The invention relates to production of proinsulin or insulin in seeds of monocot plants by transformation of monocot plants with a nucleotide sequence encoding proinsulin or insulin under the control of a seed specific promoter.

Description of the Invention

SUMMARY OF THE INVENTION

The invention is the expression of an insulin compound in plants. In a preferred embodiment, the proinsulin and/or insulin proteins are expressed in monocotyledonous plants. In a further preferred embodiment, they are expressed in maize. The proteins may be extracted from the plant, or the plant tissue used in various applications. In one such application, the plant tissue can be orally administered to an animal. In yet another embodiment, a biomass is created by expressing the proteins in a plurality of plants where at least some of the plants express the proteins, then harvesting the biomass.

DESCRIPTION

This invention relates to the expression of proinsulin and insulin in plants, where the plants express at high levels. This invention further relates to stable transformation of plants with such proteins. As used herein stable transformation refers to the transfer of a nucleic acid fragment into a genome of a host organism resulting in genetically stable inheritance.

The term "insulin" as used herein refers to mammalian insulin, such as bovine, porcine or human insulin, whose sequences and structures are known in the art. Bovine, porcine, and human insulin are preferred mammalian insulins; human insulin is more preferred. The amino acid sequence and spatial structure of human insulin are well-known. Human insulin is comprised of a twenty-one amino acid A-chain and a thirty amino acid B-chain which are cross-linked by disulfide bonds. A properly cross-linked human insulin contains three disulfide bridges: one between position 7 of the A-chain and position 7 of the B-chain, a second between position 20 of the A-chain and position 19 of the B-chain, and a third between positions 6 and 11 of the A-chain. The proinsulin molecule also contains a C-chain, which is cleaved by enzymes in the human body. Proinsulin is a single polypeptide chain containing a sequence of about thirty residues that is absent from mature insulin. Proinsulin has a B-C-A chain structure. The C or connecting peptide joins the carboxyl end of the B chain and the amino terminus of the A chain of the future insulin molecule. Biochemistry 3rd edition, pg. 995 (1988) New York, WH Freeman & Co. The mature insulin is generated by cleavage of the C peptide at dibasic residues after Arg(31)-Arg(32) and after Lys(64)-Arg(65). Two distinct processing enzymes have been defined which are specific for their respective dibasic cleavage sites in proinsulin; type I is substrate specific for the BC junction, while type II is specific for the CA junction (Weiss, Biochemistry 29, 1990). When proinsulin is used for manufacture of insulin, enzymes such as trypsin are used to cleave the C-chain and an enzyme such as carboxypeptidase B is used to further remove basic amino acids.

Use of insulin analogs is also included within the scope of the invention and refers to proteins that have an A-chain and a B-chain that have substantially the same amino acid sequences as the A-chain and B-chain of human insulin, respectively, but differ from the A-chain and B-chain of human insulin by having one or more amino acid deletions, one or more amino acid replacements, and/or one or more amino acid additions that do not destroy the insulin activity of the insulin analog. Also included in this invention are analogs of proinsulin, where the C peptide is reduced in length and/or modified in sequence.

Genes which encode insulin and insulin-like proteins are available to one skilled in the art. See for example Galloway, J. A. and Chance, R. E., Horm. Metab. Res. (1994) 26:591-598 and Genbank Access no. XP.sub.--006400; Arakawa et al., supra; Bell, G. I. et al., (1980) Nature 284: 26-32 (human insulin gene). Any gene which encodes proinsulin, insulin or insulin analogs may be used in this invention. Codon optimization for the plant in which the gene is to be inserted can be useful in obtaining high expression levels, as can be removing sequences that may destabilize the mRNA.

The methods available for putting together a gene as described above for the improved expression described above can differ in detail. However, the methods generally include the designing and synthesis of overlapping, complementary synthetic oligonucleotides, which are annealed and ligated together to yield a gene with convenient restriction sites for cloning. PCR-based approaches may be used to attach and link stretches of sequence. Also, subsequent PCR amplification of the product may be necessary before later sub-cloning steps.

