Patent Publication Number: US-7723109-B2

Title: Expression of spider silk proteins

Description:
This application is a continuation application of U.S. application Ser. No. 10/479,638, filed May 5, 2004, now U.S. Pat. No. 7,288,391, issued Oct. 30, 2007, which is a §371 filing of PCT/US02/18256, filed Jun. 6, 2002, which in turn claims priority to U.S. Provisional Application No. 60/296,184, filed Jun. 6, 2001. Each of the foregoing applications is incorporated herein by reference as though set forth in full. 
    
    
     Pursuant to 35 U.S.C. §202(c) it is acknowledged that the U.S. Government has certain rights in the invention described herein, which was made in part with funds from the Army Research Office, Grant Number DAAG55-98-1-0262 and the National Science Foundation, Grant Number MCB-9806999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the fields of molecular biology and plant biology. Specifically, the invention is directed to the methods for expressing spider silk proteins in plants and the synthesis and purification of spider silk proteins therefrom. 
     BACKGROUND OF THE INVENTION 
     Several publications and patent documents are referenced in this application in order to more fully describe the state of the art to which this invention pertains. The disclosure of each of these publications and documents is incorporated by reference herein. 
     Plants may be viewed as model systems for the large-scale production of exogenous proteins intended for therapeutic and industrial applications. Many plants are relatively easy and inexpensive to grow and are routinely produced in large quantities. Such large quantities, or crops, are harvested and processed by standard procedures utilized by the agronomic industry. The effective use of plants as bioreactors or protein factories depends on the ability to achieve high levels of expression of exogenous protein, which is stable throughout the life cycle of the transgenic plant and whose expression is maintained in subsequent generations. Silencing of an introduced transgene may occur in transformed plants and, thus, contributes to the commercial risk involved and hampers the general economic exploitation of plants as protein factories. A number of efficient strategies have been developed to avoid transgene silencing, including careful design of the transgene construct and thorough analysis of transformants at the molecular level. Recent research has focused on additional aspects related to the generation of transgenic plants intended for protein production and their influence on the stability of heterologous gene expression (De Wilde et al. 2000. Plant Mol Biol 43:347-59). 
     Of note, clinical trials are proceeding on the first biopharmaceuticals derived from transgenic plants. One transgenic plant-derived biopharmaceutical, hirudin, has been commercially produced in Canada. Product purification may, however, present a potential obstacle in the process because it is expensive. Various methods have been developed to overcome this problem, including oleosin-fusion technology, which allows extraction with oil bodies. In some cases, delivery of a biopharmaceutical product by direct ingestion of the modified plant may potentially remove the need for product purification. Such biopharmaceuticals may be stored and distributed as, for example, seeds, tubers, fruits, or ground plant material. The stability of exogenous proteins expressed in plants provides an additional benefit of such systems. (Giddings et al. 2000. Nat Biotechnol 18:1151-5). 
     The presence of silk protein producing abdominal glands is a unique feature of spiders. Spiders are also unique in the use of these silks throughout their life span and their nearly total dependence on silk for evolutionary success (14, 19). There were periods of fairly intense study of spider silk prior to World War II and in the late 1950s. Progress was relatively meager, however, particularly when compared to that related to silkworm silk. Beginning in the 1970s, interest in spider silk was revived with several papers describing physical, mechanical and chemical properties of spider silks. The composition of spider silks has been known to be predominantly protein since the 1907 studies of Fischer (5). In fact, except for the sticky spiral thread, no significant amount of any other compound but protein has been detected. 
     Typical spider webs are constructed from several different silks, each of which is produced in a different gland. Due to their large size and ease of study, major ampullate glands have received the most attention. Thus, most of what is known about the synthesis of silk proteins is based on studies of major ampullate glands. Morphological and histochemical studies of the other glands, however, have confirmed the conclusions drawn from research performed using major ampullate glands. Synthesis of the silk protein(s) takes place in specialized columnar epithelial cells (2). There appear to be at least two different types of cells producing protein (14), which correlates with findings that revealed the presence of two proteins in the silk from these glands. Newly synthesized protein droplets within the cell are secreted into the lumen of the gland, which serves as a reservoir of soluble silk protein. 
     The protein in the lumen of the gland is believed to be in a liquid crystal state (21), a structure which prevents fiber formation prior to passage through a narrow duct leading to a spinnernet. Maintenance of the liquid crystal state is likely due to physical properties related to protein structure and concentration, which serve to prevent aggregation into large protein arrays. It has been shown that silk in the lumen is not birefringent whereas silk becomes increasingly birefringent as it passes down through the duct (22). Thus, the ordered array of protein observed in the final fiber occurs during its passage through the duct. This appears to be due to the mechanical and frictional forces aligning the protein molecules and altering the secondary structure to the final fiber form. Iizuka (13) has proposed a similar mechanism for silkworm silk formation. The ability to draw silk fibers directly from the lumen of the major, minor and cylindrical glands (Hinman, M. personal comm.) implies that the physical force of drawing the solution is sufficient for fiber formation and provides experimental evidence for this mechanism. Once the fiber has reached the spinneret, a muscular valve at the exit of the spinneret is utilized to control the flow rate of the fiber and, to a small degree the fiber diameter. The silk exits the spider through the spinnerets, of which there are three pairs, anterior, median and posterior. 
     One of the features attracting researchers to study spider silks is their unusual mechanical properties. Orb-web weaving spiders use the minimum amount of silk in their webs to catch prey. The web has to stop a rapidly flying insect nearly instantly in a manner that allows the prey to become entangled and trapped. To achieve this end, the web must absorb the energy of momentum of the moving insect without breaking. Moreover, the web must also possess mechanical properties that serve to retain the insect. Gosline et al. (8) have reviewed several aspects of this property and concluded that spider silk and the web are nearly optimally designed for each other. 
     The present inventors have tested major and minor ampullate and egg case silks from both  Nephila clavipes  and  Araneus gemmoides  using standard mechanical testing methods (18). The silks were found to exceed the published data for tensile strength by a substantial margin. This was due to the use of the minimum diameter at ten points along the tested fiber for the calculation instead of the average diameter calculated from the density, length and weight. This minimum diameter is about 50% of the average diameter and since silks are likely to break at the narrowest point, these values may be more characteristic of the true properties of these silk fibers. Further examination of spider silk fibers (19) using scanning electron microscopy has confirmed the large variation in diameter of the fibers. 
     As with any polymer, especially those comprised of protein, there are numerous factors including temperature, hydration state, and rate of extension that can affect tensile strength and elasticity. Despite these caveats, it is clear that dragline silk is a unique biomaterial. Dragline silk can absorb more energy prior to breaking than nearly any commonly used material. It is nearly as strong as several of the current synthetic fibers but can outperform them in many applications where total energy absorption is required. 
     In 1990, the first spider silk protein from major ampullate silk was cloned in the form of a MaSp 1 cDNA from  N. clavipes  (23). The led to the appreciation that a second major ampullate silk protein existed which comprised a proline-containing peptide which was absent from the cDNA sequence coding for MaSp 1. This led to the cloning and sequencing of the cDNA for the second major ampullate silk protein, MaSp 2 (10). 
     The sizes of the mRNA and genes for MaSp1 and MaSp2 have been determined by analysis of Northern blots, restriction digestion patterns, and Southern blots of genomic DNA. The mRNA sizes for MaSp 1 and 2 are approximately 12.5 and 10.5 kb, respectively. The genomic DNA studies all indicate the absence of large introns in the coding regions and the lack of any detectable introns in the main portion of the gene. 
     SUMMARY OF THE INVENTION 
     Compositions and methods for producing large amounts of spider silk proteins are highly desirable give the superior quality of spider silk fibers. 
     Thus, in accordance with the present invention, methods for producing large quantities of novel spider silk proteins in plants, and transgenic plants comprising the same are provided. 
     In one embodiment, an exemplary method for expressing at least one spider silk protein in a higher plant comprises providing at least one expression vector containing a nucleic acid molecule encoding a spider silk protein, the nucleic acid sequence being operably linked to an exogenous promoter and at least one selectable marker gene which confers resistance to a selection agent. Plant cells are then contacted with the expression vector under conditions whereby the vector enters the plant cell, expresses the nucleic acid molecule, thereby producing the encoded spider silk protein. The plant cells are then incubated in the presence of a selection agent and those plant cells which survive in the presence of said agent are selected. The method optionally comprises regenerating a plant from the plant cell, a plant so isolated producing at least one spider silk protein. 
     Several nucleic acid sequences for producing natural and synthetic spider silk proteins are disclosed herein. Exemplary nucleic sequences for use in the methods of the invention include those set forth in SEQ ID NOS: 4, 5, 6, 7, 8, 9, 10, 11. The proteins encoded by these nucleic acids are also within the scope of the present invention and include SEQ ID NOS: 15, 16, 17, 18, 19, 20, 21, and 22. Additionally, nucleic acids encoding amino acid sequences of SEQ ID NOS: 23, 29-42, 43-43 and 49-54 and the amino acid sequences themselves are also encompassed by the present invention. 
     As mentioned above, the spider silk encoding nucleic acids are operably linked to a promoter element. Such promoters may be either constitutive or inducible. 
     The methods of the invention may be used to advantage to express spider silk proteins in plant cells such as those from  Arabidopsis , tobacco, tubers, sunflower, canola, alfalfa, soybean maize, sorghum, wheat, cotton, small grains, and rice. Transgenic plants comprising the spider silk proteins disclosed herein are also within the scope of the invention. 
    
    
     
       DETAILED DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic of pBT110-SS1. 
         FIG. 2  shows a flow chart of the transformation and selection of transgenic plants. 
         FIGS. 3A-3B  show a nucleic acid sequence (GenBank Accession Number M92913; SEQ ID NO: 1) which encodes an amino acid sequence of natural MaSp2 (SEQ ID NO: 7). 
         FIG. 4  shows a nucleic acid sequence (SEQ ID NO: 2) which encodes amino acid sequence of a synthetic monomeric repeat of MaSp2 (SEQ ID NO: 8). 
         FIG. 5  shows a nucleic acid sequence (SEQ ID NO: 3) which encodes an amino acid sequence of a synthetic spider silk protein (SS1) comprising sixteen monomeric repeats of MaSp2 (SEQ ID NO: 9). 
         FIG. 6  shows a Western blot of  Arabidopsis  cellular lysates probed with an antibody immunologically specific for MaSp2 protein. 
         FIG. 7  shows a Southern blot of DNA isolated from transgenic  Arabidopsis  plants and probed with a SS1 specific probe. 
         FIG. 8  shows a Northern blot of RNA isolated from transgenic  Arabidopsis  plants and probed with a SS1 specific probe. 
         FIG. 9  shows a nucleic acid sequence (SEQ ID NO: 4) which encodes an amino acid sequence of  Argiope trifasciata  aciniform fibroin 1 (SEQ ID NO: 15). 
         FIG. 10  shows a nucleic acid sequence (SEQ ID NO: 5) which encodes an amino acid sequence of  Phidippus audax  fibroin 1 (SEQ ID NO: 16). 
         FIG. 11  shows a nucleic acid sequence (SEQ ID NO: 6) which encodes an amino acid sequence of  Zoracrates  sp. fibroin 1 (SEQ ID NO: 17). 
         FIGS. 12A-12E  show a nucleic acid sequence (SEQ ID NO: 7) which encodes an amino acid sequence of a  Kukulcania  MaSp (SEQ ID NO: 18). 
         FIGS. 13A-13C  show a nucleic acid sequence (SEQ ID NO: 8) which encodes an amino acid sequence of a  Kukulcania  MaSp (SEQ ID NO: 19). 
         FIG. 14  shows a nucleic acid sequence (SEQ ID NO: 9) which encodes an amino acid sequence of a  Kukulcania  MaSp (SEQ ID NO: 20). 
         FIGS. 15A-15D  show a nucleic acid sequence (SEQ ID NO: 10) which encodes an amino acid sequence of an  Argiope  MiSp (SEQ ID NO: 21). 
         FIGS. 16A-16B  show a nucleic acid sequence (SEQ ID NO: 11) which encodes an amino acid sequence of an  Argiope  MiSp (SEQ ID NO: 22). 
         FIG. 17  shows a consensus sequence repeat of the  A. trifasciata  aciniform fibroin 1 protein (SEQ ID NO: 23) comprised of approximately 200 amino acids. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The physical characteristics of spider silk proteins confer unparalleled mechanical properties to these fibroins and, thus, render spider silk proteins ideally suited to a variety of applications. It is, therefore, desirable to generate large quantities of spider silk proteins to provide a resource for the development and execution of such applications. The expression of large quantities of spider silk proteins is, however, a technically challenging proposition. The present inventors have discovered that plants provide a model expression system for the synthesis of spider silk proteins in quantity. Accordingly, methods are provided herein for the expression of spider silk proteins in plants. 
     In a preferred embodiment of the invention, methods are provided for the transformation of plants with expression constructs encoding a spider silk protein. 
     In a particularly preferred embodiment of the invention, expression constructs are provided which comprise nucleic acid sequences encoding spider silk proteins. Such nucleic acid sequences may encode natural spider silk proteins or synthetic spider silk proteins. Nucleic acid sequences encoding synthetic spider silk proteins may be engineered to provide a spider silk protein having desirable properties optimized for a particular application. 
     In another aspect of the present invention, different expression constructs are provided. Such expression constructs are designed for expression of exogenous molecules in plant cells. One of skill in the art would appreciate that the choice of an expression construct may be based on a variety of different factors, including, but not limited to, the type of plant used, the mode of transformation, and the exogenous protein expressed. In yet another aspect of the invention, methods are provided for the transformation of a variety of different plants, including, but not limited to,  Arabidopsis thaliana , alfalfa, tobacco, tubers, sunflower, canola, soybean, maize, sorghum, wheat, cotton, small grains, and rice. 
     In one embodiment of the present invention, methods are provided for harvesting transgenic plants expressing spider silk proteins. Also provided are methods for the isolation of spider silk proteins from cellular extracts of plant cells. Methods for the expression and isolation of spider silk proteins from plants have been previously disclosed in Scheller et al. (2001, Nature Biotechnology 19:573) and PCT Publication Number WO 01/94393 A2, the entire contents of which are incorporated herein by reference. 
     Additional spider silk proteins and methods for isolating the same have been previously identified, see for example U.S. Pat. Nos. 5,245,012; 5,728,810; 5,733,771; 5,756,677; 5,989,894; 5,994,099; 6,268,169 and 6,280,747, the entire disclosures of which are incorporated herein by reference. 
     I. Definitions 
     The following definitions are provided to facilitate an understanding of the present invention: 
     With reference to nucleic acids used in the invention, the term “isolated nucleic acid” is sometimes employed. This term, when applied to DNA, refers to a DNA molecule that is separated from sequences with which it is immediately contiguous (in the 5′ and 3′ directions) in the naturally occurring genome of the organism from which it was derived. For example, the “isolated nucleic acid” may comprise a DNA molecule inserted into a vector, such as a plasmid or virus vector, or integrated into the genomic DNA of a procaryote or eucaryote. An “isolated nucleic acid molecule” may also comprise a cDNA molecule. An isolated nucleic acid molecule inserted into a vector is also sometimes referred to herein as a recombinant nucleic acid molecule. 
     With respect to RNA molecules, the term “isolated nucleic acid” primarily refers to an RNA molecule encoded by an isolated DNA molecule as defined above. Alternatively, the term may refer to an RNA molecule that has been sufficiently separated from RNA molecules with which it would be associated in its natural state (i.e., in cells or tissues), such that it exists in a “substantially pure” form. 
     With respect to single stranded nucleic acids, particularly oligonucleotides, the term “specifically hybridizing” refers to the association between two single-stranded nucleotide molecules of sufficiently complementary sequence to permit such hybridization under pre-determined conditions generally used in the art (sometimes termed “substantially complementary”). In particular, the term refers to hybridization of an oligonucleotide with a substantially complementary sequence contained within a single-stranded DNA or RNA molecule of the invention, to the substantial exclusion of hybridization of the oligonucleotide with single-stranded nucleic acids of non-complementary sequence. Appropriate conditions enabling specific hybridization of single stranded nucleic acid molecules of varying complementarity are well known in the art. 
     For instance, one common formula for calculating the stringency conditions required to achieve hybridization between nucleic acid molecules of a specified sequence homology is set forth below (Sambrook et al., 1989):
 
