Abstract:
A distributed feedback semiconductor laser in which the waveguide on one side of the active layer consists of two layers forming a corrugation pattern therebetween to create a Bragg grating. The refractive index of the inner layer, with respect to the active layer is less than that of the outer layer and the refractive index of the outer layer is less than that of the active layer.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a single longitudinal mode semiconductor laser, and more particularly to a semiconductor laser having a distributed feedback structure. 
     Optical fiber communication has now reached a stage of practical application as enabled by the reduction of losses in optical fibers and the improvement of semiconductor laser performance. The attraction of optical fiber communication lies above all in the long-distance high-capacity transmission it permits. Optical fibers, however, vary in transmission velocity with optical wavelength (wavelength dispersion), and semiconductor lasers used as light sources oscillate during high-speed modulation in multiaxial modes (in a plurality of modes having different wavelengths) and are thus susceptible to signal waveform degradation in high-capacity long-distance transmission. To solve these problems, many attempts are under way to develop a single longitudinal mode semiconductor laser oscillating in a single longitudinal mode even during high-speed modulation. 
     Single longitudinal mode semiconductor lasers include distributed-feedback semiconductor lasers (DFB lasers). A DFB laser usually has a structure in which a grating is formed in an optical guide layer provided in the vicinity of an active layer. The DFB laser, utilizing periodic variations of the refractive index resulting from the presence of the grating, oscillates in a single longitudinal mode around the Bragg wavelength of this grating. In order to achieve satisfactory single longitudinal mode oscillation, a DFB laser requires efficient coupling between the grating and the optical field. Therefore, the most suitable position where the grating is to be formed is an area in which there is a strong optical field. In other words, as light rays propagate through the active layer and the optical guide layer, their intensity is the greatest in or around the area between the active and optical guide layers, and it is desirable to form the grating in this area. However, in most of the previously produced DFB lasers, the grating is formed on the side of the optical guide layer farther from the active layer, where the optical field is relatively weak. (See, for example, T. Matsuoka et al., &#34;CW Operation of DFB-BH GaInAsP/InP Lasers in 1.5 μm Wavelength Region&#34;, ELECTRONICS LETTERS, Vol. 18 (1982), No. 1, pp. 27-28). Accordingly, the index K which represents the coupling efficiency between the grating and the optical field is no greater than approximately 40 cm -1 , resulting in the problem that increasing the injected current would invite oscillation at the Fabry-Perot mode instead of oscillation in a single wavelength. 
     SUMMARY OF THE INVENTION 
     An object of the present invention, therefore, is to provide a DFB laser having a periodic refractive index distribution in the strongest part of the optical field and capable of satisfactory oscillation in a single longitudinal mode. 
     A semiconductor laser according to the invention has a stratified structure in which a semiconductor layer having a lower refractive index than those of an active layer and an optical guide layer is buried with a periodic thickness distribution between the active layer and the optical guide layer. 
     More specifically, the structure of a distributed-feedback (DFB) semiconductor laser diode comprises: an active layer emitting light rays upon injection of an electric current; and an optical waveguide, adjoining and cooperating with the active layer, for guiding the light rays. The optical waveguide comprises a first semiconductor layer formed in the vicinity of the active layer and having a greater bandgap than that of the active layer. The thickness of the first semiconductor layer varies parallel toward the direction of the active layer according to a corrugation pattern of a prescribed. The waveguide further includes a second semiconductor layer having a bandgap smaller than that of the first semiconductor layer but greater than that of the active layer and so formed as to conform to the corrugation of the first semiconductor layer. The DFB laser diode further includes first and second cladding layers adjoining the active layer and the optical waveguide, respectively, and both cladding layers having bandgaps greater than either of the active layer and of second semiconductor layer. First and second electrodes inject an electric current into the active layer, wherein a PN junction is formed in the active layer by forward biasing. The light rays emitted from the active layer interact with the diffraction grating formed by the boundary of the corrugation to receive positive feedback, resulting in laser oscillation. 
