# Patent

## Title
DNA molecules encoding enzymes involved in starch synthesis, vectors, bacteria, transgenic plant cells and plants containing these molecules

## Abstract

The present invention relates to DNA molecules encoding enzymes which are involved in the starch synthesis of plants. These enzymes represent two different isotypes of the soluble starch synthase as well as a starch granule-bound starch synthase. This invention furthermore relates to vectors, bacteria, as well as to plant cells transformed with the DNA molecules described and to plants regenerated from them.
Furthermore, the invention relates to starch that can be isolated from plants having an increased or reduced activity of the proteins described.

## Background
This application is a divisional of U.S. application Ser. No. 09/606,304, filed Jun. 28, 2000, which is a divisional of U.S. application Ser. No. 08/836,567, filed Jul. 24, 1997, now U.S. Pat. No. 6,130,367, which is a 371 of PCT/EP95/04415, filed Nov. 9, 1995.


FIELD OF THE INVENTION
The present invention relates to DNA molecules encoding enzymes which are involved in the starch synthesis of plants. These enzymes represent two different isotypes of the soluble starch synthase as well as a starch granule-bound starch synthase.
This invention furthermore relates to vectors, bacteria, as well as to plant cells transformed with the DNA molecules described and to plants regenerated from them.
Also, processes for the production of transgenic plants are described which, due to the introduction of DNA molecules encoding soluble or starch granule-bound starch synthases, synthesize a starch which is modified as regards its properties.
BACKGROUND OF THE INVENTION
With respect to its increasing significance which has recently been ascribed to vegetal substances as regenerative sources of raw materials, one of the objects of biotechnological research is to try to adapt vegetal raw materials to the demands of the processing industry. In order to enable the use of modified regenerative raw materials in as many areas as possible, it is furthermore important to obtain a large variety of substances. Apart from oils, fats and proteins, polysaccharides constitute the essential regenerative raw materials derived from plants. Apart from cellulose, starch maintains an important position among the polysaccharides, being one of the most significant storage substances in higher plants. Besides maize, rice and wheat, potato plays an important role as starch producer.
The polysaccharide starch is a polymer made up of chemically homogeneous basic components, namely the glucose molecules. However, it constitutes a highly complex mixture from various types of molecules which differ from each other in their degree of polymerization and in the degree of branching of the glucose chains. Therefore, starch is not a homogeneous raw material. One differentiates particularly between amylose-starch, a basically non-branched polymer made up of α-1,4-glycosidically branched glucose molecules, and amylopectin-starch which in turn is a complex mixture of various branched glucose chains. The branching results from additional α-1,6-glycosidic interlinkings. In plants which are typically used for starch production, such as, e.g., maize or potato, the synthesized starch consists of about 25% of amylose starch and of about 75% of amylopectin starch.
In order to enable as wide a use of starch as possible, it seems to be desirable that plants be provided which are capable of synthesizing modified starch which is particularly suitable for various uses. A possibility of providing such plants is—apart from breeding—in the specific genetic modification of the starch metabolism of starch-producing plants by means of recombinant DNA techniques. However, a prerequisite therefor is to identify and to characterize the enzymes involved in the starch synthesis and/or the starch modification as well as to isolate the respective DNA molecules encoding these enzymes.
The biochemical pathways which lead to the production of starch are basically known. The starch synthesis in plant cells takes place in the plastids. In photosynthetically active tissues these are the chloroplasts, in photosynthetically inactive, starch-storing tissues the amyloplasts.
The most important enzymes involved in starch synthesis are starch synthases as well as branching enzymes. In the case of starch synthases various isotypes are described which all catalyze a polymerization reaction by transferring a glucosyl residue of ADP-glucose to α-1,4-glucans. Branching enzymes catalyze the introduction of α-1,6 branchings into linear α-1,4-glucans.
Furthermore, it is discussed that other enzyme activities, such as hydrolytic or phosphorolytic activities, are involved in the synthesis of starch (Preiss in Oxford Survey of Plant Molecular and Cell Biology, Oxford University Press, Vol. 7 (1991), 59-114). It can furthermore not be precluded that the “R enzyme”, or the so-called disproportionizing enzyme, and the starch phosphorylases also are involved in starch synthesis, although these enzymes so far have been connected with the degradation of starch.
Starch synthases may be divided up in two groups: the granule-bound starch synthases (GBSS), which are mainly present bound to starch granules but also in soluble form, and the soluble starch synthases (SSS). Within these classifications, various isotypes are described for various species of plants. These isotypes differ from each other in their dependency on primer molecules (so-called “primer dependent” (type II) and “primer independent” (type I) starch synthases).
So far only in the case of the isotype GBSS I its exact function during starch synthesis has been successfully determined. Plants in which this enzyme activity has been strongly or completely reduced, synthesize starch free of amylose (a so-called “waxy” starch) (Shure et al., Cell 35 (1983), 225-233; Visser et al., Mol. Gen. Genet. 225 (1991), 289-296; WO 92/11376); therefore this enzyme has been assigned a decisive role in synthesizing amylose-starch. This phenomenon is also observed in the cells of the green alga Chlamydomonas reinhardtii (Delrue et al., J. Bacteriol. 174 (1992), 3612-3620). In the case of Chlamydomonas it was furthermore demonstrated that GBSS I is not only involved in the synthesis of amylose but also has a certain influence on amylopectin synthesis. In mutants which do not show any GBSS I activity a certain fraction of the normally synthesized amylopectin, exhibiting long chain glucans, is missing.
The functions of the other isotypes of the granule-bound starch synthases, particularly GBSS II, and of the soluble starch synthases are so far not clear. It is assumed that soluble starch synthases, together with branching enzymes, are involved in the synthesis of amylopectin (see, e.g., Ponstein et al., Plant Physiol. 92 (1990), 234-241) and that they play an important role in the regulation of starch synthesis rate.
For potato, the isotypes GBSS I, GBSS II, as well as two or three isotypes of the soluble starch synthases, which so far have not been characterized further, have been identified (Ponstein et al. Plant Physiol. 92 (1990), 234-241; Smith et al., Planta 182 (1990), 599-604; Hawker et al., Phytochemistry 11 (1972), 1287-1293). Also for pea a GBSS II could be found (Dry et al., The Plant Journal 2,2 (1992), 193-202).
A cDNA encoding GBSS I from potato as well as a genomic DNA have already been described (Visser et al., Plant Sci. 64 (1989), 185-192; van der Leij et al., Mol. Gen. Genet. 228 (1991), 240-248). So far, no nucleic acid sequences encoding further granule-bound starch synthases or one of the soluble starch synthase isotypes from potato, have been reported.
Soluble starch synthases have been identified in several other plant species apart from potato. Soluble starch synthases have for example been isolated in homogeneous form from pea (Denyer and Smith, Planta 186 (1992), 609-617) and maize (WO 94/09144). In the case of pea it was found that the isotype of the soluble starch synthase identified as SSS II is identical with the granule-bound starch synthase GBSS II (Denyer et al., Plant J. 4 (1993), 191-198). In the case of other plant species the existence of several SSS-isotypes was described by means of chromatographic methods, as for example in the case of barley (Tyynelä and Schulman, Physiologia Plantarum 89 (1993) 835-841; Kreis, Planta 148 (1980), 412-416), maize (Pollock and Preiss, Arch. Biochem. Biophys. 204 (1980), 578-588) and wheat (Rijven, Plant Physiol. 81 (1986), 448-453). However, DNA sequences encoding these proteins have so far not been described.
A cDNA encoding a soluble starch synthase so far has only been described for rice (Baba et al., Plant Physiol. 103 (1993), 565-573).
SUMMARY OF THE INVENTION
In order to provide possibilities for modifying any desired starch-storing plant in such a way that they will synthesize a modified starch, respective DNA sequences encoding the various isotypes of granule-bound or soluble starch synthases have to be identified.
Therefore, it was the object of the present invention to provide DNA molecules—especially from potato-encoding enzymes involved in starch biosynthesis and by means of which genetically modified plants may be produced that show an elevated or reduced activity of those enzymes, thereby prompting a modification in the chemical and/or physical properties of the starch synthesized in these plants.
This object has been achieved by the provision of the embodiments described in the claims.
The invention therefore relates to DNA molecules encoding starch synthases, particularly such DNA molecules encoding the granule-bound starch synthases of the isotype II, as well as DNA molecules encoding soluble starch synthases.
The present invention particularly relates to DNA molecules encoding proteins with the biological activity of a granule-bound starch synthase of the isotype II (GBSSII) or a biologically active fragment of such a protein, such molecules preferably encoding proteins having the amino acid sequence indicated under Seq ID No. 8. Particularly, the invention relates to DNA molecules having the nucleotide sequence indicated under Seq ID No. 7, preferably molecules comprising the coding region indicated under Seq ID No. 7.
The subject matter of the invention are also DNA molecules encoding a GBSSII and the sequence of which differs from the nucleotide sequences of the above-described DNA molecules due to the degeneracy of the genetic code.
Furthermore, the invention relates to DNA molecules encoding GBSSII and hybridizing to any of the above-described DNA molecules. Such DNA molecules preferably are derived from starch-storing plants, particularly from dicotyledonous plants, and particularly preferred from potato.
The GBSSII proteins encoded by the DNA molecules according to the invention preferably have a molecular weight of 85±5 kD. GBSSII proteins are mainly present bound to starch granules, however, they may also be present in soluble form.
Furthermore, the invention relates to DNA molecules encoding proteins with the biological activity of a soluble starch synthase of the isotype B (SSSB) or a biologically active fragment of such a protein, with such molecules preferably encoding proteins having the amino acid sequence indicated under Seq ID No. 10. In particular, the invention relates to DNA molecules having the nucleotide sequence indicated under Seq ID No. 9, preferably molecules comprising the coding region indicated under Seq ID No. 9.
Another subject matter of the invention are DNA molecules encoding an SSSB and the sequence of which differs from the nucleotide sequences of the above-described DNA molecules due to the degeneracy of the genetic code.
Furthermore, the invention relates to DNA molecules encoding SSSB and hybridizing to any of the above-described DNA molecules. An exception are the DNA molecules from rice. The SSSB proteins encoded by the DNA molecules according to the invention preferably have a molecular weight of 78±5 kD.
The enzymatic properties of the SSSB proteins are described in the examples.
The invention furthermore relates to DNA molecules encoding proteins with the biological activity of a soluble starch synthase of the isotype A (SSSA). Such proteins can, for example, be characterized in that they are recognized by an antibody that is directed to the peptide having the amino acid sequence











NH2—GTGGLRDTVENC—COOH.
(Seq ID No.13)







