ISOMALTULOSE SYNTHASE

Abstract: Isomaltulose synthase (sucrose, EC 5.4.99.11) converts surcrose to isomaltulose (6-<i>O</i>- alpha -D-glucopyranosyl-D-fructofuranose). The enzyme has been isolated from <i>Erwinia rhapontici</i> and an additional bacterial isolate 68J. Also disclosed are methods for converting sucrose to isomaltulose, transformation of plants and bacteria such that they convert surcrose, methods of detection of isomaltulose synthase from samples and methods of identifying bacteria with isomaltulose synthase from environmental samples.

Description: NOVEL POLYPEPTIDES AND POLYNUCLEOTIDES AND USES THEREFOR FIELD OF THE INVENTION THIS INVENTION relates generally to enzymes that convert sucrose to isomaltulose. More particularly, the present invention relates to novel sucrose isomerases, to polynucleotides encoding these sucrose isomerases, to methods for isolating such polynucleotides and to nucleic acid constructs that express these polynucleotides. The invention also relates to cells, particularly transformed bacterial or plant cells, and to differentiated plants comprising cells, which contain these nucleic acid constructs. The invention further relates to the use of the polypeptides, polynucleotides, cells and plants of the invention for producing isomaltulose. BACKGROUND OF THE INVENTION The acariogenic sugar substitute, isomaltulose (palatinose), is a hetero- disaccharide composed of glucose and fructose linked together through an α-l,6-glucosidic linkage. Isomaltulose can be produced on a large scale by enzymatic rearrangement of sucrose using the bacterial enzyme sucrose isomerase. Initially, large-scale production of isomaltulose was facilitated using immobilised bacterial cells that naturally produce sucrose isomerase enzymes (eg. species of Protaminobacter rubrum, Erwinia rhapontici and Serratia plymuthica). Higher yields of isomaltulose have been achieved recently using recombinant techniques. In this respect, Mattes et al. (U.S. Patent Serial No. 5,786,140) disclose isolated polynucleotides encoding partial or full-length sucrose isomerase enzymes from Protaminobacter rubrum (CBS 547,77), Erwinia rhapontici (NCPPB 1578), the microorganism SZ 62 (Enterobacter species) and the microorganism MX-45 (FERM 11808 or FERM BP 3619). Mattes et al. also disclose conserved amino acid sequences from which degenerate oligonucleotides could be designed for cloning sucrose isomerase-encoding polynucleotides by the polymerase chain reaction (PCR). SUMMARY OF THE INVENTION In work leading up to the present invention, degenerate oligonucleotides, based on the conserved amino acid sequences disclosed by Mattes et al, were used to amplify sucrose isomerase-encoding polynucleotides by PCR from Erwinia rhapontici (Accession Number WAC2928), and from 30 independent sucrose-isomerase negative bacterial isolates. The PCR amplification yielded multiple DNA products from most tested bacteria. However, these products were found not to encode sucrose isomerase. Nucleic acid sequence analysis of 12 separate PCR products, including 6 products amplified from Erwinia rhapontici, revealed that none of the DNA products displayed significant sequence homology to sucrose isomerase genes. Instead, most of these products showed high sequence homology to known glucosidase genes. It was therefore concluded that the conserved sequences of Mattes et al. were not specific to sucrose isomerases, but were common to other classes of enzymes including glucosidases. Notwithstanding the above, the present inventors developed a novel functional screening assay for the isolation and characterisation of novel polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes. Several such novel polynucleotides were cloned using this assay and some of these were found to encode polypeptides with superior sucrose isomerase activity relative to those disclosed by Mattes et al. Comparison of the deduced polypeptide sequences with known sucrose isomerase or glucosidase polypeptide sequences revealed a number of conserved motifs, which are unique to sucrose isomerases, and which could therefore be used inter alia for designing sucrose isomerase- specific oligonucleotides. Such oligonucleotides are advantageous in that they provide for the first time facile isolation of sucrose isomerase-encoding polynucleotides using nucleic acid amplification techniques. The inventors have reduced the above discoveries to practice in new isolated molecules, recombinant cells and plants for producing isomaltulose as described hereinafter. Accordingly, in one aspect of the invention, there is provided a method for isolating novel polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes, said method comprising: (a) obtaining an environmental sample from a location in which organisms, capable of converting sucrose to isomaltulose, have a selective advantage; (b) screening for organisms producing isomaltulose from sucrose; and (c) isolating polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes from isomaltulose-producing organisms. Preferably, the method further comprises selecting or otherwise enriching for dual sucrose- and isomaltulose-metabolising organisms which are capable of using both sucrose and isomaltulose as carbon sources for growth. Suitably, the screening utilises an assay that quantifies isomaltulose production by an organism. In another aspect of the invention, there is provided an isolated polypeptide, or a biologically active fragment thereof, or a variant or derivative of these, said polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof. Suitably, the variant has at least 75%, preferably at least 80%, more preferably at least 85%, more preferably at least 90% and still more preferably at least 95% sequence identity to any one of the amino acid sequences set forth in SEQ ID NO: 2, 4, 8, 10, and 26. Suitably, the biologically active fragment is at least 6 amino acids in length. Preferably, the variant comprises the consensus sequence set forth in any one or more of SEQ ID NO: 19, 20, 21, 22, 23 and 24 or variant thereof. Suitably, said consensus sequence variant has at least 80%, preferably at least 85%, more preferably at least 90%, and still more preferably at least 95% sequence identity to any one of the amino acid sequences set forth in SEQ ID NO: 19, 20, 21, 22, 23 and 24. In another aspect, the invention provides an isolated polynucleotide encoding a polypeptide, fragment, variant or derivative as broadly described above. Preferably, the polynucleotide comprises the sequence set forth in any one of SEQ ID NO: 1, 3, 7 and 9, or a biologically active fragment thereof, or a polynucleotide variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 1, or 3 comprises a contiguous sequence of nucleotides contained within SEQ ID NO: 25 or polynucleotide variant thereof. In one embodiment, the polynucleotide variant has at least 60%, preferably at least 70%, more preferably at least 80%, and still more preferably at least 90% sequence identity to any one of the polynucleotides set forth in SEQ ID NO: 1, 3, 7 and 9. In another embodiment, the polynucleotide variant is capable of hybridising to any one of the polynucleotides identified by SEQ ID NO: 1, 3, 7 and 9 under at least low stringency conditions, preferabl under at least medium stringency conditions, and more preferably under high stringency conditions. Suitably, the biologically active fragment is at least 18 nucleotides in length. Preferably, the polynucleotide variant comprises a nucleotide sequence encoding a consensus sequence set forth in any one or more of SEQ ID NO: 19, 20, 21, 22, 23 and 24 or variant thereof. Suitably, the consensus sequence is encoded by a nucleotide sequence set forth in any one of SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 or nucleotide sequence variant thereof. In one embodiment, the nucleotide sequence variant has at least 60%, preferably at least 70%, more preferably at least 80%, and still more preferably at least 90% sequence identity to any one of the sequences set forth in SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36. In another embodiment, the nucleotide sequence variant is capable of hybridising to any one of the sequences identified by SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 under at least low stringency conditions, preferably under at least medium stringency conditions, and more preferably under high stringency conditions. In another aspect, the invention features an expression vector comprising a polynucleotide as broadly described above wherein the polynucleotide is operably linked to a regulatory polynucleotide. In a further aspect, the invention provides a host cell containing a said expression vector. Suitably, the host cell is a bacterium or other prokaryote, or a plant cell or other eukaryote. Preferably, the plant is sugarcane (Saccharum sp.) or another species capable of synthesising and/or accumulating sucrose (e.g. sugar beet). The invention also features a method of producing a recombinant polypeptide, fragment, variant or derivative as broadly described above, comprising: - culturing a host cell containing an expression vector as broadly described above such that said recombinant polypeptide, fragment, variant or derivative is expressed from said polynucleotide; and - isolating the said recombinant polypeptide, fragment, variant or derivative. In another aspect, the invention provides a method of producing a biologically active fragment of a polypeptide as broadly described above, comprising: - detecting sucrose isomerase activity associated with a fragment of a polypeptide according to any one of SEQ ID NO: 2, 4, 8 and 10, which indicates that said fragment is a said biologically active fragment. In a further aspect, the invention provides a method of producing a biologically active fragment as broadly described above, comprising: - introducing a polynucleotide from which a fragment of a polypeptide according to any one of SEQ ID NO: 2, 4, 8 and 10 can be produced into a cell; and - detecting sucrose isomerase activity, which indicates that said fragment is a said biologically active fragment. In yet a further aspect, the invention provides a method of producing a polypeptide variant of a parent polypeptide comprising the sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof, comprising: - producing a modified polypeptide whose sequence is distinguished from the parent polypeptide by substitution, deletion or addition of at least one amino acid; and - detecting sucrose isomerase activity associated with the modified polypeptide, which indicates that said modified polypeptide is a said polypeptide variant. In a further aspect, the invention contemplates a method of producing a polypeptide variant of a parent polypeptide comprising the sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof, comprising: - producing a polynucleotide from which a modified polypeptide as described above can be produced; - introducing said polynucleotide into a cell; and - detecting sucrose isomerase activity, which is indicative of the modified polypeptide being a said polypeptide variant. According to another aspect of the invention, there is provided a method for producing isomaltulose from sucrose, said method comprising contacting sucrose or a sucrose-containing substrate with the polypeptide, fragment, variant or derivative as broadly described above, or with a host cell as broadly described above, for a time and under conditions sufficient to produce isomaltulose. In another aspect, the invention resides in an antigen-binding molecule that is immuno-interactive with said polypeptide, fragment, variant or derivative according to the present invention. Preferably, said antigen-binding molecule is immuno-interactive with any one of the amino acid sequences set forth in SEQ ID NO: 19, 20, 21, 22, 23 and 24 or variant thereof. Another aspect of the invention provides a method for detecting a specific polypeptide or polynucleotide, comprising detecting the sequence of: (a) SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof at least 6 amino acids in length, or variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof; or (b) a polynucleotide encoding (a). In a preferred embodiment, the sequence of (b) is selected from SEQ ID NO: 1, 3, 7 and 9, or a biologically active fragment thereof at least 18 nucleotides in length, or a polynucleotide variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 1, or 3 comprises a contiguous sequence of nucleotides contained within SEQ ID NO: 25 or polynucleotide variant thereof. According to another aspect of the invention, there is provided a method of detecting a sucrose isomerase in a sample, comprising: - contacting the sample with an antigen-binding molecule as broadly described above; and - detecting the presence of a complex comprising the said antigen-binding molecule and the said polypeptide, fragment, variant or derivative in said contacted sample. In yet another aspect, there is provided a method for detecting a polypeptide, fragment, variant or derivative as broadly described above, comprising: - detecting expression in a cell of a polynucleotide encoding said polypeptide, fragment, variant or derivative as broadly described above. Still a further aspect of the invention provides a probe comprising a nucleotide sequence which is capable of hybridising to at least a portion of a nucleotide sequence encoding SEQ D NO: 2, 4, 8 and 10 under at least low stringency conditions. In a preferred embodiment, the probe comprises a nucleotide sequence which is capable of hybridising to at least a portion of SEQ ID NO: 1, 3, 7 and 9 under at least low stringency conditions. According to another aspect of the invention, there is provided a transformed plant cell containing an expression vector as broadly described above. In a preferred embodiment, the plant is sugarcane (Saccharum sp.). In a still further aspect, the invention provides a differentiated plant comprising plant cells containing an expression vector as broadly described above. In yet another aspect, the invention provides isomaltulose harvested from a differentiated plant as broadly described above. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1. Conversion of sucrose to isomaltulose in isolated bacteria. Peaks: 1 - sucrose, 2 - isomaltulose, 3 - fructose, 4 - glucose. "Dotted" electrophoretogram is sucrose and isomaltulose standards. Figure 2. Conversion of sucrose to isomaltulose in E.coli expressing sucrose isomerase genes cloned in SuperCos™ vector. Peaks: 1 - sucrose, 2 - isomaltulose, 3 - fructose, 4 - glucose. "Dotted" electrophoretogram is sucrose and isomaltulose standards. Figure 3. Nucleotide sequence of sucrose isomerase cloned from Erwinia rhapontici. Figure 4. Nucleotide sequence of sucrose isomerase cloned from 68J. Figure 5. Predicted amino acid sequence of sucrose isomerase cloned from Erwinia rhapontici. Figure 6. Predicted amino acid sequence of sucrose isomerase cloned from 68J. Figure 7. Efficiency of conversion from sucrose to isomaltulose by E. coli expressing cloned sucrose isomerase genes. Results are means ± standard errors derived from 3 replications. Figure 8. Conversion of sucrose to isomaltulose in stably transformed sugarcane calli expressing cloned sucrose isomerase genes. Peaks: 1 - sucrose, 2 - isomaltulose, 3 - fructose, 4 - glucose. Traces: a - pUbi Er + 2.5mM isomaltulose, b - pUbi Er, c - pUbi 14S, d - 2.5mM sucrose and isomaltulose standards, e - pUbi 68J, f - pUbi 68J+ 2.5mM isomaltulose BRIEF DESCRIPTION OF THE SEQUENCES: SUMMARY TABLE TABLE A Sequence ID Sequence Length Number SEQ UD NO: 1 Full-length sucrose isomerase coding sequence from 1899 bases Ei-winia rhapontici (Accession No. WAC2928) SEQ ID NO: 2 Full-length sucrose isomerase polypeptide sequence 632 residues from Erwinia rhapontici (Accession No. WAC2928) SEQ ID NO: 3 Polynucleotide sequence encoding mature sucrose 1791 bases isomerase from Erwinia rhapontici (Accession No. WAC2928) SEQ ID NO: 4 Mature sucrose isomerase polypeptide sequence from 596 residues Erwinia rhapontici (Accession No. WAC2928) SEQ ID NO: 5 Signal peptide coding sequence relating to sucrose 108 bases isomerase from Erwinia rhapontici (Accession No. WAC2928) SEQ ID NO: 6 Signal peptide relating to sucrose isomerase from 36 residues Erwinia rhapontici (Accession No. WAC2928) SEQ ID NO: 7 Full-length sucrose isomerase coding sequence from 1797 bases bacterial isolate 68J SEQ ID NO: 8 Full-length sucrose isomerase polypeptide sequence 598 residues from bacterial isolate 68 SEQ ID NO: 9 Polynucleotide sequence encoding mature sucrose 1698 bases isomerase from bacterial isolate 68J SEQ ID NO: 10 Mature sucrose isomerase polypeptide sequence from 565 residues bacterial isolate 681 SEQ ID NO: 11 Signal peptide coding sequence relating to sucrose 99 bases isomerase from bacterial isolate 681 SEQ ID