Polypeptide having gamma-interferon activity lacking amino acids coded by exon 4
This application is a continuation of application Ser. No. 614,130, filed on May 25, 1984, now abandoned. 
    
    
     TECHNICAL FIELD 
     The present invention is directed to a novel polypeptide having properties similar to naturally occurring human gammainterferon (IFN-γ). The present invention also relates to a novel plasmid capable of producing the novel polypeptide and a microorganism containing said plasmid. 
     BACKGROUND OF THE INVENTION 
     The gene in the human body which carries the genetic information responsible for the synthesis of gamma-interferon contains certain sequences of bases which code for the amino acid sequence of gamma-interferon (exons) and certain sequences which do not code for gamma-interferon (introns). In a human cell, the gamma-interferon gene creates RNA in the nucleus of the cell. This RNA contains both introns and exons. The introns, which do not code for a sequence of the gamma-interferon, are deleted to generate messenger RNA (mRNA). After the introns are deleted, the resulting mRNA directs the synthesis of natural gamma-interferon in the cytoplasm of the cell. 
     The amino acid sequence of human gamma-interferon has been identified, and DNA which codes for the gamma-interferon has been incorporated into, for example, E. coli which is capable of producing the gamma-interferon (See, European Patent Application Publication No. 0077670, published on 27 Apr. 1983). 
     SUMMARY OF THE INVENTION 
     The present invention relates to a novel polypeptide having human gamma-interferon activity which has an amino acid sequence which is different from naturally occurring human gamma-interferon as reported by Gray et al., Nature 295, 503 (1982). It has been surprisingly discovered that this polypeptide, which is enclosed by a polynucleotide lacking exon 4, possesses human gamma-interferon activity. 
     A radioactive probe was utilized to identify plaques from a bacteriophage lambda/human genomic library (gift from Professor T. Maniatis) containing the human gamma-interferon gene. One of the clones which appeared to contain the entire gamma-interferon gene on the basis of restriction enzyme analysis was cleaved with BamHI, and the fragment containing the gamma-interferon gene was ligated into BamHI-cleaved pBR327 (Soberon et al., Gene 9: 287 (1980)) to produce a new plasmid designated pCG5 (11.7 kb). This new plasmid was incorporated into a host microorganism to produce a transformed microorganism. pCG5 was then cleaved with MstII and SalI to excise a portion of extraneous DNA and religated to form a new plasmid pCG51 (8.7 kb). This plasmid was then further cleaved and a portion was ligated with a segment excised from pCG5 to produce a new plasmid pCG52 (6.4 kb). pCG52 in turn was cleaved and religated to yield another new plasmid pCG53 (5.6 kb). A segment from this plasmid was then inserted into a phage vector (M13mp9) and used to transfect E. coli. An attempt was made to delete introns 1, 2, and 3 from the phage containing the gamma-interferon gene. Unexpectedly, during the above procedure portions of the genetic material which code for gamma-interferon (portions of exons 1 and 4) were deleted. However, the deleted nucleotide sequence does not appear to be necessary for gamma-interferon activity. A phage capable of expressing a polypeptide having gamma-interferon activity was obtained by further genetic manipulation and utilized in E. coli to prepare a material having gamma-interferon activity. 
     The genetic sequence responsible for the production of this activity was excised from the M13mp9 bacteriophage derivative and inserted into pBR327 to prepare the plasmid pGLT104. Surprisingly, E. coli harboring this latter plasmid contains a polypeptide in the cytoplasm thereof which possesses gamma-interferon activity in a concentration which is approximately 4 to 10 times higher than the activity present in a microorganism utilized for comparison purposes which contains the entire gamma-interferon gene. Analysis of the nucleotide sequence of pGLT104 indicates that this polynucleotide contains a sequence of nine nucleotides which are the same as the first nine nucleotides of exon 4. However, these nine nucleotides are not derived from exon 4 but are derived from the deleter which is utilized to delete intron 3. 