 T   m =81.5° C.+16.6 Log [Na+]+0.41(%  G+C )−0.63(% formamide)−600/#bp in duplex
 
     As an illustration of the above formula, using [Na+]=[0.368] and 50% formamide, with GC content of 42% and an average probe size of 200 bases, the T m  is 57° C. The T m  of a DNA duplex decreases by 1-1.5° C. with every 1% decrease in homology. Thus, targets with greater than about 75% sequence identity would be observed using a hybridization temperature of 42° C. 
     The term “oligonucleotide,” as used herein refers to primers and probes of the present invention, and is defined as a nucleic acid molecule comprised of two or more ribo- or deoxyribonucleotides, preferably more than three. The exact size of the oligonucleotide will depend on various factors and on the particular application and use of the oligonucleotide. Preferred oligonucleotides comprise 15-50 consecutive bases of SEQ ID NOs: 4-11. 
     The term “probe” as used herein refers to an oligonucleotide, polynucleotide or nucleic acid, either RNA or DNA, whether occurring naturally as in a purified restriction enzyme digest or produced synthetically, which is capable of annealing with or specifically hybridizing to a nucleic acid with sequences complementary to the probe. A probe may be either single-stranded or double-stranded. The exact length of the probe will depend upon many factors, including temperature, source of probe and use of the method. For example, depending on the complexity of the target sequence, the oligonucleotide probe typically contains 15-25 or more nucleotides, although it may contain fewer nucleotides. The probes herein are selected to be complementary to different strands of a particular target nucleic acid sequence. This means that the probes must be sufficiently complementary so as to be able to “specifically hybridize” or anneal with their respective target strands under a set of pre-determined conditions. Therefore, the probe sequence need not reflect the exact complementary sequence of the target. For example, a non-complementary nucleotide fragment may be attached to the 5′ or 3′ end of the probe, with the remainder of the probe sequence being complementary to the target strand. Alternatively, non-complementary bases or longer sequences can be interspersed into the probe, provided that the probe sequence has sufficient complementarity with the sequence of the target nucleic acid to anneal therewith specifically. 
     The term “primer” as used herein refers to an oligonucleotide, either RNA or DNA, either single-stranded or double-stranded, either derived from a biological system, generated by restriction enzyme digestion, or produced synthetically which, when placed in the proper environment, is able to functionally act as an initiator of template-dependent nucleic acid synthesis. When presented with an appropriate nucleic acid template, suitable nucleoside triphosphate precursors of nucleic acids, a polymerase enzyme, suitable cofactors and conditions such as a suitable temperature and pH, the primer may be extended at its 3′ terminus by the addition of nucleotides by the action of a polymerase or similar activity to yield a primer extension product. The primer may vary in length depending on the particular conditions and requirement of the application. For example, in diagnostic applications, the oligonucleotide primer is typically 15-25 or more nucleotides in length. The primer must be of sufficient complementarity to the desired template to prime the synthesis of the desired extension product, that is, to be able anneal with the desired template strand in a manner sufficient to provide the 3′ hydroxyl moiety of the primer in appropriate juxtaposition for use in the initiation of synthesis by a polymerase or similar enzyme. It is not required that the primer sequence represent an exact complement of the desired template. For example, a non-complementary nucleotide sequence may be attached to the 5′ end of an otherwise complementary primer. Alternatively, non-complementary bases may be interspersed within the oligonucleotide primer sequence, provided that the primer sequence has sufficient complementarity with the sequence of the desired template strand to functionally provide a template-primer complex for the synthesis of the extension product. 
     Polymerase chain reaction (PCR) has been described in U.S. Pat. Nos. 4,683,195, 4,800,195, and 4,965,188, the entire disclosures of which are incorporated by reference herein. 
     Amino acid residues described herein are preferred to be in the “L” isomeric form. However, residues in the “D” isomeric form may be substituted for any L-amino acid residue, provided the desired properties of the polypeptide are retained. All amino-acid residue sequences represented herein conform to the conventional left-to-right amino-terminus to carboxy-terminus orientation. 
     The term “isolated protein” or “isolated and purified protein” is sometimes used herein. This term refers primarily to a protein produced by expression of an isolated nucleic acid molecule of the invention. Alternatively, this term may refer to a protein that has been sufficiently separated from other proteins with which it would naturally be associated, so as to exist in “substantially pure” form. “Isolated” is not meant to exclude artificial or synthetic mixtures with other compounds or materials, or the presence of impurities that do not interfere with the fundamental activity, and that may be present, for example, due to incomplete purification, addition of stabilizers, or compounding into, for example, immunogenic preparations or pharmaceutically acceptable preparations. Pharmaceutically acceptable preparations may be used in the production of fibers and synthetic polymers, for example, that may be incorporated into a variety of medical implements, including, but not limited to, sutures, wound coverings, and implants. 
     A “pharmaceutically acceptable carrier” refers to a solution in which a spider silk protein or a nucleic acid sequence encoding a spider silk protein may be maintained without altering the functional properties of the spider silk molecule therein. For administration to a mammal, for example, a spider silk protein or a nucleic acid sequence encoding a spider silk protein may be suspended in any pharmaceutically acceptable carrier, for example, HEPES buffered saline at a pH of about 7.8. 
     Other pharmaceutically acceptable carriers which are useful include, but are not limited to, glycerol, water, saline, ethanol and other pharmaceutically acceptable salt solutions such as phosphates and salts of organic acids. Examples of these and other pharmaceutically acceptable carriers are described in Remington&#39;s Pharmaceutical Sciences (1991, Mack Publication Co., New Jersey). 
     “Mature protein” or “mature polypeptide” shall mean a polypeptide possessing the sequence of the polypeptide after any processing events that normally occur to the polypeptide during the course of its genesis, such as proteolytic processing from a polyprotein precursor. In designating the sequence or boundaries of a mature protein, the first amino acid of the mature protein sequence is designated as amino acid residue 1. As used herein, any amino acid residues associated with a mature protein not naturally found associated with that protein that precedes amino acid 1 are designated amino acid −1, −2, −3 and so on. For recombinant expression systems, a methionine initiator codon is often utilized for purposes of efficient translation. This methionine residue in the resulting polypeptide, as used herein, would be positioned at −1 relative to the mature protein sequence. 
     A low molecular weight “peptide analog” shall mean a natural or mutant (mutated) analog of a protein, comprising a linear or discontinuous series of fragments of that protein and which may have one or more amino acids replaced with other amino acids and which has altered, enhanced or diminished biological activity when compared with the parent or nonmutated protein. 
     The term “biological activity” is a function or set of functions performed by a molecule in a biological context (i.e., in an organism or an in vitro surrogate or facsimile model). For spider silk proteins, biological activity is characterized by physical properties (e.g., tensile strength and elasticity) as described herein. 
     The term “substantially pure” refers to a preparation comprising at least 50-60% by weight the compound of interest (e.g., nucleic acid, oligonucleotide, polypeptide, protein, etc.). More preferably, the preparation comprises at least 75% by weight, and most preferably 90-99% by weight, the compound of interest. Purity is measured by methods appropriate for the compound of interest (e.g. chromatographic methods, agarose or polyacrylamide gel electrophoresis, HPLC analysis, mass spectrometry and the like). 
     The term “tag,” “tag sequence” or “protein tag” refers to a chemical moiety, either a nucleotide, oligonucleotide, polynucleotide or an amino acid, peptide or protein or other chemical, that when added to another sequence, provides additional utility or confers useful properties, particularly in the detection or isolation, of that sequence. Thus, for example, a homopolymer nucleic acid sequence or a nucleic acid sequence complementary to a capture oligonucleotide may be added to a primer or probe sequence to facilitate the subsequent isolation of an extension product or hybridized product. In the case of protein tags, histidine residues (e.g., 4 to 8 consecutive histidine residues) may be added to either the amino- or carboxy-terminus of a protein to facilitate protein isolation by chelating metal chromatography. Alternatively, amino acid sequences, peptides, proteins or fusion partners representing epitopes or binding determinants reactive with specific antibody molecules or other molecules (e.g., flag epitope, c-myc epitope, transmembrane epitope of the influenza A virus hemaglutinin protein, protein A, cellulose binding domain, calmodulin binding protein, maltose binding protein, chitin binding domain, glutathione S-transferase, and the like) may be added to proteins to facilitate protein isolation by procedures such as affinity or immunoaffinity chromatography. Chemical tag moieties include such molecules as biotin, which may be added to either nucleic acids or proteins and facilitates isolation or detection by interaction with avidin reagents, and the like. Numerous other tag moieties are known to, and can be envisioned by the trained artisan, and are contemplated to be within the scope of this definition. 
     A “vector” is a replicon, such as a plasmid, cosmid, bacmid, phage or virus, to which another genetic sequence or element (either DNA or RNA) may be attached so as to bring about the replication of the attached sequence or element. An “expression vector” is a specialized vector that contains a gene with the necessary regulatory regions needed for expression in a host cell. Such vectors may be obtained from various commercial sources, including Clontech Laboratories, Inc. (Palo Alto, Calif.), Stratagene (La Jolla, Calif.), Invitrogen (Carlsbad, Calif.), New England Biolabs (Beverly, Mass.) and Promega (Madison, Wis.). Exemplary vectors which may be used in the present invention include, but are not limited to, pIBT110, pBI121, and pGreen. 
     The term “operably linked” means that the regulatory sequences necessary for expression of the coding sequence are placed in the DNA molecule in the appropriate positions relative to the coding sequence so as to effect expression of the coding sequence. This same definition is sometimes applied to the arrangement of coding sequences and transcription control elements (e.g. promoters, enhancers, and termination elements) in an expression vector. This definition is also sometimes applied to the arrangement of nucleic acid sequences of a first and a second nucleic acid molecule wherein a hybrid nucleic acid molecule is generated. 
     An exogenous coding region is typically flanked by operably linked regulatory regions that regulate expression of the exogenous coding region in a transformed plant cell. A typical regulatory region operably linked to the exogenous coding region includes a promoter, i.e., a nucleic acid fragment that can cause transcription of the exogenous coding region, positioned 5′ to the exogenous coding region. The invention is not limited by the use of any particular promoter and a wide variety are known in the art. Plant-specific promoters are preferred. These include, but are not limited to, constitutive promoters, inducible promoters, and tissue-specific promoters. 
     A promoter may be, but need not be, heterologous with respect to the host. Promoters may be obtained from Ti- or Ri-plasmids, from plant cells, plant viruses or other hosts wherein the promoters are functional in plants. Such promoters include, for example, the octopine synthetase promoter, the nopaline synthase promoter, and the manopine synthetase promoter, promoters of bacterial origin which are functional in plants. Viral promoters include the cauliflower mosaic virus full length (CaMV35S) and region VI promoters, etc. Endogenous plant promoters include the ribulose-1,6-biphosphate (RUBP) carboxylase small subunit (ssu) promoter, the beta-conglycinin promoter, the phaseolin promoter, the ADH promoter, GPAL2 promoter, GPAL3 promoter, heat-shock promoters, and tissue specific promoters, e.g., promoters associated with fruit ripening. In one embodiment of the present invention, the promoter is a constitutive CaMV35S promoter. 
     As described above, expression vectors may comprise an inducible promoter operably linked to a spider silk encoding nucleic acid sequence. “Inducible” promoters may direct expression of a polynucleotide to which they are operably linked in a tissue or developmental stage specific manner or may respond to environmental conditions. In one aspect of the invention, expression vectors comprising a tightly-regulated inducible promoter operably linked to a nucleic acid encoding a spider silk protein may be used. Such expression vectors may further comprise a selectable marker gene (e.g. a gene encoding a protein which confers antibiotic resistance) operably linked to either a constitutive promoter or a tightly-regulated inducible promoter. Depending on the application, it may be beneficial to express the spider silk encoding nucleic acid sequence from a pathogen-inducible promoter. Such promoters include those derived from pathogenesis-related proteins (PR proteins), which are induced following infection by a pathogen, e.g. PR proteins, SAR proteins, beta-1,3 glucanase, chitinase, etc. See, for example, Redolfi et al.,  Neth J Plant Pathol  89: 245 (1983); Uknes et al.,  Plant Cell  4: 645 (1992); Van Loon,  Plant Mol Virol  4: 111 (1985). 
     In an aspect of the present invention, it may be advantageous to utilize promoters which are expressed locally at or near the site of pathogen infection. See, for example, Marineau et al.,  Plant Mol Biol  9: 335 (1987); Matton et al.,  Mol Plant - Microbe Interact  2: 325 (1989); Somsisch et al.,  Mol and Gen Genetics  2: 93 (1988). Yang,  Proc Natl Acad Sci  93: 14972 (1996). See also, Chen et al.,  Plant J  10: 955 (1996); Zhang and Sing,  Proc Natl Acad Sci USA  91: 2507 (1994); Warner et al.,  Plant J  3: 191 (1993); Siebertz et al.,  Plant Cell  1: 961 (1989); and the references cited therein. The inducible promoter of the maize PRms gene, whose expression is induced by the pathogen  Fusarium moniliforme , may be of particular utility for a number of applications (see, for example, Cordero et al.,  Physiol and Mol Plant Path  41: 189 (1992). 
     Additionally, because pathogens enter plants through wounds, which may result from insect damage, a wound-inducible promoter may be used in the expression vectors of the invention. Such wound inducible promoters include, but are not limited to, potato proteinase inhibitor (pin II) gene (Ryan,  Annu Rev Phytopath  28: 425; Duan et al.,  Nature Biotech  14: 494; wun1 and wun2, U.S. Pat. No. 5,428,148; win1 and win2 (Stanford et al.,  Mol Gen Genet  215: 200; systemin (McGurl et al.,  Science  225: 1570; WIPI (Rohmeier et al.,  Plant Mol Biol  22: 783; Eckelkamp et al.,  FEBS Let  323: 73; MPI gene (Corderok et al., 6(2)  Plant J  141 and references contained therein. 