NO: 12 Signal peptide relating to sucrose isomerase from 33 residues bacterial isolate 68J DETAILED DESCRIPTION OF THE INVENTION 1. Definitions Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. For the purposes of the present invention, the following terms are defined below. The articles "a" and "an" are used herein to refer to one or to more than one (i.e. to at least one) of the grammatical object of the article. By way of example, "an element" means one element or more than one element. The term "about" is used herein to refer to sequences that vary by as much as 30%, preferably by as much as 20% and more preferably by as much as 10% to the length of a reference quantity, level, value, dimension, length, position, size, or amount. "Amplification product" refers to a nucleic acid product generated by nucleic acid amplification techniques. By "antigen-binding molecule" is meant a molecule that has binding affinity for a target antigen. It will be understood that this term extends to immunoglobulins, immunoglobulin fragments and non-immunoglobulin derived protein frameworks that exhibit antigen-binding activity. As used herein, the term "binds specifically" and the like refers to antigen- binding molecules that bind the polypeptide or polypeptide fragments of the invention but do not significantly bind to homologous prior art polypeptides. By "biologically active fragment" is meant a fragment of a full-length parent polypeptide which fragment retains the activity of the parent polypeptide. A biologically active fragment will therefore comprise sucrose isomerase activity, which converts sucrose to isomaltulose. As used herein, the term "biologically active fragment" includes deletion mutants and small peptides, for example of at least 8, preferably at least 10, more preferably at least 20, and still more preferably at least 30 contiguous amino acids, which comprise the above activities. Peptides of this type may be obtained through the application of standard recombinant nucleic acid techniques or synthesised using conventional liquid or solid phase synthesis techniques. For example, reference may be made to solution synthesis or solid phase synthesis as described, for example, in Chapter 9 entitled "Peptide Synthesis" by Atherton and Shephard which is included in a publication entitled "Synthetic Vaccines" edited by Nicholson and published by Blackwell Scientific Publications. Alternatively, peptides can be produced by digestion of a polypeptide of the invention with proteinases such as endoLys-C, endoArg-C, endoGlu-C and staphylococcus V8-protease. The digested fragments can be purified by, for example, high performance liquid chromatographic (HPLC) techniques. Throughout this specification, unless the context requires otherwise, the words "comprise", "comprises" and "comprising" will be understood to imply the inclusion of a stated step or element or group of steps or elements but not the exclusion of any other step or element or group of steps or elements. By "corresponds to" or "corresponding to" is meant a polynucleotide (a) having a nucleotide sequence that is substantially identical or complementary to all or a portion of a reference polynucleotide sequence or (b) encoding an amino acid sequence identical to an amino acid sequence in a peptide or protein. This phrase also includes within its scope a peptide or polypeptide having an amino acid sequence that is substantially identical to a sequence of amino acids in a reference peptide or protein. By "derivative" is meant a polypeptide that has been derived from the basic sequence by modification, for example by conjugation or complexing with other chemical moieties or by post-translational modification techniques as would be understood in the art. The term "derivative'' also includes within its scope alterations that have been made to a parent sequence including additions, or deletions that provide for functionally equivalent molecules. Accordingly, the term derivative encompasses molecules that will have sucrose isomerase activity. "Homology" refers to the percentage number of amino acids that are identical or constitute conservative substitutions as defined in Table B infra. Homology may be determined using sequence comparison programs such as GAP (Deveraux et al. 1984, Nucleic Acids Research 12, 387-395). In this way, sequences of a similar or substantially different length to those cited herein might be compared by insertion of gaps into the alignment, such gaps being determined, for example, by the comparison algorithm used by GAP. "Hybridisation" is used herein to denote the pairing of complementary nucleotide sequences to produce a DNA-DNA hybrid or a DNA-RNA hybrid. Complementary base sequences are those sequences that are related by the base-pairing rules. In DNA, A pairs with T and C pairs with G. In RNA U pairs with A and C pairs with G. In this regard, the terms "match" and "mismatch" as used herein refer to the hybridisation potential of paired nucleotides in complementary nucleic acid strands. Matched nucleotides hybridise efficiently, such as the classical A-T and G-C base pair mentioned above. Mismatches are other combinations of nucleotides that do not hybridise efficiently. Reference herein to "immuno-interactive" includes reference to any interaction, reaction, or other form of association between molecules and in particular where one of the molecules is, or mimics, a component of the immune system. By "immuno-interactive fragment" is meant a fragment of the polypeptide set forth in any one of SEQ ID NO: 2, 4, 8 and 10, which fragment elicits an immune response, including the production of elements that specifically bind to said polypeptide, or variant or derivative thereof. As used herein, the term "immuno-interactive fragment" includes deletion mutants and small peptides, for example of at least six, preferably at least 8 and more preferably at least 20 contiguous amino acids, which comprise antigenic determinants or epitopes. Several such fragments may be joined together. By "isolated" is meant material that is substantially or essentially free from components that normally accompany it in its native state. For example, an "isolated polynucleotide", as used herein, refers to a polynucleotide, which has been purified from the sequences which flank it in a naturally occurring state, e.g., a DNA fragment which has been removed from the sequences which are normally adjacent to the fragment. By "marker gene" is meant a gene that imparts a distinct phenotype to cells expressing the marker gene and thus allows such transformed cells to be distinguished from cells that do not have the marker. A selectable marker gene confers a trait for which one can 'select' based on resistance to a selective agent (e.g., a herbicide, antibiotic, radiation, heat, or other treatment damaging to untransformed cells). A screenable marker gene (or reporter gene) confers a trait that one can identify through observation or testing, i.e., by 'screening' (e.g. β-glucuronidase, luciferase, or other enzyme activity not present in untransformed cells). By "obtained from" is meant that a sample such as, for example, a nucleic acid extract or polypeptide extract is isolated from, or derived from, a particular source. For example, the extract may be isolated directly from any sucrose-metabolising organism, preferably from a sucrose-metabolising microorganism, more preferably from microorganisms of the genera Agrobacterium, Enterobacter, Erwinia, Klebsiella, Leuconostoc, Protaminobacter, Pseudomonas and Serratia or from a microorganism obtained from a location in which organisms, capable of converting sucrose to isomaltulose, have a selective advantage as for example described herein. The term "oligonucleotide" as used herein refers to a polymer composed of a multiplicity of nucleotide units (deoxyribonucleotides or ribonucleotides, or related structural variants or synthetic analogues thereof) linked via phosphodiester bonds (or related structural variants or synthetic analogues thereof). Thus, while the term "oligonucleotide" typically refers to a nucleotide polymer in which the nucleotides and linkages between them are naturally occurring, it will be understood that the term also includes within its scope various analogues including, but not restricted to, peptide nucleic acids (PNAs), phosphoramidates, phosphorothioates, methyl phosphonates, 2-O-methyl ribonucleic acids, and the like. The exact size of the molecule may vary depending on the particular application. An oligonucleotide is typically rather short in length, generally from about 10 to 30 nucleotides, but the term can refer to molecules of any length, although the term "polynucleotide" or "nucleic acid" is typically used for large oligonucleotides. By "operably linked" is meant that transcriptional and translational regulatory nucleic acids are positioned relative to a polypeptide-encoding polynucleotide in such a manner that the polynucleotide is transcribed and optionally the polypeptide is translated. As used herein, "plant" and "differentiated plant" refer to a whole plant or plant part containing differentiated plant cell types, tissues and/or organ systems. Plantlets and seeds are also included within the meaning of the foregoing terms. Plants included in the invention are any plants amenable to transformation techniques, including angiosperms, gymnosperms, monocotyledons and dicotyledons. The term "plant cell" as used herein refers to protoplasts or other cells derived from plants, gamete-producing cells, and cells which regenerate into whole plants. Plant cells include cells in plants as well as protoplasts or other cells in culture. By "plant tissue" is meant differentiated and undifferentiated tissue derived from roots, shoots, pollen, seeds, tumour tissue, such as crown galls, and various forms of aggregations of plant cells in culture, such as embryos and calluses. "Constitutive promoter" refers to a promoter that directs expression of an operably linked transcribable sequence in many or all tissues of a plant. By "stem-specific promoter" is meant a promoter that preferentially directs expression of an operably linked transcribable sequence in culm or stem tissue of a plant, as compared to expression in leaf, root or other tissues of the plant. The term "polynucleotide" or "nucleic acid' as used herein designates mRNA, RNA, cRNA, cDNA or DNA. The term typically refers to oligonucleotides greater than 30 nucleotides in length. The terms "polynucleotide variant" and "variant" refer to polynucleotides displaying substantial sequence identity with a reference polynucleotide sequence or polynucleotides that hybridise with a reference sequence under stringent conditions that are defined hereinafter. These terms also encompass polynucleotides in which one or more nucleotides have been added or deleted, or replaced with different nucleotides. In this regard, it is well understood in the art that certain alterations inclusive of mutations, additions, deletions and substitutions can be made to a reference polynucleotide whereby the altered polynucleotide retains the biological function or activity of the reference polynucleotide. The terms "polynucleotide variant" and "variant" also include naturally occurring allelic variants. "Polypeptide", "peptide" and "protein" are used interchangeably herein to refer to a polymer of amino acid residues and to variants and synthetic analogues of the same. Thus, these terms apply to amino acid polymers in which one or more amino acid residues is a synthetic non-naturally occurring amino acid, such as a chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally-occurring amino acid polymers. The term "polypeptide variant" refers to polypeptides in which one or more amino acids have been replaced by different amino acids. It is well understood in the art that some amino acids may be changed to others with broadly similar properties without changing the nature of the activity of the polypeptide (conservative substitutions) as described hereinafter. These terms also encompass polypeptides in which one or more amino acids have been added or deleted, or replaced with different amino acids. Accordingly, polypeptide variants as used herein encompass polypeptides that have sucrose isomerase activity. By "primer" is meant an oligonucleotide which, when paired with a strand of DNA, is capable of initiating the synthesis of a primer extension product in the presence of a suitable polymerising agent. The primer is preferably single-stranded for maximum efficiency in amplification but may alternatively be double-stranded. A primer must be sufficiently long to prime the synthesis of extension products in the presence of the polymerisation agent. The length of the primer depends on many factors, including application, temperature to be employed, template reaction conditions, other reagents, and source of primers. For example, depending on the complexity of the target sequence, the oligonucleotide primer typically contains 15 to 35 or more nucleotides, although it may contain fewer nucleotides. Primers can be large polynucleotides, such as from about 200 nucleotides to several kilobases or more. Primers may be selected to be "substantially complementary" to the sequence on the template to which it is designed to hybridise and serve as a site for the initiation of synthesis. By "substantially complementary", it is meant that the primer is sufficiently complementary to hybridise with a target nucleotide sequence. Preferably, the primer contains no mismatches with the template to which it is designed to hybridise but this is not essential. For example, non-complementary nucleotides may be attached to the 5' end of the primer, with the remainder of the primer sequence being complementary to the template. Alternatively, non-complementary nucleotides or a stretch of non-complementary nucleotides can be interspersed into a primer, provided that the primer sequence has sufficient complementarity with the sequence of the template to hybridise therewith and thereby form a template for synthesis of the extension product of the primer. "Probe" refers to a molecule that binds to a specific sequence or sub-sequence or other moiety of another molecule. Unless otherwise indicated, the term "probe" typically refers to a polynucleotide probe that binds to another nucleic acid, often called the "target nucleic acid", through complementary base pairing. Probes may bind target nucleic acids lacking complete sequence complementarity with the probe, depending on the stringency of the hybridisation conditions. Probes can be labelled directly or indirectly. The term "recombinant polynucleotide" as used herein refers to a polynucleotide formed in vitro by the manipulation of nucleic acid into a form not normally found in nature. For example, the recombinant polynucleotide may be in the form of an expression vector. Generally, such expression vectors include transcriptional and translational regulatory nucleic acid operably linked to the nucleotide sequence. By "recombinant polypeptide" is meant a polypeptide made using recombinant techniques, i.e., through the expression of a recombinant polynucleotide. The term "regeneration" as used herein in relation to plant materials means growing a whole, differentiated plant from a plant cell, a group of plant cells, a plant part (including seeds), or a plant piece (e.g., from a protoplast, callus, or tissue part). By "reporter molecule" as used in the present specification is meant a molecule that, by its chemical nature, provides an analytically identifiable signal that allows the detection of a complex comprising an antigen-binding molecule and its target antigen. The term "reporter molecule" also extends to use of cell agglutination or inhibition of agglutination such as red blood cells on latex beads, and the like. Terms used to describe sequence relationships between two or more polynucleotides or polypeptides include "reference sequence", "comparison window", "sequence identity", "percentage of sequence identity" and "substantial identity". A "reference sequence" is at least 12 but frequently 15 to 18 and often at least 25 monomer units, inclusive of nucleotides and amino acid residues, in length. Because two polynucleotides may each comprise (1) a sequence (i.e., only a portion of the complete polynucleotide sequence) that is similar between the two polynucleotides, and (2) a sequence that is divergent between the two polynucleotides, sequence comparisons between two (or more) polynucleotides are typically performed by comparing sequences of the two polynucleotides over a "comparison window" to identify and compare local regions of sequence similarity. A "comparison window" refers to a conceptual segment of at least 6 contiguous positions, usually about 50 to about 100, more usually about 100 to about 150 in which