Once the gene has been made or isolated which encodes the protein, it is placed into an expression vector by standard methods. The selection of an appropriate expression vector will depend upon the method of introducing the expression vector into host cells. A typical expression vector contains prokaryotic DNA elements coding for a bacterial replication origin and an antibiotic resistance gene to provide for the growth and selection of the expression vector in the bacterial host; a cloning site for insertion of an exogenous DNA sequence, which in this context would code for the protein of interest; eukaryotic DNA elements that control initiation of transcription of the exogenous gene, such as a promoter; and DNA elements that control the processing of transcripts, such as leader and transcription termination/polyadenylation sequences. It also can contain such sequences as are needed for the eventual integration of the vector into the plant chromosome.

In a preferred embodiment, the expression vector also contains a gene encoding a selection marker, which is functionally linked to a promoter and terminator that control transcriptional initiation and termination, respectively. For a general description of plant expression vectors and reporter genes, see Gruber et al. (1993) (Gruber et al., "Vectors for Plant Transformation" in Methods of Plant Molecular Biology and Biotechnology 89-119, CRC Press, 1993).

Promoter elements employed to control expression of the enzyme encoding gene and the selection gene, respectively, can be any plant-compatible promoters. Those can be plant gene promoters, such as, for example, a polyubiquitin promoter, a promoter for the small subunit of ribulose-1,5-bis-phosphate carboxylase, or promoters from the tumor-inducing plasmids from Agrobacterium tumefaciens, such as the nopaline synthase and octopine synthase promoters, or viral promoters such as the cauliflower mosaic virus (CaMV) 19S and 35S promoters or the figwort mosaic virus 35S promoter. See Kay et al. (1987) Science 236:1299 and European patent application No. 0 342 926. See international application WO 91/19806 for a review of illustrative plant promoters. The range of available plant compatible promoters includes tissue specific and inducible promoters. In one embodiment of the present invention, the exogenous DNA is under the transcriptional control of a plant polyubiquitin promoter. Plant polyubiquitin promoters are well known in the art, as evidenced by European patent application no. 0 342 926.

Alternatively, a tissue specific promoter can be provided to direct transcription of the DNA preferentially to the seed. One such promoter is the globulin promoter. This is the promoter of the maize globulin-1 gene, described by Belanger, F. C. and Kriz, A. L. (1991) Genetics 129:863-872. It also can be found as accession number L22344 in the Genebank database. Another example is the phaseolin promoter. See, Bustos et al. (1989) The Plant Cell Vol. 1, 839-853.

One option for use of a selectable marker gene is a glufosinate-resistance encoding DNA and in an embodiment can be the phosphinothricin acetyl transferase ("PAT") or maize optimized PAT gene (Jayne et al., U.S. Pat. No. 6,096,947) under the control of the CaMV 35S promoter and terminator. The gene confers resistance to bialaphos. See, Gordon-Kamm et al. (1990) Plant Cell 2, 603-618; Uchimiya et al., (1993) Bio/Technology 11:835, and Anzai et al., (1989) Mol. Gen. Genet. 219:492.

It may also be desirable to provide for inclusion of sequences to direct expression of the protein to a particular part of the cell. A variety of such sequences are known to those skilled in the art. For example, if it is preferred that expression be directed to the cell wall, this may be accomplished by use of a signal sequence and one such sequence is the barley alpha amylase signal sequence, (Rogers, (1985) J. Biol Chem 260, 3731-3738). Another example is the brazil nut protein signal sequence when used in canola or other dicotyledons. Another alternative is to express the protein in the endoplasmic reticulum of the plant cell. This may be accomplished by use of a localization sequence, such as KDEL. This sequence contains the binding site for a receptor in the endoplasmic reticulum. Muntro, S, and Pelham, H. R. B. (1987) Cell 48:899-907.

Obviously, many variations on the promoters, selectable markers and other components of the construct are available to one skilled in the art.