     The transcriptional activity of inducible promoters may also be regulated by various environmental conditions, including, but not limited to, temperature, anaerobic stress, and light. Examples of inducible promoters include the Adh1 promoter which is induced by hypoxia or cold stress, the Hsp70 promoter which is induced by heat stress, and the PPDK promoter which is induced by light. Examples of developmentally regulated promoters include promoters which initiate transcription only, or preferentially, in certain tissues, such as leaves, roots, fruit, seeds, or flowers. An exemplary promoter is the anther specific promoter 5126 (U.S. Pat. Nos. 5,689,049 and 5,689,051). 
     Construction of vectors comprising promoters in frame with nucleic acids is known in the art, and may be accomplished according to i.e. Sambrook et al.,  Molecular Cloning. A Laboratory Manual  (Cold Spring Harbor Laboratory Press, 1989) and Ausubel et al.,  Current Protocols in Molecular Biology  (Greene Publishing Associates, Inc., 1993). 
     Another typical regulatory region operably linked to an exogenous coding region includes a terminator (i.e., a nucleic acid fragment that can cause the termination of transcription of an exogenous coding region) positioned 3′ to the exogenous coding region. The invention is not limited by the use of any particular terminator, and a wide variety are known in the art. Plant-specific terminators are preferred. These include, but are not limited to, a nopaline synthase terminator derived from the  Agrobacterium tumefaciens  Ti plasmid (Nos-ter). 
     Expression vectors which comprise nucleic acid sequences encoding spider silk proteins are within the scope of the present invention. Also included in the present invention are recombinant plant cells, recombinant seeds, recombinant plant embryos, and recombinant plants comprising the expression vectors encoding spider silk proteins described herein. 
     The phrase “consisting essentially of” when referring to a particular nucleotide or amino acid means a sequence having the properties of a given SEQ ID NO:. For example, when used in reference to an amino acid sequence, the phrase includes the sequence per se and molecular modifications that would not affect the basic and novel characteristics of the sequence. 
     A “clone” or “clonal cell population” is a population of cells derived from a single cell or common ancestor by mitosis. 
     A “cell line” is a clone of a primary cell or cell population that is capable of stable growth in vitro for many generations. 
     An “immune response” signifies any reaction produced in response to an antigen, such as a viral antigen, in a host having a functioning immune system. Immune responses may be either humoral in nature, that is, involve production of immunoglobulins or antibodies, or cellular in nature, involving various types of B and T lymphocytes, dendritic cells, macrophages, antigen presenting cells and the like, or both. Immune responses may also involve the production or elaboration of various effector molecules such as cytokines, lymphokines and the like. Immune responses may be measured both in in vitro and in various cellular or animal systems. Such immune responses may be important in protecting the host from disease and may be used prophylactically and therapeutically. 
     An “immune response” in plants refers to the ability of a plant to respond to infection by microbial pathogens, for example, through the activation of a variety of defense responses. At the sites of infection, a hypersensitive response (HR) is often initiated. The hallmark of this response is the formation of necrotic lesions, a process that is likely due to programmed host cell death. In addition, associated with the HR is the restriction of pathogen growth and spread. Frequently, defense responses are also activated in tissue distal to the sites of infection according to a phenomenon known as systemic acquired resistance (SAR). Development of SAR results in an enhanced and long-lasting resistance to secondary challenge by the same or even unrelated pathogens. Associated with both HR and SAR is the expression of pathogenesis-related (PR) genes, several of whose products have been shown to have antimicrobial activity (for review, see U.S. Pat. Nos. 6,136,552; 5,989,846; 5,977,442; and 5,939,601). 
     In accordance with the present invention, expression of spider silk proteins in plants cells does not induce an appreciable cellular immune response as evidenced by the normal phenotype exhibited by such transgenic plants. Moreover, spider silk proteins are not known to elicit an immune response when introduced as a component of a transplant into an animal. The apparent absence of immunogenicity for spider silk proteins is a feature which may be used to advantage in applications in which the plant-derived spider silk proteins of the present invention are used as components in medical devices, implants, and the like. 
     An “antibody” or “antibody molecule” is any immunoglobulin, including antibodies and fragments thereof, that binds to a specific antigen. The term includes polyclonal, monoclonal, chimeric, and bispecific antibodies. As used herein, antibody or antibody molecule contemplates both an intact immunoglobulin molecule and an immunologically active portion of an immunoglobulin molecule such as those portions known in the art as Fab, Fab′, F(ab′)2 and F(v). 
     With respect to antibodies, the term “immunologically specific” refers to antibodies that bind to one or more epitopes of a protein or compound of interest, but which do not substantially recognize and bind other molecules in a sample containing a mixed population of antigenic biological molecules. 
     “Natural allelic variants”, “mutants” and “derivatives” of particular sequences of nucleic acids refer to nucleic acid sequences that are closely related to a particular sequence but which may possess, either naturally or by design, changes in sequence or structure. By closely related, it is meant that at least about 75%, but often, more than 90%, of the nucleotides of the sequence match over the defined length of the nucleic acid sequence referred to using a specific SEQ ID NO. Changes or differences in nucleotide sequence between closely related nucleic acid sequences may represent nucleotide changes in the sequence that arise during the course of normal replication or duplication in nature of the particular nucleic acid sequence. Other changes may be specifically designed and introduced into the sequence for specific purposes, such as to change an amino acid codon or sequence in a regulatory region of the nucleic acid. Such specific changes may be made in vitro using a variety of mutagenesis techniques or produced in a host organism placed under particular selection conditions that induce or select for the changes. Such sequence variants generated specifically may be referred to as “mutants” or “derivatives” of the original sequence. 
     A “derivative” of a spider silk protein or a fragment thereof means a polypeptide modified by varying the amino acid sequence of the protein, e.g. by manipulation of the nucleic acid encoding the protein or by altering the protein itself. Such derivatives of the natural amino acid sequence may involve insertion, addition, deletion or substitution of one or more amino acids, and may or may not alter the essential activity of original the spider silk protein. 
     As mentioned above, a spider silk polypeptide or protein of utility in the methods of the invention may be any analogue, fragment, derivative or mutant which is derived from a spider silk protein and which retains at least one property or other characteristic of a spider silk protein. Different “variants” of spider silk proteins exist in nature. These variants may be alleles characterized by differences in the nucleotide sequences of the gene coding for the protein, or may involve different RNA processing or post-translational modifications. The skilled person can produce variants having single or multiple amino acid substitutions, deletions, additions or replacements. These variants may include inter alia: (a) variants in which one or more amino acids residues are substituted with conservative or non-conservative amino acids, (b) variants in which one or more amino acids are added to a spider silk protein, (c) variants in which one or more amino acids include a substituent group, and (d) variants in which a spider silk protein or fragment thereof is fused with another peptide or polypeptide such as a fusion partner, a protein tag or other chemical moiety, that may confer useful properties to a spider silk protein, such as, for example, an epitope for an antibody, a polyhistidine sequence, a biotin moiety and the like. Other spider silk proteins of the invention include variants in which amino acid residues from one species are substituted for the corresponding residue in another species, either at the conserved or non-conserved positions. In another embodiment, amino acid residues at non-conserved positions are substituted with conservative or non-conservative residues. The techniques for obtaining these variants, including genetic (suppressions, deletions, mutations, etc.), chemical, and enzymatic techniques are known to the person having ordinary skill in the art. 
     The term “functional” as used herein implies that the nucleic or amino acid sequence is functional for the recited assay or purpose. 
     A “unit repeat” constitutes a repetitive short sequence. Thus, the primary structure of the spider silk proteins is considered to consist mostly of a series of small variations of a unit repeat. The unit repeats in the naturally occurring proteins are often distinct from each other. That is, there is little or no exact duplication of the unit repeats along the length of the protein. Synthetic spider silks, however, may be made wherein the primary structure of the protein comprises a number of exact repetitions of a single unit repeat. SEQ ID NO: 3 ( FIG. 5 ), for example, encodes a synthetic spider silk protein comprising 16 repeats of the MaSp2 monomeric unit SEQ ID NO: 2 ( FIG. 4 ). Additional synthetic spider silks may be synthesized which comprise a number of repetitions of one unit repeat together with a number of repetitions of a second unit repeat. See Example IV. Such a structure would be similar to a typical block copolymer. Unit repeats of several different sequences may also be combined to provide a synthetic spider silk protein having properties suited to a particular application. 
     The term “direct repeat” as used herein is a repeat in tandem (head-to-tail arrangement) with a similar repeat. 
     The term “native or natural spider silk protein” refers to those proteins that are present in the silks produced by spiders. These proteins may be derived from the silk itself by dissolution or from the specific silk gland in the abdomen of the spider before the silk is spun. The term may also be applied to a spider silk protein produced using a variety of expression systems, which comprises substantially the same amino acid sequence as that produced by a spider 
     The term “synthetic spider silk protein” refers to a spider silk protein which has been produced by an expression system and whose sequence may be based on a natural spider silk protein sequence or an artificially produced nucleic acid sequence which encodes key amino acid motifs of spider silk proteins. 
     II. Preparation of Spider Silk-Encoding Nucleic Acid Molecules, Spider Silk Proteins, and Antibodies Thereto 
     A. Nucleic Acid Molecules 
     Nucleic acid molecules encoding the polypeptides of the invention may be prepared by two general methods: (1) synthesis from appropriate nucleotide triphosphates, or (2) isolation from biological sources. Both methods utilize protocols well known in the art. The availability of nucleotide sequence information, such as the DNA sequences encoding a natural or synthetic spider silk protein, enables preparation of an isolated nucleic acid molecule of the invention by oligonucleotide synthesis. Synthetic oligonucleotides may be prepared by the phosphoramidite method employed in the Applied Biosystems 38A DNA Synthesizer or similar devices. The resultant construct may be used directly or purified according to methods known in the art, such as high performance liquid chromatography (HPLC). 
     In accordance with the present invention, nucleic acids having the appropriate level of sequence homology with sequences encoding a spider silk protein may be identified by using hybridization and washing conditions of appropriate stringency. Such methods are useful for a variety of purposes, including the screening of libraries comprising mutated spider silk-encoding nucleic acid sequences for desired properties. For example, hybridizations may be performed, according to the method of Sambrook et al.,  Molecular Cloning , Cold Spring Harbor Laboratory (1989), using a hybridization solution comprising: 5×SSC, 5×Denhardt&#39;s reagent, 1.0% SDS, 100 μg/ml denatured, fragmented salmon sperm DNA, 0.05% sodium pyrophosphate and up to 50% formamide. Hybridization is carried out at 37-42° C. for at least six hours. Following hybridization, filters are washed as follows: (1) 5 minutes at room temperature in 2×SSC and 1% SDS; (2) 15 minutes at room temperature in 2×SSC and 0.1% SDS; (3) 30 minutes-1 hour at 37° C. in 1×SSC and 1% SDS; (4) 2 hours at 42-65° C. in 1×SSC and 1% SDS, changing the solution every 30 minutes. 
     The nucleic acid molecules described herein include cDNA, genomic DNA, RNA, and fragments thereof which may be single- or double-stranded. Thus, oligonucleotides are provided having sequences capable of hybridizing with at least one sequence of a nucleic acid sequence, such as selected segments of sequences encoding a spider silk protein. Also contemplated in the scope of the present invention are methods of use for oligonucleotide probes which specifically hybridize with DNA from sequences encoding a spider silk protein under high stringency conditions. Primers capable of specifically amplifying sequences encoding a spider silk protein are also provided. As mentioned previously, such oligonucleotides are useful as primers for detecting, isolating and amplifying sequences encoding a spider silk protein. 
     The invention also encompasses the use of nucleic acid molecules encoding synthetic spider silk proteins. Such synthetic spider silk proteins may be engineered to possess particular physical and mechanical properties that render them appropriate for different applications. A synthetic spider silk protein may be constructed by assembling spider silk protein unit repeats into a contiguous polypeptide chain as set forth in Example IV and Table II. Also provided herein is guidance regarding the types and numbers of unit repeats which may be combined and the order in which they may be assembled. Exemplary synthetic spider silk proteins comprising MaSp2, Flag, and MaSp1 analogs are also provided. See Example IV. For some applications, it may be desirable to operably link a synthetic spider silk protein to a tag moiety as described herein. 
     B. Proteins 
     The availability of nucleic acid molecules encoding spider silk proteins enables production of large quantities of spider silk protein in a suitable prokaryotic or eukaryotic system. For example, part or all of at least one DNA molecule encoding a natural or synthetic spider silk protein, such as the nucleic acid sequence of SEQ ID NO: 1, may be inserted into a plasmid vector adapted for expression in a bacterial cell, such as  E. coli . Such vectors comprise the regulatory elements necessary for expression of the DNA in the host cell positioned in such a manner as to permit expression of the DNA in the host cell. Such regulatory elements required for expression include promoter sequences, transcription initiation sequences and, optionally, enhancer sequences. Such methods may be used to evaluate constructs for expression of spider silk proteins in, for example, a bacterial system which affords a rapid and reliable screening technique. 