     In semiconductor materials, generally, the greater the bandgap energy, the shorter the light emitting wavelength corresponding to that particular bandgap, and the wider the bandgap, the smaller the refractive index. In this specification, the magnitude of the bandgap energy or the bandgap wavelength will be referred to as required in place of the level of the refractive index. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1a and 1b are longitudinal sections illustrating the fabrication process of a first preferred embodiment of the press invention; 
     FIG. 1c is a longitudinal section, along the laser oscillation axis, of the first preferred embodiment of the invention; 
     FIGS. 2a and 2b are longitudinal sections illustrating the production process of a distributed feedback laser diode (DFB LD), which a second preferred embodiment of the invention; 
     FIG. 2c is a longitudinal section of the DFB LD, the second preferred embodiment; and 
     FIGS. 3 through 5 are each longitudinal sections of laser diode structures according to alternative embodiments of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1c, a DFB laser diode (LD) according to the present invention comprises an active layer 12, emitting light rays upon injection of an electric current, an optical guide 20 for guiding the light rays emanating from the thin active layer 12, a cladding layer 15, and a contact layer 16, all formed over a substrate 11. Over the bottom of the substrate 11 and the upper face of the contact layer 16 are formed a negative electrode 18 and a positive electrode 17, respectively. Every layer other than the electrodes 17 and 18 is composed of a semiconductor material. The optical waveguide 20 comprises a low refractive index layer 13, made of a material having a lower refractive index than that of the active layer 12, and a high refractive index layer 14, made of a material having a refractive index lower than that of the active layer 12 but higher than that of the low refractive index layer 13, that is n 13  &lt;n 14  &lt;n 12 , where n i  is the refractive index of the respective numbered layer. Alternatively, in terms of bandgap energy Eg 13  &gt;Eg.sub. 14 &gt;Eg 12 . The low refractive index layer 13 consists of a first semiconductor layer 13, whose thickness periodically varies in a corrugation Eg; pattern parallel to the active layer 12. The high refractive index layer 14 consists of a second semiconductor layer 14 whose thickness periodically varies to conform to the corrugation of the first semiconductor layer 13. The first and second semiconductor layers constitute a diffraction grating at the boundary of the corrugation. As the electrodes 17 and 18 are forward biased, the light rays emanating from the active layer 12 interact with the differing diffraction index of the waveguide 20, and oscillate at a Bragg wavelength determined by the pitch of the corrugation. The cladding layer 15 has a smaller refractive index than that of second semiconductor layer 14 and, together with the substrate 11 which has a small refractive index than that of the active layer 12 and functions as a cladding region, confines the light rays. 
     Now will be described the fabrication process for the DFB LD with reference to FIGS. 1a and 1b. 
     Referring first to FIG. 1a, over the substrate 11 of n-InP (bandgap wavelength λg=0.92 micron) are epitaxially grown the active layer 12 of non-doped InGaAsP (λg=1.3 microns) active and the first semiconductor layer 13 of p-InP (λg=0.92 micron) to thicknesses of 0.1 micron and 0.2 micron, respectively. 
     Referring next to FIG. 1b, a corrugation of 4000 Å in pitch and 1500 Å in trough depth is formed in the p-InP first semiconductor layer or low refractive index layer 13 by photolithography using an interferometric exposure technique and chemical etching. At this step, the etching may be so accomplished that the troughs of the corrugation reach the active layer 12. Suitable etching liquids include Br+CH 3  OH and HCl. Use of an HCl-based etchant as etching liquid would provide the advantage of etching only the p-InP layer 13 but not the non-doped InGaAsP active layer 12. The p-InGaAsP (λg=1.2 microns) second semiconductor layer or high refractive index layer 14, the p-InP cladding layer 15 and the p +  -InGaAsP contact layer 16 are so epitaxially grown to the respective thicknesses of 0.2 micron, 3 microns and 1 micron as to bury the corrugated p-InP first semiconductor layer 13 within this structure as illustrated in FIG. 1c, which shows the completed form of the DFB LD. The thickness of the second semiconductor layer 14 is measured from the middle level between the peaks and troughs of the corrugation. Further the p-electrode 17 of AuZn alloy is formed over the p +  -InGaAsP contact layer 16, and the n-electrode 18 of AuGeNi alloy is formed underneath the n-InP substrate 11. In the DFB LD thereby produced, the p-InP first semiconductor layer 13, buried and having its thickness varying periodically between the active layer 12 (which has the strongest optical field) and the second semiconductor layer 14, provides an extended periodic distribution of refractive index, so that a satisfactory coupling is attained between the optical field and the grating, resulting in a satisfactory single longitudinal mode oscillation at or around the wavelength of 1.3 microns. 