In accordance with the present invention, a transgenic plant is produced that contains a DNA molecule, comprised of elements as described above, integrated into its genome so that the plant expresses a heterologous protein-encoding DNA sequence. In order to create such a transgenic plant, the expression vectors containing the gene can be introduced into protoplasts, into intact tissues, such as immature embryos and meristems, into callus cultures, or into isolated cells. Preferably, expression vectors are introduced into intact tissues. General methods of culturing plant tissues are provided, for example, by Miki et al., (1993) "Procedures for Introducing Foreign DNA into Plants" in Methods in Plant Molecular Biology and Biotechnology, Glick et al. (eds) pp. 67-68 (CRC Press 1993) and by Phillips et al., (1988) "Cell/Tissue Culture and In Vitro Manipulation" in Corn and Corn Improvement 3d Edit. Sprague et al. (eds) pp. 345-387 (American Soc. Of Agronomy 1988). The selectable marker incorporated in the DNA molecule allows for selection of transformants.

Methods for introducing expression vectors into plant tissue available to one skilled in the art are varied and will depend on the plant selected. Procedures for transforming a wide variety of plant species are well known and described throughout the literature. See, for example, Miki et al., supra; Klein et al., (1992) Bio/Technology 10:286-291; and Weisinger et al., (1988) Ann. Rev. Genet. 22: 421-477. For example, the DNA construct may be introduced into the genomic DNA of the plant cell using techniques such as microprojectile-mediated delivery, Klein et al., (1992) supra; electroporation, Fromm et al., (1985) Proc. Natl. Acad. Sci. USA 82: 5824-5828; polyethylene glycol (PEG) precipitation, Paszkowski et al., (1984) EMBO J. 3: 2717-2722; direct gene transfer, WO 85/01856 and EP No. 0 275 069; in vitro protoplast transformation, U.S. Pat. No. 4,684,611; and microinjection of plant cell protoplasts or embryogenic callus, Crossway, (1985) Mol. Gen. Genet. 202:179-185. Co-cultivation of plant tissue with Agrobacterium tumefaciens is another option, where the DNA constructs are placed into a binary vector system, Ishida et al., (1996) Nature Biotechnology 14, 745-750. The virulence functions of the Agrobacterium tumefaciens host will direct the insertion of the construct into the plant cell DNA when the cell is infected by the bacteria. See, for example Horsch et al., (1984) Science 233: 496-498, and Fraley et al. (1983) Proc. Natl. Acad. Sci. USA 80: 4803-4807.

Standard methods for transformation of canola are described by Moloney et al., (1989) Plant Cell Reports 8:238-242. Corn transformation is described by Fromm et al., (1990) Bio/Technology 8:833-839, and Gordon-Kamm et al., The Plant Cell 2:603-618. Agrobacterium is primarily used in dicots, but certain monocots such as maize can be transformed by Agrobacterium, U.S. Pat. No. 5,550,318. Rice transformation is described by Hiei et al., (1994) The Plant Journal 6(2), 271-282, Christou et al., (1991) Trends in Biotechnology 10:239. Wheat can be transformed by techniques similar to those used for transforming corn or rice. Sorghum transformation is described by Wan et al., (1994) Plant Physiol. 104:37. Soybean transformation is described in a number of publications, including U.S. Pat. No. 5,015,580.

In one preferred method, the Agrobacterium transformation methods of Ishida supra and also described in U.S. Pat. No. 5,591,616, are generally followed, with modifications that allow the inventors to recover transformants from Hill maize. The Ishida method uses the A188 variety of maize that produces Type I callus in culture. In one preferred embodiment the High II maize line is used which initiates Type II embryogenic callus in culture. While Ishida recommends selection on phosphinothricin when using the bar or PAT gene for selection, another preferred embodiment provides for use of bialaphos instead.

The bacterial strain used in the Ishida protocol is LBA4404 with the 40 kb super binary plasmid containing three vir loci from the hypervirulent A281 strain. This strain is resistant to tetracycline. The cloning vector cointegrates with the super binary plasmid. Since the cloning vector has an E. coli specific replication origin but not an Agrobacterium specific replication origin, it cannot survive in Agrobacterium without cointegrating with the super binary plasmid. Since the LBA4404 strain is not highly virulent, and has limited application without the super binary plasmid, the inventors have found in yet another embodiment that the EHA101 strain is preferred. It is a disarmed helper strain derived from the hypervirulent A281 strain. The cointegrated super binary/cloning vector from the LBA4404 parent is isolated and electroporated into EHA 101, selecting for spectinomycin resistance. The plasmid is isolated to assure that the EHA101 strain contains the plasmid.