a sequence is compared to a reference sequence of the same number of contiguous positions after the two sequences are optimally aligned. The comparison window may comprise additions or deletions (i.e., gaps) of about 20% or less as compared to the reference sequence (which does not comprise additions or deletions) for optimal alignment of the two sequences. Optimal alignment of sequences for aligning a comparison window may be conducted by computerised implementations of algorithms (GAP, BESTFIT, FASTA, and TFASTA in the Wisconsin Genetics Software Package Release 7.0, Genetics Computer Group, 575 Science Drive Madison, WI, USA) or by inspection and the best alignment (i.e., resulting in the highest percentage homology over the comparison window) generated by any of the various methods selected. Reference also may be made to the BLAST family of programs as for example disclosed by Altschul et al, 1997, Nucl. Acids Res. 25:3389. A detailed discussion of sequence analysis can be found in Unit 19.3 of Ausubel et al, "Current Protocols in Molecular Biology", John Wiley & Sons Inc, 1994-1998, Chapter 15. The term "sequence identity" as used herein refers to the extent that sequences are identical on a nucleotide-by-nucleotide basis or an amino acid-by-amino acid basis over a window of comparison. Thus, a "percentage of sequence identity" is calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical nucleic acid base (e.g., A, T, C, G, I) or the identical amino acid residue (e.g., Ala, Pro, Ser, Thr, Gly, Nal, Leu, lie, Phe, Tyr, Tip, Lys, Arg, His, Asp, Glu, Asn, Gin, Cys and Met) occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity. For the purposes of the present invention, "sequence identity" will be understood to mean the "match percentage" calculated by the DΝASIS computer program (Version 2.5 for windows; available from Hitachi Software engineering Co., Ltd., South San Francisco, California, USA) using standard defaults as used in the reference manual accompanying the software. "Stringency" as used herein, refers to the temperature and ionic strength conditions, and presence or absence of certain organic solvents, during hybridisation and washing procedures. The higher the stringency, the higher will be the degree of complementarity between immobilised target nucleotide sequences and the labelled probe polynucleotide sequences that remain hybridised to the target after washing. "Stringent conditions" refers to temperature and ionic conditions under which only nucleotide sequences having a high frequency of complementary bases will hybridise. The stringency required is nucleotide sequence dependent and depends upon the various components present during hybridisation and subsequent washes, and the time allowed for these processes. Generally, in order to maximise the hybridisation rate, non-stringent hybridisation conditions are selected; about 20 to 25° C lower than the thermal melting point (T m ). The T m is the temperature at which 50% of specific target sequence hybridises to a perfectly complementary probe in solution at a defined ionic strength and pH. Generally, in order to require at least about 85% nucleotide complementarity of hybridised sequences, highly stringent washing conditions are selected to be about 5 to 15° C lower than the T m . In order to require at least about 70% nucleotide complementarity of hybridised sequences, moderately stringent washing conditions are selected to be about 15 to 30° C lower than the T m . Highly permissive (low stringency) washing conditions may be as low as 50° C below the T m , allowing a high level of mis-matching between hybridised sequences. Those skilled in the art will recognise that other physical and chemical parameters in the hybridisation and wash stages can also be altered to affect the outcome of a detectable hybridisation signal from a specific level of homology between target and probe sequences. Other examples of stringency conditions are described in section 3.3. The term "transformation" means alteration of the genotype of an organism, for example a bacterium or a plant, by the introduction of a foreign or endogenous nucleic acid. By "transgenote" is meant an immediate product of a transformation process. By "vector" is meant a nucleic acid molecule, preferably a DNA molecule derived, for example, from a plasmid, bacteriophage, or plant virus, into which a nucleic acid sequence may be inserted or cloned. A vector preferably contains one or more unique restriction sites and may be capable of autonomous replication in a defined host cell including a target cell or tissue or a progenitor cell or tissue thereof, or be integrable with the genome of the defined host such that the cloned sequence is reproducible. Accordingly, the vector may be an autonomously replicating vector, i.e., a vector that exists as an extrachromosomal entity, the replication of which is independent of chromosomal replication, e.g., a linear or closed circular plasmid, an extrachromosomal element, a minichromosome, or an artificial chromosome. The vector may contain any means for assuring self -replication. Alternatively, the vector may be one which, when introduced into a cell, is integrated into the genome of the recipient cell and replicated together with the chromosome(s) into which it has been integrated. A vector system may comprise a single vector or plasmid, two or more vectors or plasmids, which together contain the total DNA to be introduced into the genome of the host cell, or a transposon. The choice of the vector will typically depend on the compatibility of the vector with the cell into which the vector is to be introduced. The vector may also include a selection marker such as an antibiotic resistance gene that can be used for selection of suitable transformants. Examples of such resistance genes are well known to those of skill in the art. 2. Isolated polypeptides, biologically active fragments, polypeptide variants and derivatives 2J Polypeptides of the invention The present invention is predicated in part on the determination of the full-length sequence of a sucrose isomerase from Erwinia rhapontici (Accession No. WAC2928) and the full-length sequence of a novel sucrose isomerase from a bacterial isolate designated 68J. The full-length amino acid sequence of the Erwinia rhapontici sucrose isomerase extends 632 residues and includes 197 additional residues of carboxyl terminal sequence (set forth in SEQ ID NO: 26) relative to the sequence disclosed by Mattes et al. (supra). The E. rhapontici polypeptide includes a leader or signal peptide, set forth in SEQ ID NO: 6, which extends from residues 1 to about 36 of SEQ ID NO: 2. The signal peptide is necessary only for correct localisation of the mature polypeptide in a particular cell compartment (e.g., in the outer membrane, in the inner membrane or in the periplasmic space between the outer membrane and the inner membrane). The mature polypeptide, set forth in SEQ ID NO: 4, extends from about residue 37 to residue 632. Accordingly, in one embodiment, the invention provides an isolated precursor polypeptide according to SEQ ID NO: 2, which comprises a leader peptide according to SEQ ID NO: 6 fused in frame with a polypeptide according to SEQ ID NO: 4. In another embodiment, the invention provides an isolated mature polypeptide comprising the sequence set forth in SEQ ID NO: 4. The full-length amino acid sequence of the 68J sucrose isomerase extends 598 residues set forth in SEQ ID NO: 8, and comprises a signal peptide, set forth in SEQ ID NO: 12, extending from residues 1 to about 33 of SEQ ID NO: 8. The mature polypeptide, set forth in SEQ ID NO: 10, extends from about residue 34 to residue 598 of SEQ ID NO: 8. Thus, in one embodiment, the present invention features an isolated precursor polypeptide according to SEQ ID NO: 8, which comprises a leader peptide according to SEQ ID NO: 12 fused in frame with a polypeptide according to SEQ ID NO: 10. In another embodiment, the invention contemplates an isolated mature polypeptide comprising the sequence set forth in SEQ ID NO: 10. 2.2 Biologically active fragments Biologically active fragments may be produced according to any suitable procedure known in the art. For example, a suitable method may include first producing a fragment of said polypeptide and then testing the fragment for the appropriate biological activity. In one embodiment, the fragment may be tested for sucrose isomerase activity. Any assay that detects or preferably measures sucrose isomerase activity is contemplated by the present invention. Preferably, sucrose isomerase activity is determined by an aniline/diphenylamine assay and capillary electrophoresis as described herein. In another embodiment, biological activity of the fragment is tested by introducing a polynucleotide from which a fragment of the polypeptide can be translated into a cell, and detecting sucrose isomerase activity, which is indicative of said fragment being a said biologically active fragment. The invention also contemplates biological fragments of the above polypeptides of at least 6 and preferably at least 8 amino acids in length, which can elicit an immune response in an animal for the production of antibodies that are immuno-interactive with a sucrose isomerase enzyme of the invention. For example exemplary polypeptide fragments of 8 residues in length, which could elicit an immune response, include but are not limited to residues 1-8, 9-16, 17-24, 25-32, 33-40, 41-48, 49-56, 57-64, 65-72, 73-80, 81-88, 89- 96, 97-104, 105-112, 113-120, 121-128, 129-136, 137-144, 145-152, 153-160, 161-168, 169-176, 177-184, 185-192, 193-200, 201-208, 209-216, 217-224, 225-232, 223-240, 241- 248, 249-256, 257-264, 265-272, 273-280, 281-288, 289-296, 297-304, 305-312, 313-320, 321-328, 329-336, 337-344, 345-352, 353-360, 361-368, 369-376, 377-384, 385-392, 393- 400, 401-408, 409-416, 417-424, 425-432, 423-440, 441-448, 449-456, 457-464, 465-472, 473-480, 481-488, 489-496, 497-504, 505-512, 513-520, 521-528, 529-536, 537-544, 545- 552, 553-560, 561-568, 569-576, 577-584, 585-592 and 589-596 of SEQ ID NO: 2, or residues 1-8, 9-16, 17-24, 25-32, 33-40, 41-48, 49-56, 57-64, 65-72, 73-80, 81-88, 89-96, 97-104, 105-112, 113-120, 121-128, 129-136, 137-144, 145-152, 153-160, 161-168, 169- 176, 177-184, 185-192, 193-200, 201-208, 209-216, 217-224, 225-232, 223-240, 241-248, 249-256, 257-264, 265-272, 273-280, 281-288, 289-296, 297-304, 305-312, 313-320, 321- 328, 329-336, 337-344, 345-352, 353-360, 361-368, 369-376, 377-384, 385-392, 393-400, 401-408, 409-416, 417-424, 425-432, 423-440, 441-448, 449-456, 457-464, 465-472, 473- 480, 481-488, 489-496, 497-504, 505-512, 513-520, 521-528, 529-536, 537-544, 545-552, 553-560 and 559-566 of SEQ ID NO: 4. In a preferred embodiment of this type, the biologically active fragment comprises at least one sucrose isomerase consensus sequence selected from SEQ ID NO: 19, 20, 21, 22, 23 or 24. 2.3 Polypeptide variants The invention also contemplates polypeptide variants of the polypeptides of the invention wherein said variants have sucrose isomerase activity. Suitable methods of producing polypeptide variants include, for example, producing a modified polypeptide whose sequence is distinguished from a parent polypeptide by substitution, deletion and/or addition of at least one amino acid, wherein the parent polypeptide comprises a sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or a biologically active fragment thereof. The modified polypeptide is then tested for sucrose isomerase activity, wherein the presence of that activity indicates that said modified polypeptide is a said variant. In another embodiment, a polypeptide variant is produced by introducing into a cell a polynucleotide from which a modified polypeptide can be translated, and detecting sucrose isomerase activity associated with the cell, which is indicative of the modified polypeptide being a said polypeptide variant. In general, variants will have at least 60%, more suitably at least 70%, preferably at least 80%, and more preferably at least 90% homology to a polypeptide as for example shown in SEQ ID NO: 2, 4, 8 and 10, or biologically active fragments thereof. It is preferred that variants display at least 60%, more suitably at least 70%, preferably at least 75%, more preferably at least 80%, more preferably at least 85%, more preferably at least 90% and still more preferably at least 95% sequence identity with a polypeptide as for example shown in SEQ ID NO: 2, 4, 8 and 10, or biologically active fragments thereof. In this respect, the window of comparison preferably spans about the full length of the polypeptide or of the biologically active fragment. Suitable variants can be obtained from any suitable sucrose-metabolising organism. Preferably, the variants are obtained from a sucrose-metabolising bacterium as for example described in Section 3.3 infra. 2.4 Methods of producing polypeptide variants 2.4.1 Mutagenesis Polypeptide variants according to the invention can be identified either rationally, or via established methods of mutagenesis (see, for example, Watson, J. D. et al, "MOLECULAR BIOLOGY OF THE GENE", Fourth Edition, Benjamin/Cummings, Menlo Park, Calif., 1987). Significantly, a random mutagenesis approach requires no a priori information about the gene sequence that is to be mutated. This approach has the advantage that it assesses the desirability of a particular mutant based on its function, and thus does not require an understanding of how or why the resultant mutant protein has adopted a particular conformation. Indeed, the random mutation of target gene sequences has been one approach used to obtain mutant proteins having desired characteristics (Leatherbarrow, R. 1986, /. Prot. Eng. 1: 7-16; Knowles, J. R., 1987, Science 236: 1252- 1258; Shaw, W. V., 1987, Biochem. J. 246: 1-17; Gerit, J. A. 1987, Chem. Rev. 87: 1079- 1105). Alternatively, where a particular sequence alteration is desired, methods of site- directed mutagenesis can be employed. Thus, such methods may be used to selectively alter only those amino acids of the protein that are believed to be important (Craik, C. S., 1985, Science 228: 291-297; Cronin, et al, 1988, Biochem. 27: 4572-4579; Wilks, et al, 1988, Science 242: 1541-1544). Variant peptides or polypeptides, resulting from rational or established methods of mutagenesis or from combinatorial chemistries as hereinafter described, may comprise conservative amino acid substitutions. Exemplary conservative substitutions in a polypeptide or polypeptide fragment according to the invention may be made according to the following table: TABLE B Substantial changes in function are made by selecting substitutions that are less conservative than those shown in TABLE B. Other replacements would be non- conservative substitutions and relatively fewer of these may be tolerated. Generally, the substitutions which are likely to produce the greatest changes in a polypeptide' s properties are those in which (a) a hydrophilic residue (e.g., Ser or Thr) is substituted for, or by, a hydrophobic residue (e.g., Ala, Leu, He, Phe or Val); (b) a cysteine or proline is substituted for, or by, any other residue; (c) a residue having an electropositive side chain (e.g., Arg, His or Lys) is substituted for, or by, an electronegative residue (e.g., Glu or Asp) or (d) a residue having a bulky side chain (e.g., Phe or Trp) is substituted for, or by, one having a smaller side chain (e.g., Ala, Ser) or no side chain (e.g., Gly). What constitutes suitable variants may be determined by conventional techniques. For example, nucleic acids encoding a polypeptide according to SEQ ID NO: 2, 4, 8 and 10 can be mutated using either random mutagenesis for example using transposon mutagenesis, or site-directed mutagenesis as described, for example, in Section 3.3 infra. 