     Whereas the periodic p-InP first semiconductor layer 13 is buried beneath the p-InGaAsP secondinagawa; Hideo 
     Minoo 
     JPX 
    
    
     The present invention relates to an expression vector, the production of the same and the use of the same. More particularly, the present invention relates to an expression vector having a strong gene expression which carries a gene coding for a phosphate-binding protein, a method for producing the same and a method for producing a desired polypeptide by means of gene expression using said expression vector. 
     The term &#34;gene expression&#34; as used herein is intended to mean the translation to polypeptide from a gene coding for the sequence of polypeptide. In the gene expression, a DNA chain coding for the sequence of polypeptide is first transcribed to a complementary RNA which is called a messenger RNA and, then, the thus transcribed messenger RNA is translated into the above-mentioned polypeptide. 
     In 1970, the transformation technique of Escherichia coli was established [M. Mandel and A. Higa, Journal of Molecular Biology, 53, 159-162 (1970)], and a restriction enzyme was found [H. O. Smith and K. W. Wilcox, Journal of Molecular Biology, 51, 379-391 (1970)]. These findings brought rapid progress in gene engineering and cell technology. For example, the fundamental technique of genetic recombination disclosed in U.S. Pat. No. 4,237,224 has been established based on such findings. Further, novel vectors have been produced and methods for producing useful substances by gene engineering have been proposed. The above-mentioned novel vectors are disclosed in, for example, German patent application Laid-Open Specification No. 2712615, French patent Specification No. 2441659, U.S. Pat. No. 4,273,875, PCT patent application Publication No. WO 79/01169, British patent Specification No. 2052516 and EPC patent Specification No. 32238. The above-mentioned methods for producing useful substances are disclosed in, for example, U.S. Pat. No. 4,375,514. 
     However, all of the above publications only disclose that desired useful substances may be produced by means of gene expression. The techniques of the above-mentioned prior art publications have such a drawback that the yield of desired useful substances is not sufficiently high. Therefore, the development of an expression vector capable of producing useful substances in high yield is earnestly desired in the art. 
     Accordingly, from the standpoint of industrial application of gene engineering and reduction of cost for the production of useful substances, it has been desired in the art to develop an expression vector excellent in power of gene expression. The fundamental techniques of genetic recombination are disclosed in Proceedings of the National Academy of Science, U.S.A., 70, 3240-3244 (1973) and ibid., 71, 1743-1747 (1974), but there has not been proposed a general method for isolating a gene excellent in power of gene expression from a large number of genes present in nature, leading to difficulty in solving the above-mentioned problem. 
     The present inventors have made extensive and intensive studies to settle the above-mentioned problem. As a result, it has been found that a gene coding for a phosphate-binding protein can provide an expression vector having an extremely strong gene expression as compared with those of the conventionally known vectors. The present inventors then succeeded in preparing such an expression vector carrying a gene coding for a phosphate-binding protein by the technique of cloning, with high efficiency. The present invention has been made, based on such a novel finding and success. 
     Therefore, it is an object of the present invention to provide an expression vector which is excellent in power of gene expression. 
     It is another object of the present invention to provide a method for preparing an expression vector which is excellent in power of gene expression. 
     It is a further object of the present invention to provide a method for producing polypeptides in high yield by means of gene expression using an expression vector of the kind as mentioned above. 
    
    
     The foregoing and other objects, features and advantages of the present invention will be apparent from the following detailed description taken in connection with the accompanying drawings in which: 
     FIG. 1 illustrates the restriction maps of plasmids pSN401, pSN507, pSN508 and pSN518; and 
     FIG. 2 illustrates the flow-sheet of the method for the preparation of the plasmid pSN2507, shown with the restriction maps of the plasmids pKN402A, pSH101 and pSN2507. 
    
    
     In FIGS. 1 and 2, the symbol phoS designates a gene coding for a phosphate-binding protein of Enterobacteriaceae (on which a detailed explanation will be given later). The symbols phoT, pstA and phoU each designate genes which are clustered on chromosomal DNA of E. coli together with phoS (on which a detailed explantion will be given later). The following symbols are used to denote recognition sequences for restriction enzymes: 
     EcoRI, HpaI, PstI, BglII, MluI 
     HindIII, AvaI, HincII and SmaI 
     (the suffix numbers attached to the above-mentioned symbol as shown in FIGS. 1 and 2 are used to identify the respective recognition sites for a restriction enzyme). 