     The present invention is therefore directed to a novel polypeptide having gamma-interferon activity wherein the portion of said polypeptide which is coded for by all but the first nine nucleotides of exon 4 is deleted. Thus, the polypeptide of the present invention contains a sequence of amino acids coded for by the last 18 amino acids of exon 1, exon 2, exon 3 and the first nine nucleotides of exon 4, respectively. In addition, the polypeptide contains an additional terminal sequence of amino acids which is coded for by a portion of the nucleotides of intron 3. Thus, the present invention is further directed to a polypeptide coded for by a nucleotide sequence in which a substantial thermal portion by exon 4 is deleted and is replaced with a nucleotide sequence derived from intron 3. The polynucleotide which codes for the polypeptide is representable by the general formula (a terminal portion of exon 1) (exon 2)-(exon 3)-(an initial portion of exon 4)-(a portion of intron 3), (the last fifty-four nucleotides of exon 1)-(exon 2)-(exon 3)-(the first nine nucleotides of exon 4)-(twenty-four nucleotides derived from intron 3) or (Cys Tyr Cys Gln Asp Pro Tyr Val Lys Glu Ala Glu Asn Leu Lys Lys Tyr Phe)-(exon 2)-(exon 3)-(GTC ACT GAC)-(CAT CAT GAC ATT AGC AGA ATA TCC). The specific amino sequence of the polypeptide is shown in FIG. 7 by the amino acids coded for by nucleotides 1579 through 1241. 
     The invention is also directed to a novel polynucleotide sequence which codes for the new polypeptide and the complementary strand thereof, a novel replicable expression vehicle such as a plasmid containing said nucleotide sequence, a novel microorganism containing said plasmid, mutants of said microorganism and various intermediate products utilized in the preparation of said microorganism. For example, one aspect of the present invention is directed to a vector, such as a bacteriophage, which contains the nucleotide sequence of the present invention. 
     The present invention also is directed to a method for preparing the above-discussed plasmids and microorganisms, a method for preparing a polypeptide having gamma-interferon activity, and pharmaceutical compositions containing the polypeptide produced in accordance with the present invention. Genetic material capable of producing polypeptides having interferon activity in accordance with the present invention may be incorporated into various types of replicable expression vehicles that are compatible with different strains of bacteria, such as E. coli, or vehicles which are compatible with yeast, mammalian cells, or other cells capable of producing or encoding the polypeptide of the present invention after transformation. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is a flow chart showing construction of pCG5; 
     FIGS. 2 and 3 are a schematic diagram illustrating the construction of pCG53; 
     FIG. 4 shows the portion of the nucleotide sequence of pCG53 which contains the inserted gene of interest; 
     FIG. 5 summarizes the events leading to the nucleotide sequence of mp9-TGdl; 
     FIG. 6 shows the nucleotide sequence of mp9-TGΔ123-1; and 
     FIG. 7 shows the nucleotide sequence of pGLT104. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Synthesis of DNA Oligomers Using Nucleotide Monomers 
     To a stainless steel 500 μl reaction vessel with stainless steel filters at each end is added 20 mg of a polystyrene resin to which a nucleoside (2.0 μmoles) is connected via a succinate linkage. The resin is treated with zinc bromide (1M) in dichloromethane/isopropanol (85:15) to remove the dimethoxytrityl (DMT) protecting group, washed with dimethylformamide, pyridine, and acetonitrile, and dried with a stream of nitrogen. To the dried resin is added a solution of DMT-nucleotide (20 μM) and mesitylenesulfonylnitrotriazole (60 μM) in 200 μl pyridine. The coupling reaction is allowed to proceed at 45° C. for 20 minutes. This cycle of deprotection and coupling is repeated for successive nucleotides until the desired DNA oligomer has been assembled on the resin. The resin is then treated to remove the DNA oligomer from the resin and purified as described by Ito et al, (Nucl. Acids Res. 10: 1755 (1982)). 
     Isolation of the Human Genomic Gamma-Interferon Gene 
     As illustrated in FIG. 1, a  32  P-labelled synthetic 25-mer oligonucleotide, 5`CCATTATCCGCTACATCTGAATGAC3`, complementary to a sequence in exon 2, is used as a hybridization probe to screen 10 6  plaques of bacteriophage Charon 4A/human genomic library prepared by insertion into the EcoRI ligation site of Charon 4A vector (Blattner et al., Science 196: 161 (1977)) sized fragments from EcoRI digested hhuman DNA (Maniatis et al., Cell 15: 687 (1978)). The plaque hybridization method of Benton et al, (Science 196: 180 (1977)) is used. Since not all of the bacteriophage in culture contain the necessary genetic material for preparing gamma-interferon a probe which has a base sequence complementary to the exon 2 portion of the gamma-interferon gene is used. Plaques having the desired genetic material are identified by the binding of the radioactive probes to their DNA. Two hybridizing plaques are isolated from the library. DNA from each recombinant phage is isolated and digested with five restriction endonucleases (EcoRI, BamHI, PvuII, HindIII, and HincII). Each phage DNA is analyzed by gel hybridization with the 25-mer probe, after digestions with the above enzymes. Comparison of these hybridization patterns with those of Gray et al, (Nature 298: 859 (1982)) indicates that one phage DNA (clone 5-1) appears to contain the entire gamma-interferon gene. Clone 5-1 is selected for further study. 