The enzymatic properties of the SSSA proteins are described in the examples.
An example of a DNA molecule encoding such a protein is a DNA molecule having the coding region depicted under Seq ID No. 11. This DNA molecule may be used to isolate from other organisms, in particular plants, DNA molecules encoding the SSSA proteins.
Thus, the present invention also relates to DNA molecules encoding proteins with the biological activity of a soluble starch synthase of the isotype A (SSSA) or a biologically active fragment of such a protein, with such molecules preferably encoding proteins having the amino acid sequence indicated under Seq ID No. 12. The invention particularly relates to DNA molecules having the nucleotide sequence indicated under Seq ID No. 11, preferably molecules comprising the coding region indicated under Seq ID No. 11.
Another subject matter of the invention are DNA molecules encoding SSSA and the sequence of which differs from the nucleotide sequences of the above-described DNA molecules due to a degeneracy of the genetic code.
Furthermore, the present invention relates to DNA molecules encoding SSSA and hybridizing to any of the above-described DNA molecules.
The SSSA protein preferably has an apparent molecular weight of about 120 to 140 kD, particularly of about 135 kD, in SDS gel electrophoresis.
DETAILED DESCRIPTION OF THE INVENTION
In this invention the term “hybridization” signifies hybridization under conventional hybridizing conditions, preferably under stringent conditions as described for example in Sambrook et al., Molecular Cloning, A Laboratory Manual, 2nd Edition (1989) Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.). DNA molecules hybridizing to the DNA molecules according to the invention can basically be derived from any organism (i.e., prokaryotes or eukaryotes, particularly from bacteria, fungi, algae, plants or animal organisms) which possesses such DNA molecules. Preferably, they originate from monocotyledonous or dicotyledonous plants, in particular from useful plants, and particularly preferred from starch-storing plants.
DNA molecules hybridizing to the molecules according to the invention may be isolated, e.g., from genomic or from cDNA libraries from various organisms.
The identification and isolation of such DNA molecules from plants or other organisms may take place by using the DNA molecules according to the invention or parts of these DNA molecules or, as the case may be, the reverse complement strands of these molecules, e.g., by hybridization according to standard methods (see, e.g., Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).
As a probe for hybridization, e.g., DNA molecules may be used which exactly or basically contain the nucleotide sequences indicated under Seq ID No. 7, 9 or 11 or parts thereof. The fragments used as hybridization probe may also be synthetic DNA fragments which were produced by means of the conventional DNA synthesizing methods and the sequence of which is basically identical with that of a DNA molecule according to the invention. After identifying and isolating the genes hybridizing to the DNA sequences according to the invention, the sequence has to be determined and the properties of the proteins encoded by this sequence have to be analyzed.
The molecules hybridizing to the DNA molecules of the invention also comprise fragments, derivatives and allelic variants of the above-described DNA molecules which encode one of the proteins described above. Thereby, fragments are defined as parts of the DNA molecules, which are long enough in order to encode one of the described proteins. In this context, the term derivatives means that the DNA sequences of these molecules differ from the sequences of the above-mentioned DNA molecules at one or more positions and that they exhibit a high degree of homology to these DNA sequences. Hereby, homology means a sequence; identity of at least 40%, in particular an identity of at least 60%, preferably of more than 80% and still more preferably a sequence identity of more than 90%. The deviations occurring when comparing with the above-described DNA molecules might have been caused by deletion, substitution, insertion or recombination. Moreover, homology means that functional and/or structural equivalence exists between the respective DNA molecules or the proteins they encode. The DNA molecules, which are homologous to the above-described DNA molecules and represent derivatives of these DNA molecules, are generally variations of these molecules, that constitute modifications which exert the same biological function. These variations may be naturally occurring variations, for example sequences derived from other organisms, or mutations, whereby these mutations may have occurred naturally or they may have been introduced by means of a specific mutagenesis. Moreover, the variations may be synthetically produced sequences. The allelic variants may be naturally occurring as well as synthetically produced variants or variants produced by recombinant DNA techniques.
The proteins encoded by the various variants of the DNA molecules according to the invention exhibit certain common characteristics. Enzyme activity, molecular weight, immunologic reactivity, conformation etc. may belong to these characteristics as well as physical properties such as the mobility in gel electrophoresis, chromatographic characteristics, sedimentation coefficients, solubility, spectroscopic properties, stability; pH-optimum, temperature-optimum etc.
Significant characteristics of a starch synthase are: i) their localization within the stroma of the plastids of plant cells; ii) their capability of synthesizing linear α-1,4-linked polyglucans using ADP-glucose as substrate. This activity can be determined as shown in Denyer and Smith (Planta 186 (1992), 606-617) or as described in the examples.
The DNA molecules according to the invention may basically originate from any organism expressing the proteins described, preferably from plants, particularly from starch-synthesizing or starch-storing plants. These plants may be monocotyledonous but also dicotyledonous plants. Particularly preferred are, e.g., cereals (such as barley, rye, oats, wheat, etc.), maize, rice, pea, cassava, potato, etc.
Furthermore, the invention relates to vectors, especially plasmids, cosmids, viruses, bacteriophages and other vectors common in genetic engineering, which contain the above-mentioned DNA molecules of the invention.
In a preferred embodiment the DNA molecules contained in the vectors are linked to DNA elements that ensure the transcription and synthesis of a translatable RNA in prokaryotic and eukaryotic cells.
The expression of the DNA molecules of the invention in prokaryotic cells, e.g., in Escherichia coli, is interesting insofar as this enables a more precise characterization of the enzymatic activities of the enzymes encoding these molecules. In particular, it is possible to characterize the product being synthesized by the respective enzymes in the absence of other enzymes which are involved in the starch synthesis of the plant cell. This makes it possible to draw conclusions about the function, which the respective protein exerts during the starch synthesis within the plant cell.
Moreover, it is possible to introduce various mutations into the DNA molecules of the invention by means of conventional molecular-biological techniques (see, e.g., Sambrook et al., 1989, Molecular Cloning, A Laboratory Manual, 2nd Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.), whereby the synthesis of proteins with possibly modified biological properties is induced. By means of this it is on the one hand possible to produce deletion mutants, in which DNA molecules are produced by continuing deletions at the 5′- or the 3′-end of the encoding DNA-sequence. These DNA molecules may lead to the synthesis of correspondingly shortened proteins. Such deletions at the 5′-end of the nucleotide sequence make it possible, for example, to identify amino acid sequences which are responsible for the translocation of the enzyme in the plastids (transit peptides). This allows for the specific production of enzymes which due to the removal of the respective sequences are no longer located in the plastids but within the cytosol, or which due to the addition of other signal sequences are located in other compartments.
On the other hand, point mutations might also be introduced at positions where a modification of the amino acid sequence influences, for example, the enzyme activity or the regulation of the enzyme. In this way, e.g., mutants with a modified Km-value may be produced, or mutants which are no longer subject to the regulation mechanisms by allosteric regulation or covalent modification usually occurring in cells.
Furthermore, mutants may be produced exhibiting a modified substrate or product specificity such as mutants that use ADP-glucose-6-phosphate instead of ADP-glucose as substrate. Moreover, mutants with a modified activity-temperature-profile may be produced.
For the genetic manipulation in prokaryotic cells the DNA molecules of the invention or parts of these molecules may be integrated into plasmids which allow for a mutagenesis or a sequence modification by recombination of DNA sequences. By means of standard methods (cf. Sambrook et al., 1989, Molecular Cloning: A laboratory manual, 2nd edition, Cold Spring Harbor Laboratory Press, NY, USA) base exchanges may be carried out or natural or synthetic sequences may be added. In order to connect the DNA fragments, adapters or linkers may be attached to the fragments. Moreover, use can be made of manipulations which offer suitable restriction sites or which remove superfluous DNA or restriction sites. Wherever use is made of inserts, deletions or substitutions, in vitro mutagenesis, “primer repair”, restriction or ligation may be used. For analyzing use is usually made of a sequence analysis, a restriction analysis or further biochemico-molecularbiological methods.
In a further embodiment the invention relates to host cells, in particular prokaryotic or eukaryotic cells, which contain a DNA molecule of the invention as described above or a vector of the invention. These are preferably bacterial cells or plant cells.
Furthermore, the proteins encoded by the DNA molecules of the invention are the subject-matter of the invention as well as methods for their production whereby a host cell of the invention is cultivated under conditions that allow for a synthesis of the protein and whereby the protein is then isolated from the cultivated cells and/or the culture medium.
It was found that by making available the nucleic acid molecules of the invention it is now possible—by means of recombinant DNA techniques—to interfere with the starch metabolism of plants in a way so far impossible and to modify it in such a way that a starch is synthesized which, e.g., is modified, compared to the starch synthesized in wild-type plants, with respect to its physico-chemical properties, especially the amylose/amylopectin ratio, the degree of branching, the average chain length, the phosphate content, the pastification behavior, the size and/or the shape of the starch granule. Soluble starch synthases, play, e.g., a central role in the regulation of the synthesis rate of starch. There is the possibility of increasing the yield of genetically modified plants by increasing the activity of these enzymes or by making mutants available which are no longer subject to cell-specific regulation schemes and/or different temperature-dependencies with respect to their activity. The economic significance of the chance to interfere with the starch synthesis, namely of potato plants, is obvious: In Europe, for example, potato is one of the most important plants for producing starch apart from maize and wheat. About 20% of the starch produced in Europe per year is obtained from potatoes. Furthermore, potato starch exhibits some advantageous properties as compared to starch from maize or wheat, such as, e.g., a low protein and lipid content as well as relatively large starch granules and phosphate content. Therefore, if possible, potato starch is preferably used.
Therefore, it is possible to express the DNA molecules of the invention in plant cells in order to increase the activity of one or more starch synthases. Furthermore, the DNA molecules of the invention may be modified by means of methods known to the skilled person, in order to produce starch synthases which are no longer subject to the cell-specific regulation mechanisms or show modified temperature-dependencies or substrate or product specificities.
The synthesized protein may in principle be located in any desired compartment within the plant cell. In order to locate it within a specific compartment, the sequence ensuring the localization in the plastids must be deleted and the remaining coding regions optionally have to be linked to DNA sequences which ensure localization in the respective compartment. Such sequences are known (see, e.g., Braun et al., 1992, EMBO J. 11:3219-3227; Wolter et al., 1988, Proc. Natl. Acad. Sci. USA 85: 846-850; Sonnewald et al., 1991, Plant J. 1:95-106).
Thus, the present invention also relates to transgenic plant cells containing a DNA molecule of the invention, this DNA molecule being linked to regulatory DNA elements, which ensure the transcription in plant cells, especially with a promoter which is heterologous with respect to the DNA molecule.
By means of methods known to the skilled person the transgenic plant cells can be regenerated to whole plants. Thus, the plants obtained by regenerating the transgenic plant cells of the invention are also the subject-matter of the present invention. A further subject-matter of the invention are plants which contain the above-described transgenic plant cells. The transgenic plants may in principle be plants of any desired species, i.e., they may be monocotyledonous as well as dicotyledonous plants. These are preferably useful plants, such as cereals (rye, barley, oats, wheat etc.), rice, maize, peas, cassava or potatoes.
The invention also relates to propagation material of the plants of the invention, e.g., fruits, seeds, tubers, cuttings etc.
Due to the expression or, as the case may be, additional expression of a DNA molecule of the invention, the transgenic plant cells and plants of the invention synthesize a starch which compared to starch synthesized in wild-type plants, i.e., non-transformed plants, is modified, in particular with respect to the viscosity of aqueous solutions of this starch and/or the phosphate content. Thus, the starch derived from transgenic plant cells and plants according to the invention is the subject-matter of the present invention.
A further subject-matter of the invention are transgenic plant cells, in which the activity of a protein according to the invention is reduced when compared to non-transformed plants. It was found that plant cells exhibiting a reduced activity of a protein of the invention synthesize a starch having modified chemical and/or physical properties as compared to that of wild-type plant cells.
The production of plant cells with a reduced activity of a protein of the invention may for example be achieved by using the DNA molecules of the invention. Possibilities are the expression of a corresponding antisense-RNA, of a sense-RNA for achieving a cosupression effect or the expression of a correspondingly constructed ribozyme, which specifically cleaves transcripts encoding a protein of the invention.