     The spider silk proteins produced by gene expression in a recombinant prokaryotic or eukaryotic system may be purified according to methods known in the art. In a preferred embodiment, a commercially available expression/secretion system may be used, whereby the recombinant protein is expressed and thereafter secreted from the host cell, to be easily purified from the surrounding medium. If expression/secretion vectors are not used, an alternative approach involves purifying the recombinant protein from cell lysates (remains of cells following disruption of cellular integrity) derived from prokaryotic or eukaryotic cells in which a protein was expressed. Methods for generation of such cell lysates are known to those of skill in the art. Recombinant protein may be purified by affinity separation, such as by immunological interaction with antibodies that bind specifically to the recombinant protein or nickel columns for isolation of recombinant proteins tagged with 6-8 histidine residues at their N-terminus or C-terminus. Alternative tags may comprise the FLAG epitope or the hemagglutinin epitope. Such methods are commonly used by skilled practitioners. 
     Alternatively, standard purification strategies designed to differentially isolate silk protein from plant homogenates may be used to advantage. Purification of a plant-expressed spider silk protein may be facilitated by its extreme stability under conditions that denature typical proteins, such as, for example, high heat and low pH. Accordingly, general protein purification strategies may be adapted to optimize silk purification from leaves. Above-ground portions of transgenic plants may be harvested and allowed to air dry as per normal production practices. The “hay” may be homogenized in an appropriate buffer followed by various treatments designed to differentially eliminate contaminants. Silk protein recovery may be optimized following treatments in which plant extracts are subject to any one or a combination of the following: 1) boiling in the presence or absence of detergent; 2) differential centrifugation; 3) progressively decreasing the pH; and 4) precipitation with varying concentrations of urea or ammonium sulfate. One of ordinary skill in the art may vary the above treatments to optimize the yield and efficiency of purification of spider silk proteins from plants. 
     The level of silk protein may be determined by immunoblotting and the purity and concentration assessed definitively by amino acid analysis. Purified silk protein may be analyzed for mechanical properties as previously described (18) to ensure that the recombinant protein possesses the desired properties. 
     The spider silk proteins of the invention, prepared by the aforementioned methods, may be analyzed according to standard procedures. For example, such proteins may be subjected to amino acid sequence analysis, according to known methods. 
     A protein produced according to the present invention may be chemically modified after synthesis of the polypeptide. The presence of several carboxylic acid side chains (Asp or Glu) in the spacer regions facilitates the attachment of a variety of different chemical groups to silk proteins including amino acids having such side chains. The simplest and easiest procedure is to use a water-soluble carbodiimide to attach the modifying group via a primary amine. If the group to be attached has no primary amine, a variety of linking agents can be attached via their own primary amines and the modifying group attached via an available chemistry. Jennes, L. and Stumpf, W. E. Neuroendocrine Peptide Methodology, Chapter 42. P. Michael Conn, editor. Academic Press, 1989. 
     Desirable chemical modifications include, but are not limited to, derivatization with peptides that bind to cells, e.g. fibroblasts, derivatization with antibiotics and derivatization with cross-linking agents so that cross-linked fibers can be made. The selection of derivatizing agents for a particular purpose is within the skill of the ordinary practitioner of the art. 
     Exemplary Methods for Generation of Spider Silk Proteins 
     In view of the unique properties of spider silk proteins, special considerations should be applied to the generation of synthetic spider silk proteins. The repetitive nature of amino acid sequences encoding these proteins may render synthesis of a full length spider silk protein, or fragments thereof, technically challenging. To facilitate production of full length silk protein molecules, the following protocol is provided. 
     The polypeptides of the present invention can be made by direct synthesis or by expression from cloned DNA. Means for expressing cloned DNA are set forth above and are generally known in the art. The following considerations are recommended for the design of expression vectors used to express DNA encoding spider silk proteins. 
     First, since spider silk proteins are highly repetitive in structure, cloned DNA should be propagated and expressed in host cell strains that can maintain repetitive sequences in extrachromosomal elements (e.g. SURE™ cells, Stratagene). The prevalence of specific amino acids (e.g., alanine, glycine, proline, and glutamine) also suggests that it might be advantageous to use a host cell that over-expresses tRNA for these amino acids or in which these specific tRNAs are known to be in high abundance. 
     The proteins of the present intention can otherwise be expressed using vectors providing for high level transcription of a spider silk protein or, fusion proteins thereof. Fusion protein tags (e.g., His tags) facilitate affinity purification of epitope-tagged proteins. The hosts may be either bacterial or eukaryotic cells. Eukaryotic cells such as yeast, especially  Saccharomyces cerevisisae , or insect cells might be particularly useful eukaryotic hosts. In a particularly preferred embodiment of the invention, host cells in which spider silk proteins are expressed are plant cells. Such plant cells may be maintained in vitro or in vivo in a plant transformed with an expression construct encoding a spider silk protein. Expression of an engineered minor ampullate silk protein is described in U.S. Pat. No. 5,756,677, herein incorporated by reference. Such an approach may be used to express proteins of the present invention. 
     Exemplary Methods for Plant Transformation 
     Growth Conditions of the Plant Material Pre-Infiltration 
     Plants which may be transformed using the methods of the present invention include, but are not limited to  Arabidopsis thaliana , alfalfa, tobacco, tubers, sunflower, canola, soybean, maize, sorghum, wheat, cotton, small grains, and rice. Sow ˜50 individual seeds onto the surface of moderately wet compost. Incubate at 4° C. for 64 hours for stratification (optional). Incubate in a greenhouse (sixteen-hour day photoperiod, 15° C. night and 20 to 25° C. day temperature with additional artificial light (105 μE m 2 /s) and sub-irrigate until germination. Water moderately for 4 to 6 weeks. Plants should be as vigorous as possible. Optimal development may be achieved by growth during the rosette stage under conditions corresponding to relatively short days (13 h). To avoid etiolation, sufficient lighting may be provided. The optimal stage for infiltration tends to be coincident with the formation of the first siliques and the appearance of secondary floral stems. 
       Agrobacterium  Culture and Preparation 
     Precultures may be prepared by inoculating 10 ml of Luria Broth (LB) medium containing the appropriate antibiotics with 100 μl of a fresh culture or a glycerol stock, or with a colony taken from a dish. Cultures may be maintained for approximately 1 month at 4° C. and used as an inoculum for a larger culture. Precultures may be grown at 28° C. with good aeration overnight. Two liter flasks containing 1 liter of LB medium and the appropriate antibiotics may be inoculated with 10 ml of preculture and grown at 28° C. with good aeration until an OD(600 nm ) of at least 0.8 is obtained. The MP5-1 strain of  Agrobacteria , for example, requires 15 hours to reach the desired optical density (OD), but it may take longer for other strains. Bacteria may be pelleted by centrifugation at 8000 g for 8 minutes and gently resuspended in 1 liter of the initial volume of infiltration medium for vacuum infiltration and in one volume of 5% sucrose—200 μl 1 Silwet L-77 for floral dipping. 
     The LBA 4404 strain, which was used herein, may be prepared following a protocol essentially the same as that described above for the MP5-1 strain. 
     Vacuum Infiltration 
     Four to six week-old plants are gently removed from the soil with the roots intact. Alternatively plants may remain in the tray and infiltration may be performed on the leaves and stems only (see below). The roots may be rinsed briefly in water to eliminate any adhering soil particles. Immerse 25 to 50 plants in 300 ml of fresh bacterial suspension. The plants (in trays) may be placed in a vacuum chamber and vacuum pressure applied (˜10 4  Pa (0.1 atm) for 20 minutes, after which the vacuum may be released slowly. Replant the infiltrated plants (T 0 ) in the trays filled with compost (watered and treated). Cover with a perforated plastic wrap or a seed tray incubator and water from below. Remove the cover 3-4 days later. Water the plants moderately until maturity (4-6 weeks) and dry the plant progressively to allow the leaves to dry while the floral stems hold up and continue to flower. Harvest the seeds from 50 plants in bulk. Let the siliques dry at 27° C. for 2 days, then thresh and clean the seeds. 
     Floral Dipping 
     The bacterial suspension may be placed in an appropriate recipient which is chosen based on the length of the floral stems. The whole plant or just the floral stems may be dipped in the suspension for 2 minutes, after which the plants may be grown in a greenhouse or growth chamber for two days under the cover of a seed tray incubator. Plants may be cultivated following standard protocols. 
     Screening of Transformants 
     In the greenhouse. Seed transformed with pIBT110, for example, may be plated on agarose plates (50 μg/ml kanamycin) to select for positive transformant seedlings. Such seedlings may be transferred to soil after approximately 1-2 weeks. 
     For applications in which vectors that confer herbicide resistance have been used, each bulk of seeds may be sowed in an appropriately sized tray (e.g., 55×36) containing Perlite and a top layer of fine sand, which has been previously wet with water containing a herbicide. The herbicide used will vary according to the selection marker used. In preferred embodiments, the herbicide used may be phosphinothricin (at a final concentration of ˜7.5 mg/l) or glyphosate (Roundup, at a final concentration of ˜18 mg/l). Perlite may be used to reduce the weight of the trays. Sand particles of a sufficiently small size may be used to inhibit sowing of seeds on the surface. Transformed plants may be supplemented with a nutrient solution at the 2-leaf stage. Germination may be synchronized by incubation at 4° C. for 64 hours, after which the trays may be transferred to a greenhouse and sub-irrigated permanently with water containing an appropriate herbicide, as described above for 4 weeks. Transformants (T 1 ) having normal green cotyledons and first leaves formed may be observed after two weeks. The growth of untransformed plantlets is inhibited following germination and generally is indicated by the absence of cotyledon expansion, the rudiments of which rapidly turn yellow. 
     When resistant plantlets have reached the 4-5 leaf-stage, they may be transferred into individual pots (˜5.5 cm diameter) containing compost (watered and treated) and covered to facilitate rooting. Plantlets may be watered moderately, alternating tap water and a nutrient solution until the flowering stage. The frequency of watering may be progressively reduced as described above, while the T 1  plants finish producing flowers. Plants may be staked and individualized with perforated transparencies rolled up around the pot. When siliques have dried, the T 2  seeds from each T 1  plant may be harvested and cleaned. The in vitro segregation of the T-DNA selectable markers and Southern blotting analysis may be performed to estimate the number of loci and the number of copies of T-DNA. Generally, more than 50% of transformants with a T-DNA insertion occur at a single Mendelian locus and 70% of the T-DNA insertions occur in tandem. 
     In Vitro Culture 
     The seeds may be divided into 1.5 ml microtubes, at approximately 100 μl of seeds per tube. One ml of sterilization solution may be added to the tubes prior to closure of the tubes and mixing of the contents. The tubes may incubated on their sides in a laminar flow cabinet for 8 minutes to disperse the seeds into the solution to facilitate sterilization. The solution may be removed with a pipette and rinsed twice with 1 ml of pure 95% ethanol. The ethanol may be removed as effectively as possible and the seeds dried in the flow cabinet overnight. No more than 500 seeds may be sowed under sterile conditions on a 10 cm Petri dish containing the selective medium, which may be closed and sealed appropriately. The dishes may be incubated at 4° C. for 64 hours, after which they are transferred to a growth chamber (16 hour day length; 20° C.). Transformants (green rooted plants) may be scored 10 days later for kanamycin selection, for example. Resistant plantlets may be planted individually in pots when sufficiently developed (4-5 leaf-stage), transferred to a growth chamber, and covered to facilitate rooting. The process may be continued as described above. 
     Control of  Arabidopsis  Pests 
     Several insect and fungal pests are known to adversely affect  Arabidopsis  plants. Below are some of the pests that may affect  Arabidopsis , and a description of the pesticides used to combat them. 
     Thrips may infect flowers and reduce fertility, and can transmit virus. Spraying abamecin (9 mg/l) or formetanate (500 mg/l) (homogeneously wet the plants) may be used to control them. 
     Aphids may accumulate on the stems and leaves. These may be controlled with regular nicotine (150 mg/m 2 ) fumigation in the greenhouse. 
     Sciarid flies may lay eggs in the soil and their larvae eat the vegetative leaves. A sciarid infestation is indicated by the presence of large numbers of small black flies around the plants. Abamecin (9 mg/l) or cyromazine (300 mg/l) may be sprayed on the trays after sowing or on the plants after floral induction to control such infestations. 
     Gray rot is a fungal infection that may affect the plants when high levels of humidity are maintained. This may be controlled by spraying with vinchlozoline (750 mg/l). 
     The following materials are provided to facilitate the practice of the present invention: 22×16 cm aluminum trays (Bourgeat, 38490 Les Abrets, France), net pots diameter=5.5 cm (TEKU, D2842 Lohne/Oldb, Germany), 45×33×3.5 cm incubator for seed trays (BHR, 71370 St Germain du Plain, France), 28×38 cm carrying tray (KIB NL5140 AD Waalwijk, Netherlands), 40-well multipot trays (KIB, Netherlands), perforated plastic wrap (1000 holes/m2) subirrigation potting mix (WOGEGAL, 27700 St Pierre-des-Corps, France), 0.5 mm sieved sand, perlite, Hypnol (nicotine) plant louse treatment (CP Jardin 59570 Bavay, France) Vertimec (abamecin) sciarid flies and thrips treatment (Merck-Sharp), Dicarzol 200 (formetanate) thrips treatment (Hoechst), Trigard 75 WP (cyromazine) sciarid flies treatment (Merck-Sharp), Ronilan (vinchlozoline) gray rot treatment (BASF), Dedevap (dichlorvos) empty greenhouse disinfection (Bayer), FINAL™ (phosphinothricin) transformed plant selection (Hoechst), Roundup (glyphosate) transformed plant selection (Monsanto). 
     