     Construction of Bacterial Clones Containing Human Genomic Gamma-Interferon Gene 
     The method of Landy et al. (Biochemistry 13: 2134 (1974)) is used to obtain DNA of clone 5-1. This DNA is digested with BamHI and the digest is electrophoresed on 1% low melting agarose gel. The 8.5 kb band is isolated from the agarose gel as described by T. Maniatis (p. 377, Molecular Cloning, Cold Spring Harbor Laboratory, 1982). This fragment is cloned to BamHI-cleaved pBR327. The plasmid obtained is used to transform E. coli K12 strain MC1061. Ten bacterial clones are screened using the miniprep technique of Holmes et al, (Anal. Biochem. 114: 193 (1981)). In this way, a colony which contains the desired genomic segment is obtained. The plasmid DNA prepared from this colony is designated pCG5. 
     Construction of pCG53 Containing Human Mini-Genomic Gamma-Interferon Gene 
     The DNA in the plasmid pCG5 is found to read in the counterclockwise direction. From pCG5, pCG53 is constructed as indicated in FIGS. 2 and 3. pCG5 (5 μg) is cut with MstII (5 units) and SalI (8 units). After digestion, the Klenow fragment of E. coli DNA polymerase I (14 units) is added, and the mixture is adjusted to 0.5 mM with dATP, dGTP, dCTP, and dTTP. The reaction is allowed to proceed for 5 minutes at room temperature. By this treatment, the single-stranded end of the fragment is filled in with the complementary nucleotides to form blunt double-stranded ends. After electrophoresis on 1% low melting agarose gel, the 8.7 kb size DNA is eluted. Two μg of this fragment is recovered. This fragment (60 ng) is treated with T4 DNA ligase (10 units) to ligate the ends of the fragment and form a circular product. The product is used to transform E. coli MC1061. In this way,  41 transformants are obtained. Unexpectedly, the plasmid DNA of one of these transformants has a SalI site located at the ligation site. The plasmid DNA from this clone is designated pCG51. 
     To the plasmid DNA (10 μg) (FIG. 2) of pCG51 clone is added AccI restriction enzyme (10 units). After digestion, the Klenow fragment of E. coli DNA polymerase (14 units) is added, and the mixture is adjusted to 0.5 mM with dATP, dGTP, dCTP and dTTP. The reaction is allowed to continue for 30 minutes at room temperature. The product is then digested with EcoRI (20 units). The final digest is electrophoresed on 1% low melting agarose gel, and 4.5 kb size DNA is eluted. The plasmid pCG5 (5 μg) is then digested with AvaII (10 units). After digestion, the Klenow fragment of E. coli DNA polymerase I (14 units) is added, and the reaction mixture is adjusted to 0.5 mM with dATP, dGTP, dCTP and dTTP. After reaction for 30 minutes at room temperature, the product is cut with EcoRI (10 units). After electrophoresis on 1% low melting agarose gel, 1.9 kb DNA is eluted. These two fragments (the 4.5 kb and the 1.9 kb fragments) are then ligated with T4 ligase for 2 hours at 15° C. The ligated product is used to transform E. coli MC10761 and E. coli D1210. The plasmid DNA isolated from the clones of the transformed E. coli is examined by restriction enzyme digestion. Plasmid DNA with the desired digestion pattern is obtained from a clone and designated pCG52. A culture of E. coli containing the plasmid pCG52 has been deposited with the American Type Culture Collection (ATCC) in Rockville, Md. in accordance with the provisions of the Budapest Treaty and has been given Deposit No. ATCC 39665. The plasmid pCG52 carries a gene coding for ampicillin resistance. E. coli containing this plasmid should be cultivated in L-broth containing 20 μg/ml ampicillin. The E. coli deposited at ATCC 39665 is a gram negative rod which is lac - , leu - , ara -  and str r . 