Preferably, an antisense RNA is expressed to reduce the activity of a protein of the invention in plant cells.
For this purpose, a DNA molecule can be used which comprises the complete sequence encoding a protein of the invention, including possibly existing flanking sequences as well as DNA molecules, which only comprise parts of the encoding sequence whereby these parts have to be long enough in order to prompt an antisense-effect within the cells. Basically, sequences with a minimum length of 15 bp, preferably with a length of 100-500 bp and for an efficient antisense-inhibition, in particular sequences with a length of more than 500 bp may be used. Generally DNA-molecules are used which are shorter than 5000 bp, preferably sequences with a length of less than 2500 bp. Preferably, use is made of DNA molecules that are homologous with respect to the plant species to be transformed.
Use may also be made of DNA sequences which are highly homologous, but not completely identical to the sequences of the DNA molecules of the invention. The minimal homology should be more than about 65%. Preferably, use should be made of sequences with homologies between 95 and 100%.
The transgenic plant cells of the invention can be regenerated to whole plants by means of methods known to the skilled person. Thus, plants containing the transgenic plant cells of the invention are also the subject-matter of the present invention. These plants generally are plants of any species, i.e., monocotyledonous and dicotyledonous plant. Preferably these plants are useful plants, especially starch-storing plants such as cereals (rye, barley, oats, wheat, etc.), rice, maize, peas, cassava or potatoes. The invention also relates to propagation material of the plants of the invention, such as fruit, seeds, tubers, cuttings, etc.
Due to the reduction of the activity of one of the proteins of the invention, the transgenic plant cells and plants of the invention synthesize a starch which is modified, compared to the starch from non-transformed plant cells or plants, in its chemical and/or physical properties. This starch exhibits for example a modified viscosity of its aqueous solutions and/or a modified phosphate content.
Thus, starch derived from the above-mentioned transgenic plant cells and plants is also the subject-matter of the invention.
The starches of the invention may be modified according to techniques known to the skilled person; in unmodified as well as in modified form they are suitable for use in foodstuffs or non-foodstuffs.
Basically, the possibilities of uses of the starch can be subdivided into two major fields. One field comprises the hydrolysis products of starch which mainly include glucose and glucan components obtained by enzymatic or chemical processes. They serve as starting materials for further chemical modifications and processes such as fermentation. In this context, it might be of importance that the hydrolysis process can be carried out simply and inexpensively. Currently, it is carried out substantially enzymatically using amyloglucosidase. It is thinkable that costs might be reduced by using lower amounts of enzymes for hydrolysis due to changes in the starch structure, e.g., increased surface of the grain, improved digestibility due to less branching or a steric structure, which limits the accessibility for the used enzymes.
The other area in which starch is used due to its polymer structure as so-called native starch, can be subdivided into two further areas:
1. Use in Foodstuffs
Starch is a classic additive for various foodstuffs, in which it essentially serves the purpose of binding aqueous additives and/or causes an increased viscosity or an increased gel formation. Important characteristic properties are flowing and sorption behavior, swelling and pastification temperature, viscosity and thickening performance, solubility of the starch, transparency and paste structure, heat, shear and acid resistance, tendency to retrogradation, capability of film formation, resistance to freezing/thawing, digestibility as well as the capability of complex formation with, e.g., inorganic or organic ions.
2. Use in Non-Foodstuffs
The other major field of application is the use of starch as an adjuvant in various production processes or as an additive in technical products. The major fields of application for the use of starch as an adjuvant are, first of all, the paper and cardboard industry. In this field, the starch is mainly used for retention (holding back solids), for sizing filler and fine particles, as solidifying substance and for dehydration. In addition, the advantageous properties of starch with regard to stiffness, hardness, sound, grip, gloss, smoothness, tear strength as well as the surfaces are utilized.
2.1 Paper and Cardboard Industry
Within the paper production process, a differentiation can be made between four fields of application, namely surface, coating, mass and spraying.
The requirements on starch with regard to surface treatment are essentially a high degree of brightness, corresponding viscosity, high viscosity stability, good film formation as well as low formation of dust. When used in coating the solid content, a corresponding viscosity, a high capability to bind as well as a high pigment affinity play an important role. As an additive to the mass rapid, uniform, loss-free dispersion, high mechanical stability and complete retention in the paper pulp are of importance. When using the starch in spraying, corresponding content of solids, high viscosity as well as high capability to bind are also significant.
2.2 Adhesive Industry
A major field of application is, for instance, in the adhesive industry, where the fields of application are subdivided into four areas: the use as pure starch glue, the use in starch glues prepared with special chemicals, the use of starch as an additive to synthetic resins and polymer dispersions as well as the use of starches as extenders for synthetic adhesives. 90% of all starch-based adhesives are used in the production of corrugated board, paper sacks and bags, composite materials for paper and aluminum, boxes and wetting glue for envelopes, stamps, etc.
2.3 Textile and Textile Care Industry
Another possible use as adjuvant and additive is in the production of textiles and textile care products. Within the textile industry, a differentiation can be made between the following four fields of application: the use of starch as a sizing agent, i.e., as an adjuvant for smoothing and strengthening the burring behavior for the protection against tensile forces active in weaving as well as for the increase of wear resistance during weaving, as an agent for textile improvement mainly after quality-deteriorating pretreatments, such as bleaching, dying, etc., as a thickener in the production of dye pastes for the prevention of dye diffusion and as an additive for warping agents for sewing yarns.
2.4 Building Industry
The fourth area of application of starch is its use as an additive in building materials. One example is the production of gypsum plaster boards, in which the starch mixed in the thin plaster pastifies with the water, diffuses at the surface of the gypsum board and thus binds the cardboard to the board. Other fields of application are admixing it to plaster and mineral fibers. In ready-mixed concrete, starch may be used for the deceleration of the sizing process.
2.5 Ground Stabilization
Furthermore, the starch is advantageous for the production of means for ground stabilization used for the temporary protection of ground particles against water in artificial earth shifting. According to state-of-the-art knowledge, combination products consisting of starch and polymer emulsions can be considered to have the same erosion- and incrustation-reducing effect as the products used so far; however, they are considerably less expensive.
2.6 Use of Starch in Plant Protectives and Fertilizers
Another field of application is the use of starch in plant protectives for the modification of the specific properties of these preparations. For instance, starches are used for improving the wetting of plant protectives and fertilizers, for the dosed release of the active ingredients, for the conversion of liquid, volatile and/or odorous active ingredients into microcristalline, stable, deformable substances, for mixing incompatible compositions and for the prolongation of the duration of the effect due to a reduced disintegration.
2.7 Drugs, Medicine and Cosmetics Industry
Starch may also be used in the fields of drugs, medicine and in the cosmetics industry. In the pharmaceutical industry, the starch may be used as a binder for tablets or for the dilution of the binder in capsules. Furthermore, starch is suitable as disintegrant for tablets since, upon swallowing, it absorbs fluid and after a short time it swells so much that the active ingredient is released. For qualitative reasons, medicinal flowance and dusting powders are further fields of application. In the field of cosmetics, the starch may for example be used as a carrier of powder additives, such as scents and salicylic acid. A relatively extensive field of application for the starch is toothpaste.
2.8 Starch as an Additive in Coal and Briquettes
The use of starch as an additive in coal and briquettes is also thinkable. By adding starch, coal can be quantitatively agglomerated and/or briquetted in high quality, thus preventing premature disintegration of the briquettes. Barbecue coal contains between 4 and 6% added starch, calorated coal between 0.1 and 0.5%. Furthermore, the starch is suitable as a binding agent since adding it to coal and briquette can considerably reduce the emission of toxic substances.
2.9 Processing of Ore and Coal Slurry
Furthermore, the starch may be used as a flocculant in the processing of ore and coal slurry.
2.10 Starch as an Additive in Casting
Another field of application is the use as an additive to process materials in casting. For various casting processes cores produced from sands mixed with binding agents are needed. Nowadays, the most commonly used binding agent is bentonite mixed with modified starches, mostly swelling starches.
The purpose of adding starch is increased flow resistance as well as improved binding strength. Moreover, swelling starches may fulfill more prerequisites for the production process, such as dispersability in cold water, rehydratisability, good mixability in sand and high capability of binding water.
2.11 Use of Starch in Rubber Industry
In the rubber industry starch may be used for improving the technical and optical quality. Reasons for this are improved surface gloss, grip and appearance. For this purpose, the starch is dispersed on the sticky rubberized surfaces of rubber substances before the cold vulcanization. It may also be used for improving the printability of rubber.
2.12 Production of Leather Substitutes
Another field of application for the modified starch is the production of leather substitutes.
2.13 Starch in Synthetic Polymers
In the plastics market the following fields of application are emerging: the integration of products derived from starch into the processing process (starch is only a filler, there is no direct bond between synthetic polymer and starch) or, alternatively, the integration of products derived from starch into the production of polymers (starch and polymer form a stable bond).
The use of the starch as a pure filler cannot compete with other substances such as talcum. This situation is different when the specific starch properties become effective and the property profile of the end products is thus clearly changed. One example is the use of starch products in the processing of thermoplastic materials, such as polyethylene. Thereby, starch and the synthetic polymer are combined in a ratio of 1:1 by means of coexpression to form a ‘master batch’, from which various products are produced by means of common techniques using granulated polyethylene. The integration of starch in polyethylene films may cause an increased substance permeability in hollow bodies, improved water vapor permeability, improved antistatic behavior, improved anti-block behavior as well as improved printability with aqueous dyes. Present disadvantages relate to insufficient transparency, reduced tensile strength as well as reduced extensibility.
Another possibility is the use of the starch in polyurethane foams. Due to the adaptation of starch derivatives as well as due to the optimization of processing techniques, it is possible to specifically control the reaction between synthetic polymers and the starch's hydroxy groups. The results are polyurethane films having the following property profiles due to the use of starch: a reduced coefficient of thermal expansion, decreased shrinking behavior, improved pressure/tension behavior, increased water vapor permeability without a change in water acceptance, reduced flammability and cracking density, no drop off of combustible parts, no halides and reduced aging. Disadvantages that presently still exist are reduced pressure and impact strength.
Product development of film is not the only option. Also solid plastics products, such as pots, plates and bowls can be produced by means of a starch content of more than 50%. Furthermore, the starch/polymer mixtures offer the advantage that they are much easier biodegradable.
Furthermore, due to their extreme capability to bind water, starch graft polymers have gained utmost importance. These are products having a backbone of starch and a side lattice of a synthetic monomer grafted on according to the principle of radical chain mechanism. The starch graft polymers available nowadays are characterized by an improved binding and retaining capability of up to 1000 g water per g starch at a high viscosity. The fields of application of these super absorbers have extended over the last few years and they are used mainly in the hygiene field, e.g., in products such as diapers and sheets, as well as in the agricultural sector, e.g., in seed pellets.
What is decisive for the use of the new starch modified by recombinant DNA techniques are, on the one hand, structure, water content, protein content, lipid content, fiber content, ashes/phosphate content, amylose/amylopectin ratio, distribution of the relative molar mass, degree of branching, granule size and shape as well as crystallization, and on the other hand, the properties resulting in the following features: flow and sorption behavior, pastification temperature, viscosity, thickening performance, solubility, paste structure, transparency, heat, shear and acid resistance, tendency to retrogradation, capability of gel formation, resistance to freezing/thawing, capability of complex formation, iodine binding, film formation, adhesive strength, enzyme stability, digestibility and reactivity.
The production of modified starch by genetically operating with a transgenic plant may modify the properties of the starch obtained from the plant in such a way as to render further modifications by means of chemical or physical methods superfluous. On the other hand, the starches modified by means of recombinant DNA techniques might be subjected to further chemical modification, which will result in further improvement of the quality for certain of the above-described fields of application. These chemical modifications are principally known to the person skilled in the art. These are particularly modifications by means of