     Other abbreviations are respectively: 
     Tn3(Ap R ) (indicated by    ): transponson carrying ampicillin resistance coding gene 
     Rep: replication gene 
     ori: replication origin 
     Kb: 1,000 nucleotide pairs 
        : deleted portion 
     In one aspect of the present invention, there is provided an expression vector which comprises a DNA fragment including a gene coding for a phosphate-binding protein and a replicon selected from plasmids and bacteriophages, said DNA fragment being ligated to said replicon. The term &#34;replicon&#34; used herein is intended to mean a vector which is replicable and has a gene for a phenotypical trait and a restriction site adopted to ligate a donor DNA fragment thereto. 
     A replicon is a carrier of genetic information which is known as a replication unit of a chain of nucleotides such that, once replication begins, replication is successively carried out to the end of nucleotide sequence. As such a replicon, there can be mentioned plasmids, bacteriophages, viruses and the like. The replicons include DNA replicons and RNA replicons, but in the present invention the RNA replicons are not preferred from a practical point of view. As the DNA replicons, there can be mentioned plasmids and replicable DNA fragments derived from DNA viruses, procaryotic cells and eucaryotic cells. Among them, as a replicon to be used in the present invention, plasmids and bacteriophages are preferred. Conventionally, DNA replicons carrying a gene conferring antibiotic resistance are widely used for cloning various genes. The widely employed method of cloning a gene comprises digesting chromosomal DNA with a restriction enzyme to obtain a DNA fragment carrying the desired gene, ligating said DNA fragment to said DNA replicon, transforming a unicellular organism with said DNA replicon including said DNA fragment to form transformants, isolating the transformants from parent unicellular organism by means of the antibiotic resistance imparted by said DNA replicon, and incubating the isolated transformants, whereby said DNA replicon including said DNA fragment is cloned. 
     The gene coding for a phosphate-binding protein is responsible for phosphate transport system and phosphate metabolism system and, evolutionally, exist widely in procaryotic cells and unicellular eucaryotic cells. Aong them, the gene coding for a phosphate-binding protein of Escherichia coli has been comparatively well studied in detail, and especially, the gene of Escherichia coli is called phoS gene. The phoS gene exists not only in Escherichia coli but also in bacteria belonging to Enterobacteriaceae. Accordingly, the phoS gene of any of bacteria belonging to Enterobacteriaceae may be used in the present invention. The phoS of Escherichia coli is clustered together with phoT, pstA and phoU at 83 minutes on E. coli standard genetic map [Backman, B. J. and K. B. Low, Microbiol. Rev. 44, 1-56 (1980)]. The phoT, pstA and phoU are genes whose function is related to phosphate transport and phosphate metabolism. As mentioned above, the phoS, phoT, pstA and phoU genes are clustered on a chromosome of E. coli and, therefore, when a DNA fragment carrying the phoS gene is prepared from chromosomal DNA of E. coli by means of a restriction enzyme, the phoT, pstA and phoU genes are also included in the DNA fragment. The gene coding for a phosphate-binding protein of λglmS phage has also been studied, and may be used in the present invention. 
     Further, genes, whose function is related to phosphate transport system and phosphate metabolism system, of yeast and a bacterium other than a bacterium belonging to Enterobacteriaceae may be used. In this case, it is necessary to choose a replicon and a host suitable for genes, whose function is related to phosphate transport system and phosphate metabolism system, of yeast and a bacterium other than a bacterium belonging to Enterobactericeae. 
     In another aspect of the present invention, there is provided a method for preparing an expression vector which comprises: 
     (1) preparing a chromosomal DNA containing a gene coding for a phosphate-binding protein from bacteria belonging to Enterobacteriaceae; 
     (2) cleaving the chromosomal DNA with a restriction enzyme to provide DNA fragments; 
     (3) ligating said DNA fragments to a replicon selected from plasmids and bacteriophages; 
     (4) transforming cells of a bacterium belonging to Enterobacteriaceae with said replicon having the DNA fragment ligated thereto to form transformants including those which contain a recombinant vector including the DNA fragments carrying a gene coding for a phosphate-binding protein; 
     (5) selecting transformants containing the recombinant vector including the DNA fragments carrying a gene coding for a phosphate-binding protein from said transformants; and 
     (6) isolating the recombinant vector including the DNA fragments carrying the gene coding for a phosphate-binding protein from the selected transformants. 