2.4.2 Peptide libraries produced by combinatorial chemistry A number of facile combinatorial technologies can be utilised to synthesise molecular libraries of immense diversity. In the present case, variants of a polypeptide, or preferably a polypeptide fragment according to the invention, can be synthesised using such technologies. Variants can be screened subsequently using the methods described in Section 2.3. Preferably, soluble synthetic peptide combinatorial libraries (SPCLs) are produced which offer the advantage of working with free peptides in solution, thus permitting adjustment of peptide concentration to accommodate a particular assay system. SPCLs are suitably prepared as hexamers. In this regard, a majority of binding sites is known to involve four to six residues. Cysteine is preferably excluded from the mixture positions to avoid the formation of disulfides and more difficult-to-define polymers. Exemplary methods of producing SPCLs are disclosed by Houghten et al. (1991, Nature 354: 84-86; 1992, BioTechniques 13: 412-421), Appel et al. (1992, Immunomethods 1: 17- 23), and Pinilla et al. (1992, BioTechniques 13: 901-905; 1993, Gene 128: 71-76). Preparation of combinatorial synthetic peptide libraries may employ either t- butyloxycarbonyl (t-Boc) or 9-fluorenylmethyloxycarbonyl (Fmoc) chemistries (see Chapter 9.1, of Coligan et al, supra; Stewart and Young, 1984, Solid Phase Peptide Synthesis, 2nd ed. Pierce Chemical Co., Rockford, HI; and Atherton and Sheppard, 1989, Solid Phase Peptide Synthesis: A Practical Approach. IRL Press, Oxford) preferably, but not exclusively, using one of two different approaches. The first of these approaches, suitably termed the "split-process-recombine" or "split synthesis" method, was described first by Furka et al. (1988, 14th Int. Congr. Biochem., Prague, Czechoslovakia 5: 47; 1991, Int. J. Pept. Protein Res. 37: 487-493) and Lam et al. (1991, Nature 354: 82-84), and reviewed later by Eichler et al. (1995, Medicinal Research Reviews 15(6): 481-496) and Balkenhohl et al. (1996, Angew. Chem. Int. Ed. Engl. 35: 2288-2337). Briefly, the split synthesis method involves dividing a plurality of solid supports such as polymer beads into n equal fractions representative of the number of available amino acids for each step of the synthesis (e.g., 20 L-amino acids), coupling a single respective amino acid to each polymer bead of a corresponding fraction, and then thoroughly mixing the polymer beads of all the fractions together. This process is repeated for a total of x cycles to produce a stochastic collection of up to N x different compounds. The peptide library so produced may be screened for sucrose isomerase activity. Upon detection, some of the positive beads are selected for sequencing to identify the active peptide. Such a peptide may be subsequently cleaved from the beads, and assayed as above. The second approach, the chemical ratio method, prepares mixed peptide resins using a specific ratio of amino acids empirically defined to give equimolar incorporation of each amino acid at each coupling step. Each resin bead contains a mixture of peptides. Approximate equimolar representation can be confirmed by amino acid analysis (Dooley and Houghten, 1993, Proc. Natl. Acad. Sci. U.S.A. 90: 10811-10815; Eichler and Houghten, 1993, Biochemistry 32: 11035-11041). Preferably, the synthetic peptide library is produced on polyethylene rods, or pins, as a solid support, as for example disclosed by Geysen et al. (1986, Mol. Immunol. 23: 709-715). An exemplary peptide library of this type may consist of octapeptides in which the third and fourth position represent defined amino acids selected from natural and unnatural amino acids, and in which the remaining six positions represent a randomised mixture of amino acids. This peptide library can be represented by the formula Ac-XXOiO 2 XXXX-S s [SEQ ID NO: 37], where S s is the solid support. Peptide mixtures remain on the pins for assaying purposes. For example, a peptide library can be first screened for the ability to convert sucrose to isomaltulose. The most active peptides are then selected for an additional round of testing comprising linking, to the starting peptide, an additional residue (or by internally modifying the components of the original starting peptide) and then screening this set of candidates for sucrose isomerase activity. This process is reiterated until the peptide with the desired sucrose isomerase activity is identified. One identified, the identity of the peptide attached to the solid phase support may be determined by peptide sequencing. 2.4.3 Alanine scanning mutagenesis In one embodiment, the invention herein utilises a systematic analysis of a polypeptide or polypeptide fragment according to the invention to determine the residues in the polypeptide or fragment that are involved in catalysis of sucrose to isomaltulose. Such analysis is conveniently performed using recombinant DNA technology. In general, a DNA sequence encoding the polypeptide or fragment is cloned and manipulated so that it may be expressed in a convenient host. DNA encoding the polypeptide or fragment can be obtained from a genomic library, from cDNA derived from mRNA in cells expressing the said polypeptide or fragment, or by synthetically constructing the DNA sequence (Sambrook et al, supra; Ausubel et al, supra). The wild-type DNA encoding the polypeptide or fragment is then inserted into an appropriate plasmid or vector as described herein. In particular, prokaryotes are preferred for cloning and expressing DNA sequences to produce variants of the polypeptide or fragment. For example, E. coli K12 strain 294 (ATCC No. 31446) may be used, as well as E. coli B, E. coli X1776 (ATCC No. 31537), and E. coli c600 and c600hfl, and E. coli W3110 (F " , γ " , prototrophic, ATCC No. 27325), bacilli such as Bacillus subtilis, and other enterobacteriaceae such as Salmonella typhimurium or Serratia marcescens, and various Pseudomonas species. A preferred prokaryote is E. coli W3110 (ATCC 27325). Once the polypeptide or fragment is cloned, site-specific mutagenesis as for example described by Carter et al. (1986, Nucl. Acids. Res., 13: 4331) or by Zoller et al. (1987, Nucl. Acids Res., 10: 6487), cassette mutagenesis as for example described by Wells et al. (1985, Gene, 34: 315), restriction selection mutagenesis as for example described by Wells et al. (1986, Philos. Trans. R. Soc. London SerA, 317: 415), or other known techniques may be performed on the cloned DNA to produce the variant DNA that codes for the changes in amino acid sequence defined by the residues being substituted. When operably linked to regulatory polynucleotides in an appropriate expression vector, variant polypeptides are obtained. In some cases, recovery of the variant may be facilitated by expressing and secreting such molecules from the expression host by use of an appropriate signal sequence operably linked to the DNA sequence encoding the variant. Such methods are well known to those skilled in the art. Of course, other methods may be employed to produce such polypeptides or fragments such as the in vitro chemical synthesis of the desired polypeptide variant (Barany et al. In The Peptides, eds. Ε. Gross and J. Meienhofer (Academic Press: N.Y. 1979), Vol. 2, pp. 3-254). Once the different variants are produced, they are contacted with sucrose or a sucrose-containing substrate and the conversion to isomaltulose, if any, is determined for each variant. These sucrose isomerase activities are compared to the activity of the parent polypeptide or fragment to determine which of the amino acid residues in the active site a involved in sucrose isomerisation. The sucrose isomerase activity of the parent and variant, respectively, can be measured by any convenient assay as for example described herein. While any number of analytical measurements may be used to compare activities, a convenient one for enzymic activity is the Michaelis constant K m of the variant as compared to the m for the parent polypeptide or fragment. Generally, a two-fold increase or decrease in K m per analogous residue substituted by the substitution indicates that the substituted residue(s) is active in the interaction of the parent polypeptide or fragment with the substrate. When a suspected or known active amino acid residue is subjected to scanning amino acid analysis, the amino acid residues immediately adjacent thereto should be scanned. The scanning amino acid used in such an analysis may be any different amino acid from that substituted, i.e., any of the 19 other naturally occurring amino acids. Three residue-substituted polypeptides can be made. One contains a scanning amino acid, preferably alanine, at position N that is the suspected or known active amino acid. The two others contain the scanning amino acid at position N+l and N-l. If each substituted polypeptide or fragment causes a greater than about two-fold effect on K m for the substrate, the scanning amino acid is substituted at position N+2 and N-2. This is repeated until at least one, and preferably four, residues are identified in each direction which have less than about a two-fold effect on K m or until either of the ends of the parent polypeptide or fragment are reached. In this manner, along a continuous amino acid sequence one or more amino acids that are involved in the catalysis of sucrose to isomaltulose can be identified. The active amino acid residue identified by amino acid scan is typically one that contacts sucrose directly. However, active amino acids may also indirectly contact sucrose through salt bridges formed with other residues or small molecules such as H 2 O or ionic species such as Na + , Ca +2 , Mg +2 , or Zn +2 . In some cases, the substitution of a scanning amino acid at one or more residues results in a residue-substituted polypeptide which is not expressed at levels that allow for the isolation of quantities sufficient to carry out analysis of its sucrose isomerase activity. In such cases, a different scanning amino acid, preferably an isosteric amino acid, can be used. Among the preferred scanning amino acids are relatively small, neutral amino acids. Such amino acids include alanine, glycine, serine, and cysteine. Alanine is the preferred scanning amino acid among this group because it eliminates the side-chain beyond the beta-carbon and is less likely to alter the main-chain conformation of the variant. Alanine is also preferred because it is the most common amino acid. Further, it is frequently found in both buried and exposed positions (Creighton, The Proteins, W. H. Freeman & Co., N.Y.; Chothia, 1976, J. Mol. Biol, 150: 1). If alanine substitution does not yield adequate amounts of variant, an isosteric amino acid can be used. Alternatively, the following amino acids in decreasing order of preference may be used: Ser, Asn, and Leu. Once the active amino acid residues are identified, isosteric amino acids may be substituted. Such isosteric substitutions need not occur in all instances and may be performed before any active amino acid is identified. Such isosteric amino acid substitution is performed to minimise the potential disruptive effects on conformation that some substitutions can cause. Isosteric amino acids are shown in the table below: TABLE C Polypeptide Amino Acid Isosteric Scanning Amino Acid His (H) Asn, Gin The method herein can be used to detect active amino acid residues within different domains of a polypeptide or fragment according to the invention. Once this identification is made, various modifications to the parent polypeptide or fragment may be made to modify the interaction between the parent polypeptide or fragment and its substrate. 2.4.4 Polypeptide or peptide libraries produced by phage display The identification of variants can also be facilitated through the use of a phage (or phagemid) display protein ligand screening system as for example described by Lowman, et al. (1991, Biochem. 30: 10832-10838), Markland, et al. (1991, Gene 109: 13-19), Roberts, et al. (1992, Proc. Natl. Acad. Sci. (U.S.A.) 89: 2429-2433), Smith, G. P. (1985, Science 228: 1315-1317), Smith, et al. (1990, Science 248: 1126-1128) and Lardner et al. (U.S. Patent 5,223,409). In general, this method involves expressing a fusion protein in which the desired protein ligand is fused to the N-terminus of a viral coat protein (such as the M13 Gene HI coat protein, or a lambda coat protein). In one embodiment, a library of phage is engineered to display novel peptides within the phage coat protein sequences. Novel peptide sequences are generated by random mutagenesis of gene fragments encoding a polypeptide of the invention or biologically active fragment using error-prone PCR, or by in vivo mutation by E. coli mutator cells. The novel peptides displayed on the surface of the phage are placed in contact with sucrose or a sucrose-containing substrate. Phage that display coat protein having peptides that are capable of isomerising sucrose to isomaltulose are then selected. The selected phage can be amplified, and the DNA encoding their coat proteins can be sequenced. In this manner, the amino acid sequence of the embedded peptide or polypeptide can be deduced. In more detail, the method involves (a) constructing a replicable expression vector comprising a first gene encoding a polypeptide or fragment of the invention, a second gene encoding at least a portion of a natural or wild-type phage coat protein wherein the first and second genes are heterologous, and a transcription regulatory element operably linked to the first and second genes, thereby forming a gene fusion encoding a fusion protein; (b) mutating the vector at one or more selected positions within the first gene thereby forming a family of related plasmids; (c) transforming suitable host cells with the plasmids; (d) infecting the transformed host cells with a helper phage having a gene encoding the phage coat protein; (e) culturing the transformed infected host cells under conditions suitable for forming recombinant phagemid particles containing at least a portion of the plasmid and capable of transforming the host, the conditions adjusted so that no more than a minor amount of phagemid particles displays more than one copy of the fusion protein on the surface of the particle; (f) contacting the phagemid particles with sucrose or a sucrose- containing substrate; and (g) separating the phagemid particles that isomerise sucrose to isomaltulose from those that do not. Preferably, the method further comprises transforming suitable host cells with recombinant phagemid particles that isomerise sucrose to isomaltulose and repeating steps (d) through (g) one or more times. Preferably, in this method the plasmid is under tight control of the transcription regulatory element, and the culturing conditions are adjusted so that the amount or number of phagemid particles displaying more than one copy of the fusion protein on the surface of the particle is less than about 20%. More, preferably, the number of phagemid particles displaying more than one copy of the fusion protein is less than 10% of the number of phagemid particles displaying a single copy of the fusion protein. Most preferably, , the number is less than 1%. Typically in this method, the expression vector will further contain a secretory signal sequence fused to the DNA encoding each subunit of the polypeptide and the transcription regulatory element will be a promoter system. Preferred promoter systems are selected from lac Z, λ PL , tac, T7 polymerase, tryptophan, and alkaline phosphatase promoters and combinations thereof. Normally the method will also employ a helper phage selected from M13K07, M13R408, M13-VCS, and Phi X 174. The preferred helper phage is M13K07, and the preferred coat protein is the M13 Phage. gene HI coat protein. The preferred host is E. coli, and protease-deficient strains of E. coli. Repeated cycles of variant selection are used to select for higher and higher affinity binding by the phagemid selection of multiple amino acid changes that are selected by multiple selection cycles. Following a first round of phagemid selection, involving a first region or selection of amino acids in the ligand polypeptide, additional rounds of phagemid selection in other regions or amino acids of the ligand polypeptide are conducted. The cycles of phagemid selection are repeated until the desired affinity properties of the polypeptide are achieved. It will be appreciated that the amino acid residues that form the active site of the polypeptide or fragment may not be sequentially linked and may reside on different subunits of the polypeptide or fragment. That is, the binding domain tracks with the particular secondary structure at the active site and not the primary structure. Thus, generally, mutations will be introduced into codons encoding amino acids within a particular secondary structure at sites directed away from the interior of the polypeptide so that they will have the potential to interact with sucrose or a sucrose-containing substrate. The phagemid-display method herein contemplates fusing a polynucleotide encoding the polypeptide or fragment (polynucleotide 1) to a second polynucleotide (polynucleotide 2) such that a fusion protein is generated during transcription. Polynucleotide 2 is typically a coat protein gene of a phage, and preferably it is the phage Ml 3 gene HI coat protein, or a fragment thereof. Fusion of polynucleotides 1 and 2 may be accomplished by inserting polynucleotide 2 into a particular site on a plasmid that contains polynucleotide 1, or by inserting polynucleotide 1 into a particular site on a plasmid that contains polynucleotide 2. Between polynucleotide 1 and polynucleotide 2, DNA encoding a termination codon may be inserted, such termination codons being UAG (amber), UAA (ocher), and UGA (opel) (see for example, Davis et al, Microbiology (Harper