    
    
        heat treatment
        acid treatment
        oxidation and
        esterification

leading to the formation of phosphate, nitrate, sulfate, xanthate, acetate and citrate starches. Other organic acids may also be used for the esterification:

        formation of starch ethers starch alkyl ether, O-allyl ether, hydroxylalkyl ether, O-carboxylmethyl ether, N-containing starch ethers, P-containing starch ethers and S-containing starch ethers.
        formation of branched starches
        formation of starch graft polymers.
    
    


In order to express the DNA molecules of the invention in sense- or antisense-orientation in plant cells, these are linked to regulatory DNA elements which ensure the transcription in plant cells. Such regulatory DNA elements are particularly promoters.
The promoter may be selected in such a way that the expression takes place constitutively or in a certain tissue, at a certain point of time of the plant development or at a point of time determined by external circumstances. With respect to the plant the promoter may be homologous or heterologous. A suitable promoter for a constitutive expression is, e.g., the 35S RNA promoter of the Cauliflower Mosaic Virus. For a tuber-specific expression in potatoes the patatin gene promoter B33 (Rocha-Sosa et al., EMBO J. 8 (1989), 23-29) or a promoter which ensures expression only in photosynthetically active tissues, e.g., the ST-LS1 promoter (Stockhaus et al., Proc. Natl. Acad. Sci. USA 84 (1987), 7943-7947; Stockhaus et al., EMBO J. 8 (1989), 2445-2451) may be used. For an endosperm-specific expression the HMG promoter from wheat, or promoters from zein genes from maize are suitable.
Furthermore, a termination sequence may exist which serves to correctly end the transcription and to add a poly-A-tail to the transcript which is believed to stabilize the transcripts. Such elements are described in the literature (cf. Gielen et al., EMBO J. 8 (1989), 23-29) and can be exchanged as desired.
According to the invention, it is basically possible to produce plants in which only the activity of one isotype of the SSS or the GBSS II is modified, and also plants in which the activities of several starch synthase forms are simultaneously modified. Thereby, all kinds of combinations and permutations are thinkable.
By modifying the activities of one or more isotypes of the starch synthases in plants, a synthesis of a starch modified in its structure is brought about.
By increasing the activity of one or more isotypes of the starch synthases in the cells of the starch-storing tissue of transformed plants such as in the potato tuber or in the endosperm of maize or wheat, increased yields may be the result.
Since the DNA sequence encoding the GBSS I from potato is already known (Visser et al., Plant Sci. 64 (1989), 185-192), DNA sequences encoding all starch synthases so far identified in potato are available. This allows for the identification of the function of the individual isotypes in the starch biosynthesis as well as for the production of genetically modified plants in which the activity of at least one of these enzymes is modified. This enables the synthesis of starch with a modified structure and therefore with modified physico-chemical properties in the plants manipulated in such a way.
The DNA molecules of the invention may be used in order to produce plants in which the activity of the starch synthases mentioned is elevated or reduced and in which at the same time the activities of other enzymes involved in the starch biosynthesis are modified. Thereby, all kinds of combinations and permutations are thinkable. For example, DNA molecules encoding the SSS proteins or GBSS II may be introduced into plant cells according to the process described above in which the synthesis of endogenous GBSS I-proteins is already inhibited due to an antisense-effect (as described in Visser et al., Mol. Gen. Genet. 225 (1991), 289-296), or in which the synthesis of the branching enzyme is inhibited (as described in WO92/14827).
If the inhibition of the synthesis of several starch synthases in transformed plants is to be achieved, DNA molecules can be used for transformation, which at the same time contain several regions in antisense-orientation controlled by a suitable promoter and encoding the corresponding starch synthases. Hereby, each sequence may be controlled by its own promoter or else the sequences may be transcribed as a fusion of a common promoter. The last alternative will generally be preferred as in this case the synthesis of the respective proteins should be inhibited to approximately the same extent.
Furthermore, it is possible to construct DNA molecules in which apart from DNA sequences encoding starch synthases other DNA sequences are present encoding other proteins involved in the starch synthesis or modification and coupled to a suitable promoter in antisense orientation. Hereby, the sequences may again be connected up in series and be transcribed by a common promoter. For the length of the individual coding regions used in such a construct the above-mentioned facts concerning the production of antisense-construct are also true. There is no upper limit for the number of antisense fragments transcribed from a promoter in such a DNA molecule. The resulting transcript, however, should not be longer than 10 kb, preferably 5 kb.
Coding regions which are located in antisense-orientation behind a suitable promoter in such DNA molecules in combination with other coding regions, may be derived from DNA sequences encoding the following proteins: granule-bound starch synthases (GBSS I and II), other soluble starch synthases (SSS I and II), branching enzymes (KoBmann et al., Mol. Gen. Genet. 230 (1991) 39-44), debranching enzymes (R enzymes), disproportionizing enzymes (Takaha et al., J. Biol. Chem. 268 (1993), 1391-1396) and starch phosphorylases. This enumeration merely serves as an example. The use of other DNA sequences within the framework of such a combination is also thinkable.
By means of such constructs it is possible to inhibit the synthesis of several enzymes at the same time within the plant cells transformed with these molecules.
In order to prepare the integration of foreign genes into higher plants a high number of cloning vectors are at disposal, containing a replication signal for E. coli and a marker gene for the selection of transformed bacterial cells. Examples for such vectors are pBR322, pUC series, M13mp series, pACYC184 etc. The desired sequence may be integrated into the vector at a suitable restriction site. The obtained plasmid is used for the transformation of E. coli cells. Transformed E. coli cells are cultivated in a suitable medium and subsequently harvested and lysed. The plasmid is recovered. As an analyzing method for the characterization of the obtained plasmid DNA use is generally made of restriction analysis, gel electrophoresis and other biochemico-molecularbiological methods. After each manipulation the plasmid DNA may be cleaved and the obtained DNA fragments may be linked to other DNA sequences. Each plasmid DNA may be cloned into the same or in other plasmids.
In order to integrate DNA into plant host cells a wide range of techniques are at disposal. These techniques comprise the transformation of plant cells with T-DNA by using Agrobacterium tumefaciens or Agrobacterium rhizogenes as transformation medium, the fusion of protoplasts, the injection and the electroporation of DNA, the integration of DNA by means of the biolistic method as well as further possibilities.
In the case of injection and electroporation of DNA into plant cells, there are no special demands made to the plasmids used. Simple plasmids such as pUC derivatives may be used. However, in case that whole plants are to be regenerated from cells transformed in such a way, a selectable marker gene should be present.
Depending on the method of integrating desired genes into the plant cell, further DNA sequences may be necessary. If the Ti- or Ri-plasmid is used, e.g., for the transformation of the plant cell, at least the right border, more frequently, however, the right and left border of the Ti- and Ri-plasmid T-DNA has to be connected to the foreign gene to be integrated as a flanking region.
If Agrobacteria are used for the transformation, the DNA which is to be integrated must be cloned into special plasmids, namely either into an intermediate vector or into a binary vector. Due to sequences homologous to the sequences within the T-DNA, the intermediate vectors may be integrated into the Ti- or Ri-plasmid of the Agrobacterium due to homologous recombination. This also contains the vir-region necessary for the transfer of the T-DNA. Intermediate vectors cannot replicate in Agrobacteria. By means of a helper plasmid the intermediate vector may be transferred to Agrobacterium tumefaciens (conjugation). Binary vectors may replicate in E. coli as well as in Agrobacteria. They contain a selectable marker gene as well as a linker or polylinker which is framed by the right and the left T-DNA border region. They may be transformed directly into the Agrobacteria (Holsters et al. Mol. Gen. Genet. 163 (1978), 181-187). The Agrobacterium acting as host cell should contain a plasmid carrying a vir-region. The vir-region is necessary for the transfer of the T-DNA into the plant cell. Additional T-DNA may be present. The Agrobacterium transformed in such a way is used for the transformation of plant cells.
The use of T-DNA for the transformation of plant cells was investigated intensely and described sufficiently in EP 120 516; Hoekema, In: The Binary Plant Vector System Offsetdrukkerij Kanters B. V., Alblasserdam (1985), Chapter V; Fraley et al., Crit. Rev. Plant. Sci., 4, 1-46 and An et al. EMBO J. 4 (1985), 277-287.
For transferring the DNA into the plant cells, plant explants may suitably be co-cultivated with Agrobacterium tumefaciens or Agrobacterium rhizogenes. From the infected plant material (e.g. pieces of leaves, stem segments, roots, but also protoplasts or suspension-cultivated plant cells) whole plants may then be regenerated in a suitable medium which may contain antibiotics or biozides for the selection of transformed cells. The plants obtained in such a way may then be examined as to whether the integrated DNA is present or not. Other possibilities in order to integrate foreign DNA by using the biolistic method or by transforming protoplasts are known to the skilled person (cf., e.g., Willmitzer, L., 1993 Transgenic plants. In: Biotechnology, A Multi-Volume Comprehensive Treatise (H. J. Rehm, G. Reed, A. Pühler, P. Stadler, editors), Vol. 2, 627-659, VCH Weinheim-New York-Basel-Cambridge).
Once the introduced DNA has been integrated in the genome of the plant cell, it usually continues to be stable there and also remains within the descendants of the originally transformed cell. It usually contains a selectable marker which confers resistance against a biozide or against an antibiotic such as kanamycin, G 418, bleomycin, hygromycin or phosphinotricine, etc. to the transformed plant cells. The individually selected marker should therefore allow for a selection of transformed cells to cells lacking the integrated DNA.
The transformed cells grow in the usual way within the plants (see also McCormick et al., 1986, Plant Cell Reports 5: 81-84).
The resulting plants can be cultivated in the usual way and cross-bred with plants having the same transformed genetic heritage or another genetic heritage. The resulting hybrid individuals have the corresponding phenotypic properties.
Two or more generations should be grown in order to ensure whether the phenotypic feature is kept stably and whether it is transferred. Furthermore, seeds should be harvested in order to ensure that the corresponding phenotype or other properties will remain.
The plasmid pBinARHyg used in this invention was deposited with Deutsche Sammlung von Mikroorganismen (DSM) [German collection of microorganisms] in Brunswick, Federal Republic of Germany, as international recognized depositary authority in accordance with the stipulations of the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure on Jan. 20, 1994 under deposit no. DSM 9505.
Abbreviations used
















bp
base pair



GBSS
granule-bound starch synthase



IPTG
isopropyl β-D-thiogalacto-pyranoside



SSS
soluble starch synthase



PMSF
phenylmethylsulfonylfluoride



VK
full-length clone










Media and solutions used in the examples:

