     In the step (1) of the method of the present invention, chromosomal DNA containing a gene coding for a phosphate-binding protein (hereinafter referred to simply as &#34;phoS gene&#34;) is prepared from bacteria belonging to Enterobacteriaceae by a customary isolation technique such as lysis, centrifugation or the like. 
     In the step (2), the chromosomal DNA as prepared in the above step (1) is cleaved with a restriction enzyme to provide DNA fragments containing a DNA fragment carrying the phoS gene. 
     In the step (3) of the method of the present invention, as mentioned above, any of plasmids and bacteriophages may be used as the replicon. The ligation of the DNA fragments to the replicon may be performed by using a ligase according to the customary method (see, for example, U.S. Pat. No. 4,237,224). At least one of the DNA fragment carrying a phoS gene and replicon has a gene for a phenotypical trait. 
     In the above step (4), cells of a bacterium belonging to Enterobacteriaceae are transformed with the replicon carrying the DNA fragment as prepared in the above step (3). As the bacterium belonging to Enterobacteriaceae, there can be mentioned Escherichia, Enterobacter, Erwinia, Klebsiella, Proteus, Salmonella, Serratia and Shigella. Further, besides the above-mentioned bacteria, there may be used mutant cells of any of the above-mentioned bacteria. It, of course, is necessary to choose the suitable bacterium, taking into consideration replicability of the replicon in the cells of a bacteria. For example, Escherichia coli, Serratia species or Salmonella species can be suitably used as the host, when well-known relaxed type plasmid such as pBR322, pBR325, pACYA177 or pKN410 is used as the replicon. 
     In the above step (5), the transformants are first isolated from parent cells of a bacterium belonging to Enterobacteriaceae by means of the phenotypical trait such as drug resistance imparted by the replicon having gene for a phenotypical trait. Then, transformants containing the recombinant vector including the DNA fragment carrying the phoS gene are selected from the above-isolated transformants using as a criterion the amount of phosphate-binding protein in the transformants. The amount of phosphate-binding protein in each of the transformants is determined as follows. First, the phosphate-binding protein is isolated from the transformant and purified by a customary technique such as lysis, centrifugation, or salting-out. Then, the amount of the purified phosphate-binding protein is determind by a customary technique such as SDS-agarose gel electrophoresis. The amount of the phosphate-binding protein in each of the transformants is compared with the amount of the phosphate-binding protein produced from a parent cell of a bacterium belonging to Enterobacteriaceae not transformed with the replicon carrying the DNA fragment. The transformants having larger amount of phosphate-binding protein than the parent cell of bacterium belonging to Enterobacteriaceae are selected. 
     In case a mutant cell which produces alkaline phosphatase not only under a phosphate starvation condition but also under a phosphate-rich condition is used as the cell of a bacterium belonging to Enterobacteriaceae in the above step (4), the selection of transformants containing the replicon including the DNA fragment carrying the phoS gene in the above step (5) can be very easily carried out without determination of the amount of the phosphate-binding protein in each of the transformants. The reason for this is as follows. When such a mutant cell is transformed with the replicon carrying the DNA fragment as prepared in the above step (3), the mutant cell produces alkaline phosphatase under a phosphate starvation condition but do not produce alkaline phosphatase under a phosphate-rich condition by means of function of the phoS gene contained in the DNA fragment ligated to the replicon. Therefore, when the transformation is carried out under a phosphate-rich condition, alkaline phosphatase negative transformants are selected. 
     In the above step (6), the recombinant vector including the DNA fragment carrying the phoS gene is isolated from the transformants selected in the step (5) by a customary technique such as lysis, centrifugation or the like. 
     On the other hand, it should be noted that an expression vector which carries a gene, whose function is related to phophate transport system and phosphate metabolism system, of yeast or a bacterium other than a bacterium belonging to Enterobacteria may be prepared according to substantially the same method as described above. Further, it is noted that instead of the DNA fragment prepared according to the step (1) and (2) of the above-mentioned method, a DNA fragment prepared from phages such as λglmS phage may be used. 