     pCG52 (50 μg) is cut with SacI (50 units) and XbaI (100 units), and the single-stranded ends are rendered blunt by digestion with S1 nuclease (2.9 units) for 5 minutes at 37° C. After electrophoresis on 1% low melting agarose gel, the 5.6 kb size DNA is eluted. Six μg of DNA is recovered. This DNA (100 ng) is circularized by treatment with T4 ligase (10 units) for 2 hours at 15° C. The circularized DNA is used to transform E. coli MC1061 and E. coli D1210. The plasmid DNA isolated from the clones of the transformed E. coli is examined by restriction enzyme digestion. Plasmid DNA with the desired digestion pattern is obtained from a clone and designated pCG53. The portion of the nucleotide sequence of pCG53 which contains the desired gene is shown in FIG. 4. A culture of E. coli containing the plasmid pCG53  has been deposited with the ATCC in accordance with the provisions of the Budapest Treaty and has been given Deposit No. ATCC 39666. This bacteria should be cultivated in the same manner as ATCC 39665. The E. coli deposited as ATCC 39666 is lac - , leu - , ara -  and str r . 
     Construction of M13mp9-TG2 Containing the Human Minigene for Gamma-Interferon from pCG53 
     Plasmid pCG53 (10 μg) is digested with SalI (20 units) and BamHI (20 units). After electrophoresis on 1% low melting agarose gel, the 2.6 kb fragment is eluted. This fragment is inserted into the BamHI/SalI fragment from the replicative form of M13mp9 phage. The product is transfected to E. coli JM103 (BRL User Manual/M13mp7 cloning/`Dideoxy` sequencing, 1980). The product is designated M13mp9-TG2. 
     Deletion of Introns 1, 2, and 3, Using M13mp9-TG2 Single Strand DNA and Deleters E1-2, E2-3, and E3-4 
     The single-strand DNA and M13mp9-TG2 is prepared by the method of BRL User Manual/M13mp7 cloning/`Dideoxy` sequencing, 1980. E1-2, (5`ACCTGCATTAAAATATTTCTTAAGG3`), is a deleter for intron 1. E2-3, (5`GTCACTCTCCTCTTTCCAATT3`), is a deleter for intron 2. E3-4, (5`GTCAGTTACCGAATAATTAGT3`), is a deleter for intron 3. 
     The deleters have a base sequence which is complementary to the base sequence of the bases before and after the intron to be deleted. Thus, the deleters should attach to the appropriate base sequences before and after the intron causing the intron to be deleted to loop out. Fill-in of the second strand comprising the deleters would delete the introns as described in Wallace et al., Science 209: 1396 (1980). 
     E1-2 (124 ng, 15 pmole), E2-3 (104 ng, 15 pmole) and E3-4 (104 ng, 15 pmole) are phosphorylated using T4 kinase (10 units) and ATP (3 mM) and are added to the template M13mp9-TG2 (1.65  μg, 0.5 pmole). The reaction mixture is heated at 65° C. for 5 minutes, cooled to room temperature for 5 minutes, and finally cooled in ice water. To dATP, dCTP, dGTP, dTTP and ATP (0.4 mM) is added Klenow fragment of E. coli DNA polymerase I (5 units), T4 DNA ligase (10 units) in Hin buffer (Wallace et al., Nuc. Ac. Res. 9: 3647 (1981)), 10 mM Tris.HCl (pH 7.2), 2 mM MgCl 2  and 1 mM beta-mercaptoethanol. The reaction mixture (final volume 50 μl) is incubated for 30 minutes at 4° C. and then for 30 minutes at room temperature. The DNA from the oligonucleotide-primed reaction is used to transfect E. coli JM103 by the procedure of the BRL User Manual/M13mp7 cloning/`Dideoxy` sequencing, 1980. From 2 μl of the 50 μl reaction volume, 10,000 plaques are obtained. From these, 150 plaques are picked to YT plates (J. H. Miller, p. 433, Experiments in Molecular Genetics, Cold Spring Harbor Laboratory (1972)). The colonies obtained are hybridized at 55° C. for 2 hours with  32  P-labelled E1-2, E2-3, and E3-4. For this step the deleters are used as probes to identify sequences of DNA having the corresponding complementary base sequence after the introns have been deleted. The numbers of plaques out of the 150 plaques hybridizing with each of the three deleters or combinations of them are given below. 