20 × SSC
175.3
g NaCl




88.2
g sodium citrate




ad 1000
ml with ddH2O




pH 7.0
with 10 N NaOH



Buffer A
50
mM Tris-HCl pH 8.0




2.5
mM DTT




2
mM EDTA




0.4
mM PMSF




10%
glycerol




0.1%
sodium dithionite



Buffer B
50
mM Tris-HCl pH 7.6




2.5
mM DTT




2
mM EDTA



Buffer C
0.5
M sodium citrate pH 7.6




50
mM Tris-HCl pH 7.6




2.5
mM DTT




2
mM EDTA



10 × TBS
0.2
M Tris-HCl pH 7.5




5.0
M NaCl



10 × TBST
10 ×
TBS




0.1%
(vol./vol.) Tween 20



Elution buffer
25
mM Tris pH 8.3




250
mM glycine



Dialysis buffer
50
mM Tris-HCl pH 7.0




50
mM NaCl




2
mM EDTA




14.7
mM β-mercaptoethanol




0.5
mM PMSF



Protein buffer
50
mM sodium phosphate buffer pH 7.2




10
mM EDTA




0.5
mM PMSF




14.7
mM B-mercaptoethanol













BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows plasmid pSSSA
The thin line corresponds to the sequence of pBluescript II SK(−). The thick line represents the cDNA encoding the SSS A isotype from Solanum tuberosum. The restriction sites of the insert are indicated. The cDNA insert is ligated between the EcoR I and Xho I restriction sites of the polylinker of the plasmid. The DNA sequence of the cDNA insert is indicated under Seq ID No. 1.
FIG. 2 shows plasmid pSSSB
The thin line corresponds to the sequence of pBluescript II SK(−). The thick line represents the cDNA encoding the SSS B isotype from Solanum tuberosum. The restriction sites of the insert are indicated. The cDNA insert is ligated between the EcoR I and Xho I restriction sites of the polylinker of the plasmid. The DNA sequence of the cDNA insert is indicated under Seq ID No. 2.
FIG. 3 shows plasmid p35S-anti-SSSA

    
    
        Structure of the plasmid:
        A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al., Cell 21 (1980), 285-294)
        B=fragment B: cDNA from Solanum tuberosum encoding soluble starch synthase; SSSA isotype; Xba I/Asp718 fragment from pSSSA, about 2.1 kb orientation with respect to the promoter: antisense
        C=fragment C: nt 11748-11939 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846)
    
    


FIG. 4 shows plasmid p35S-anti-SSSB

    
    
        Structure of the plasmid:
        A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al., Cell 21 (1980), 285-294)
        B=fragment B: cDNA from Solanum tuberosum encoding soluble starch synthase; SSSB isotype; Xho I/Spe I fragment from pSSSB, about 1.8 kb orientation with respect to the promoter: antisense
        C=fragment C: nt 11748-11939 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846)
    
    


FIG. 5 shows plasmid pGBSSII
The thin line corresponds to the sequence of pBluescript II SK(−). The thick line represents the cDNA encoding the GBSS II isotype from Solanum tuberosum. The restriction sites of the insert are indicated. The cDNA insert is ligated between the EcoR I and Xho I restriction sites of the polylinker of the plasmid. The DNA sequence of the cDNA insert is indicated under Seq ID No. 3.
FIG. 6 shows plasmid p35S-anti-GBSSII

    
    
        Structure of the plasmid:
        A=fragment A: CaMV 35S promoter, nt 6909-7437 (Franck et al., Cell 21 (1980), 285-294)
        B=fragment B: cDNA from Solanum tuberosum encoding granule-bound starch synthase; GBSS II isotype; Sma I/Asp 718 fragment from pGBSS II, about 1.9 kb orientation with respect to the promoter: antisense
        C=fragment C: nt 11748-11939 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846)
    
    


FIG. 7 shows a partial comparison of the amino acid sequences of prokaryotic glycogen synthases, granule-bound starch synthases and soluble starch synthases from various organisms.















a: glycogen synthase from E. coli



b: GBSS I from barley



c: GBSS I from wheat



d: GBSS I from maize



e: GBSS I from rice



f: GBSS I from cassava



g: GBSS I from potato



h: GBSS II from pea



i: GBSS II from potato



k: SSS from rice



l: SSS A from potato



m: SSS B from potato










The marked regions (I), (II) and (III) indicate three peptide sequences which are strongly conserved between the various starch synthases and glycogen synthases.
FIG. 8 shows activity gels of the soluble starch synthase isotypes from tuber extracts from wild-type and starch synthase “antisense” potato plants.

    A) GBSS II “antisense” plant, lines 14 and 35, K=wild-type plant
    B) SSS-A “antisense” plant, lines 25 and 39, K=wild-type plant
    C) SSS B “antisense” plant, lines 1 and 4, K=wild-type plant


50 μg each of the protein extracts were separated on a 7.5% native gel and the activities of the synthase isotypes were determined in the citrate-stimulated mixture with 0.1% amylopectin as primer. The synthesized glucans were dyed with Lugol's solution.



The examples serve to illustrate the invention.
In the examples, the following methods were used:
1. Cloning Methods
Vector pBluescript II SK (Stratagene) was used for cloning in E. coli. 
For plant transformation, the gene constructs were cloned into the binary vector pBinAR Hyg (DSM 9505).
2. Bacterial Strains
For the Bluescript vector and for the pBinAR Hyg constructs the E. coli strain DH5α (Bethesda Research Laboratories, Gaithersburg, USA) was used. For the in vivo excision the E. coli strain XL1-Blue was used.
The transformation of the plasmids in the potato plants was carried out using the Agrobacterium tumefaciens strain C58C1 pGV2260 (Deblaere et al., Nucl. Acids Res. 13 (1985), 4777-4788).
3. Transformation of Agrobacterium tumefaciens 
The transfer of the DNA was carried out by direct transformation according to the method by Höfgen & Willmitzer (Nucl. Acids Res. 16 (1988), 9877). The plasmid DNA of transformed Agrobacteria was isolated according to the method by Birnboim & Doly (Nucl. Acids Res. 7 (1979), 1513-1523) and was analyzed gel electrophoretically after suitable restriction digestion.
4. Transformation of Potatoes
Ten small leaves of a potato sterile culture (Solanum tuberosum L.cv. Désirée) were wounded with a scalpel and placed in 10 ml MS medium (Murashige & Skoog, Physiol. Plant. 15 (1962), 473) containing 2% sucrose which contained 50 μl of a selectively grown overnight culture of Agrobacterium tumefaciens. After gently shaking the mixture for 3-5 minutes it was further incubated in the dark for 2 days. For callus induction the leaves were placed on MS medium containing 1.6% glucose, 5 mg/l naphthyl acetic acid, 0.2 mg/l benzyl aminopurine, 250 mg/l claforan, 50 mg/l kanamycin, and 0.80% Bacto Agar. After incubation at 25° C. and 3,000 lux for one week the leaves were placed for shoot induction on MS medium containing 1.6% glucose, 1.4 mg/l zeatin ribose, 20 mg/l naphthyl acetic acid, 20 mg/l giberellic acid, 250 mg/l claforan, 50 mg/l kanamycin and 0.80% Bacto Agar.
5. Radioactive Labeling of DNA Fragments
The DNA fragments were radioactively labeled using a DNA Random Primer Labelling Kit of Boehringer (Germany) according to the manufacturer's information.
6. Determination of the Starch Synthase Activity
The starch synthase activity was determined via the determination of the incorporation of 14C glucose from ADP [14C glucose] into a product insoluble in methanol/KCl as described by Denyer and Smith (Planta 186 (1992), 609-617).
7. Detection of Soluble Starch Synthases in the Native Gel
In order to detect the activity of soluble starch synthases by non-denaturing gel electrophoresis tissue samples of potato tubers were extracted with 50 mM Tris-HCl pH 7.6, 2 mM DTT, 2.5 mM EDTA, 10% glycerol and 0.4 mM PMSF. Electrophoresis was carried out in a MiniProtean II chamber (BioRAD). The monomer concentration of the gels having 1.5 mm thickness was 7.5% (wt./vol.). 25 mM Tris-glycine pH 8.4 served as gel and running buffer. Equal amounts of protein extract were applied and separated for 2 hrs at 10 mA per gel.
The activity gels were subsequently incubated in 50 mM tricine NaOH pH 8.5, 25 mM potassium acetate, 2 mM EDTA, 2 mM DTT, 1 mM ADP glucose, 0.1% (wt./vol.) amylopectin and 0.5 M sodium citrate. The glucans formed were dyed with Lugol's solution.
8. Starch Analysis
The starch produced by the transgenic potato plants was characterized using the following methods:
a) Determination of the phosphate content

    
    
        In potato starch some glucose units may be phosphorylated at the carbon atoms at positions C3 and C6. In order to determine the phosphorylation degree at the C6 position of the glucose 100 mg starch were hydrolyzed in 1 ml 0.7 M HCl at 95° C. for 4 hours (Nielsen et al., Plant Physiol. 105 (1994), 111-117). After neutralization with 0.7 M KOH, 50 μl of the hydrolysate were subjected to a photometric-enzymatic test to determine the glucose-6-phosphate content. The alteration of the absorption of the test mixture (100 mM imidazole/HCl; 10 mM MgCl2; 0.4 mM NAD; 2 units glucose-6-phosphate dehydrogenase from Leuconostoc mesenteroides; 30° C.) was measured at 334 nm.
    
    


b) Analysis of the side chain length distribution

    
    
        For an analysis of the side chains of the starch molecules 1 ml of a 0.1% starch solution was digested with about 1 unit isoamylase overnight at 37° C. in 100 mM sodium citrate buffer, pH 4.0 (Y. C. Lee, Analytical Biochemistry 189 (1990), 151-162). The individual glucan chains were separated via a complex gradient on HPLC (column PA1; elution with 150 mM NaOH with sodium acetate gradients).
    
    


c) Determination of granule size

    
    
        The granule size was determined with a photosedimentometer of the “Lumosed” type by Retsch GmbH, Germany. For this purpose, 0.2 g starch were suspended in about 150 ml water and measured immediately. The program supplied by the manufacturer together with the photosedimentometer calculated the average diameter of the starch granules based on an average density of the starch of 1.5 g/l.
    