     The recombinant vector prepared according to the method as mentioned above may be partially deleted, with respect to the DNA fragment portion and/or the replicon portion, using a restriction enzyme while retaining the phoS gene and subjecting the resulting deleted recombinant vector to religation by means of a ligase, thereby to prepare a reconstructed recombinant vector having a reduced size. The reconstructed recombinant vector becomes easier to manipulate because the number of restriction sites of the reconstructed recombinant vector is decreased as compared with that of the original recombinant vector. Further, when a gene coding for a certain polypeptide is ligated to the reconstructed recombinant vector, the ratio of the length of the gene coding for the polypeptide to the total length of the recombinant vector becomes higher than that in the case where the gene coding for the polypeptide is ligated to the original recombinant vector which is not reconstructed. It is conceivable that the higher the ratio of the length of the gene coding for a polypeptide to the total length of the recombinant vector is, the larger the amount of the protein produced per cell becomes and the less the amount of byproducts produced per cell becomes. Therefore, when the above-mentioned reconstructed recombinant vector is used for producing a certain desired polypeptide, not only the high productivity of the polypeptide can be attained but also the purification of the polypeptide obtained can be achieved with ease. 
     A DNA fragment carrying phoS gene may be cut out from the recombinant vector as prepared according to the method of the present invention and ligated to a different kind of replicon from the kind of replicon of the original recombinant vector to prepare another type of reconstructed recombinant vector. Such another type of reconstructed recombinant vector can advantageously be used when easy selection of the transformants is intended by changing a phenotypical trait of the replicon and/or an increase is intended in copy number of a recombinant vector. The use of such another type of reconstructed recombinant vector is desirable when the transformants obtained by means of the original recombinant vector are those from which the desired transformants are difficult to select and/or the replicability of the original recombinant vector in the transformants is low. 
     The reconstruction of the recombinant vector as mentioned above is carried out by a customary technique (see, for example, U.S. Pat. Nos. 4,340,674, 4,349,629 and 4,356,270). 
     The expression vector prepared according to the method of the present invention can be identified by constructing the restriction map as follows. The expression vector is partially or completely digested with various restriction enzymes to obtain DNA fragments. The obtained fragments are subjected to agarose-gel electrophoresis, thereby to determin the size and restriction pattern of the expression vector. The position of the phoS gene on the above mentioned expression vector is determined by constructing a variety of deletion expression vectors of the present invention by means of partial or complete digestion with an appropriate restriction enzyme and subsequent self-ligation, and examining said deletion expression vectors by the complementation tests using the phoS gene-negative mutant as the host. 
     The expression vector carrying the phoS gene of the present invention has the following advantages when the gene coding for a desired polypeptide is ligated to said expression vector at its portion downstream of the DNA sequence of a promoter of the phoS gene and the production of the desired polypeptide is conducted by gene techniques: 
     (1) The yield of the above-mentioned desired polypeptide is as high as 1×10 5  -10 6  molecules/cell which is several to 100 times that obtained using the conventionally employed vector. 
     (2) The gene expression of the expression vector of the present invention can be freely controlled by adjusting the concentration of phosphate in culture medium. Therefore, the production of the desired polypeptide can also be controlled by adjusting the concentration of phosphate in the culture medium. 
     (3) The desired polypeptide may be secreted together with phosphate-binding protein portion into periplasm of the host. Therefore, the above-mentioned desired polypeptide can be easily isolated from the host. 
     Therefore, the expression vector of the present invention can be widely used in the industrial application field. An example of the method of producing a desired polypeptide using the expression vector of the present invention will now be explained. The gene coding for the desired polypeptide is prepared from the genes of eucaryotic cells, procaryotic cells or viruses, and the thus prepared gene is ligated to the expression vector of the present invention. Then, the host is transformed with the above-obtained expression vector to form transformants. Subsequently, the thus obtained transformants are incubated on a large scale, thereby to produce the desired polypeptide in extremely high yield. 