       
         
           
               
            
               
                   
               
               
                 Media  Agrobacterium  culture medium 
               
            
           
           
               
               
               
            
               
                   
                 LB (Luria-Bertani) medium 
                 (g/l) 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                   
                 Bacto-tryptone 
                 10 
               
               
                   
                 Bacto-Yeast extract 
                 5 
               
               
                   
                 NaCl 
                 10 
               
               
                   
                 pH = 7 
               
               
                   
                   
               
               
                   
                 pH adjusted with 1M NaOH. The medium is sterilized by autoclaving at 115° C. for 20 min. 
               
            
           
         
       
     
     
       
         
           
               
            
               
                   
               
               
                 Infiltration medium 
               
            
           
           
               
               
            
               
                   
                 (mg/l) 
               
               
                   
                   
               
            
           
           
               
            
               
                 Macroelements 
               
            
           
           
               
               
               
            
               
                   
                 NH 4 NO 3   
                 1650 
               
               
                   
                 KNO 3   
                 1900 
               
               
                   
                 CaCl 2 , 2H 2 O 
                 440 
               
               
                   
                 MgSO 4 , 7H 2 O 
                 370 
               
               
                   
                 KH 2 PO 4   
                 170 
               
            
           
           
               
            
               
                 Microelements 
               
            
           
           
               
               
               
            
               
                   
                 H 3 BO 3   
                 6.3 
               
               
                   
                 MnSO 4 , 4H 2 O 
                 22.3 
               
               
                   
                 ZnSO 4 , 7H 2 O 
                 8.6 
               
               
                   
                 KI 
                 0.83 
               
               
                   
                 Na 2 MoO 4 , 2H 2 O 
                 0.25 
               
               
                   
                 CuSO 4 , 5H 2 O 
                 0.025 
               
               
                   
                 CoCl 2 , 6H 2 O 
                 0.025 
               
               
                   
                 BA 
                 0.010 
               
               
                   
                 Sucrose 
                 50000 
               
               
                   
                 pH 
                 5.8 
               
               
                   
                   
               
            
           
         
       
     
     The microelements and 6-benzylaminopurine (BA) may be made as concentrated stock solutions (at 1000× and 1 mg/l respectively) and stored at 4° C. The pH is adjusted with KOH and the medium is sterilized by autoclaving at 115° C. 
     Floral Dipping Medium 
     Combine H 2 O+Sucrose 5%+200 μl/l Silwet L-77 (OSI Specialties S.A. 7 rue du Pré-Bouvier CH-1217 MEYRIN SWITZERLAND). 
     Sterilization Solution 
     Dissolve 1 tablet of Bayrochlore (contains sodium dichlorocyanate and releases 1.5 g of active chlorine; Bayrol GMBH, D-800 München 70) in 40 ml of distilled water and add some drops of 1% Tween. Use 5 ml in 45 ml Ethanol 95%. 
     
       
         
           
               
            
               
                   
               
               
                 In vitro culture medium 
               
            
           
           
               
               
            
               
                   
                 (mg/l) 
               
               
                   
                   
               
            
           
           
               
            
               
                 Macroelements 
               
            
           
           
               
               
               
            
               
                   
                 KNO 3   
                 506 
               
               
                   
                 KH 2 PO 4   
                 340 
               
               
                   
                 MgSO 4 (7H 2 O) 
                 493 
               
               
                   
                 Ca(NO 3 )2(4H 2 O) 
                 472 
               
            
           
           
               
            
               
                 Microelements 
               
            
           
           
               
               
               
            
               
                   
                 H 3 BO 3   
                 4.3 
               
               
                   
                 MnCl 2 (4H 2 O) 
                 2.8 
               
               
                   
                 CuSO 4 (5H 2 O) 
                 0.13 
               
               
                   
                 Na 2 MoO 4 (2H 2 O) 
                 0.05 
               
               
                   
                 NaCl 
                 0.58 
               
               
                   
                 ZnSO 4 (7H 2 O) 
                 0.29 
               
               
                   
                 CoCl 2 (6H 2 O) 
                 0.0025 
               
            
           
           
               
            
               
                 Morel and Wetmore vitamins 
               
            
           
           
               
               
               
            
               
                   
                 Myo-inositol 
                 100 
               
               
                   
                 Calcium panthothenate 
                 1 
               
               
                   
                 Niacine 
                 1 
               
               
                   
                 Pyridoxine 
                 1 
               
               
                   
                 Thiamine Hcl 
                 1 
               
               
                   
                 Biotin 
                 0.01 
               
               
                   
                 MES 
                 700 
               
               
                   
                 Sucrose 
                 10000 
               
               
                   
                 Agar BIOMAR 
                 7000 
               
               
                   
                 pH = 5.8 
               
               
                   
                   
               
            
           
         
       
     
     5 ml/l of a filter sterilized ammoniacal iron citrate stock solution (1%) and for kanamycin selection 1 ml/l of a filter sterilized kanamycin stock solution (100 mg/ml) may be added after autoclaving at 115° C. 
     Macroelements, microelements, vitamins and MES are made up as concentrated stock solutions and stored at room temperature or at 4° C. for the microelements and the vitamins. The stock solutions are made as followed: 200×KNO 3  (1M), 400×KH 2 PO 4  (1M), 500×MgSO 4  (7H 2 O) (1M), 500×Ca(NO 3 )2(4H 2 O) (1M), 1000× microelements, 500× Vitamins, 100×MES (14%). Agar is added in each bottle before autoclaving. The pH is adjusted with KOH. The medium is sterilized by autoclaving at 115° C. 
     Plant Materials 
       Arabidopsis thaliana  (L.) Heyn., ecotype Wassilevskija (WS) may be used. [Ecotype Columbia (Co10), for example]. Nossen (No0) and Landsberg erecta may also is be used with good efficiency. 
       Agrobacterium  Strains and Vectors 
     The  Agrobacterium  strain MP5-1 may be used in the methods of the present invention. This strain carries the binary vector pGKB5 (Bouchez et al., 1993), which was constructed for T-DNA insertional mutagenesis. This plasmid is very stable in  Agrobacterium  under non-selective conditions and confers resistance to kanamycin. It was introduced into the helper strain C58Cl (pMP90) (Bouchez et al., 1993), which contains a disarmed C58 Ti plasmid, to produce strain MP5-1. The T-DNA contains a promoterless GUS reporter gene fused to the right border, and kanamycin and Basta resistance genes as plant selection markers. 
     Other binary vectors and helper strains may also be used, including, but not limited to the strains LBA 4404, ABI, ASE, GV3101; vectors pBin19, pOCA18, pCGN, and pDE1001. Commonly used binary vectors confer resistance to kanamycin and selection of transformants is performed in vitro under sterile conditions. 
     References which may be of utility in practicing the methods of the present invention include the following: Bechtold et al. (1993) C R Acad Sci Paris, Life Sciences 316:1194-1199; Bent et al. (1994) Science 265:1856-1860; Bouchez et al. (1993) C R Acad Sci Paris, Life Sciences 316:1188-1193; Chang et al. (1990) The Plant Journal 5:551-558; Clough and Bent. (1998) The Plant Journal 16:735-743; Damm et al. (1989) Mol. Gen. Genet. 213:15-20; Feldmann and Marks. (1987) Mol Gen Genet 208:1-9; Hooykaas and Schilperoort. (1992) Plant Mol Biol. 35:205-218; Koncz and Schell. (1986) Mol Gen Genet 204:383-396; Tinland. (1996) Trends in Plant Science. 1:178-184; Valvekens et al. (1988) Proc. Natl. Acad. Sci. USA. 85:5536-5540; Zupan and Zambryski (1997) Critical Reviews in Plant Sciences. 16:279-295, the entire contents of which are incorporated herein by reference. 
     Chloroplast Transformation for High Level Expression of Transgenes 
     A gene of interest (e.g., the SS1 gene) may be inserted into a chloroplast expression vector such as those described by DeGray et al. (Plant Physiology 127: 852-862) and Lutz et al. (Plant Physiology 125, 1585-11590). Transplastomic tobacco plants transformed by a chloroplast expression vector comprising the SS1 gene may be generated according to DeGray et al. and Lutz et al. Additional methodology pertaining to chloroplast transformation of plants has been described in U.S. Pat. Nos. 5,451,513; 5,545,818; and 6,376,744, the entire contents of which are incorporated herein by reference. Target gene integration, copy number, and transcription levels may be determined by Southern and Northern analyses. Western blotting may be used to quantify levels of protein expression. 
     Harvesting and Purification of Spider Silk Proteins from Plants 
     A useful spider silk protein or fragment thereof may be (1) insoluble inside a cell in which it is expressed and (2) capable of being formed into an insoluble fiber under normal conditions by which fibers are made. Preferably, the protein is insoluble under conditions (1) and (2). Specifically, the protein or fragment may be insoluble in a solvent such as water, alcohol (methanol, ethanol, etc.), acetone and/or organic acids, etc. The spider silk protein or fragment thereof should be capable of being formed into a fiber having high tensile strength, e.g., a tensile strength of 0.5x to 2x wherein x is the tensile strength of a fiber formed from a corresponding natural silk or whole protein. A spider silk protein or fragment thereof should also be capable of being formed into a fiber possessing high elasticity, e.g., at least 15%, more preferably about 25%. 
     Variants of a spider silk protein may be formed into a fiber having a tensile strength and/or elasticity which is greater than that of the natural spider silk or natural protein. The elasticity may be increased up to 100%. Variants may also possess properties of protein fragments. 
     A fragment or variant may have substantially the same characteristics as a natural spider silk. The natural protein may be particularly insoluble when in fiber form and resistant to degradation by most enzymes. 
     Recombinant spider silk proteins may be recovered from cultures by lysing cells to release spider silk proteins expressed therein. Initially, cell debris can be separated by centrifugation. Clarified cell lysate comprised of debris and supernatant may be repeatedly extracted with solvents in which spider silk proteins are insoluble, but cellular debris is soluble. A differential solubilization process such as described above may be used to facilitate isolation of a purified spider silk protein precipitate. These procedures may be repeated and combined with other procedures including filtration, dialysis and/or chromatography to obtain a pure product. 
     Fibrillar aggregates may form from solutions by spontaneous self-assembly of spider silk proteins when the protein concentration exceeds a critical value. The aggregates may be gathered and mechanically spun into macroscopic fibers according to the method of O&#39;Brien et al. [I. O&#39;Brien et al., “Design, Synthesis and Fabrication of Novel Self-Assembling Fibrillar Proteins”, in Silk Polymers: Materials Science and Biotechnology, pp. 104-117, Kaplan, Adams, Farmer and Viney, eds., c. 1994 by American Chemical Society, Washington, D.C.; Lazaris et al., 2002, Science 295:472-476]. 
     Exemplary Methods for Preparation of Fibers from Spider Silk Proteins 
     As noted above, the spider silk proteins may be viewed as derivatized polyamides. Accordingly, methods for producing fiber from soluble spider silk proteins are similar to those used to produce typical polyamide fibers, e.g. nylons, and the like. 
     O&#39;Brien et al. supra describe fiber production from adenovirus fiber proteins. Following general methods for fiber production, spider silk proteins may be solubilized in a strongly polar solvent. The protein concentration of such a protein solution should typically be greater than 5% and is preferably between 8 and 20%. 
     Fibers may preferably be spun from solutions having properties characteristic of a liquid crystal phase. The fiber concentration at which phase transition can occur is dependent on the polypeptide composition of a protein or combination of proteins present in the solution. Phase transition, however, may be detected by monitoring the clarity and birefringence of the solution. Onset of a liquid crystal phase may be detected when the solution acquires a translucent appearance and registers birefringence when viewed through crossed polarizing filters. 
     The solvent used to dissolve a spider silk protein may be polar, and is preferably highly polar. Such solvents are exemplified by di- and tri-haloacetic acids, haloalcohols (e.g. hexafluoroisopropanol). In some instances, co-solvents such as acetone are useful. Solutions of chaotropic agents, such as lithium thiocyanate, guanidine thiocyanate or urea may also be used. 
     In one fiber-forming technique, fibers are first extruded from the protein solution through an orifice into methanol, until a length sufficient to be picked up by a mechanical means is produced. A fiber may then be pulled by such mechanical means through a methanol solution, collected, and dried. Methods for drawing fibers are considered well-known in the art. For example, fibers made from a 58 kDa synthetic MaSp consensus polypeptide were drawn by methods similar to those used for drawing low molecular weight nylons. Such methods are described in U.S. Pat. No. 5,994,099 and Lazaris et al. (2002, Science 295:472-476) the entire contents of which are incorporated herein by reference. 
     Of note, spider silk proteins have primary structures dominated by imperfect repetition of a short sequence of amino acids. A “unit repeat” constitutes one such short sequence. Thus, the primary structure of a spider silk protein may be thought to consist mostly of a series of small variations of a unit repeat. Unit repeats in a naturally occurring protein are often distinct from each other. In other words, there is little or no exact duplication of a unit repeat along the length of a protein. Synthetic spider silks, however, may be generated wherein the primary structure of a synthetic spider silk protein may be described as a number of exact repetitions of a single unit repeat. See  FIG. 5  (SEQ ID NO: 3), which provides a nucleic acid sequence encoding a synthetic spider silk protein (SS1) comprised of sixteen repeats of a MaSp2 monomeric unit ( FIG. 4 ; SEQ ID NO: 2). Additional synthetic spider silks may be designed comprising a number of repetitions of one unit repeat together with a number of repetitions of a second unit repeat. Such a structure would be similar to a typical block copolymer. The present invention also encompasses generation of synthetic spider silk proteins comprising unit repeats derived from several different spider silk sequences (naturally occurring variants or genetically engineered variants thereof). 
     Such synthetic hybrid spider silk proteins may each have 900 to 2700 amino acids with 25 to 100, preferably 30 to 90 repeats. A spider silk or fragment or variant thereof usually has a molecular weight of at least about 16,000 daltons, preferably 16,000 to 150,000 daltons, more preferably 50,000 to 120,000 daltons for fragments and greater than 100,000 but less than 500,000 daltons, preferably 120,000 to 350,000 for a full length protein. 
     C. Antibodies 
     The methods of the present invention also utilize antibodies capable of immunospecifically binding to spider silk proteins. Such antibodies may comprise polyclonal or monoclonal antibodies immunologically specific for a spider silk protein or functional fragments or derivatives thereof. Such antibodies may be used to advantage to identify and/or purify spider silk proteins. For example, antibodies may be utilized for affinity separation of a spider silk protein with which it immunospecifically interacts. Antibodies may also be used to immunoprecipitate a spider silk protein from a sample containing a mixture of proteins and other biological molecules. Other uses of anti-spider silk protein antibodies are described below. 
     III. Uses of Spider Silk-Encoding Nucleic Acids and Spider Silk Proteins 
     A. Spider Silk-Encoding Nucleic Acids 
     Spider silk protein-encoding nucleic acids may be used for a variety of purposes in accordance with the present invention. Spider silk protein-encoding DNA, RNA, or fragments thereof may be used as probes to detect the presence of and/or expression of genes encoding spider silk proteins. Methods in which spider silk protein-encoding nucleic acids may be utilized as probes for such assays include, but are not limited to: (1) in situ hybridization; (2) Southern hybridization; (3) northern hybridization; and (4) assorted amplification reactions such as polymerase chain reactions (PCR). 
     Host cells comprising at least one spider silk protein encoding DNA molecule are encompassed in the present invention. Host cells contemplated for use in the present invention include but are not limited to bacterial cells, fungal cells, insect cells, mammalian cells, and plant cells. In a preferred aspect of the present invention, spider silk proteins are expressed in plant cells in vitro. In a particularly preferred aspect of the present invention, spider silk proteins are expressed in plant cells in vivo. The spider silk protein-encoding DNA molecules may be introduced singly into such host cells or in combination to assess the phenotype of cells conferred by such expression. Methods for introducing DNA molecules are also well known to those of ordinary skill in the art. Such methods are set forth in Ausubel et al. eds.,  Current Protocols in Molecular Biology , John Wiley &amp; Sons, NY, N.Y. 1995, the disclosure of which is incorporated by reference herein. 
     As described above, spider silk protein-encoding nucleic acids may also used to advantage to produce large quantities of substantially pure spider silk proteins, or selected portions thereof. 
     B. Proteins and Antibodies 
     Purified spider silk protein, or fragments thereof, produced by methods of the present invention may be used to advantage in a variety of different applications, including, but not limited to, production of fabric, sutures, medical coverings, high-tech clothing, rope, reinforced plastics, and other applications in which various combinations of strength and elasticity are required. 
     Table I lists physical properties of various biological and manmade materials 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                   
                 Material Strength 
                 Elasticity 
                 Energy to Break 
               