    


d) Pastification properties

    
    
        The pastification curves of the starch were recorded with a Viskograph E of Brabender oHG, Germany, or with a Rapid Visco Analyser, Newport Scientific Pty Ltd, Investment Support Group, Warriewood NSW 2102, Australia. When the Viskograph E was used, a suspension of 30 g starch in 450 ml water was subjected to the following heating regimen: heating up from 50° C. to 96° C. at 3°/min, maintaining constant for 30 minutes, cooling off to 30° C. at 3°/min and maintaining constant for another 30 minutes. The temperature profile yielded characteristic pastification properties.
        When the Rapid Visco Analyser was used, a suspension of 2 g starch in 25 ml water was subjected to the following heating regimen: suspending at 50° C. for 50 s, heating up from 50° C. to 95° C. at 12°/min, maintaining constant for 2.5 minutes, cooling off to 50° C. at 16.4°/min and maintaining constant for another 2 minutes. The temperature profile yielded the maximum and the final viscosity as well as the pastification temperature.
    
    


EXAMPLE 1 
Identification, Isolation and Characterization of Two cDNAs Encoding the Isotypes SSS B and GBSS II of the Starch Synthase from Solanum tuberosum 
Although SSS proteins have already been detected in a variety of plant species, inter alia in potato, and cDNA sequences have been described for SSS proteins from rice (Baba et al., supra), the purification of these proteins from potato or other plants as well as the identification of such DNA sequences has not been successful. The problem in isolating such DNA sequences resides in that the homogeneous purification of soluble starch synthases so far has not been successful due to technical reasons, although it has been attempted many times. The soluble synthases co-purify in all purification steps with the branching enzyme and other impurities. Therefore, these proteins have not been amenable to the detection of partial amino acid sequences. It is hence extremely difficult to identify cDNA sequences by hybridization to degenerate oligonucleotides derived from the amino acid sequence. For the same reasons, it is not possible to develop antibodies which specifically recognize these enzymes and thus could be used to screen expression libraries.
The prerequisite for the isolation of DNA sequences encoding SSS proteins from potato by hybridization to heterologous probes encoding the soluble starch synthases from other plant species is that there is sufficiently high homology and at the same time no significant homologies to other encoding DNA sequences. In the case of the only heterologous DNA sequence from rice available (Baba et al., supra), however, it was known that it has high homologies to the granule-bound starch synthases from rice as well as to GBSS I and therefore presumably also to GBSS II from potato. Due to these high homologies to GBSS I and II cross-hybridizations occur to GBSS I and II cDNAs when screening cDNA libraries. The identification of cDNAs which encode SSS proteins can therefore only be achieved by differential screening. This, however, requires the availability of cDNA sequences for GBSS I and II proteins from potato. cDNA sequences encoding GBSS II from potato, however, have not been available so far.
In the following, the isolation of a cDNA encoding a soluble starch synthase from potato is described.
For this purpose, a DNA fragment from a cDNA from rice encoding a soluble starch synthase (Baba et al., 1993, Plant Physiol. 103:565-573) was amplified using the polymerase chain reaction. The following oligonucleotides were used as primers:












Oligonucleotide 1:
(Seq ID No.14)












5′-ACAGGATCCTGTGCTATGCGGCGTGTGAAG-3′




 












Oligonucleotide 2:
(Seq ID No.15)












5′-TTGGGATCCGCAATGCCCACAGCATTTTTTTC-3′







The fragment resulting from PCR was 1067 bp long. This DNA fragment was later on used as heterologous probe for the identification of cDNA sequences from potato encoding soluble starch synthases.
For the preparation of a cDNA library, poly(A+) mRNA was isolated from potato tubers of the potato variety “Berolina”. Starting from the poly(A+) mRNA cDNA was prepared according to the method of Gubler and Hoffmann (1983, Gene 25:263-269) using an Xho I oligo d(t)18 primer. This cDNA was first provided with an EcoR I linker and then digested with Xho I and ligated in a specific orientation into a lambda ZAP II vector (Stratagene) which had been digested with EcoR I and Xho I.
500,000 plaques of a thus constructed cDNA library were screened for DNA sequences which are homologous to the heterologous probe of rice using said probe. Since the probe from rice used strongly cross-hybridizes to various sequences from potato, a direct identification of cDNA molecules encoding soluble starch synthases was not possible. From homology comparisons it was known that the cDNA encoding the SSS protein from rice has a high homology to the GBSS I cDNA already isolated-from potato. Since GBSS I and GBSS II exhibit high homologies in other organisms, it could be presumed that the probe from rice would also exhibit a high homology to GBSS II sequences from potato. In order to make an identification of cDNA sequences possible which encode a soluble starch synthase from potato, it was therefore necessary to have sequences available encoding GBSS I and II from potato. DNA sequences encoding GBSS I from potato had already been described, however, none encoding GBSS II from potato. Therefore, a cDNA was isolated encoding the GBSS II from potato.
For this purpose, granule-bound proteins from potato starch were isolated. The isolation was carried out by electroelution in an elution device which was constructed in analogy to the “Model 422 Electro-Eluter” (BIORAD Laboratories Inc., USA) but had a substantially greater volume (about 200 ml). 25 g dried starch were dissolved in elution buffer (final volume 80 ml). The suspension was heated in a water bath to 70-80° C. 72.07 g urea were added (final concentration 8 M) and the volume was filled up with elution buffer to give 180 ml. The starch was dissolved under constant stirring and developed a glue-like consistency. The proteins were electroeluted overnight from the solution using the elution device (100 V; 50-60 mA). The proteins eluted were carefully removed from the device. Suspended matter was removed by short centrifugation. The supernatant was dialyzed 2-3 times for one hour each at 4° C. against dialysis buffer. Then, the volume of the protein solution was determined. The proteins were precipitated by adding ammonium sulfate (90% final concentration) while constantly stirring the solution at 0° C. The proteins precipitated were sedimented by centrifugation and dissolved in protein buffer.
The proteins isolated were used to prepare polyclonal antibodies from rabbits which specifically detect granule-bound proteins. With the help of such antibodies a cDNA expression library was then screened by standard methods for sequences encoding the granule-bound proteins. The expression library was prepared as described above.
Positive phage clones were purified further using standard techniques. By way of the in vivo excision method E. coli clones were obtained from positive phage clones which contain a double-stranded pBluescript plasmid exhibiting the respective cDNA insert. After ascertaining the size and the restriction pattern of the inserts suitable clones were analyzed further. A clone cGBSSII was identified as a clone encoding the GBSSII protein.
From this clone, plasmid pGBSSII (FIG. 5) was isolated and its cDNA insert was determined by standard techniques by the didesoxy method (Sanger et al., Proc. Natl. Acad. Sci. USA 84 (1977), 5463-5467). The insert is 1925 bp long and is merely a partial cDNA sequence. The nucleotide sequence is indicated under Seq ID No. 5. Sequence comparisons showed that this DNA sequence, too, in various sites exhibited high homologies to the cDNA from rice encoding soluble starch synthase. Therefore, these sequences hybridize to the probe from rice when the cDNA library is screened.
The insert of this plasmid was later on used as probe in the screening of a cDNA library from potato to identify sequences encoding GBSS II proteins.
When screening the expression library with the polyclonal antibodies which are directed to the granule-bound proteins clones were isolated besides the clone cGBSSII that exhibited the cDNA inserts encoding GBSS I from potato. From one of these clones, cGBSSI, plasmid pGBSSI was isolated and the sequence of the cDNA insert was determined. This sequence substantially corresponded to the known DNA sequences encoding GBSSI from potato (Visser et al., Plant Sci. 64 (1989), 185-192; van der Leij et al., Mol. Gen. Genet. 228 (1990), 240-248). This cDNA insert, obtained in plasmid pGBSS I, was therefore later on used as probe when screening a cDNA library from potato tubers in order to identify sequences encoding the GBSS I proteins.
The above-described cDNA library from potato was first screened for sequences encoding GBSS I or GBSS II from potato. For this purpose, the phage plaques were transferred to nitrocellulose filters, the DNA was denatured by NaOH treatment, the filters were neutralized and the DNA was fixated on the filters by heat treatment. The filters were prehybridized for 2 hours at 42° C. in 0.25 M NaHPO4, pH 7.2, 0.25 M NaCl, 7% SDS, 1 mM EDTA, 25% formamide, 10% PEG. Then the filters were hybridized overnight at 42° C. in 0.25 M NaHPO4, pH 7.2, 0.25 M NaCl, 7% SDS, 1 mM EDTA, 25% formamide, 10% PEG after the respective radioactively labeled probe had been added. As probe on the one hand the cDNA insert from plasmid pGBSSII was used and one the other hand the cDNA insert from plasmid pGBSSI.
The filters were subsequently washed 2×30 min in 0.1×SSC, 0.5% SDS at 65° C. and exposed on X-ray films.
In a parallel procedure, filters of the same cDNA library were hybridized under the same conditions as described for GBSS I and GBSS II with the radioactively labeled cDNA probe derived from rice. The washing of the filters was carried out in this case for 2×30 min at 40° C. with 2×SSC, 0.5% SDS. Phage clones that did not hybridize to GBSS I or GBSS II from potato but to the rice cDNA were purified further using standard techniques. By way of the in vivo excision method E. coli clones were obtained from positive phage clones, which contain a double-stranded pBluescript plasmid exhibiting the respective cDNA insert. After ascertaining the size and the restriction pattern of the inserts suitable clones were subjected to a sequence analysis.
EXAMPLE 2
Sequence Analysis of the cDNA Insert of Plasmid pSSSB
Plasmid pSSSB (FIG. 2) was isolated from an E. coli clone obtained according to Example 1 and its cDNA insert was determined by standard techniques using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert is 1758 bp long and represents a partial cDNA. The nucleotide sequence is indicated under Seq ID No. 3.
The corresponding amino acid sequence is depicted under Seq ID No. 4.
EXAMPLE 3 
Isolation of the Full-Length cDNA Encoding the GBSS II Isotype of the Granule-Bound Starch Synthase from Solanum tuberosum 
A leaf-specific cDNA expression library from Solanum tuberosum L. cv. Désirée (KoBmann et al., Planta 186 (1992), 7-12) was screened for full-length clones by standard techniques using hybridization to a 5′ fragment of the cDNA insert of plasmid pGBSS II (1.9 kb). As a result, plasmid pGBSS II-VK could be isolated that contains a cDNA insert having a length of about 2.8 kb.
EXAMPLE 4
Sequence Analysis of the cDNA Insert of Plasmid pGBSS II-VK
Plasmid pGBSS II-VK was isolated from the E. coli clone obtained according to Example 3 and its cDNA insert was determined by standard techniques using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert is about 2.8 kb long. The nucleotide sequence is indicated under Seq ID No. 7 and comprises besides flanking regions the entire coding region for the GBSSII protein from potato. The molecular weight derived from the amino acid sequence of the protein is about 85.1 kD.
EXAMPLE 5 
Isolation of the Full-Length cDNA Encoding the SSS B Isotype of the Soluble Starch Synthase from Solanum tuberosum 
A leaf-specific cDNA expression library from Solanum tuberosum L. cv. Désirée (KoBmann et al., Planta 186 (1992), 7-12) was screened for full-length clones by standard techniques using hybridization to a 5′ fragment of the cDNA insert of plasmid pSSS B (1.6 kb). As a result, plasmid pSSS B-VK could be isolated that contains a cDNA insert having a length of about 2.3 kb.
EXAMPLE 6 
Sequence Analysis of the cDNA Insert of Plasmid pSSS B-VK
Plasmid pSSS B-VK was isolated from the E. coli clone obtained according to Example 5 and its cDNA insert was determined by standard techniques using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert is about 2.3 kb long. The nucleotide sequence is indicated under Seq ID No. 9 and comprises besides flanking regions the entire coding region for the B isotype of the soluble starch synthase from potato. The molecular weight derived from the amino acid sequence of the protein is about 78.6 kD.
EXAMPLE 7
Identification, Isolation and Characterization of a cDNA Encoding the SSS A Isotype of the Soluble Starch Synthase from Solanum tuberosum 
A sequence comparison between the sequences encoding soluble and granule-bound starch synthase from plants known so far (FIG. 7) showed that there are three strongly conserved regions between the various proteins (regions (I), (II) and (III) in FIG. 7).
In order for a soluble starch synthase from potato to be isolated, these three regions were selected to generate polyclonal peptide antibodies. For this purpose, three synthetic polypeptides having the following amino acid sequences were prepared:










Peptide 1:
NH2—PWSKTGGLGDVC—COOH
(Seq ID No.16)



 


Peptide 2:
NH2—PSRFEPCGLNQLY—COOH
(Seq ID No.17)


 


Peptide 3:
NH2—GTGGLRDTVENC—COOH
(Seq ID No.13)






These peptides were coupled to the KLH carrier (keyhole limpet homocyanin) and then used to prepare polyclonal antibodies in rabbits (Eurogentec, Seraing, Belgium).
The resulting antibodies were designated as follows:
anti-SS1 polyclonal antibody against peptide 1
anti-SS2 polyclonal antibody against peptide 2
anti-SS3 polyclonal antibody against peptide 3.
The antibodies were examined for their specificity with partially purified soluble starch synthase from potato.
The purification of the soluble starch synthases was carried out as follows:
2.5 kg potatoes were processed in 2 1 buffer A. After removal of the starch by centrifugation at 1000 g for 5 min the protein extract was bound to DEAE-FastFlow column material (Pharmacia LKB)(equilibrated with buffer B). After washing the column with a five-fold column volume of buffer B, bound proteins were eluted with 300 mM NaCl in buffer B. The eluted proteins were collected fractionwise and fractions having a high starch synthase activity were pooled. The pooled fractions were desalted by chromatography on a gel filtration column (G25) which was equilibrated with buffer B. 1 volume sodium citrate, 50 mM Tris-HCl pH 7.6, 2.5 mM DTT, 2 mM EDTA were added to the eluate. The protein solution was applied to an amylose resin column (AR column) equilibrated with buffer C. The column was washed with the 20-fold column volume of buffer C. Bound proteins were then-eluted with buffer B.
The fractions exhibiting high starch synthase activity were pooled and desalted by gel filtration on a G25 column.
The fractions having high starch synthase activity were applied to a MonoQ column equilibrated with buffer B. The column was washed with a five-fold column volume of buffer B. Bound proteins were eluted using a linear NaCl gradient of 0-300 mM and pooled fractionwise.
The analysis of the fractions for their starch synthase activity and for their molecular weight was carried out using various methods:

    a) analysis of the fractions on a native polyacrylamide gel
    b) analysis of the fractions on a denaturing SDS polyacrylamide gel and subsequent silver staining
    c) determination of the synthase activity by incorporation of radioactively labeled ADP glucose (Amersham, UK) in newly synthesized starch
    d) analysis of the fractions in a Western blot.


For a Western blot analysis, 50 μg, 5 μg and 0.5 μg protein of a protein crude extract were electrophoretically separated on an SDS polyacrylamide gel along with 15 μg protein of the fractions eluted from the DEAE FastFlow column, 10 μg protein of the factions eluted from the AR column and 3 μg protein of the fractions eluted from the MonoQ column. The proteins were transferred onto a nitrocellulose membrane using the semidry electroblot method.
Proteins that were recognized by the antibodies anti-SS1, anti-SS2 or anti-SS3 were identified using the “Blotting detection kit for rabbit antibodies RPN 23” (Amersham, UK) according to the manufacturer's instructions.
Three parallel Western blot analyses were performed with the above-described polyclonal antibodies anti-SS1, anti-SS2 and anti-SS3. It was found that the antibody anti-SS1 specifically recognized GBSS I and GBSS II and that the antibody anti-SS2 exhibited no specificity. Only antibody anti-SS3 specifically recognized in the Western blot new proteins, particularly proteins with molecular weights of 120-140 kD, besides GBSS I and GBSS II.
Antibody anti-SS3 was then used to screen a cDNA library from potato tubers for sequences encoding the soluble starch synthases from potato. For this purpose, a cDNA library prepared as described in Example 1 was used. For an analysis of the phage plaques they were transferred onto nitrocellulose filters which were previously incubated for 30-60 min in a 10 mM IPTG solution and then dried on filter paper. The transfer was carried out for 3 hrs at 37° C. The filters were then incubated for 30 min at room temperature in block reagent and washed twice for 5-10 min in TBST buffer. The filters were shaken for 1 hr at room temperature or for 16 hrs at 4° C. with the polyclonal antibody anti-SS3 in suitable dilution. Plaques expressing a protein that was recognized by antibody anti-SS3 were identified using the “Blotting detection kit for rabbit antibodies RPN 23” (Amersham, UK) according to the manufacturer's instructions.
Phage clones of the cDNA library expressing a protein that was recognized by antibody anti-SS3 were further purified using standard techniques. With the help of the in vivo excision method (Stratagene) E. coli clones were obtained from positive phage clones, which contain a double-stranded pBluescript II SK plasmid with the corresponding cDNA insert between the EcoRI and the Xho I restriction site of the polylinker. After ascertaining the size and the restriction pattern of the inserts a suitable clone was subjected to sequence analysis.
EXAMPLE 8
Sequence Analysis of the cDNA Insert of Plasmid pSSSA
Plasmid pSSA (FIG. 1) was isolated from an E. coli clone obtained according to Example 7 and its cDNA insert was determined by standard techniques using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert is 2303 bp long. The nucleotide sequence is indicated under Seq ID No. 1. The corresponding amino acid sequence is depicted under Seq ID No. 2.
A sequence analysis and a sequence comparison with known DNA sequences showed that the sequence depicted under Seq ID No. 1 is new and comprises a partial coding region encoding a protein having homology to starch synthases from various organisms. The protein encoded by this cDNA insert or by sequences hybridizing thereto is designated SSSA within this application.
This DNA sequence differs from the DNA sequence depicted under Seq ID NO. 2 which likewise encodes a soluble starch synthase from potato and could not be isolated from a cDNA library from potato tubers using the method described in Example 1.
EXAMPLE 9
Isolation of the Full-Length cDNA Encoding the SSS A Isotype of the Soluble Starch Synthase from Solanum tuberosum 
A leaf-specific cDNA expression library from Solanum tuberosum L. cv. Désirée (KoBmann et al., Planta 186 (1992), 7-12) was screened for full-length clones by standard techniques using hybridization to a 5′ fragment of the cDNA insert of plasmid pSSSA (2.3 kb). As a result, a clone could be isolated that contains a cDNA insert that is about 1.86 kb longer in the 5′ region. The cDNA insert had an entire length of about 4.16 kb.
EXAMPLE 10 
Sequence Analysis of the cDNA Insert of Plasmid pSSSA-VK
Plasmid pSSSA-VK was isolated from an E. coli clone obtained according to Example 9 and its cDNA insert was determined by standard techniques using the didesoxynucleotide method (Sanger et al., Proc. Natl. Acad. Sci. USA 74 (1977), 5463-5467). The insert is about 4.16 kb long. The nucleotide sequence is indicated under Seq ID No. 11. The corresponding amino acid sequence is depicted under Seq ID No. 12. The molecular weight derived from the amino acid sequence of the SSSA protein is about 135 kD.
EXAMPLE 11 
Construction of Plasmid p35S-anti-SSSA and Introduction of the Plasmid into the Genome of Potato Plants
From plasmid pSSSA a DNA fragment of about 2.1 kb was isolated using the restriction endonucleases Xba I and Asp 718 which comprises the coding region for the A isotype of the soluble starch synthase from potato, and was ligated into vector pBinAR Hyg (DSM 9505) which was digested with Xba I and Asp 718.
The insertion of the cDNA fragment results in an expression cassette which is composed of fragments A, B and C as follows (FIG. 3):
Fragment A (529 bp) contains the 35S promoter of the Cauliflower mosaic virus (CaMV). The fragment comprises nucleotides 6909 to 7437 of the CaMV (Franck et al., Cell 21 (1980), 285-294).
Fragment B contains besides flanking regions the protein-encoding region of the A isotype of the soluble starch synthase from Solanum tuberosum. This region was isolated as Xba I/Asp 718 fragment from pSSSA as described above and was fused to the 35S promoter in pBinAR Hyg in antisense orientation.
Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).
The size of plasmid p35S-anti-SSSA is about 13 kb.
The plasmid was transferred to potato plants using Agrobacteria-mediated transformation as described above. Whole plants were regenerated from the transformed cells.
As a result of transformation the transgenic potato plants exhibited a reduced activity of A isotype of the soluble starch synthase (cf. FIG. 8).
The starch reduced by these plants differs from the starch synthesized by wild-type plants in its phosphate content, in the viscosity of aqueous solutions, its pastification properties and the mean granule size. The results are depicted in Table I.
The phosphate content of the starch produced in transgenic plants is at least 30%, preferably 50%, particularly 70% higher than that of the starch synthesized by the wild-type plants.
The final viscosity of the starch from SSS A “antisense” plants exhibits values that are at least 10%, preferably 20%, particularly 30% lower than those of the starch synthesized by wild-type plants.
The pastification temperature, the maximum viscosity and the mean granule size of the modified starch is clearly lower than that of the starch produced in wild-type plants (see Table I).







TABLE I







Characteristics of the starch from wild-type and SSS A


“antisense” potato plants











Wild-type
Line 25
Line 39














Phosphate content [nmol
8.50 ± 0.4
14.61 ± 0.3
14.54 ± 0.2


mg−1 starch−1]


Pastification temperature
69.5
67.4
66.2


[° C.]