     Thus, in a further aspect of the present invention, there is provided a method for producing a polypeptide by means of gene expression using the expression vector of the present invention which comprises: 
     (1) ligating a DNA fragment carrying a gene coding for a polypeptide to said expression vector at its portion downstream of the DNA sequence of a promoter of the gene coding for a phosphate-binding protein; 
     (2) transforming cells of a bacterium belonging to Enterobacteriaceae with said expression vector having the gene coding for a polypeptide ligated thereto to form transformants; 
     (3) selecting said transformants from parent cells of a bacterium belonging to Enterobacteriaceae; 
     (4) incubating said transformants, causing said transformants to express the gene coding for the polypeptide and produce the polypeptide; and 
     (5) isolating the polypeptide from the incubated transformants. 
     In practicing the above-mentioned method, a DNA fragment carrying a gene coding for a polypeptide is first prepared by organic synthesis, or isolated from organisms by a customary technique such as lysis, centrifugation, digestion with a restriction enzyme or the like. It is also possible to prepare a DNA fragment carrying a gene coding for a desired polypeptide from a RNA fragment prepared from messenger RNA and RNA viruses by converting a RNA fragment carrying a desired gene to complementary DNA fragment using well-known reverse transcriptase. The DNA fragment carrying a gene coding for a polypeptide is ligated to an expression vector carrying the phoS gene of the present invention at its portion downstream of the DNA sequence of a promoter of the phoS gene. At least one of the DNA fragment carrying a gene coding for a polypeptide, DNA fragment carrying the phoS gene of the expression vector and repliconval of 11 nanoseconds is selected so that there are not an integral number of shifts within a bit interval. This insures that the clock will never recycle to the same position. Thus, if by chance, a leading edge of the sampling signal occurred exactly at a transition in the biphase signal, that coincidence would be lost after cycling through four or five bits. 
     The specific circuitry utilized for providing decoding of the biphase signal is illustrated with respect to FIG. 3. As shown there, a 20 MHz clock or local oscillator 50 has its output signal applied through a succession of 11 nanosecond delay circuits 52. The output of each delay circuit 52 is applied to a distinct input of a multiplexer 54. The multiplexer 54 is controlled by a counter 56, of a design which produces a single bit transition between each count state to avoid the possibility of glitch ambiguities from multiple transitions. The counter 56 controls which of the delayed inputs from the delay circuits 52 will be provided as a clock output by multiplexer 54. In this manner the clock signal from the 20 megahertz clock 50 can be successively delayed or advanced in 11 nanosecond steps. 
     The incoming biphase signal is applied within the decoder circuitry to data inputs of first and second flip-flops 60 and 62. The output of the multiplexer 54 is applied to the clock input of the first flip-flop 60 and, after an 11 nanosecond delay in a delay circuit 64, to the clock input of the second flip-flop 62. The delay circuit 64 essentially establishes the interval between leading edges of the sampling signal of FIGS. 2C&#39; and 2C&#34;. 
     The state outputs of the flip-flops 60 and 62 are applied through a NAND gate 66 whose output is in turn applied to set a register 67 that in turn controls one input of an AND gate 68. Gate 68 advances counter 56 to shift the clock in 11 nanosecond steps through multiplexer 54. The AND gate 68 also receives a clock input from multiplexer 54 defining a single shift with each bit interval. When both flip-flops 60 and 62 detect the high level biphase signal at 11 nanosecond intervals, register 67 is set, freezing the AND gate 68 from further advances on counter 56. 
     The biphase input signal is applied to the data input of a first flip-flop 72 while the output of that flip-flop is applied as the data input to a second flip-flop 74. Both flip-flops 72 and 74 are clocked by the output of the multiplexer 54 after a 5 nanosecond delay in a delay circuit 76. 
     In operation, the flip-flops 72 and 74 detect the initial occurrence of the stream of binary ones in the synching bytes and set the inputs to an AND gate 70, in turn setting a register 77 enabling one input of an AND gate 78. 
     The biphase signal is also applied to clear inputs of a further set of flip-flops 80, 82 and 84. The delayed clock output from the delay circuit 76 is applied to the clock inputs of the flip-flops 80, 82 and 84. The data input of the flip-flop 80 is set at binary 1 while the flip-flops 82 and 84 receive data inputs from the state output of the preceding flip-flop in this series. The state output of the flip-flop 84 will be at binary 1 if there is a lack of any signal for at least three bit intervals. Its output functions as a system reset or stop signal which is applied to reset the counter 56 and registers 67 and 77. Because the biphase signal insures a high signal during at least a half of the bit interval during signal transmission, the presence of three bit intervals with no signal is an indication of the termination of the packet. 