     
         ______________________________________                                    
E1-2               6                                                      
E2-3               22                                                     
E3-4               14                                                     
E1-2 + E2-3        2                                                      
E1-2 + E3-4        1                                                      
E2-3 + E3-4        7                                                      
E1-2 + E2-3 + E3-4 0                                                      
______________________________________                                    
 
    
     One of the two clones hybridizing with both E1-2 and E2-3, mp9-TGdl, is used in an attempted deletion of intron 3. Subsequent experiments indicate that the exon 4 portion of the DNA of mp9-TGdl is missing. The events leading to mp9-TGdl are summarized in FIG. 5. Deleter E3-4 (104 ng, 15 pmole) was kinased using T4 kinase (10 units) and ATP (3 mM). The template mp9-TGdl single strand DNA (1.6 μg, 0.5 pmole) is added, an the mixture is heated at 65° C. for 5 minutes, cooled at room temperature for 5 minutes, and then cooled in ice water. To a 0.4 mM solution of dATP, dCTP, dGTP, dTTP and ATP is added Klenow fragment of E. coli DNA polymerase I (5 units) and T4 DNA ligase (10 units) in Hin buffer and treated as described above. The resultant phage is plated, and 150 plaques are picked to YT plates. The clones are allowed to hybridize at 55° C. for 2 hours with  32  P-labelled E3-4. Two positive clones, mp9-TGΔ123-1, and mp9-TGΔ123-2 are obtained. When clone mp9-TGΔ123-1 is sequenced, intron 1 is found to be completely deleted, but this clone does not hybridize with the 23-mer primer/mutator (5`GCAGGTCGACCATTACTGGGATG3`) of which the terminal 19 nucleotides are complementary to the terminal 8 nucleotides of exon 4 and the 11 subsequent nucleotides. This result suggests that a portion of exon 4 is missing. Examination of mp9-TGdl reveals that this also does not complement the 23-mer primer mutator. Apparently a portion of exon 4 is lost prior to the second treatment with E3-4. The phage mp9-TGΔ123-1 has been deposited with the ATCC in accordance with the provisions of the Budapest Treaty and has been given deposit number ATCC 40117. The nucleotide sequence of the relevant portion of mp9-TGΔ123-1 is shown in FIG. 6. This phage can be grown under conditions which are normally used to cultivate M13mp9 phage. The ATCC deposit 40117 is a deposit of the phage in a lysate supernate. 
     Construction of mp9-TGd-Δ123-1 for Expression 
     The sequence between the ATG start codon at the termination of the promoter and the second cysteine codon of the gamma-interferon sequence is deleted fro mp9-TGdl using D-4 deleter, (5`GGTCCTGGCAGTAACACATAGCTGTTTCC3`), by an analogous procedure to obtain mp9-TGΔ123-1. Sequencing confirms that deletion of the desired portion between the ATG start codon of the lac promoter and the second cysteine codon has been been achieved. As can be seen from FIG. 5, the D-4 deleter deleted the codons for the first 19 amino acids of exon 1. 