and Row: New York, 1980), pages 237, 245-247, and 274). The termination codon expressed in a wild-type host cell results in the synthesis of the polynucleotide 1 protein product without the polynucleotide 2 protein attached. However, growth in a suppressor host cell results in the synthesis of detectable quantities of fused protein. Such suppressor host cells contain a tRNA modified to insert an amino acid in the termination codon position of the mRNA, thereby resulting in production of detectable amounts of the fusion protein. Suppressor host cells of this type are well known and described, such as E. coli suppressor strain, such as JM101 or XLl-Blue (Bullock et al, 1987, BioTechniques, 5: 376-379). Any acceptable method may be used to place such a termination codon into the mRNA encoding the fusion polypeptide. The suppressible codon may be inserted between the polynucleotide encoding the polypeptide or fragment and a second polynucleotide encoding at least a portion of a phage coat protein. Alternatively, the suppressible termination codon may be inserted adjacent to the fusion site by replacing the last amino acid triplet in the polypeptide/fragment or the first amino acid in the phage coat protein. When the phagemid containing the suppressible codon is grown in a suppressor host cell, it results in the detectable production of a fusion polypeptide containing the polypeptide or fragment and the coat protein. When the phagemid is grown in a non-suppressor host cell, the polypeptide or fragment is synthesised substantially without fusion to the phage coat protein due to termination at the inserted suppressible triplet encoding UAG, UAA, or UGA. In the non-suppressor cell the polypeptide is synthesised and secreted from the host cell due to the absence of the fused phage coat protein which otherwise anchored it to the host cell. The polypeptide or fragment may be altered at one or more selected codons. An alteration is defined as a substitution, deletion, or insertion of one or more codons in the gene encoding the polypeptide or fragment that results in a change in the amino acid sequence as compared with the unaltered or native sequence of the said polypeptide or fragment. Preferably, the alterations will be by substitution of at least one amino acid with any other amino acid in one or more regions of the molecule. The alterations may be produced by a variety of methods known in the art, as for example described in Section 2.3 and 2.4.1. These methods include, but are not limited to, oligonucleotide-mediated mutagenesis and cassette mutagenesis as described for example herein. The library of phagemid particles is then contacted with sucrose or a sucrose- containing substrate under suitable conditions. Normally, the conditions, including pH, ionic strength, temperature, and the like will mimic physiological conditions. Phagemid particles having high sucrose isomerase activity are then selected from those having low activity. Suitable host cells are infected with the selected phagemid particles and helper phage, and the host cells are cultured under conditions suitable for amplification of the phagemid particles. The phagemid particles are then collected and the selection process is repeated one or more times until binders having the desired affinity for the target molecule are selected. 2.4.5 Rational drug design Variants of an isolated polypeptide according to the invention, or a biologically active fragment thereof, may also be ^ obtained using the principles of conventional or of rational drug design as for example described by Andrews, et al. (In: "PROCEEDINGS OF THE ALFRED BENZON SYMPOSIUM", volume 28, pp. 145-165, Munksgaard, Copenhagen, 1990), McPherson, A. (1990, Eur. J. Biochem. 189: 1-24), Hoi,, et al. (In: "MOLECULAR RECOGNITION: CHEMICAL AND BIOCHEMICAL PROBLEMS", Roberts, S. M. (ed.); Royal Society of Chemistry; pp. 84-93, 1989), Hoi, W. G. J. (1989, Arzneim-Forsch. 39: 1016-1018), Hoi, W. G. J. (1986, Agnew Chem. Int. Ed. Engl. 25: 767-778). In accordance with the methods of conventional drug design, the desired variant molecules are obtained by randomly testing molecules whose structures have an attribute in common with the structure of a parent polypeptide or biologically active fragment according to the invention. The quantitative contribution that results from a change in a particular group of a binding molecule can be determined by measuring the capacity of competition or cooperativity between the parent polypeptide or polypeptide fragment and the candidate polypeptide variant. In one embodiment of rational drug design, the polypeptide variant is designed to share an attribute of the most stable three-dimensional conformation of a polypeptide or polypeptide fragment according to the invention. Thus, the variant may be designed to possess chemical groups that are oriented in a way sufficient to cause ionic, hydrophobic, or van der Waals interactions that are similar to those exhibited by the polypeptide or polypeptide fragment of the invention. In a second method of rational design, the capacity of a particular polypeptide or polypeptide fragment to undergo conformational "breathing" is exploited. Such "breathing" - the transient and reversible assumption of a different molecular conformation - is a well-appreciated phenomenon, and results from temperature, thermodynamic factors, and from the catalytic activity of the molecule. Knowledge of the 3-dimensional structure of the polypeptide or polypeptide fragment facilitates such an evaluation. An evaluation of the natural conformational changes of a polypeptide or polypeptide fragment facilitates the recognition of potential hinge sites, potential sites at which hydrogen bonding, ionic bonds or van der Waals bonds might form or might be eliminated due to the breathing of the molecule, etc. Such recognition permits the identification of the additional conformations that the polypeptide or polypeptide fragment could assume, and enables the rational design and production of mimetic polypeptide variants that share such conformations. The preferred method for performing rational mimetic design employs a computer system capable of forming a representation of the three-dimensional structure of the polypeptide or polypeptide fragment (such as those obtained using RIBBON (Priestle, J., 1988, J. Mol. Graphics 21: 572), QUANTA (Polygen), InSite (Biosyn), or Nanovision (American Chemical Society)). Such analyses are exemplified by Hoi, et al. (In: "MOLECULAR RECOGNITION: CHEMICAL AND BIOCHEMICAL PROBLEMS", supra, Hoi, W. G. J. (1989, supra) and Hoi, W. G. J., (1986, supra). In lieu of such direct comparative evaluations of candidate polypeptide variants, screening assays may be used to identify such molecules. Such assays will preferably exploit the capacity of the variant to catalyse the conversion of sucrose to isomaltulose. 2.5 Polypeptide derivatives With reference to suitable derivatives of the invention, such derivatives include amino acid deletions and/or additions to a polypeptide, fragment or variant of the invention, wherein said derivatives catalyse the conversion of sucrose to isomaltulose. "Additions" of amino acids may include fusion of the polypeptides, fragments and polypeptide variants of the invention with other polypeptides or proteins. For example, it will be appreciated that said polypeptides, fragments or variants may be incorporated into larger polypeptides, and that such larger polypeptides may also be expected to catalyse the conversion of sucrose to isomaltulose as mentioned above. The polypeptides, fragments or variants of the invention may be fused to a further protein, for example, which is not derived from the original host. The further protein may assist in the purification of the fusion protein. For instance, a polyhistidine tag or a maltose binding protein may be used in this respect as described in more detail below. Other possible fusion proteins are those which produce an immunomodulatory response. Particular examples of such proteins include Protein A or glutathione S-transferase (GST). Other derivatives contemplated by the invention include, but are not limited to, modification to side chains, incorporation of unnatural amino acids and/or their derivatives during peptide, polypeptide or protein synthesis and the use of crosslinkers and other methods which impose conformational constraints on the polypeptides, fragments and variants of the invention. Examples of side chain modifications contemplated by the present invention include modifications of amino groups such as by acylation with acetic anhydride; acylation of amino groups with succinic anhydride and tetrahydrophthalic anhydride; amidination with methylacetimidate; carbamoylation of amino groups with cyanate; pyridoxylation of lysine with pyridoxal-5-phosphate followed by reduction with NaBHj.; reductive alkylation by reaction with an aldehyde followed by reduction with NaBE ; and trinitrobenzylation of amino groups with 2, 4, 6-trinitrobenzene sulphonic acid (TNBS). The carboxyl group may be modified by carbodiimide activation via O- acylisourea formation followed by subsequent derivatisation, by way of example, to a corresponding amide. The guanidine group of arginine residues may be modified by formation of heterocyclic condensation products with reagents such as 2,3-butanedione, phenylglyoxal and glyoxal. i Sulphydryl groups may be modified by methods such as performic acid oxidation to cysteic acid; formation of mercurial derivatives using 4-chloromercuriphenylsulphonic acid, 4-chloromercuribenzoate; 2-chloromercuri-4-nitrophenol, phenylmercury chloride, and other mercurials; formation of a mixed disulphides with other thiol compounds; reaction with maleimide, maleic anhydride or other substituted maleimide; carboxymethylation with iodoacetic acid or iodoacetamide; and carbamoylation with cyanate at alkaline pH. Tryptophan residues may be modified, for example, by alkylation of the indole ring with 2-hydroxy-5-nitrobenzyl bromide or sulphonyl halides or by oxidation with N- bromosuccinimide. Tyrosine residues may be modified by nitration with tetranitromethane to form a 3-nitrotyrosine derivative. The imidazole ring of a histidine residue may be modified by N-carbethoxylation with diethylpyrocarbonate or by alkylation with iodoacetic acid derivatives. Examples of incorporating unnatural amino acids and derivatives during peptide synthesis include but are not limited to, use of 4-amino butyric acid, 6-aminohexanoic acid, 4-amino-3-hydroxy-5-phenylpentanoic acid, 4-amino-3-hydroxy-6-methylheptanoic acid, t-butylglycine, norleucine, norvaline, phenylglycine, ornithine, sarcosine, 2-thienyl alanine and/or D-isomers of amino acids. A list of unnatural amino acids contemplated by the present invention is shown in TABLE D. TABLED Non-conventional amino acid Non-conventional amino acid α-aminobutyric acid L-N-methylalanine α-amino-α-methylbutyrate L-N-methylarginine aminocyclopropane-carboxylate L-N-methylasparagine aminoisobutyric acid L-N-methylaspartic acid aminonorbornyl-carboxylate L-N-methylcysteine cyclohexylalanine L-N-methylglutamine cyclopentylalanine L-N-methylglutamic acid L-N-methylisoleucine L-N-methylhistidine D-alanine L-N-methylleucine D-arginine L-N-methyllysine D-aspartic acid L-N-methylmethionine D-cysteine L-N-methylnorleucine D-glutamate L-N-methylnorvaline D-glutamic acid L-N-methylornithine D-histidine L-N-methylphenylalanine D-isoleucine L-N-methylproline D-leucine L-N-medlylserine D-lysine L-N-methylthreonine D-methionine L-N-methyltryptophan D-ornithine L-N-methyltyrosine D-phenylalanine L-N-methylvaline D-proline L-N-methylethylglycine D-serine L-N-methyl-t-butylglycine ' fm l u. r l H-U- J.„ en, - — j — ■ — ■ Also contemplated is the use of crosslinkers, for example, to stabilise 3D conformations of the polypeptides, fragments or variants of the invention, using homo- bifunctional crosslinkers such as bifunctional imido esters having (CH 2 ) n spacer groups with n = 1 to n = 6, glutaraldehyde, N-hydroxysuccinimide esters and hetero-bifunctional reagents which usually contain an amino-reactive moiety such as N-hydroxysuccinimide and another group specific-reactive moiety such as maleimido or dithio moiety or carbodiimide. In addition, peptides can be conformationally constrained, for example, by introduction of double bonds between C α and C β atoms of amino acids, by incorporation of C α and N α -methylamino acids, and by formation of cyclic peptides or analogues by introducing covalent bonds such as forming an amide bond between the N and C termini between two side chains or between a side chain and the N or C terminus of the peptides or analogues. For example, reference may be made to: Marlowe (1993, Biorganic & Medicinal Chemistry Letters 3: 437-44) who describes peptide cyclisation on TFA resin using trimethylsilyl (TMSE) ester as an orthogonal protecting group; Pallin and Tam (1995, J. Chem. Soc. Chem. Comm. 2021-2022) who describe the cyclisation of unprotected peptides in aqueous solution by oxime formation; Algin et al (1994, Tetrahedron Letters 35: 9633-9636) who disclose solid-phase synthesis of head-to-tail cyclic peptides via lysine side-chain anchoring; Kates et al (1993, Tetrahedron Letters 34: 1549-1552) who describe the production of head-to-tail cyclic peptides by three- dimensional solid phase strategy; Tumelty et al (1994, J. Chem. Soc. Chem. Comm. 1067- 1068) who describe the synthesis of cyclic peptides from an immobilised activated intermediate, wherein activation of the immobilised peptide is carried out with the N- protecting group intact and the N-protecting group is subsequently removed leading to cyclisation; McMurray et al (1994, Peptide Research 7: 195-206) who disclose head-to-tail cyclisation of peptides attached to insoluble supports by means of the side chains of aspartic and glutamic acid; Hruby et al (1994, Reactive Polymers 22: 231-241) who teach an alternate method for cyclising peptides via solid supports; and Schmidt and Langer (1997, J. Peptide Res. 49: 67-73) who disclose a method for synthesising cyclotetrapeptides and cyclopentapeptides. The foregoing methods may be used to produce conformationally constrained polypeptides that catalyse the conversion of sucrose to isomaltulose. The invention also contemplates polypeptides, fragments or variants of the invention that have been modified using ordinary molecular biological techniques so as to improve their resistance to proteolytic degradation or to optimise solubility properties or to render them more suitable as an immunogenic agent. 