               
                 Material 
                 (N m −2 ) 
                 (%) 
                 (J kg −1 ) 
               
               
                   
               
             
            
               
                 Dragline Silk 
                 4 × 10 9   
                 35  
                 1 × 10 5   
               
               
                 Minor Silk 
                 1 × 10 9   
                 5 
                 3 × 10 4   
               
               
                 Flagelliform 
                 1 × 10 9   
                 200+  
                 1 × 10 5   
               
               
                 Silk 
               
               
                 KEVLAR 
                 4 × 10 9   
                 5 
                 3 × 10 4   
               
               
                 Rubber 
                 1 × 10 9   
                 600  
                 8 × 10 4   
               
               
                 Tendon 
                 1 × 10 9   
                 5 
                 5 × 10 3   
               
               
                   
               
               
                 Data derived from Gosline et al (9) and Stauffer et al (18). 
               
            
           
         
       
     
     As shown in Table I, spider silks are characterized by advantageous physical properties, including, but not limited to, high tensile strength and pronounced elasticity, that are highly desirable for numerous applications. It is significant to note that spider silks possess these physical properties in aggregation which renders them unique proteins having unparalleled utility. For example, spider dragline silk has a tensile strength greater than steel or carbon fibers (200 ksi), elasticity as great as some nylon (35%), a stiffness as low as silk (0.6 msi), and the ability to supercontract in water (up to 60% decrease in length). In view of its high tensile strength and elasticity, the energy required to break dragline silk exceeds that required to break any known fiber including Kelvar™ and steel. These properties are unmatched by any known natural or manmade material. Moreover, the new materials of the present invention would also provide unique combinations of such desirable features in a very low weight material. 
     In view of the foregoing advantageous properties, incorporation of spider silk proteins expressed in plant cells into materials used to generate a product would produce a superior product. When spider silk is dissolved in an appropriate solvent and forced through a small orifice to generate spider silk fibers, such fibers may be woven into a fabric/material or added into a composite fabric/material. For example, spider silk fibers may be woven into fabrics to modulate the strength and elasticity of a fabric, thus rendering materials comprising such modified fabric optimized for different applications. Spider silk fibers may be of particular utility when incorporated into materials used to make high-tech clothing, rope, sails, parachutes, wings on aerial devices (e.g., hang gliders), flexible tie downs for electrical components, sutures, and even as a biomaterial for implantation (e.g., artificial ligaments or aortic banding). 
     Biomedical applications involve use of natural and/or synthetic spider silk fibers produced by the methods of the present invention in sutures used in surgical procedures, including, but not limited to: eye surgery, reconstructive surgery (e.g., nerve or tympanic membrane reconstruction), vascular closure, bowel surgery, cosmetic surgery, and central nervous system surgery. Natural and synthetic spider silk fibers may also be of utility in the generation of antibiotic impregnated sutures and implant material and matrix material for reconstruction of bone and connective tissue. Implants and matrix material for reconstruction may be impregnated with aggregated growth factors, differentiation factors, and/or cell attractants to facilitate incorporation of the exogenous material and optimize recovery of a patient. Plant-expressed spider silk proteins and fibers may be used for any application in which various combinations of strength and elasticity are required. Moreover, spider silk proteins may be modified to optimize their utility in any application. As described above, sequences of spider silk proteins may be modified to alter various physical properties of a fibroin and different spider silk proteins and variants thereof may be woven in combination to produce fibers comprised of at least one spider silk protein or variant thereof. In a preliminary study designed to evaluate the potential for an immune response to a natural spider silk protein, natural dragline silk was implanted into mice and rats intramuscularly, intraperitoneally, or subcutaneously. Animals into which natural dragline silk was introduced did not mount an immune response to the spider silk protein, irrespective of the site of implantation. Of note, tissue sections surrounding spider silk protein implants were essentially identical to tissue sections derived from implantation sites into which a polyethylene rod was inserted. Since a polyethylene rod was used as the solid matrix about which the dragline spider silk protein was wrapped prior to implantation, introduction of a polyethylene rod alone serves as a negative control for such experiments. In view of the above, spider silk proteins of the present invention are expected to elicit minimal immunological responses when introduced into vertebrate animals. 
     Synthetic spider silk fibers are of utility in any applications for which natural spider silk fibers may be used. For example, synthetic fibers may be mixed with various plastics and/or resins to prepare a fiber-reinforced plastic and/or resin product. Because spider silk is stable up to 180° C., spider silk protein fibers would be of utility as structural reinforcement material in thermal injected plastics. 
     It should be apparent from the foregoing that spider silk proteins expressed in plants may be generated in large quantities by means generally known to those of skill in the art and described herein. Spider silk proteins and derivatives thereof can be made into fibers for any intended use. Moreover, mixed composites of fibers are also of interest as a consequence of their unique combined properties. Such mixed composites may confer characteristics of flexibility and strength to any material into which they can be incorporated. 
     Polyclonal or monoclonal antibodies immunologically specific for a spider silk protein may be used in a variety of assays designed to detect and quantitate spider silk proteins. Such assays include, but are not limited to: (1) flow cytometric analysis; and (2) immunoblot analysis (e.g., dot blot, Western blot) of extracts from various cells. Additionally, anti-spider silk protein antibodies may be used for purification of a spider silk protein and any associated subunits (e.g., affinity column purification, immunoprecipitation). 
     From the foregoing discussion, it can be seen that the methods of the present invention may be used, for example, to: 1) transform plant cells from which transgenic plants may be derived; 2) express large quantities of spider silk proteins (e.g. natural or synthetic spider silk proteins) in plant cells in vitro and in vivo; 3) purify large quantities of these spider silk proteins; 4) draw fibers comprised of plant-expressed spider silk proteins; 5) weave these spider silk fibers, singly and in combination, into fabric; and 6) detect expression of spider silk proteins in cells and/or organisms. 
     Methods of Use for Nucleic and Amino Acid Sequences and Antibodies 
     Synthetic genes have been constructed which encode 4, 8, 16, and 32 units of the consensus repeat sequence of MaSp1 and MaSp 2 (17). The method involved starts with a DNA representing the consensus sequence for the silk, which is repeatedly doubled using compatible but non-regenerable restriction enzymes until the desired size is reached. Each of these constructs has been shown to produce a protein of the expected size in  E. coli . The 16 monomeric repeat MaSp2-derived protein has been over-expressed in  E. coli  to a level that has facilitated the synthesis of over 25 grams of purified protein from a series of 10 liter cultures. Although this is useful for analytical purposes, insufficient amounts were produced by this approach for commercial applications. Methods are provided herein for the generation of additional synthetic spider silk proteins. See Example IV. 
     The following examples are provided to illustrate embodiments of the invention. They are not intended to limit the scope of the invention in any way. 
     Example I 
     Generation of  Arabidopsis  Plants Expressing Synthetic Spider Silk Protein (SS1) 
     In order to express a spider silk protein in a transgenic plant, a nucleic acid sequence (SEQ ID NO: 3) which encodes a synthetic spider silk protein (SS1) was constructed and subcloned via multiple intermediate vectors into pIBT110. pIBT110 is a plant expression vector which has been shown to drive the expression of high levels of exogenous molecules (e.g., RNA and protein) in a variety of plant cells in vitro and in vivo, including  Arabidopsis  cells. As described below, pIBT110 comprising SEQ ID NO: 3 was utilized in the transformation of  Arabidopsis  plants, which resulted in the expression of synthetic spider silk protein in these plants. Similar methods may be used to express other synthetic spider silk proteins, such as but not limited to, the synthetic spider silk proteins described herein. See Example IV. 
     The following protocols are provided to facilitate practice of the methods of the present invention. 
     Methods and Materials 
     Western blotting to demonstrate steady state SS1 protein in transgenic  Arabidopsis . Leaves from individual transgenic  Arabidopsis  plants were ground in liquid nitrogen followed by addition of buffer (50 mM Tris-Cl, pH 8.0, 10 mM MgCl, 100 mM NaCl) at a ratio of 1 g tissue:1 ml buffer. The lysate then centrifuged at 10,000×g for 10 minutes at 5° C. The supernatant was removed and subjected to SDS polyacrylamide gel electrophoresis. The gel was blotted onto nitrocellulose and air dried for 5 minutes. The blot was incubated in Blotto (1 M Tris-Cl, pH 7.8, 2 M NaCl, 3% non-fat dried milk) for 2 hours at room temperature. The wet blot was then incubated in fresh Blotto plus MaSp2 antiserum at a 1:2000 dilution overnight at 4° C. with shaking. Subsequently, the blot was washed three times for 15 minutes each in Blotto at room temperature. The blot was then incubated in Blotto minus dried milk plus anti-IgG-peroxidase (1:15,000 dilution) for 1 hour at room temperature with shaking. The blot was then washed six times in Blotto minus milk for 15 minutes each wash. Subsequently, the blot was immersed in chemiluminescence substrate for 1 minute followed by rinsing with distilled water for 10 sec and exposure to X-ray film. 
     Northern blotting to demonstrate SS1 mRNA production in transgenic  Arabidopsis . Total RNA from transgenic lines was extracted using a Gentra kit. The RNA samples were resolved on a 1.0% denaturing formaldehyde gel. The gel was rinsed in deionized water and blotted onto a Zeta probe membrane. The blot was rinsed in 6×SSC, allowed to air dry and baked in vacuo at 80° C. for 30 minutes. The SS1 probe was prepared and hybridized to the blot as described below for Southern blotting. 
     Southern blotting to demonstrate SS1 gene insertion into  Arabidopsis . Genomic DNA from putative transgenic T 2    Arabidopsis  plants was purified using a DNA kit from Gentra, Inc. DNA was then digested with BamH1. As a positive control the synthetic MaSp2 gene (SS1) was digested from pET19. The DNAs were resolved on an agarose gel, followed by depurination in 0.25M HCl for 25 minutes and washing in deionized water. DNA was then denatured in 1.5M NaCl/0.5M NaOH for 30 minutes, washed in sterile deionized water, and neutralized in 1.5M NaCl/0.5M Tris-Cl (pH 7) for 30 minutes followed by a final wash in deionized water. The gel was blotted onto a Zeta probe membrane, washed in 2×SSC and dried in vacuo. 
     The probe was prepared by digesting pET19 with BamH1 to release the SS1 sixteen repeat fragment followed by purification of the SS1 gene. The resultant DNA was radiolabeled using a random priming kit (Sigma). The labeled probe was purified by spin column chromatography. Blots were pre-hybridized in 6×SSC, 5×Denhardt&#39;s solution, 0.5% SDS, 100 mg/ml ssDNA for 3 hours at 45° C. The labeled probe was then added to the solution and incubation continued overnight at 45° C. Subsequently the blot was washed in 2×SSC, 0.2% SDS with shaking for 10 minutes and then in 0.2×SSC/0.1% SDS at room temperature for 10 minutes. The blot was then exposed to X-ray film. 
     Results 
     In order to determine if spider silk could be produced in a large scale and stable manner by transgenic plants, a nucleic acid sequence (SEQ ID NO: 3) which encodes a synthetic spider silk protein was constructed and subcloned via multiple intermediate vectors into pIBT110. As described above, SEQ ID NO: 3 comprises sixteen repeats of a monomeric unit (SEQ ID NO: 2) which was based on a unit repeat of MaSp2 ( FIG. 3 ; SEQ ID NO: 1). The final construct (pIBT110-SS1) was determined to be in the proper reading frame and flanked by multiple in-frame histidine (His) sequences that allowed affinity purification of the expressed protein. The vector pIBT110 was used because it is commercially available and drives high level expression of proteins in plants. In addition to the required signal sequences for foreign gene incorporation, the vector also comprises the cauliflower mosaic virus (CaMV) 35S promoter, duplicated enhancers linked to the tobacco etch 5′untranslated leader region (UTR), the soybean vspB 3′ UTR and polyadenylation signal. The presence of a kanamycin resistance gene selectable marker also facilitates selection of transformed plants with kanamycin. 
     The presence of the fragment containing the synthetic spider silk DNA in pIBT110 was confirmed by restriction endonuclease analysis of isolated plasmid. The pIBT110 plasmid was then shuttled into  Agrobacterium tumefaciens . This bacterium is a plant pathogen which has the ability to insert DNA that is flanked by specific targeting sequences into the plant genome. Once inserted, the foreign DNA is typically replicated concordant with that of the genomic DNA of the transformed plant, generation to generation in a stable inherited fashion. The  A. tumefaciens  used in this study has been disarmed such that only essential transfer sequences remain. Thus, transformation with such vectors does not incite disease. 
     The pIBT110 construct comprising the synthetic spider silk (pIBT110-SS1;  FIG. 1 ) was transformed into  Agrobacterium tumefaciens  and then introduced into  Arabidopsis thaliana  by standard techniques (11).  Arabidopsis  is a model plant system because these plants are easily transformed and regenerated, and grow very rapidly. In this example,  A. thaliana  was infected by the floral dipping method. See above protocol and the following website, www.arabidopsis.org, the entire contents of which is incorporated herein by reference. Putative transformed seeds were selected by plating on kanamycin plates, followed by transplantation to soil mix under greenhouse conditions. 
     Western blotting of extracts derived from T 1  and T 2  plants established that spider silk was stably expressed. See  FIG. 6 . In  FIG. 6 , the expression of synthetic spider silk protein in Transgenic  Arabidopsis  is shown. Lanes 1-5 were loaded with varying concentrations of recombinant MaSp2 protein purified from  E. coli . Lanes 6-7 contained the protein crude extracts from transgenic  Arabidopsis  transformed with the synthetic silk gene using pIBT110. 1× was no dilution, 0.2× was 5 times dilution from the crude extract. Lane 8 was the control plant transformed with pIBT110 plasmid only. 
     In  FIG. 7 , a Southern blot analysis of the silk transgene and its copy number in transgenic  Arabidopsis  plants is shown. Genomic DNA extracted from T 2  transgenic lines #5, #6 (lane 4, 5 respectively) and from plants transformed with pIBT110 plasmid only (lane 6) was digested with BamHI. Plasmid DNA from the pET19 vector containing the synthetic silk gene was also digested with BamH1. The silk gene fragment was random labeled and used as a probe in Southern hybridization. After calculating the ratio between pET19SS (7.1 kb) and the  Arabidopsis  genome (125,000 kb), the loading amount of genomic DNA and plasmid DNA was used to show a single copy. About 18 ug of digested genomic DNA was loaded in lane 4-6, and 0.2 ng, 1 ng and 5 ng digested plasmid DNA were loaded in lanes 1-3, respectively, as a standard. The intensity of the bands was compared between standards and plant samples to determine copy number. 
     In  FIG. 8 , Northern blot analysis of transgene expression is shown. Total RNA was extracted from  Arabidopsis  leaves and about 1.0 ug of total RNA per lane was blotted onto a zeta-probe membrane (lane 1-3). Probe was the same as in Southern blot. The total RNA from plant transformed with pIBT110 plasmid only was also prepared as a control shown in lane 4. The mRNA size was marked as an arrow. 
     Notably,  Arabidopsis  plants transformed to express SS1 protein were phenotypically normal. This study has provided the foundation to engineer additional crop plants using the methods of the present invention. 
     Example II 
     Generation of  Arabidopsis  Plants Expressing Native Spider Silk Proteins 
     Natural or native nucleic acid sequences, such as  Nephila clavipes  dragline silk fibroin mRNA (SEQ ID NO: 1;  FIG. 3 ),  Argiope trifasciata  aciniform fibroin 1 (SEQ ID NO: 4;  FIG. 9 ),  Phidippus audax  fibroin 1 (SEQ ID NO: 5;  FIG. 10 );  Zorocrates  sp. fibroin 1 (SEQ ID NO: 6;  FIG. 11 );  Kukulcania  MaSp (SEQ ID NO: 7;  FIG. 12 );  Kukulcania  MaSp (SEQ ID NO: 8;  FIG. 13 );  Kukulcania  MaSp (SEQ ID NO: 9;  FIG. 14 );  Argiope  MiSp (SEQ ID NO: 10;  FIG. 15 ); and  Argiope  MiSp (SEQ ID NO: 11;  FIG. 16 ) may be subcloned into a plant expression vector (e.g., pIBT110). Such methods may be used to express amino acid sequences comprising  Argiope trifasciata  aciniform fibroin 1 (SEQ ID NO: 15;  FIG. 9 ),  Phidippus audax  fibroin 1 (SEQ ID NO: 16:  FIG. 10 ),  Zorocrates  sp. fibroin 1 (SEQ ID NO: 17;  FIG. 11 );  Kukulcania  MaSps (SEQ ID NOs: 7, 8, 9;  FIGS. 12 ,  13 , and  14 , respectively); and  Argiope  MiSps (SEQ ID NOs: 10 and 11;  FIGS. 15 and 16 , respectively). Spider silk proteins may also be expressed in plants as fusion proteins comprising a spider silk protein operably linked to a tag moiety (e.g. His or myc tag). Methods for producing fusion proteins are disclosed herein and known to those skilled in the art. The incorporation of such tags facilitates detection and purification of a spider silk fusion protein expressed in a plant cell. A pIBT110 construct comprising SEQ ID NO: 1, 4, 5, 6, 7, 8, 9, 10, or 11 may be transformed into  Agrobacterium tumefaciens  and positive transformants selected on antibiotic media as described above.  Arabidopsis , for example, may be infected with  Agrobacterium  transformed with a pIBT110 expression vector comprising SEQ ID NO: 1, 4, 5, 6, 7, 8, 9, 10, or 11 by the floral dipping method. Seed may be collected and positive transformants selected on kanamycin plates. T 1  plants may be generated, allowed to self-fertilize, and seed collected therefrom for analysis. Extracts may be isolated from T 1  plants transformed with pIBT110-MaSp2, for example, and analyzed by immunoblotting using antiserum immunologically specific for the spider silk protein MaSp2. Extracts isolated from T 1  plants transformed with a pIBT110 expression vector comprising SEQ ID NO: 4, 5, 6, 7, 8, 9, 10, or 11 may be analyzed by immunoblotting with the appropriate immunologically specific antibodies. For embodiments in which spider silk proteins are expressed as fusion proteins, antibodies specific for the tag moiety may be used to detect the fusion protein on immunoblots. As described above, standard procedures may be used to perform the above Western blotting experiments. Expression of natural spider silk proteins in plants may provide an additional resource for the production of spider silk proteins having desirable qualities for diverse applications. 