Further, the Ishida protocol as described provides for growing fresh culture of the Agrobacterium on plates, scraping the bacteria from the plates, and resuspending in the co-culture medium as stated in the '616 patent for incubation with the maize embryos. This medium includes 4.3 gl.sup.-1 MS salts, 0.5 mgl.sup.-1 nicotinic acid, 0.5 mgl.sup.-1 pyridoxine hydrochloride, 1.0 mgl.sup.-1 thiamine hydrochloride, 1.0 gl.sup.-1 casamino acids, 1.5 mgl.sup.-1 2,4-Dichlorophenoxyacetic Acid (2,4-D), 68.5 gl.sup.-1 sucrose and 36 gl.sup.-1 glucose, all at a pH of 5.8. In a further preferred method, the bacteria are grown overnight in a 1 ml culture, then a fresh 10 ml culture is re-inoculated the next day when transformation is to occur. The bacteria grow into log phase, and are harvested at a density of no more than OD600=0.6 and preferably between 0.2 and 0.5. The bacteria are then centrifuged to remove the media and resuspended in the co-culture medium. Since Hi II is used, medium preferred for Hi II is used. This medium is described in considerable detail by Armstrong, C. I. and Green C. E. "Establishment and maintenance of friable, embryogenic maize callus and involvement of L-proline" Planta (1985) 154:207-214. The resuspension medium is the same as that described above. All further Hi II media are as described in Armstrong supra. The result is redifferentiation of the plant cells and regeneration into a plant. Redifferentiation is sometimes referred to as dedifferentiation, but the former term more accurately describes the process where the cell begins with a form and identity, is placed on a medium in which it loses that identity, and becomes "reprogrammed" to have a new identity. Thus the scutellum cells become embryogenic callus.

The levels of expression of the gene of interest can be enhanced by the stable maintenance of a protein encoding gene on a chromosome of the transgenic plant. Use of linked genes, with herbicide resistance in physical proximity to the proinsulin or insulin encoding gene, would allow for maintaining selective pressure on the transgenic plant population and for those plants where the genes of interest are not lost.

With transgenic plants according to the present invention, protein can be produced in commercial quantities. Thus, the selection and propagation techniques described above yield a plurality of transgenic plants, which are harvested in a conventional manner. The plant with the protein can be used in the processing, or the protein extracted. Protein extraction from biomass can be accomplished by known methods which are discussed, for example, by Heney and Orr, (1981) Anal. Biochem. 114: 92-96.

It is evident to one skilled in the art that there can be loss of material in any extraction method used. Thus, a minimum level of expression is required for the process to be economically feasible. For the relatively small number of transgenic plants that show higher levels of expression, a genetic map can be generated, via conventional RFLP and PCR analysis, which identifies the approximate chromosomal location of the integrated DNA molecule. For exemplary methodologies in this regard, see Glick and Thompson (1993), in Methods in Plant Molecular Biology and Biotechnology, 269-84 (CRC Press 1993).

One of skill will recognize that after the expression cassette is stably incorporated in transgenic plants and confirmed to be operable, it can be introduced into other plants by sexual crossing. Any of a number of standard breeding techniques can be used, depending upon the species to be crossed.

In one embodiment of the invention, a biomass is created by producing a plurality of plants by the methods described above, where at least some of the plants express the proinsulin or insulin proteins. The biomass created is then harvested. The plants may be used as the source of the proteins, with all or part of the plant used as the protein source. In a preferred embodiment of the invention, seed is used as the source of the proteins. This is particularly preferred when a promoter preferentially expressing the proteins in the seed is used. Alternatively, the protein may be extracted by wet milling, dry milling or any one of numerous procedures available.