     The conversion from the biphase to a nonreturn to zero signal is accomplished by a flip-flop 88 functioning in a conventional manner. Its output, the nonreturn to zero signal, is applied through an AND gate 78, when initial binary zeros are detected, to the serial-to-parallel converter 32 as discussed above. 
     The clock 50 is selected to have a frequency stability sufficient to insure maintenance of synchronization over the short data packet length of typically 80 bytes, or at 50 nanoseconds per bit, typically 32 microseconds per packet. With currently available oscillator circuitry it is possible to specify and obtain oscillators 50 which have sufficient frequency accuracy and stability to stay in sync within the parameters for synchronization discussed above, reliably over the 32 microsecond range, and substantially greater. By running open loop after a course range synchronization, as discussed above, very rapid synchronization, requiring at most 200 nanoseconds of the 32 microsecond data package, is readily achievable. This permits efficient channel sharing for real time transmission such as in voice communication requiring a conversational quality that precludes the loss of coherency that would otherwise result with long data packets. 
     The fast settling amplifier receiver 28 illustrated in FIG. 1 is more fully illustrated with respect to FIG. 4. Since the incoming biphase or Manchester code signal has two nonzero signal states, a high and a low signal level, the fast settling amplifier must rapidly identify whether a given signal level is a high or low level in order for the present invention to permit rapid phase locking to a data burst. Since the range of acceptable signal levels can be very large, the incoming signal level representing the lower level may, for strong signals, be above the signal level representing the high level state for incoming signals of low signal strength. The amplifier and receiver 28, as more completely illustrated in FIG. 4, rapidly responds to the incoming signal to establish a discrimination threshold so as to accurately identify the low level and high level components of the incoming Manchester, biphase signal. 
     The input signal from the detector 26 is AC coupled to the base of a suitably biased transistor 102 connected in a differential arrangement with a transistor 104, sharing a common emitter resistor 106 through independent emitter resistors 108 and 110 respectively. The collector of the transistor 104 is supplied through a resistor 112 from the positive power supply, via a decoupling resistor 114. This same signal is applied to the collector of the transistor 102 and is regulated by a transistor 116 driven at its base from the negative voltage supply. 
     The collector of the transistor 104 is applied through a low-pass filter 120 to the base of a transistor 122 whose collector is powered through a decoupling resistor 124 from the positive supply voltage. The emitter is clamped by a diode 126 and biased through resistors 128 and 130 to the negative voltage supply. A capacitator 132 is driven from the emitter of the transistor 122 coupling into the center of the low-pass filter 120. The low-pass filter 120 typically has a 30 MHz cutoff point. The transistor 122 operates as an emitter follower and the emitter base diode provides a level shift in the emitter follower output which is applied to a noninverting input of a high speed comparator 134. The output of the comparator 134 is applied to a line driver 136 which in turn provides buffering for the output signal to be applied to the decoder 30 of FIG. 1. 
     The emitter follower output from the transistor 122 is applied to an integrator 140 which in turn drives a low-pass filter 142, typically having a 100 kHz cuttoff point, and which in turn drives the base of a transistor 144. The collector of transistor 144 is driven by the negative voltage supply while the emitter is driven through a resistor 146 from the positive voltage supply. The emitter signal is fed back through a capacitor 148 into the low-pass filter 142 and is applied to the base of the transistor 104. 
     The integrator 140 and low-pass filter 142 establish a threshold signal level which is applied to the base of the collector 104 and which tracks the average value of the biphase input signal. Since each binary digit in the biphase signal has effectively an equal time of high signal level and low signal level, the DC average will settle and maintain a value approximately half way between the high and low level signals. This threshold, as applied to the transistor 104, sets a reference value for the differential amplifier consisting of the transistors 102 and 104 and adjusts the average level at the output of the collector 104 so that the incoming high and low signal levels span a preselected average. This signal is low-pass filtered to remove high frequency transients and level shifted in the transistor 122 before application to the comparator 134. Comparator 134 effectively squares up the signal into a conventional appearing biphase signal format. 
     The circuitry of FIG. 4 utilizes level shifting rather than gain adjustment to provide more reliable and noise free fast attack capabilities for accomplishing the signal level normalization required in the present system. 
     The specific implementation illustrated above is exemplary only, the scope of the invention being solely as indicated in the following claims.