     Example III 
     Generation of Alfalfa Plants Expressing High Levels of Spider Silk Protein 
     As described above, the  Nephila clavipes  dragline silk fibroin synthetic DNA (SEQ ID NO: 3) was subcloned into the plant expression vector pIBT110. Alfalfa may be transformed with the pIBT110 empty vector as a control and with pIBT110-SS1, for example, according to the procedure of D. A. Samac (University of Minnesota, pers. communication). A flow chart depicting a process for transforming alfalfa and selecting positive transformants is provided in  FIG. 2 . Briefly, excised leaf discs may be soaked in an  Agrobacterium  suspension (described elsewhere herein) and subsequently replated on different media to generate callus, shoots, and roots. 
     In a preferred embodiment, alfalfa may be transformed to express any natural and/or synthetic spider silk protein described herein. This procedure is based upon co-cultivation of surface sterilized alfalfa leaves (variety Regen SY27) with  A. tumefaciens  for 15 minutes. Regen SY27 was selected because it is easily agro-transformed and amenable to tissue culture. Inoculated leaves may be transferred to B5h plates to allow plant cell growth in vitro (1, 3). Following incubation for seven days, leaf pieces may be rinsed in sterile water and transferred to B5hKTc selection plates (1, 3). B5hKTc medium contains 25 mg/L kanamycin and 500 mg/L carbenicillin to effect selection of transformants and kill remaining bacterial cells associated with the leaves, respectively. Within two weeks of infection by  A. tumefaciens , callus begin to grow from transformed cells that are resistant to the kanamycin. These cells may be transferred to B5hOKTc media (1, 3) that lack hormones, thus allowing embryo and shoot growth. 
     Green plantlets may be transferred to MMSTc rooting medium (1, 3) that lacks kanamycin and allows root generation. Following formation of a vigorous root system, plants may be transplanted to soil mix and grown under growth chamber conditions. Fifty individual kanamycin resistant plantlets that represent apparent separate transformations may be selected for further testing. Insertion of the target gene from pIBT110 into the plant genome occurs in an essentially random fashion, thus, screening must be done to establish incorporation of the gene and determine the number of inserts or fragments incorporated. Additionally, the expression level of the encoded spider silk protein may vary between independent transformants and must be quantified. Multiple transformations may be examined to ensure that sufficient plants are available for selection procedures. 
     Regeneration and selection of transformants. Plants into which pIBT110-SS1, for example, has been incorporated are rendered resistant to kanamycin and referred to herein as primary transformants (T 0 ). T 0  plants may be analyzed to evaluate their phenotype and vegetatively propagated to obtain sufficient plant material for testing. Initial assays to establish SS1 gene expression may be performed by isolating extracts prepared from plant leaves transformed with empty pIBT110 vector (control) or pIBT110-SS1 and analyzing them by immunoblotting with antibodies immunologically specific for MaSp2. Additional controls may comprise purified MaSp2 protein alone and in the presence of plant extracts. 
     In brief, leaf tissue from transformants may be frozen in liquid nitrogen and ground into a fine powder. Buffer may be added (as described in reference 4) and the homogenate filtered through four layers of sterile cheesecloth followed by centrifugation at 10,000×g for 15 minutes. Proteins in the supernatant may be resolved by SDS-polyacrylamide gel electrophoresis (SDS-PAGE) followed by blotting onto nitrocellulose membranes. Membranes may be incubated with pre-hybridization blocking buffer (6, 7) followed by incubation with antiserum immunologically specific for MaSp2 (provided by Nexia Biotechnologies). Following hybridization, the membranes may be thoroughly washed and incubated in hybridization buffer containing secondary antibody conjugated to horseradish peroxidase. Unbound antibody may be removed by washing. Washed membranes may be immersed in chemiluminescence solution and the chemiluminescent signal detected by exposing the membranes to X-ray film. This technique is used routinely in research laboratories and is well known to those skilled in the art. See references 6 and 7. This method affords an estimation of the relative SS1 expression levels among transformed plants. Since spider silk protein is not effectively detected by Coomassie Blue staining techniques, the immunoblotting approach as described above provides workable means with which to compare SS1 expression levels among transformed plants. 
     The phenotype of transgenic plants may be evaluated based upon comparisons among non-transgenic parent plants, pIBT110 only transformed plants, and silk-expressing transformed (e.g., pIBT110-SS1) plants based on the following parameters: germination time, percentage germination, plant height at two weeks, one and three months, time to flowering, flower number and dry matter at harvest. In addition, visual examinations may be done throughout the growth cycle. Data may be collected for 30 individuals in each line and statistically evaluated to determine if silk expression and/or the level of silk expression affects phenotype. As described in Example I, measurements of transgenic  Arabidopsis  plants transformed to express SS1 revealed that spider silk expression did not effect phenotype in these plants. In brief, ten transgenic alfalfa lines shown to express high levels of protein and exhibit a normal phenotype as compared to controls will be selected for further analyses. 
     Molecular assays of transgenic plants for gene insertion and expression. Following propagation of transgenic plants, detailed molecular analyses of the transgene insert may be performed to ensure continued, appropriate expression and to identify any events (such as multiple insertion or insertion of SS1 fragments) that may be unacceptable from a functional and/or regulatory perspective. These experiments may be used to establish the copy number of inserted spider silk encoding DNA (e.g. SS1) in a transformed alfalfa genome, determine the level of mRNA production, and confirm protein expression levels. Southern blotting may be used to evaluate the presence and copy number of spider silk DNA in the genome. 
     Plant DNA may be isolated using the cetyl trimethyl ammonium bromide (CTAB) procedure wherein leaf tissue homogenate in CTAB buffer is incubated in 5% Sarkosyl for 60 minutes at 60° C. with mixing, followed by chloroform/octanol extraction and ethanol precipitation of DNA (12). As described above, the treated DNA may be digested with BamH1, resolved on an agarose gel, and transferred to a nylon membrane. The DNA may be crosslinked to the membrane by exposure to UV light. The membrane may be incubated with prehybridization buffer followed by hybridization buffer containing a denatured complementary SS1 oligonucleotide probe. The probe may be labeled by nick translation to a specific activity &gt;1×10 8  dpm/mg. Following hybridization, the membrane may be thoroughly washed and signal developed by autoradiography. Such methods may be used to establish the productive insertion of spider silk DNA into the alfalfa genome. 
     For Northern blotting, total RNA may be isolated as previously described (12) and resolved on a 1.5% agarose gel under denaturing conditions followed by electro-blotting onto a Zeta-Probe membrane. RNA may be fixed to the filter by ultraviolet crosslinking. Hybridizations may be performed as described (10) using a degenerate synthetic oligonucleotide probe of the spider silk nucleotide sequence. The probe may be labeled by nick translation using  32 P-DATP according to standard procedures (10). Following extensive washing of the filter, bands may be visualized by autoradiography. Each of these procedures may be performed according to established protocols. 
     Protocols for the isolation and purification of spider silk from alfalfa. Protocols for the efficient isolation of spider silk from plants may be necessary for large-scale industrial and medical uses. The SS1 protein, for example, may be expressed as a fusion protein in which the SS1 sequences have been operably linked to multiple His sequences to facilitate affinity purification of spider silk protein on nickel columns. Such purification methods may be used to advantage for small scale characterization studies. 
     Affinity purification strategies amenable for large-scale applications, however, are desirable. Affinity purification gauged for large-scale applications may be performed as follows: leaves may be homogenized by grinding in extraction buffer (50 mM Tris, pH 6.5, 10 mM MgCl 2  and 100 mM NaCl) and filtered through four layers of sterile cheesecloth. Following centrifugation at 10,000×g for 15 minutes, the supernatant may be incubated for 30 minutes with the nickel resin. Unbound supernatant may be removed by vacuum filtration and the resin washed in 10 volumes of binding buffer (5 mM imidazole, 500 mM NaCl, 20 mM Tris-Cl, pH 7.9) followed by 6 volumes of wash buffer (60 mM imidazole, 500 mM NaCl and 20 nM Tris-Cl, pH 7.9). Following filtration to remove any residual liquid, the protein may be eluted using 300 ml of stripping buffer (100 mM EDTA, 500 mM NaCl and 20 mM Tris-HcL, pH 7.9). The elutant may be dialyzed, lyophilized, and re-solubilized in sterile water prior to immunoblotting analysis as described above. This procedure may be used to yield an analytical amount of silk protein for quantitation by amino acid analysis. This procedure has been utilized extensively to analyze silk expression from  E. coli . See Lewis et al. (1996, Protein Expression and Purification 7:400). Using this approach, silk protein expression in transgenic plants may be determined as a percentage of total protein and as a percentage of dry and fresh weight. 
     Alternatively, standard purification strategies designed to differentially isolate silk protein from plant homogenates may be used to advantage. Such methods are described in Scheller et al. (2001, Nature Biotechnology 19:573), the entire contents of which is incorporated herein by reference. Purification of the expressed spider silk protein may be facilitated by its extreme stability under conditions that denature typical proteins, such as, for example, high heat and low pH. Accordingly, general protein purification strategies may be adapted to optimize silk purification from leaves. Above-ground portions of transgenic plants may be harvested and allowed to air dry as per normal production practices. The “hay” may be homogenized in buffer (see reference 4) followed by various treatments designed to differentially eliminate contaminants. 
     Development of a specific purification protocol is essentially an empirical process and silk protein recovery may be optimized following treatments in which homogenates are subject to individual and/or combinations of 1) boiling for various times in the presence or absence of detergent; 2) differential centrifugation; 3) progressively decreasing the pH; and 4) precipitation with varying concentrations of urea or ammonium sulfate. One of ordinary skill in the art may vary the above treatments to optimize the yield and efficiency of purification of spider silk proteins from plants. 
     The level of silk protein may be determined by immunoblotting and the purity and concentration assessed definitively by amino acid analysis. Purified silk protein may be analyzed for mechanical properties as previously described (18) to ensure that the recombinant protein possesses the desired properties. 
     The material remaining after silk extraction may be subject to compositional analyses according to standard protocols. Such data may be used to advantage in evaluating the suitability of the material as a feed crop. 
     Introduction of the trait into cultivars adaptable to geographic regions. Following regeneration of shoots and roots, positive transformants (T 0  transgenic plants) may be selected, grown to flowering, and hybridized to any alfalfa strain optimized for growth in a particular geographical region. In one embodiment, for example, the transformants may be hybridized to diverse alfalfa breeding materials specifically adapted to various environmental conditions (as described in 1, 16). These materials are available from the Groose collection. Backcrossing may be used to advantage to (a) confirm stable Mendelian transmission of the SS1 gene in alfalfa chromosomes, (b) remove any unwanted somoclonal variation (deleterious mutations) arising from the cell culture/transformation process, and (c) take the first step towards concentrating the spider silk gene from primary transformants (hemizygote simplex genotype Sxxx, where S=silk) to develop a population (cultivar) of autotetraploid alfalfa plants that are primarily triplex (SSSx) or quadriplex (SSSS) to ensure high levels of silk protein production in a finished cultivar. Backcrossing may be done under greenhouse conditions and progeny qualitatively and quantitatively evaluated for silk expression by immunoblotting to provide information pertaining to statistical analysis and further selection. 
     Example IV 
     Exemplary Methods for Designing Synthetic Spider Silk Proteins for Expression in Plants 
     The following methods for designing synthetic spider silk proteins are based on the amino acid composition of spider silk proteins and how repetitive regions of amino acid sequences contribute to the structural/physical properties of spider silk proteins. 
     Synthetic spider silk proteins may be comprised of a series of tandem exact repeats of amino acid sequence regions identified as possessing a particular spectrum of physical properties. Exact repeats comprise regions of amino acid sequences that are duplicated precisely. Alternatively, synthetic spider silk proteins may be comprised of a series of tandem inexact repeats identified as having a spectrum of physical properties. Inexact repeats may comprise regions of amino acid sequences in which at least one amino acid in the basic inexact repeat unit has been altered as long as the alteration does not change the spectrum of physical properties characteristic of the basic inexact repeat unit remain the same. 
     In order to increase the tensile strength of a minor ampullate silk, for example, to adapt it for applications in which strength and very little elasticity are needed, such as bulletproof vests, the (GA)n regions may be replaced by (A)n regions. This change would increase the tensile strength. The typical MiSp1 protein has sixteen (GA) units. Replacing eight (GA) regions, for example, with (A) regions would increase the tensile strength from 100,000 psi to at least 400,000 psi. Moreover, if the (A)n regions were as long as the (GA)n regions the tensile strength would increase to greater than 600,000 psi. 
     To create a fiber with high tensile strength and greater elasticity than major ampullate silk, the number of (GPGXX; SEQ ID NO: 24) regions may be increased from 4-5 regions, the range of (GPGXX; SEQ ID NO: 24) regions typically found in naturally occurring major ampullate spider silk proteins, to a larger number of regions. For example, if the number were increased to 10-12 (GPGXX; SEQ ID NO: 24) regions, the elasticity would increase to 50-60%. If the number were further increased to 25-30 regions, the elasticity would be near 100%. Such fibers may be used to advantage in coverings for wounds (for example, burn wounds) to facilitate easier placement and provide structural support. Such fibers may also be used for clothing and as fibers in composite materials. 
     The tensile strength of a very elastic flagelliform silk may be increased by replacing some of the (GPGXX; SEQ ID NO: 24) units with (A)n regions. A flagelliform silk protein contains an average of 50 (GPGXX; SEQ ID NO: 24) units per repeat. Replacing two units in each repeat with (A) regions may, therefore, increase the tensile strength of a flagelliform silk by a factor of four to achieve a tensile strength of about 400,000 psi. Uses for such flagelliform silk proteins are similar to those described for major ampullate proteins having augmented elasticity (as described hereinabove). The flagelliform proteins have additional utility in that the spacer regions therein confer the ability to attach functional molecules like antibiotics and/or growth factors (or combinations thereof) to composites comprising flagelliform proteins. 
     Synthetic spider proteins may also comprise the following elastic sequence motifs: GPGQQGPGQQ (SEQ ID NO: 25), from  Araneus  dragline; GPGGYGPGPGGQQG (SEQ ID NO: 26) from  Lactrodectus  dragline; and GPGAGQQGPGSQGPGSGGQQGPGQQ (SEQ ID NO: 27), from  Argiope  dragline. Genes comprising 2, 4, 8 and 16 repeats of these motifs may be constructed. The naturally occurring linker, GPYGPGS (SEQ ID NO: 28), connected to a poly-alanine segment of eight residues may be used to flank each repeat unit. The poly-alanine segment may be used as in the natural protein for fiber formation. This entire unit may be increased up to 16 repeat units to generate an encoded protein of 70-80 kD. Varying the number of these motifs alters the amount of elasticity from about 30% (for a synthetic spider silk protein comprising two repeats derived from  Araneus ) to nearly 200% (for a synthetic spider silk protein comprising sixteen repeats derived from  Argiope ). Varying the sequence of the motifs modifies the elastic modulus (higher with  Araneus , lower with  Argiope ). 
     Genes encoding synthetic spider proteins derived from one of the  Araneus  MaSp2 protein analog genes may also be constructed. Such  Araneus  MaSp2 protein analog genes comprise β-sheet motifs from poly-alanine segments of 5 and 14 residues that are the smallest and largest poly-alanine tracts found in the major ampullate silk proteins. These segments may also be constructed the novel sequence motif (gly-ala or gly-val) n  with the numerical value of n ranging from 3 to 8, the range observed in natural spider silk proteins. Varying the length and sequence of the β-sheet region alters the tensile strength from approximately that of the typical minor ampullate silk (100,000 psi) to at least 600,000 psi, double that of dragline silk. Moreover, the specific sequence of the repeat influences the tensile strength of the fiber. 
     Table II shows spider silk protein unit repeats of utility in the construction of synthetic spider silk protein. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 Elastic 
                 Hard 
                 Linker 
                 Helix 
               