Maximum viscosity [cP]
4044
3720
3756


Final viscosity at 50° C.
3312
2904
2400


[cP]


Mean granule size [μm]
29
24
27









EXAMPLE 12 
Construction of Plasmid p35S-anti-SSSB and Introduction of the Plasmid Into the Genome of Potato Plants
From plasmid pSSSB a DNA fragment of about 1.8 kb was isolated using the restriction endonucleases Xho I and Spe I which comprises the coding region for the B isotype of the soluble starch synthase from potato, and was ligated into vector pBinAR Hyg (DSM 9505) which was digested with Sma I.
The insertion of the cDNA fragment results in an expression cassette which is composed of fragments A, B and C as follows (FIG. 4):
Fragment A (529 bp), contains the 35S promoter of the Cauliflower mosaic virus (CaMV). The fragment comprises nucleotides 6909 to 7437 of-the CaMV (Franck et al., Cell 21 (1980), 285-294).
Fragment B contains besides flanking regions the protein-encoding region of the B isotype of the soluble starch synthase from Solanum tuberosum. This region was isolated as Xho I/Spe I fragment from pSSSB as described above and was fused to the 35S promoter in pBinAR Hyg in antisense orientation.
Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).
The size of plasmid p35S-anti-SSSB is about 13 kb.
The plasmid was transferred to potato plants using Agrobacteria-mediated transformation as described above. Whole plants were regenerated from the transformed cells.
As a result of transformation the transgenic potato plants exhibited a reduced activity of B isotype of the soluble starch synthase (cf. FIG. 8).
EXAMPLE 13
Construction of Plasmid p35S-anti-GBSS I and Introduction of the Plasmid into the Genome of Potato Plants
From plasmid pGBSS II a DNA fragment of about 1.9 kb was isolated using the restriction endonucleases Asp 718 and Sma I which comprises the coding region for the GBSS II isotype of the soluble starch synthase from potato. The ends of the fragment were filled in with the T4 polymerase and the fragment was ligated into vector pBinAR Hyg (DSM 9505) which was digested with Sma I.
The insertion of the cDNA fragment results in an expression cassette which is composed of fragments A, B and C as follows (FIG. 6):
Fragment A (529 bp) contains the 35S promoter of the Cauliflower mosaic virus (CaMV). The fragment comprises nucleotides 6909 to 7437 of the CaMV (Franck et al., Cell 21 (1980), 285-294).
Fragment B contains besides flanking regions part of the protein-encoding region of the GBSS II isotype of the starch synthase from Solanum tuberosum. This region was isolated as Asp 718/Sma I fragment from pGBSS II as described above and was fused to the 35S promoter in pBinAR Hyg in antisense orientation once the ends of the fragment had been filled in.
Fragment C (192 bp) contains the polyadenylation signal of gene 3 of the T-DNA of the Ti plasmid pTiACH5 (Gielen et al., EMBO J. 3 (1984), 835-846).
The size of plasmid p35S-anti-GBSS II is about 13 kb.
The plasmid was transferred to potato plants using Agrobacteria-mediated transformation as described above. Whole plants were regenerated from the transformed cells.
As a result of transformation the transgenic potato plants exhibited a reduced activity of GBSS II isotype of the starch synthase (cf. FIG. 8).
The starch produced by these plants differs from the starch synthesized by wild-type plants in its phosphate content, in the viscosity, its pastification properties and the mean granule size. The results are depicted in Table II.







TABLE II







Characteristics of the starch from wild-type and GBSS II


“antisense” potato plants












Wild-type
Line 14
Line 35
Line 44















Phosphate
6.99 ± 0.19
4.52 ± 0.2
4.13 ± 0.06
3.76 ± 0.12


content [nmol


mg−1 starch−1]


Pastification
64.1
62.55
63.25
63.55


temperature


[° C.]


Maximum
4057
2831
2453
2587


viscosity [cP]


Final
2849
2816
2597
2587


viscosity at


50° C. [cP]


Mean granule
37
32
31
32


size [μm]









The phosphate content of the starch produced in transgenic plants is at least 35%, preferably 40%, particularly 45% lower than that of the starch synthesized by the wild-type plants.
The maximum viscosity of the starch from GBSS II “antisense” plants exhibits values that are at least 30%, preferably 35%, particularly 40% lower than those of the starch synthesized by wild-type plants.
The pastification temperature and the final viscosity of the modified starch is below that of the starch produced in wild-type plants. The mean granule size of the starch produced in transgenic plants is clearly smaller than that of wild-type starch.
EXAMPLE 14 
Overexpression of the Soluble Starch Synthases SSS A and SSS B in E. coli 
For an overexpression of soluble starch synthases in E. coli strain G6MD2 was cultivated, which is a mutant which exhibits a deletion both in the glg and in the mal operon. Hence it possesses neither the glycogen synthase (glgA), the branching enzyme (glgB) and the AGPase (glgC) nor the amylomaltase (malQ), the maltodextrine phosphorylase (malp) nor the other proteins involved in the metabolization of maltose. Therefore, mutant G6MD2 is not capable of synthesizing glycogen via the ADP glucose pathway nor α-1,4 glucans starting from maltose.
Cells of this mutant were transformed with the cDNA clones pSSSA-VK and pSSSB-VK. The E. coli cells expressing starch synthases were broken up after 2 hrs induction with IPTG in 50 mM Tris-HCl pH 7.6, 10% glycerol, 2 mM EDTA, 2 mM DTT and 0.4 mM PMSF by ultrasonification. As a control, cells transformed with pBluescript were used. Intact cells and cell wall material were removed by centrifugation for 10 min at 13,000 g. Then, the protein concentration of the supernatant was determined. 100 μg protein extract were added to the reaction buffer (final concentration: 50 mM tricine NaOH pH 8.5, 25 mM potassium acetate, 2 mM EDTA and 2 mM DTT, 1 mM ADP glucose). For an examination of the citrate-stimulated reaction (primer-independent) the reaction buffer additionally contained 0.5 M sodium citrate, while the primer-dependent reaction was performed in the presence of 0.02% (wt./vol.) maltooligosaccharides (Glucidex 19; 1-30 glucose units). The reaction was carried out overnight at room temperature. The synthesized glucans were detected via Lugol's solution and examined spectralphotometrically for further characterization.
Both the SSS A isotype and the SSS B isotype synthesized glucans in the primer-dependent reaction (absence of citrate). The absorption maximum of the glucan synthesized by SSS A was at 614 nm which corresponds to a glucan of about 150 glucose units. The glucan produced by SSS A absorbed at 575 nm, which points to the synthesis of short-chain glucans having a polymerization degree of about 50 glucose units.
In the primer-independent, i.e., citrate-stimulated, reaction SSS B isotype alone yielded a glucan which absorbed at 612 nm after dyeing with Lugol's solution. The SSS A isotype showed no activity in the primer-independent reaction and therefore did not synthesize any glucan.
The protein extracts from the cells transformed with pBluescript did not yield any products in any of the reactions.

## Claims

1. Starch obtainable from a transgenic plant cell or from a plant comprising said plant cell, said plant cell comprising an isolated DNA molecule comprising a DNA sequence or a part thereof, wherein expression of said DNA molecule or part thereof reduces the activity of a protein having starch synthase activity, and wherein the DNA sequence is:
(a) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 8;
(b) the coding region of SEQ ID NO: 7;
(c) a nucleotide sequence that has more than 90% sequence identity to the nucleotide sequence of (a) or (b);
(d) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 10;
(e) the coding region of SEQ ID NO: 9;
(f) a nucleotide sequence that has more than 90% sequence identity to the nucleotide sequence of (d) or (e);
(g) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 12;
(h) the coding region of SEQ ID NO: 11; or
(i) a nucleotide sequence that has more than 90% sequence identity to the nucleotide sequence of (g) or (h).

2. The starch according to claim 1, wherein said plant cell is from a starch-storing plant.

3. The starch according to claim 2, wherein said starch-storing plant is a potato plant.

4. The starch according to claim 1, wherein the DNA sequence encodes a protein with the biological activity of a soluble starch synthase of isotype A (SSSA), wherein the protein encoded by said DNA sequence is recognized by an antibody that is directed against the peptide
NH2-GTGGLRDTVENC-COOH (SEQ ID NO: 13).

5. The starch according to claim 1, wherein the DNA sequence has more than 95% sequence identity to:
(a) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 8;
(b) the coding region of SEQ ID NO: 7;
(c) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 10;
(d) the coding region of SEQ ID NO: 9;
(e) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 12; or
(f) the coding region of SEQ ID NO: 11.

6. The starch according to claim 5, wherein the DNA sequence is:
(a) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 8;
(b) the coding region of SEQ ID NO: 7;
(c) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 10;
(d) the coding region of SEQ ID NO: 9;
(e) a nucleotide sequence encoding a protein comprising the amino acid sequence of SEQ ID NO: 12; or
(f) the coding region of SEQ ID NO: 11.

7. The starch according to claim 1, wherein the DNA molecule is expressed in antisense orientation with respect to an operably linked plant promoter.

8. The starch according to claim 1, wherein the DNA sequence encodes a protein with the biological activity of a soluble starch synthase of isotype A (SSSA), wherein the starch comprises:
(a) a phosphate content that is at least 30% higher than that of starch synthesized by a non-transformed plant;
(b) a final viscosity of the starch is at least 10% lower than that of starch synthesized by a non-transformed plant; or
(c) a mean granule size that is reduced compared to that of starch synthesized by a non-transformed plant.

9. The starch according to claim 8, wherein the starch comprises:
(a) a phosphate content that is at least 30% higher than that of starch synthesized by a non-transformed plant;
(b) a final viscosity of the starch is at least 10% lower than that of starch synthesized by a non-transformed plant; and
(c) a mean granule size that is reduced compared to that of starch synthesized by a non-transformed plant.

10. The starch according to claim 8, wherein the starch has a phosphate content that is at least 35% higher than that of starch synthesized by a non-transformed plant.

11. The starch according to claim 10, wherein the starch has a phosphate content that is at least 50% higher than that of starch synthesized by a non-transformed plant.

12. The starch according to claim 11, wherein the starch has a phosphate content that is at least 70% higher than that of starch synthesized by a non-transformed plant.

13. The starch according to claim 1, wherein the DNA sequence encodes a protein with the biological activity of a granule-bound starch synthase of isotype II (GBSS II), wherein the starch comprises:
(a) a phosphate content that is at least 35% lower than that of starch synthesized by a non-transformed plant;
(b) a maximum viscosity of the starch is at least 30% lower than that of starch synthesized by a non-transformed plant;
(c) a pastification temperature that is reduced compared to that of starch synthesized by a non-transformed plant;
(d) a final viscosity that is reduced compared to that of starch synthesized by a non-transformed plant; or
(e) a mean granule size that is reduced compared to that of starch synthesized by a non-transformed plant.

14. The starch according to claim 13, wherein the starch comprises:
(a) a phosphate content that is at least 35% lower than that of starch synthesized by a non-transformed plant;
(b) a maximum viscosity of the starch is at least 30% lower than that of starch synthesized by a non-transformed plant;
(c) a pastification temperature that is reduced compared to that of starch synthesized by a non-transformed plant;
(d) a final viscosity that is reduced compared to that of starch synthesized by a non-transformed plant; and
(e) a mean granule size that is reduced compared to that of starch synthesized by a non-transformed plant.

15. The starch according to claim 13, wherein the starch comprises a phosphate content that is at least 40% lower than that of starch synthesized by a non-transformed plant.

16. The starch according to claim 15, wherein the starch comprises a phosphate content that is at least 45% lower than that of starch synthesized by a non-transformed plant.