EXAMPLE 1

Introduction of DNA into Plants

Introduction of a nucleotide sequence encoding proinsulin protein in maize seed was achieved as follows.

The proinsulin-encoding nucleotide sequence used in this example is set forth in FIG. 1 (see Original Patent) (SEQ ID NO: 1), having been synthesized for codon optimization in maize, and ensuring no sequences were present that are predicted may destabilize the mRNA.

The protein encoded by the sequence is set forth in FIG. 2 (see Original Patent) along with 24 extra N-terminal amino acids encoding the barley alpha amylase signal sequence (SEQ ID NO: 2). Thus, the proinsulin protein consists of the 86 C-terminal amino acids of the 110 amino acids given in FIG. 2. Once the signal sequence is lost and proinsulin cleaved, 51 amino acids comprising insulin will remain. the human form of proinsulin is given.

Immature embryos of corn (Zea mays L.) were isolated from greenhouse-grown ears at 9-13 days after pollination depending on embryo size, generally 1.5-2.0 mm long. The embryos were treated with A. tumefaciens containing the DNA construct PGN9079 which carries the proinsulin gene under the control of maize globulin 1 promoter sequence (PGN9066; Belanger & Kriz, supra). The construct also contained a barley .alpha.-amylase signal sequence (BAASS; Rogers et al. 1985 J. Biol. Chem. 260, 3731-3738), for targeting the protein into the cell wall, and the plant transcription unit (PTU) was terminated by the pinII terminator, (An et al., 1989, Plant Cell 1:115-122. The PTU was positioned 5' of a CAMV 35S-pat-35S PTU encoding resistance to the selective agent bialaphos. See FIG. 3 (see Original Patent).

The treated embryos were plated onto callus induction medium and incubated in the dark at 19.degree. C. for four days. The embryos were then transferred to callus maintenance medium (CMM) and cultured in the dark at 28.degree. C. They were transferred every two weeks to fresh CMM medium. The callused embryos ceased growing after about two weeks on bialaphos and eventually turned brown. Transgenic calli appeared as early as six weeks following treatment but the majority of transformation events appeared at seven to nine weeks after treatment. The transgenic calli were easily spotted due to their white to pale yellow color, Type II callus phenotype, and rapid growth rate.

The transgenic events were grown for approximately four more weeks on bialaphos selection and then plated onto regeneration medium in the dark at 28.degree. C. for somatic embryo production. The somatic embryos were removed after three weeks and plated onto germination medium in the light (20-30 .mu.moles sec.sup.-1m.sup.-2) at 25 embryos per plate at 28.degree. C. The embryos germinated after 7-21 days and the T.sub.0 plantlets were moved into 25 mm.times.150 mm tubes containing 40 ml of minimal medium and left in the light as above for at least one week for further shoot and root development.

The plants were transferred into flats filled with equal parts of SunGro High Porosity (SunGro Horticulture Inc.) and Metro Mix 700 (Scott's-Sierra Horticultural Products Co.), covered with humidomes and placed in growth chambers for three to four weeks at 28.degree. C. and 90 .mu.molessec.sup.-1m.sup.-2. Humidomes were removed after one week. Plants were transplanted into 2 gal pots filled with High Porosity potting media and 27 g of Sierra 17-6-12 slow release fertilizer mixed into the top media surface. Plants were moved to the greenhouse floor (27.degree. C. and 195 .mu.molessec.sup.-1m.sup.-2). The T.sub.0 plants were pollinated with pollen from greenhouse-grown maize plants of elite germplasm.
 

Claim 1 of 5 Claims

1. A method of producing a protein comprising a protein selected from the group consisting of proinsulin and insulin protein, the method comprising providing biomass from a plurality of monocotyledonous plants comprising plants stably transformed with a nucleotide sequence encoding said protein, wherein expression of said protein is directed to seed cells of said transformed plants, and wherein said protein in at least one of said transformed plants is expressed at levels of at least 0.20% total soluble seed protein; said protein not fused to cholera toxin B subunit; and producing said protein in said transformed plant seed.

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