               
                   
               
             
            
               
                 GPGGYGPGQQ (29) 
                 (A) n   
                 GPSGPGS (36) 
                 20) (GGX) n   
               
               
                 GPGQQGPGQQ (25) 
                 (GA) n   
                 GPYGPS (37) 
                 X = Y, L, Q, V, A, S 
               
               
                 GPGGYGPGPGXQQGY (30) 
                 (GV) n   
                 GPYGPG (38) 
               
               
                 GPGAGQQGPSQGPGSGGQQGPGGQ (31) 
                 (AQ) n   
                 GPGGPG* (39) 
               
               
                 GPGSGQQGPGQQGPGSGGQQGPGGQ (32) 
                 (AY) n   
                 GPGGPGSS* (40) 
               
               
                 GPGGYGPGSQGPSGPGAY* (33) 
                   
                 GPSGPGGAS* (41) 
               
               
                 GPGGQGPGQQGPGGY* (34) 
                   
                 GPGSG (42) 
               
               
                 GPGGX (35) with X = A, Y, S, V 
               
               
                   
               
               
                 *= a novel motif; n = 4-14, more likely 4-8. 
               
               
                 Numbers in parenthesis are utilized to indicate SEQ ID NO:. 
               
            
           
         
       
     
     Elastic spider silk protein unit repeats include SEQ ID NOs: 25 and 29-35. Hard spider silk protein unit repeats include (A) n ; (GA) n ; (GV) n ; (AQ) n ; and (AY) n . Linker unit repeats include SEQ ID NOs: 36-42. Helix spider silk protein unit repeats include (GGX) n , wherein X may be Y, L, Q, V, A, or S. Unit repeats may be combined as indicated herein to produce a larger repeat, multimers of which may be assembled to produce a spider silk protein. 
     General Guidelines: 
     1) Elastic units or combinations of elastic units (n=2-63) may be combined with any linker and any hard unit and any helix in that order to create a repeat unit for a MaSp2 protein except:
         a) The following combinations are excluded:
 
SEQ ID NO: 29 operably linked to (A) n  operably linked to SEQ ID NO: 36;
 
SEQ ID NO: 25 operably linked to (A) n  operably linked to SEQ ID NO: 38;
 
SEQ ID NO: 30 operably linked to (A) n  operably linked to SEQ ID NO: 42;
 
SEQ ID NO: 31 operably linked to (A) n  operably linked to SEQ ID NO: 37; and
 
SEQ ID NO: 32 operably linked to (A) n  operably linked to SEQ ID NO: 37.
       

     The above excluded combinations may be used to generate an amino acid sequence consisting essentially of the amino acid sequence of one of the natural spider silk proteins. The number of repeats (N) may range from 4-9, depending on the specific repeat unit, since they vary within the silk protein. 
     2) Any single hard segment or combination of segments may be combined with any helix segment to create a repeat unit for a MaSp1 protein. 
     3) Single repeat units or combinations of repeat units may be used to create proteins with molecular weights ranging from 50,000 dal to 500,000. 
     The composition of six exemplary synthetic spider silk proteins is provided herein below. 
     1) three copies of SEQ ID NO: 29 with 1 copy of (A) n  (n=8) with SEQ ID NO: 40 to produce a repeat (SEQ ID NO: 43) and 24 copies of repeat SEQ ID NO: 43 to produce a synthetic spider silk protein with a M.W. of approximately 110,000 dal (SEQ ID NO: 49). 
     2) four copies of SEQ ID NO: 25 with 1 copy of (GA) n  (n=7) with SEQ ID NO: 38 to produce a repeat (SEQ ID NO: 44) and 50 copies of repeat SEQ ID NO: 44 to produce a synthetic spider silk protein with a M.W. of approximately 300,000 dal (SEQ ID NO: 50). 
     3) 30 copies of SEQ ID NO: 35 with X=S, Y, S, Y, A repeating in that order with five copies of (GGX) n  with X=A with SEQ ID NO: 37 to produce a repeat (SEQ ID NO: 45) and 20 copies of repeat SEQ ID NO: 45 to produce a synthetic spider silk protein with a M.W. of approximately 330,000 dal (SEQ ID NO: 51). 
     4) 10 copies of SEQ ID NO: 35 with X=alternating S and Y with 1 copy of (A) n  (n=6) with SEQ ID NO: 36 to produce a repeat (SEQ ID NO: 46) and 32 copies of repeat SEQ ID NO: 46 to produce a synthetic spider silk protein with a M.W. of approximately 202,000 dal (SEQ ID NO: 52). 
     5) six copies of (GGX) n  with X=Y, L, and Q in that order with 1 copy of (GA) n  (n=9) to produce a repeat (SEQ ID NO: 47) and 64 copies of repeat SEQ ID NO: 47 to produce a synthetic spider silk protein with a M.W. of approximately 205,000 dal (SEQ ID NO: 53). 
     6) six copies of (GGX) n  with X=alternating V and A with 1 copy of (GV) n  with n=7 to produce a repeat (SEQ ID NO: 48) and 48 copies of repeat SEQ ID NO: 48 to produce a synthetic spider silk protein with a M.W. of approximately 154,000 dal (SEQ ID NO: 54). 
     Synthetic spider silk proteins 1-3 are provided to exemplify MaSp2 analogs. Synthetic spider silk protein 4 illustrates a Flag analog. Examples of MaSp1 analogs are provided in synthetic spider silk proteins 5 and 6. One of skill in the art, given the guidance and examples provided herein, would be able to design and produce a variety of synthetic spider silk proteins. 
     Plant expression constructs comprising any of the above modified synthetic spider silk proteins may be constructed and used to transform higher plants as described hereinabove. Modified synthetic spider silk proteins may be isolated from plant cell extracts and manipulated as described hereinabove to produce spider silk protein fibers. Plant-expressed spider silk protein fibers may be woven singly or in combination, as proscribed by a particular application. Such fibers have utility in a variety of applications, including, but not limited to, production of fabric, sutures, artificial ligaments and tendons, medical coverings, flexible casts, high-tech clothing, rope, and reinforced plastics. 
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     While certain of the preferred embodiments of the present invention have been described and specifically exemplified above, it is not intended that the invention be limited to such embodiments. Various modifications may be made thereto without departing from the scope and spirit of the present invention, as set forth in the following claims.