and collecting and purifying the streptokinase derivative protein expressed.

Inventors: FUJII SETSURO, KATANO TAMIKI, MAJIMA EIJI, OGINO KOICHI, ONO KENJI, SAKATA YASUYO, UENOYAMA TSUTOMU, FUJII, SETSURO, KATANO, TAMIKI, MAJIMA, EIJI, OGINO, KOICHI, ONO, KENJI, SAKATA, YASUYO, UENOYAMA, TSUTOMU
Applicants: OTSUKA PHARMA CO LTD, OTSUKA PHARMACEUTICAL FACTORY, INC.
Application year: 1990
Main classification: C12N15
IPC codes: C12N9, C12N15, C07K14