2.6 Methods of preparing the polypeptides of the invention Polypeptides of the invention may be prepared by any suitable procedure known to those of skill in the art. For example, the polypeptides may be prepared by a procedure including the steps of: (a) preparing a recombinant polynucleotide comprising a nucleotide sequence encoding a polypeptide comprising the sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or variant or derivative of these, which nucleotide sequence is operably linked to transcriptional and translational regulatory nucleic acid; (b) introducing the recombinant polynucleotide into a suitable host cell; (c) culturing the host cell to express recombinant polypeptide from said recombinant polynucleotide; and (d) isolating the recombinant polypeptide. Suitably, said nucleotide sequence comprises the sequence set forth in any one of SEQ ID NO: 1, 3, 7 and 9. The recombinant polynucleotide is preferably in the form of an expression vector that may be a self -replicating extra-chromosomal vector such as a plasmid, or of a vector that integrates into a host genome. The transcriptional and translational regulatory nucleic acid will generally need to be appropriate for the host cell used for expression. Numerous types of appropriate expression vectors and suitable regulatory sequences are known in the art for a variety of host cells. Typically, the transcriptional and translational regulatory nucleic acid may include, but is not limited to, promoter sequences, leader or signal sequences, ribosomal binding sites, transcriptional start and stop sequences, translational start and termination sequences, and enhancer or activator sequences. Constitutive or inducible promoters as known in the art are contemplated by the invention. The promoters may be either naturally occurring promoters, or hybrid promoters that combine elements of more than one promoter. In a preferred embodiment, the expression vector contains a selectable marker gene to allow the selection of transformed host cells. Selectable marker genes are well known in the art and will vary with the host cell used. The expression vector may also include a fusion partner (typically provided by the expression vector) so that the recombinant polypeptide of the invention is expressed as a fusion polypeptide with said fusion partner. The main advantage of fusion partners is that they assist identification and/or purification of said fusion polypeptide. In order to express said fusion polypeptide, it is necessary to ligate a polynucleotide according to the invention into the expression vector so that the translational reading frames of the fusion partner and the polynucleotide coincide. Well known examples of fusion partners include, but are not limited to, glutathione-S-transferase (GST), Fc potion of human IgG, maltose binding protein (MBP) and hexahistidine (HIS 6 ), which are particularly useful for isolation of the fusion polypeptide by affinity chromatography. For the purposes of fusion polypeptide purification by affinity chromatography, relevant matrices for affinity chromatography include, but are not restricted to, glutathione-, amylose-, and nickel- or cobalt-conjugated resins. Many such matrices are available in "kit" form, such as the QIAexpress™ system (Qiagen) useful with (HIS 6 ) fusion partners and the Pharmacia GST purification system. In a preferred embodiment, the recombinant polynucleotide is expressed in the commercial vector pFLAG as described more fully hereinafter. Another fusion partner well known in the art is green fluorescent protein (GFP). This fusion partner serves as a fluorescent "tag" which allows the fusion polypeptide of the invention to be identified by fluorescence microscopy or by flow cytometry. The GFP tag is useful when assessing subcellular localisation of the fusion polypeptide of the invention, or for isolating cells which express the fusion polypeptide of the invention. Flow cytometric methods such as fluorescence activated cell sorting (FACS) are particularly useful in this latter application. Preferably, the fusion partners also have protease cleavage sites, such as for Factor X a or Thrombin, which allow the relevant protease to partially digest the fusion polypeptide of the invention and thereby liberate the recombinant polypeptide of the invention therefrom. The liberated polypeptide can then be isolated from the fusion partner by subsequent chromatographic separation. Fusion partners according to the invention also include within their scope "epitope tags", which are usually short peptide sequences for which a specific antibody is available. Well known examples of epitope tags for which specific monoclonal antibodies are readily available include c-Myc, influenza virus, haemagglutinin and FLAG tags. The step of introducing into the host cell the recombinant polynucleotide may be effected by any suitable method including transfection, and transformation, the choice of which will be dependent on the host cell employed. Such methods are well known to those of skill in the art. Recombinant polypeptides of the invention may be produced by culturing a host cell transformed with an expression vector containing nucleic acid encoding a polypeptide, biologically active fragment, variant or derivative according to the invention. The conditions appropriate for protein expression will vary with the choice of expression vector and the host cell. This is easily ascertained by one skilled in the art through routine experimentation. Suitable host cells for expression may be prokaryotic or eukaryotic. One preferred host cell for expression of a polypeptide according to the invention is a bacterium. The bacterium used may be Escherichia coli. Alternatively, the host cell may be an insect cell such as, for example, SF9 cells that may be utilised with a baculovirus expression system. The recombinant protein may be conveniently prepared by a person skilled in the art using standard protocols as for example described in Sambrook, et al, MOLECULAR CLONING. A LABORATORY MANUAL (Cold Spring Harbor Press, 1989), in particular Sections 16 and 17; Ausubel et al, CURRENT PROTOCOLS IN MOLECULAR BIOLOGY (John Wiley & Sons, Inc. 1994-1998), in particular Chapters 10 and 16; and Coligan et al, CURRENT PROTOCOLS IN PROTEIN SCIENCE (John Wiley & Sons, Inc. 1995-1997), in particular Chapters 1, 5 and 6. Alternatively, the polypeptide, fragments, variants or derivatives of the invention may be synthesised using solution synthesis or solid phase synthesis as described, for example, in Chapter 9 of Atherton and Shephard (supra) and in Roberge et al (1995, Science 269: 202). 3. Polynucleotides of the invention 3.1 Method of isolating polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes The present invention features a method of isolating novel polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes. The method comprises obtaining an environmental sample from a location in which organisms capable of converting sucrose to isomaltulose have a selective advantage. The environmental sample may comprise, for instance, soil or plant matter including plant surfaces or tissues (e.g., flowers). The environmental sample is preferably obtained from a location that is subject to periodic or constant availability of substantial sucrose concentrations including, but not restricted to, a factory involved in processing or storage sugar-containing plants or plant parts and a field containing remnants of harvested sugar-containing plants. Preferably, but not exclusively, the sugar-containing plant is sugar beet or sugarcane. The method preferably further comprises selecting or otherwise enriching for dual sucrose- and isomaltulose-metabolising organisms that are capable of using both sucrose and isomaltulose as carbon sources for growth. For example, the organisms may be grown on an isomaltulose-containing medium for a time and under conditions sufficient to select or enrich for isomaltulose-metabolising organisms. Organisms thus selected or enriched may be grown subsequently on a sucrose-containing medium for a time and under conditions sufficient to select or enrich for dual isomaltulose- and sucrose-metabolising organisms. The order in which the organisms are grown on the aforesaid media may be reversed if desired. Organisms are screened for those that produce isomaltulose from sucrose using at least one assay that quantifies the production of isomaltulose. Preferably, but not exclusively, the assay is an aniline/diphenylamine assay such as, for example, disclosed in Examples 3 and 4 infra. Alternatively, or in addition thereto, an assay is preferably employed which quantifies the conversion of sucrose to isomaltulose. A suitable assay of this type may quantify the isomaltulose product relative to sucrose and/or related metabolites. For example the capillary electrophoresis assay described in Examples 5 and 6 infra may be used in this regard. Sucrose isomerase-encoding polynucleotides are then isolated from isomaltulose- producing organisms. This isolation preferably comprises screening a nucleic acid library derived from an isomaltulose-producing organism and optionally subclones of this library for polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes. The screening is suitably facilitated using primers or probes that are specific for sucrose isomerase-encoding polynucleotides, as for example disclosed herein. The nucleic acid library is preferably an expression library, which is suitably produced from genomic nucleic acid or cDNA. Desired polynucleotides may be detected using assays that quantify the production of isomaltulose such as, for example, described above. An exemplary protocol for functional screening of polynucleotides is described in Examples 7 to 12. Clones testing positive for isomaltulose production may then be subjected to nucleic acid sequence analysis to identify genes and/or gene products novel in relation to known sucrose isomerases. Enzymatic activities, yields and purities of desired products may then be compared to known reference enzymes under suitable conditions, to identify isolated polynucleotides that encode polypeptides with superior sucrose isomerase activity. 3.2 Polynucleotides encoding polypeptides of the invention The invention further provides a polynucleotide that encodes a polypeptide, fragment, variant or derivative as defined above. In one embodiment, the polynucleotide comprises the entire sequence of nucleotides set forth in SEQ ID NO: 1. SEQ ID NO: 1 corresponds to the full-length E. rhapontici 1899 bp sucrose isomerase coding sequence. This sequence defines: (1) a first region encoding a signal peptide, from nucleotide 1 through about nucleotide 108; and (2) a second region encoding a mature sucrose isomerase enzyme from about nucleotide 109 through nucleotide 1899. Suitably, the polynucleotide comprises the sequence set forth in SEQ ID NO: 3, which defines the region encoding the mature sucrose isomerase polypeptide without the signal sequence. The coding sequence of the present invention comprises an additional 594 bp of sequence at the 3' end relative to the E. rhapontici sucrose isomerase-encoding polynucleotide of Mattes et al. (supra). In another embodiment, the polynucleotide comprises the entire sequence of nucleotides set forth in SΕQ ID NO: 8. SΕQ ID NO: 8 corresponds to the 1791-bp full- length sucrose isomerase coding sequence of the bacterial isolate 68J. SΕQ ID NO: 12 defines: (1) a first region encoding a signal peptide, from nucleotide 1 through about nucleotide 99; and (2) a second region encoding a mature sucrose isomerase enzyme from about nucleotide 100 through nucleotide 1791. Suitably, the polynucleotide comprises the sequence set forth in SΕQ ID NO: 10, which defines the region encoding the mature sucrose isomerase polypeptide without the signal sequence. 3.3 Polynucleotide variants In general, polynucleotide variants according to the invention comprise regions that show at least 60%, more suitably at least 70%, preferably at least 80%, and more preferably at least 90% sequence identity over a reference polynucleotide sequence of identical size (" comparison window") or when compared to an aligned sequence in which the alignment is performed by a computer homology program known in the art. What constitutes suitable variants may be determined by conventional techniques. For example, a polynucleotide according to any one of SΕQ ID NO: 1, 3, 7 and 9 can be mutated using random mutagenesis (e.g., transposon mutagenesis), oligonucleotide-mediated (or site- directed) mutagenesis, PCR mutagenesis and cassette mutagenesis of an earlier prepared variant or non-variant version of an isolated natural promoter according to the invention. Oligonucleotide-mediated mutagenesis is a prefeπed method for preparing nucleotide substitution variants of a polynucleotide of the invention. This technique is well known in the art as, for example, described by Adelman et al. (1983, NA 2:183). Briefly, a polynucleotide according to any one of SΕQ ID NO: 1, 3, 7 or 9 is altered by hybridising an oligonucleotide encoding the desired mutation to a template DNA, wherein the template is the single-stranded form of a plasmid or bacteriophage containing the unaltered or parent DNA sequence. After hybridisation, a DNA polymerase is used to synthesise an entire second complementary strand of the template that will thus incorporate the oligonucleotide primer, and will code for the selected alteration in said parent DNA sequence. Generally, oligonucleotides of at least 25 nucleotides in length are used. An optimal oligonucleotide will have 12 to 15 0 nμcle.otides^t,hat 1 are completely complementary to the template on either side of the nucleotide(s) coding for the mutation. This ensures that the oligonucleotide will hybridise properly to the single-stranded DNA template molecule. The DNA template can be generated by those vectors that are either derived from bacteriophage M13 vectors, or those vectors that contain a single-stranded phage origin of replication as described by Niera et al. (1987, Methods Enzymol. 153:3). Thus, the DΝA that is to be mutated may be inserted into one of the vectors to generate single-stranded template. Production of single-stranded template is described, for example, in Sections 4.21-4.41 of Sambrook et al. (1989, supra). Alternatively, the single-stranded template may be generated by denaturing double-stranded plasmid (or other DΝA) using standard techniques. For alteration of the native DΝA sequence, the oligonucleotide is hybridised to the single-stranded template under suitable hybridisation conditions. A DΝA polymerising enzyme, usually the Klenow fragment of DΝA polymerase I, is then added to synthesise the complementary strand of the template using the oligonucleotide as a primer for synthesis. A heteroduplex molecule is thus formed such that one strand of DΝA encodes the mutated form of the polypeptide or fragment under test, and the other strand (the original template) encodes the native unaltered sequence of the polypeptide or fragment under test. This heteroduplex molecule is then transformed into a suitable host cell, usually a prokaryote such as E. coli. After the cells are grown, they are plated onto agarose plates and screened using the oligonucleotide primer having a detectable label to identify the bacterial colonies having the mutated DΝA. The resultant mutated DΝA fragments are then cloned into suitable expression hosts such as E. coli using conventional technology and clones that retain the desired sucrose isomerase activity are detected. Where the clones have been derived using random mutagenesis techniques, positive clones would have to be sequenced in order to detect the mutation. Alternatively, linker-scanning mutagenesis of DΝA may be used to introduce clusters of point mutations throughout a sequence of interest that has been cloned into a plasmid vector. For example, reference may be made to Ausubel et al, supra, (in particular, Chapter 8.4) which describes a first protocol that uses complementary oligonucleotides and requires a unique restriction site adjacent to the region that is to be mutagenised. A nested series of deletion mutations is first generated in the region. A pair of complementary oligonucleotides is synthesised to fill in the gap in the sequence of interest between the linker at the deletion endpoint and the nearby restriction site. The linker sequence actually provides the desired clusters of point mutations as it is moved or "scanned" across the region by its position at the varied endpoints of the deletion mutation series. An alternate protocol is also described by Ausubel et al, supra, which makes use of site directed mutagenesis procedures to introduce small clusters of point mutations throughout the target region. Briefly, mutations are introduced into a sequence by annealing a synthetic oligonucleotide containing one or more mismatches to the sequence of interest cloned into a single-stranded Ml 3 vector. This template is grown in an E. coli dut ung " strain, which allows the incorporation of uracil into the template strand. The oligonucleotide is annealed to the purified template and extended with T4 DNA polymerase to create a double-stranded heteroduplex. Finally, the heteroduplex is introduced into a wild-type E. coli strain, which will prevent replication of the template strand due to the presence of uracil in template strand, thereby resulting in plaques containing only mutated DNA. Region-specific mutagenesis and directed mutagenesis using PCR may also be employed to construct polynucleotide variants according to the invention. In this regard, reference may be made, for example, to Ausubel et al, supra, in particular Chapters 8.2A and 8.5. Alternatively, suitable polynucleotide sequence variants of the invention may be prepared according to the following procedure: (i) creating primers which are optionally degenerate wherein each comprises a portion of a reference polynucleotide encoding a reference polypeptide or fragment of the invention, preferably encoding the sequence set forth in any one of SΕQ ID NO: 1, 3, 7 or 9; (ii) obtaining a nucleic acid extract from a sucrose-metabolising organism, which is preferably a bacterium, more preferably from a species obtained from a location in which organisms capable of converting sucrose to isomaltulose could obtain a selective advantage as described herein; and (iii) using said primers to amplify, via nucleic acid amplification techniques, at least one amplification product from said nucleic acid extract, wherein said amplification product corresponds to a polynucleotide variant. Suitable nucleic acid amplification techniques are well known to the skilled addressee, and include polymerase chain reaction (PCR) as for example described in Ausubel et al. (supra); strand displacement amplification (SDA) as for example described in U.S. Patent No 5,422,252; rolling circle replication (RCR) as for example described in Liu et al, (1996, J. Am. Chem. Soc. International application WO SUBSTITUTE SHEET7RULE " 26) 92/01813) and Lizardi et al, (International Application WO 97/19193); nucleic acid sequence-based amplification (NASBA) as for example described by Sooknanan et al, (1994, Biotechniques 17:1077-1080); and Q-β replicase amplification as for example described by Tyagi et al, (1996, Proc. Natl. Acad. Sci. USA 93: 5395-5400). Typically, polynucleotide variants that are substantially complementary to a reference polynucleotide are identified by blotting techniques that include a step whereby nucleic acids are immobilised on a matrix (preferably a synthetic membrane such as nitrocellulose), followed by a hybridisation step, and a detection step. Southern blotting is used to identify a complementary DNA sequence; northern blotting is used to identify a complementary RNA sequence. Dot blotting and slot blotting can be used to identify complementary DNA/DNA, DNA/RNA or RNA/RNA polynucleotide sequences. Such techniques are well known by those skilled in the art, and have been described in Ausubel et al. (1994-1998, supra) at pages 2.9J through 2.9.20. According to such methods, Southern blotting involves separating DNA molecules according to size by gel electrophoresis, transferring the size-separated DNA to a synthetic membrane, and hybridising the membrane-bound DNA to a complementary nucleotide sequence labelled radioactively, enzymatically or fluorochromatically. In dot blotting and slot blotting, DNA samples are directly applied to a synthetic membrane prior to hybridisation as above. An alternative blotting step is used when identifying complementary polynucleotides in a cDNA or genomic DNA library, such as through the process of plaque or colony hybridisation. A typical example of this procedure is described in Sambrook et al. ("Molecular Cloning. A Laboratory Manual", Cold Spring Harbour Press, 1989) Chapters 8-12. Typically, the following general procedure can be used to determine hybridisation conditions. Polynucleotides are blotted/transferred to a synthetic membrane, as described above. A reference polynucleotide such as a polynucleotide of the invention is labelled as described above, and the ability of this labelled polynucleotide to hybridise with an immobilised polynucleotide is analysed. A skilled addressee will recognise that a number of factors influence hybridisation. The specific activity of radioactively labelled polynucleotide sequence should typically be greater than or equal to about 10 8 dpm/mg to provide a detectable signal. A radiolabelled nucleotide sequence of specific activity 10 8 to 10 9 dpm/mg can detect approximately 0.5 pg of DNA. It is well known in the art that sufficient DNA must be immobilised on the membrane to permit detection. It is desirable to have excess immobilised DNA, usually 10 μg. Adding an inert polymer such as 10% (w/v) dextran sulfate (MW 500,000) or polyethylene glycol 6000 during hybridisation can also increase the sensitivity of hybridisation (see Ausubel supra at 2.10.10). To achieve meaningful results from hybridisation between a polynucleotide immobilised on a membrane and a labelled polynucleotide, a sufficient amount of the labelled polynucleotide must be hybridised to the immobilised polynucleotide following washing. Washing ensures that the labelled polynucleotide is hybridised only to the immobilised polynucleotide with a desired degree of complementarity to the labelled polynucleotide. It will be understood that polynucleotide variants according to the invention will hybridise to a reference polynucleotide under at least low stringency conditions. Reference herein to low stringency conditions includes and encompasses from at least about 1% v/v to at least about 15% v/v formamide and from at least about 1 M to at least about 2 M salt for hybridisation at 42°C, and at least about 1 M to at least about 2 M salt for washing at 42°C. Low stringency conditions also may include 1% Bovine Serum Albumin (BSA), 1 mM EDTA, 0.5 M NaHPO 4 (pH 7.2), 7% SDS for hybridisation at 65°C, and (i) 2xSSC, 0.1% SDS; or (ii) 0.5% BSA, 1 mM EDTA, 40 mM NaHPO 4 (pH 7.2), 5% SDS for washing at room temperature. Suitably, the polynucleotide variants hybridise to a reference polynucleotide under at least medium stringency conditions. Medium stringency conditions include and encompass from at least about 16% v/v to at least about 30% v/v formamide and from at least about 0.5 M to at least about 0.9 M salt for hybridisation at 42°C, and at least about 0J M to at least about 0.2 M salt for washing at 55°C. Medium stringency conditions also may include 1% Bovine Serum Albumin (BSA), 1 mM EDTA, 0.5 M NaHPO 4 (pH 7.2), 7% SDS for hybridisation at 65°C, and (i) 2 x SSC, 0.1% SDS; or (ii) 0.5% BSA, 1 mM EDTA, 40 mM NaHPO 4 (pH 7.2), 5% SDS for washing at 60-65°C. Preferably, the polynucleotide variants hybridise to a reference polynucleotide under high stringency conditions. High stringency conditions include and encompass from at least about 31% v/v to at least about 50% v/v formamide and from about 0.01 M to about 0J5 M salt for hybridisation at 42°C, and about 0.01 M to about 0.02 M salt for washing at 55°C. High stringency conditions also may include 1% BSA, 1 mM EDTA, 0.5 M NaHPO 4 (pH 7.2), 7% SDS for hybridisation at 65°C, and (i) 0.2 x SSC, 0.1% SDS; or (ii) 0.5% BSA, lmM EDTA, 40 mM NaHPO 4 (pH 7.2), 1% SDS for washing at a temperature in excess of 65°C. Other stringent conditions are well known in the art. A skilled addressee will recognise that various factors can be manipulated to optimise the specificity of the hybridisation. Optimisation of the stringency of the final washes can serve to ensure a high degree of hybridisation. For detailed examples, see Ausubel et al, supra at pages 2J0J to 2J0J6 and Sambrook et al. (1989, supra) at sections 1.101 to 1J04. While stringent washes are typically carried out at temperatures from about 42°C to 68°C, one skilled in the art will appreciate that other temperatures may be suitable for stringent conditions. Maximum hybridisation rate typically occurs at about 20°C to 25°C below the T m for formation of a DNA-DNA hybrid. It is well known in the art that the T m is the melting temperature, or temperature at which two complementary polynucleotide sequences dissociate. Methods for estimating T m are well known in the art (see Ausubel et al, supra at page 2J0.8). In general, the T m of a perfectly matched duplex of DNA may be predicted as an approximation by the formula: T m = 81.5 + 16.6 (log 10 M) + 0.41 (%G+C) - 0.63 (% formamide) - (600/length) wherein: M is the concentration of Na + , preferably in the range of 0.01 molar to 0.4 molar; %G+C is the sum of guanosine and cytosine bases as a percentage of the total number of bases, within the range between 30% and 75% G+C; % formamide is the percent formamide concentration by volume; length is the number of base pairs in the DNA duplex. The T m of a duplex DNA decreases by approximately 1°C with every increase of 1% in the number of randomly mismatched base pairs. Washing is generally carried out at T m - 15 °C for high stringency, or T m - 30 °C for moderate stringency. In a preferred hybridisation procedure, a membrane (e.g., a nitrocellulose membrane or a nylon membrane) containing immobilised DNA is hybridised overnight at 42°C in a hybridisation buffer (50% deionised formamide, 5xSSC, 5x Denhardt's solution (0.1% ficoll, 0.1% polyvinylpyrollidone and 0.1% bovine serum albumin), 0.1% SDS and 200 mg/mL denatured salmon sperm DNA) containing labelled probe. The membrane is then subjected to two sequential medium stringency washes (i.e., 2xSSC, 0.1% SDS for 15 min at 45°C, followed by 2xSSC, 0.1% SDS for 15 min at 50°C), followed by two sequential higher stringency washes (i.e., 0.2xSSC, 0.1% SDS for 12 min at 55°C followed by 0.2xSSC and 0J%SDS solution for 12 min at 65-68°C). Methods for detecting a labelled polynucleotide hybridised to an immobilised polynucleotide are well known to practitioners in the art. Such methods include autoradiography, phosphorimaging, and chemiluminescent, fluorescent and colorimetric detection. 4. Antigen-binding molecules The invention also contemplates antigen-binding molecules that bind specifically to the aforementioned polypeptides, fragments, variants and derivatives. Preferably, an antigen-binding molecule according to the invention is immuno-interactive with any one or more of the amino acid sequences set forth in SEQ ID NO: 2, 4, 8, 10, 19, 20, 21, 22, 23 and 24 or variants thereof. For example, the antigen-binding molecules may comprise whole polyclonal antibodies. Such antibodies may be prepared, for example, by injecting a polypeptide, fragment, variant or derivative of the invention into a production species, which may include mice or rabbits, to obtain polyclonal antisera. Methods of producing polyclonal antibodies are well known to those skilled in the art. Exemplary protocols which may be used are described for example in Coligan et al, CURRENT PROTOCOLS IN IMMUNOLOGY, (John Wiley & Sons, Inc, 1991), and Ausubel et al, (1994-1998, supra), in particular Section HI of Chapter 11. In lieu of the polyclonal antisera obtained in the production species, monoclonal antibodies may be produced using the standard method as described, for example, by Kohler and Milstein (1975, Nature 256, 495-497), or by more recent modifications thereof as described, for example, in Coligan et al, (1991, supra) by immortalising spleen or other antibody producing cells derived from a production species which has been inoculated with one or more of the polypeptides, fragments, variants or derivatives of the invention. The invention also contemplates as antigen-binding molecules Fv, Fab, Fab' and F(ab')2 immunoglobulin fragments. Alternatively, the antigen-binding molecule may comprise a synthetic stabilised Fv fragment. Exemplary fragments of this type include single chain Fv fragments (sFv, frequently termed scFv) in which a peptide linker is used to bridge the N terminus or C terminus of a V # domain with the C terminus or N-terminus, respectively, of a V_ domain. ScFv lack all constant parts of whole antibodies and are not able to activate complement. Suitable peptide linkers for joining the V # and V L domains are those which allow the V # and V domains to fold into a single polypeptide chain having an antigen binding site with a three dimensional structure similar to that of the antigen binding site of a whole antibody from which the Fv fragment is derived. Linkers having the desired properties may be obtained by the method disclosed in U.S. Patent No 4,946,778. However, in some cases a linker is absent. ScFvs may be prepared, for example, in accordance with methods outlined in Kreber et al (Kreber et al. 1997, /. Immunol. Methods; 201(1): 35-55). Alternatively, they may be prepared by methods described in U.S. Patent No 5,091,513, European Patent No 239,400 or the articles by Winter and Milstein (1991, Nature 349:293) and Pluckthun et al (1996, In Antibody engineering: A practical approach. 203-252). Alternatively, the synthetic stabilised Fv fragment comprises a disulphide stabilised Fv (dsFv) in which cysteine residues are introduced into the V and V domains such that in the fully folded Fv molecule the two residues will form a disulphide bond therebetween. Suitable methods of producing dsFv are described for example in (Glockscuther et al. Biochem. 29: 1363-1367; Reiter et al. 1994, /. Biol. Chem. 269: 18327-18331; Reiter et al. 1994, Biochem. 33: 5451-5459; Reiter et al. 1994. Cancer Res. 54: 2714-2718; Webber et al. 1995, Mol. Immunol. 32: 249-258). Also contemplated as antigen-binding molecules are single variable region domains (termed dAbs) as for example disclosed in Ward et al. (1989, Nature 341: 544- 546); Hamers-Casterman et al. (1993, Nature. 363: 446-448); Davies & Riechmann, (1994, FEBS Lett. 339: 285-290). Alternatively, the antigen-binding molecule may comprise a "minibody". In this regard, minibodies are small versions of whole antibodies, which encode in a single chain the essential elements of a whole antibody. Suitably, the minibody is comprised of the V H and V L domains of a native antibody fused to the hinge region and CH3 domain of the immunoglobulin molecule as, for example, disclosed in U.S. Patent No 5,837,821. In an alternate embodiment, the antigen binding molecule may comprise non- immunoglobulin derived, protein frameworks. For example, reference may be made to Ku & Schultz, (1995, Proc. Natl. Acad. Sci. USA, 92: 652-6556) which discloses a four-helix bundle protein cytochrome b562 having two loops randomised to create complementarity determining regions (CDRs), which have been selected for antigen binding. The antigen-binding molecule may be multivalent (i.e., having more than one antigen binding site). Such multivalent molecules may be specific for one or more antigens. Multivalent molecules of this type may be prepared by dimerisation of two antibody fragments through a cysteinyl-containing peptide as, for example disclosed by Adams et al, (1993, Cancer Res. 53: 4026-4034) and Cumber et al. (1992, J. Immunol. 149: 120-126). Alternatively, dimerisation may be facilitated by fusion of the antibody fragments to amphiphilic helices that naturally dimerise (Pack P. Plϋnckthun, 1992, Biochem. 31: 1579-1584), or by use of domains (such as the leucine zippers jun and fos) that preferentially heterodimerise (Kostelny et al, 1992, /. Immunol. 148: 1547-1553). In an alternate embodiment, the multivalent molecule may comprise a multivalent single chain antibody (multi-scFv) comprising at least two scFvs linked together by a peptide linker. In this regard, non-covalently or covalently linked scFv dimers termed "diabodies" may be used. Multi-scFvs may be bispecific or greater depending on the number of scFvs employed having different antigen binding specificities. Multi-scFvs may be prepared for example by methods disclosed in U.S. Patent No. 5,892,020. The antigen-binding molecules of the invention may be used for affinity chromatography in isolating a natural or recombinant polypeptide or biologically active fragment of the invention. For example reference may be made to immunoaffinity chromatographic procedures described in Chapter 9.5 of Coligan et al., (1995-1997, supra). The antigen-binding molecules can be used to screen expression libraries for variant polypeptides of the invention as described herein. They can also be used to detect and/or isolate the polypeptides, fragments, variants and derivatives of the invention. Thus, the invention also contemplates the use of antigen-binding molecules to isolate sucrose isomerase enzymes using , for example, any suitable immunoaffinity based method including, but not limited to, immunochromatography and immunoprecipitation. A preferred method utilises solid phase adsorption in which anti-sucrose isomerase antigen- binding molecules are attached to a suitable resin, the resin is contacted with a sample suspected of containing sucrose isomerases, and the sucrose isomerases, if any, are subsequently eluted from the resin. Preferred resins include: Sepharose® (Pharmacia), Poros® resins (Roche Molecular Biochemicals, Indianapolis), Actigel Superflow™ resins (Sterogene Bioseparations Inc., Carlsbad Calif.), and Dynabeads™ (Dynal Inc., Lake Success, N.Y.). 