     Construction of pGLT104 
     The replicative form of mp9-TGΔ123-1 is digested with AvaI and BamHI. The fragment from the AvaI at position 5826 extending to the single BamHI site 3&#39; to the interferon gene sequence is isolated and cloned to BamHI/AvaI-cleaved pBR327 to yield the plasmid pGLT104. The plasmid pGLT104 is incorporated into E. coli strains MC1061 and D1210. A culture of E. coli D1210 containing the plasmid pGLT104 has been deposited with the ATCC in accordance with the provisions of the Budapest Treaty and has been given Deposit Number ATCC 39667. The plasmid pGLT104 carries a gene coding for ampicillin resistance. A culture of E. coli containing this plasmid should be cultivated in L-broth containing 20 μg/ml ampicillin. The E. coli containing pGLT104 which is identified as ATCC 39667 is a gram negative rod which is lac - , leu - , ara -  and str r . The nucleotide sequence of pGLT 104 is set forth in FIG. 7. The amino acid sequence of the polypeptide of the present invention is shown in FIG. 7. The polynucleotide sequence shown in FIG. 7 is the sequence of the complementary strand of DNA. The nucleotide sequence of the coding strand of the DNA is easily determined by determining the complementary nucleotides. Thus, the coding strand of the DNA has the nucleotide sequence GTC ACT GAC CAT CAT GAC ATT AGC AGA ATA TCC from nucleotides 1273 to 1241 whereas the corresponding complementary strand (reading in the 3&#39; to 5&#39; direction) has the sequence CAG TGA CTG GTA GTA CTG TAA TCG TCT TAT AGG. As shown in FIG. 7, the nucleotide base sequence &#34;CAG TGA CTG&#34; which begins at base pair 1273 has a sequence which is the same as the first nine nucleotides of the complementary strand of exon 4. As noted above, these nucleotides are derived from the E 3-4  deleter. Therefore, the resulting polypeptide having gamma-interferon activity lacks the amino acids coded by the terminal portion of exon 4 but instead contains a sequence of eight amino acids coded by twenty-four nucleotides derived from intron 3. 
     This clone has a BamHI site, but the AvaI site is no longer evidenced. E. coli containing pGLT104 is cultured in a conventional nutrient medium. Bioassay of the product of gamma-interferon activity indicates 4 to 10 times higher activity measured in terms of units/liter (u/l) than is obtained with a plasmid (pGLY102) containing the complete gamma-interferon gene under control of the lac promoter. Bioassay is conducted using a line (WISH) of human amnion cells by the procedure of Rubinstein et al. (J. Virology 37: 755 (1981)). 
     
         ______________________________________                                    
              Exp. 1     Exp. 2                                           
______________________________________                                    
pGLY102 (E. coli MC1061)                                                  
                1.2 × 10.sup.3 U/1                                  
                             5.0 × 10.sup.3 U/1                     
pGLY102 (E. coli D1210)                                                   
                1.0 × 10.sup.4 U/1                                  
                             7.6 × 10.sup.3 U/1                     
pGLT104 (E. coli MC1061)                                                  
                1.0 × 10.sup.4 U/1                                  
                             --                                           
pGLT104 (E. coli D1210)                                                   
                --           2.0 × 10.sup.4 U/1                     
______________________________________                                    
 
    
     Production of Polypeptide 
     E. coli containing a plasmid capable of coding for the polypeptide of the present invention (e.g., pGLT104) is inoculated into a conventional nutrient medium such as L-broth containing an antibiotic, a sugar source and essential vitamins. The culture is incubated until a sufficient quantity of interferon is produced by the E. coli. The E. coli cells are harvested by centrifugation and resuspended in a phosphate buffered saline solution. The cells are ruptured by sonication and the resulting solution is cleared by centrifugation. The interferon activity of the polypeptide is assayed in a conventional manner and the polypeptide is purified in a conventional manner. 
     Pharmaceutical Composition 
     Purified polypeptide is dissolved in a pharmaceutically acceptable carrier, excipient or diluent according to known methods in an amount sufficient to exhibit gamma-interferon activity when administered to a host. Suitable carriers and their formulations for the preparation of pharmaceutical compositions are described in Remington&#39;s Pharmaceutical Sciences by E. W. Martin, which is hereby incorporated by reference. The pharmaceutical compositions will contain an effective amount of the polypeptide of the invention in combination with a suitable amount of a carrier for proper administration to a host. 
     Practical Utility 
     The polypeptide is administered to a subject or host in a manner and in an amount sufficient to exhibit antitumor, antiviral or immunosuppressive activity. The amount and manner of administration is similar to that currently being employed in the art. The polypeptide of the invention may be administered to subjects requiring anti-tumor, or antiviral treatment, and to those patients exhibiting immunosuppressive conditions. Dosage and dose rate may vary according to the particular application but may essentially follow that currently in use in clinical investigations on humans with human interferon, e.g., about (1-10 )×10 6  units daily, and in the case of materials having a purity greater than 1 percent, likely up to, e.g., 50×10 6  units daily. The dosages of the polypeptide could be significantly elevated for greater effect owing to the essential absence of contaminating proteins when produced by genetic engineering techniques. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as may be obvious to those skilled in the art are intended to be included within the scope of the following claims.