5. Methods of Detection 5J Detection of polypeptides according to the invention The invention also extends to a method of detecting in a sample a polypeptide, fragment, variant or derivative as broadly described above, comprising contacting the sample with an antigen-binding molecule as described in Section 4 and detecting the presence of a complex comprising the said antigen-binding molecule and the said polypeptide, fragment, variant or derivative in said contacted sample. Any suitable technique for determining formation of the complex may be used. For example, an antigen-binding molecule according to the invention, having a reporter molecule associated therewith may be utilised in immunoassays. Such immunoassays include, but are not limited to, radioimmunoassays (RIAs), enzyme-linked immunosorbent assays (ELISAs) and immunochromatographic techniques (ICTs), Western blotting which are well known those of skill in the art. For example, reference may be made to "CURRENT PROTOCOLS IN IMMUNOLOGY" (1994, supra) which discloses a variety of immunoassays that may be used in accordance with the present invention. Immunoassays may include competitive assays as understood in the art or as for example described infra. It will be understood that the present invention encompasses qualitative and quantitative immunoassays. Suitable immunoassay techniques are described for example in US Patent Nos. 4,016,043, 4, 424,279 and 4,018,653. These include both single-site and two-site assays of the non-competitive types, as well as the traditional competitive binding assays. These assays also include direct binding of a labelled antigen-binding molecule to a target antigen. Two site assays are particularly favoured for use in the present invention. A number of variations of these assays exist, all of which are intended to be encompassed by the present invention. Briefly, in a typical forward assay, an unlabelled antigen-binding molecule such as an unlabelled antibody is immobilised on a solid substrate and the sample to be tested brought into contact with the bound molecule. After a suitable period of incubation, for a period of time sufficient to allow formation of an antibody-antigen complex, another antigen-binding molecule, suitably a second antibody specific to the antigen, labelled with a reporter molecule capable of producing a detectable signal is then added and incubated, allowing time sufficient for the formation of another complex of antibody-antigen-labelled antibody. Any unreacted material is washed away and the presence of the antigen is determined by observation of a signal produced by the reporter molecule. The results may be either qualitative, by simple observation of the visible signal, or may be quantitated by comparing with a control sample containing known amounts of antigen. Variations on the forward assay include a simultaneous assay, in which both sample and labelled antibody are added simultaneously to the bound antibody. These techniques are well known to those skilled in the art, including minor variations as will be readily apparent. In accordance with the present invention, the sample is one that might contain a sucrose isomerase such as from a sucrose-metabolising organism. Preferably, the sucrose-metabolising organism is a bacterium, which is suitably obtained from a location in which organisms that are capable of converting sucrose to isomaltulose have a selective advantage. In the typical forward assay, a first antibody having specificity for the antigen or antigenic parts thereof is either covalently or passively bound to a solid surface. The solid surface is typically glass or a polymer, the most commonly used polymers being cellulose, polyacrylamide, nylon, polystyrene, polyvinyl chloride or polypropylene. The solid supports may be in the form of tubes, beads, discs of microplates, or any other surface suitable for conducting an immunoassay. The binding processes are well known in the art and generally consist of cross-linking, covalently binding or physically adsorbing. The polymer-antibody complex is washed in preparation for the test sample. An aliquot of the sample to be tested is then added to the solid phase complex and incubated for a period of time sufficient and under suitable conditions to allow binding of any antigen present to the antibody. Following the incubation period, the antigen-antibody complex is washed and dried and incubated with a second antibody specific for a portion of the antigen. The second antibody has generally a reporter molecule associated therewith that is used to indicate the binding of the second antibody to the antigen. The amount of labelled antibody that binds, as determined by the associated reporter molecule, is proportional to the amount of antigen bound to the immobilised first antibody. An alternative method involves immobilising the antigen in the biological sample and then exposing the immobilised antigen to specific antibody that may or may not be labelled with a reporter molecule. Depending on the amount of target and the strength of the reporter molecule signal, a bound antigen may be detectable by direct labelling with the antibody. Alternatively, a second labelled antibody, specific to the first antibody is exposed to the target-first antibody complex to form a target-first antibody-second antibody tertiary complex. The complex is detected by the signal emitted by the reporter molecule. From the foregoing, it will be appreciated that the reporter molecule associated with the antigen-binding molecule may include the following: (a) direct attachment of the reporter molecule to the antigen-binding molecule; (b) indirect attachment of the reporter molecule to the antigen-binding molecule; i.e., attachment of the reporter molecule to another assay reagent which subsequently binds to the antigen-binding molecule; and (c) attachment to a subsequent reaction product of the antigen-binding molecule. The reporter molecule may be selected from a group including a chromogen, a catalyst, an enzyme, a fluorochrome, a chemiluminescent molecule, a lanthanide ion such as Europium (Eu 34 ), a radioisotope and a direct visual label. In the case of a direct visual label, use may be made of a colloidal metallic or non- metallic particle, a dye particle, an enzyme or a substrate, an organic polymer, a latex particle, a liposome, or other vesicle containing a signal producing substance and the like. A large number of enzymes suitable for use as reporter molecules is disclosed in United States Patent Specifications U.S. 4,366,241, U.S. 4,843,000, and U.S. 4,849,338. Suitable enzymes useful in the present invention include alkaline phosphatase, horseradish peroxidase, luciferase, β-galactosidase, glucose oxidase, lysozyme, malate dehydrogenase and the like. The enzymes may be used alone or in combination with a second enzyme that is in solution. Suitable fluorochromes include, but are not limited to, fluorescein isothiocyanate (FIT C), tetramethylrhodamine isothiocyanate (TRITC), R-Phycoerythrin (RPE), and Texas Red. Other exemplary fluorochromes include those discussed by Dower et al. (International Publication WO 93/06121). Reference also may be made to the fluorochromes described in U.S. Patents 5,573,909 (Singer et al), 5,326,692 (Brinkley et al). Alternatively, reference may be made to the fluorochromes described in U.S. Patent Nos. 5,227,487, 5,274,113, 5,405,975, 5,433,896, 5,442,045, 5,451,663, 5,453,517, 5,459,276, 5,516,864, 5,648,270 and 5,723,218. In the case of an enzyme immunoassay, an enzyme is conjugated to the second antibody, generally by means of glutaraldehyde or periodate. As will be readily recognised, however, a wide variety of different conjugation techniques exist which are readily available to the skilled artisan. The substrates to be used with the specific enzymes are generally chosen for the production of, upon hydrolysis by the corresponding enzyme, a detectable colour change. Examples of suitable enzymes include those described supra. It is also possible to employ fluorogenic substrates, which yield a fluorescent product rather than the chromogenic substrates noted above. In all cases, the enzyme-labelled antibody is added to the first antibody-antigen complex. It is then allowed to bind, and excess reagent is washed away. A solution containing the appropriate substrate is then added to the complex of antibody-antigen-antibody. The substrate will react with the enzyme linked to the second antibody, giving a qualitative visual signal, which may be further quantitated, usually spectrophotometrically, to give an indication of the amount of antigen which was present in the sample. Fluorescent compounds, such as fluorescein, rhodamine and the lanthanide, europium (EU), may be alternately chemically coupled to antibodies without altering their binding capacity. When activated by illumination with light of a particular wavelength, the fluorochrome-labelled antibody adsorbs the light energy, inducing a state to excitability in the molecule, followed by emission of the light at a characteristic colour visually detectable with a light microscope. The fluorescent-labelled antibody is allowed to bind to the first antibody-antigen complex. After washing off the unbound reagent, the remaining tertiary complex is then exposed to light of an appropriate wavelength. The fluorescence observed indicates the presence of the antigen of interest. Irnmunofluorometric assays (IFMA) are well established in the art. However, other reporter molecules, such as radioisotope, chemiluminescent or bioluminescent molecules may also be employed. 5.2 Detection of polynucleotides according to the invention In another embodiment, the method for detection comprises detecting expression in a cell of a polynucleotide encoding said polypeptide, fragment, variant or derivative. Expression of the said polynucleotide may be determined using any suitable technique. For example, a labelled polynucleotide encoding a said member may be utilised as a probe in a Northern blot of a RNA extract obtained from the muscle cell. Preferably, a nucleic acid extract from the animal is utilised in concert with oligonucleotide primers corresponding to sense and antisense sequences of a polynucleotide encoding a said member, or flanking sequences thereof, in a nucleic acid amplification reaction such as RT PCR. A variety of automated solid-phase detection techniques is also appropriate. For example, very large scale immobilised primer arrays (VLSIPS™) are used for the detection of nucleic acids as for example described by Fodor et al. (1991, Science 251:161-111) and Kazal et al. (1996, Nature Medicine 2:753-759). The above generic techniques are well known to persons skilled in the art. 6. Chimeric nucleic acid constructs 6.1 Prokaryotic expression The present invention further relates to a chimeric nucleic acid construct designed for genetic transformation of prokaryotic cells, comprising a polynucleotide, fragment or variant according to the invention operably linked to a promoter sequence. Preferably, the chimeric construct is operable in a Gram-negative prokaryotic cell. A variety of prokaryotic expression vectors, which may be used as a basis for constructing the chimeric nucleic acid construct, may be utilised to express a polynucleotide, fragment or variant according to the invention. These include but are not limited to a chromosomal vector (e.g., a bacteriophage such as bacteriophage λ), an extrachromosomal vector (e.g., a plasmid or a cosmid expression vector). The expression vector will also typically contain an origin of replication, which allows autonomous replication of the vector, and one or more genes that allow phenotypic selection of the transformed cells. Any of a number of suitable promoter sequences, including constitutive and inducible promoter sequences, may be used in the expression vector (see e.g., Bitter, et al., 1987, Methods in Enzymology 153: 516-544). For example, inducible promoters such as pL of bacteriophage γ, plac, ptrp, ptac ptrp-lac hybrid promoter and the like may be used. The chimeric nucleic acid construct may then be used to transform the desired prokaryotic host cell to produce a recombinant prokaryotic host cell for producing a recombinant polypeptide as described above or for producing isomaltulose as described hereinafter. 6.2 Eukaryotic expression The invention also contemplates a chimeric nucleic acid construct designed for expressing a polynucleotide, fragment or variant of the invention in a eukaryotic host cell. A variety of eukaryotic host-expression vector systems may be utilised in this regard. These include, but are not limited to, yeast transformed with recombinant yeast expression vectors; insect cell systems infected with recombinant virus expression vectors (e.g., baculovirus); or animal cell systems infected with recombinant virus expression vectors (e.g., retroviruses, adenovirus, Vaccinia virus), or transformed animal cell systems engineered for stable expression. Preferably, the chimeric nucleic acid construct is designed for genetic transformation of plants as described hereinafter. 6.3 Plant expression In a preferred embodiment, a polynucleotide, fragment or variant according to the invention is fused to a promoter sequence and a 3' non-translated sequence to create a chimeric DNA construct, designed for genetic transformation of plants. 6.3.1 Plant promoters Promoter sequences contemplated by the present invention may be native to the host plant to be transformed or may be derived from an alternative source, where the region is functional in the host plant. Other sources include the Agrobacterium T-DNA genes, such as the promoters for the biosynthesis of nopaline, octapine, mannopine, or other opine promoters; promoters from plants, such as the ubiquitin promoter; tissue specific promoters (see, e.g., U.S. Pat. No. 5,459,252 to Conkling et al; WO 91/13992 to Advanced Technologies); promoters from viruses (including host specific viruses), or partially or wholly synthetic promoters. Numerous promoters that are functional in mono- and dicotyledonous plants are well known in the art (see, for example, Greve, 1983, J. Mol. Appl. Genet. 1: 499-511; Salomon et al, 1984, EMBO J. 3: 141-146; Garfinkel et al, 1983, Cell 27: 143-153; Barker et al, 1983, Plant Mol. Biol. 2: 235-350); including various promoters isolated from plants (such as the Ubi promoter from the maize ubi-1 gene, Christensen and Quail, 1996) (see, e.g., U.S. Pat. No. 4,962,028) and viruses (such as the cauliflower mosaic virus promoter, CaMV 3„5S). The promoters sequences may include regions which regulate transcription, where the regulation involves, for example, chemical or physical repression or induction (e.g., regulation based on metabolites, light, or other physicochemical factors; see, e.g., WO 93/06710 disclosing a nematode responsive promoter) or regulation based on cell differentiation (such as associated with leaves, roots, seed, or the like in plants; see, e.g., U.S. Pat. No. 5,459,252 disclosing a root-specific promoter). Thus, the promoter region, or the regulatory portion of such region, is obtained from an appropriate gene that is so regulated. For example, the 1,5-ribulose biphosphate carboxylase gene is light-induced and may be used for transcriptional initiation. Other genes are known which are induced by stress, temperature, wounding, pathogen effects, etc. The preferred promoter for expression in cultured cells is a strong constitutive promoter, or a promoter that responds to a specific inducer (Gatz and Lenk, 1998, Trends Plant Science 3: 352-8). The preferred promoter for expression in intact plants is a promoter expressed in sucrose storage tissues (such as the mature stems of sugarcane and the tubers of sugar beet), or an inducible promoter to drive conversion of sucrose to isomaltulose at a late stage before harvest with minimal disruption to other plant growth and development processes. 6.3.2 3' Non-translated region The chimeric gene construct of the present invention can comprise a 3' non- translated sequence. A 3' non-translated sequence refers to that portion of a gene comprising a DNA segment that contains a polyadenylation signal and any other regulatory signals capable of effecting mRNA processing or gene expression. The polyadenylation signal is characterised by effecting the addition of polyadenylic acid tracts to the 3' end of the mRNA precursor. Polyadenylation signals are commonly recognised by the presence of homology to the canonical form 5' AATAAA-3' although variations are not uncommon. The 3' non-translated regulatory DNA sequence preferably includes from about 50 to 1,000 nucleotide base pairs and may contain plant transcriptional and translational termination sequences in addition to a polyadenylation signal and any other regulatory signals capable of effecting mRNA processing or gene expression. Examples of suitable 3' non-translated sequences are the 3' transcribed non-translated regions containing a polyadenylation signal from the nopaline synthase (nos) gene of Agrobacterium tumefaciens (Bevan et al, 1983, Nucl. Acid Res., 11:369) and the terminator for the T7 transcript from the octopine synthase gene of Agrobacterium tumefaciens. Alternatively, suitable 3' non-translated sequences may be derived from plant genes such as the 3' end of the protease inhibitor I or II genes from potato or tomato, the soybean storage protein genes and the pea E9 small subunit of the ribulose-l,5-bisphosphate carboxylase (ssRUBISCO) gene, although other 3' elements known to those of skill in the art can also be employed. Alternatively, 3' non-translated regulatory sequences can be obtained de novo as, for example, described by An (1987, Methods in Enzymology, 153:292), which is incorporated herein by reference. 6.3.3 Optional sequences The chimeric DNA construct of the present invention can further include enhancers, either translation or transcription enhancers, as may be required. These enhancer regions are well known to persons skilled in the art, and can include the ATG initiation codon and adjacent sequences. The initiation codon must be in phase with the reading frame of the coding sequence relating to the foreign or endogenous DNA sequence to ensure translation of the entire sequence. The translation control signals and initiation codons can be of a variety of origins, both natural and synthetic. Translational initiation regions may be provided from the source of the transcriptional initiation region, or from the foreign or endogenous DNA sequence. The sequence can also be derived from the source of the promoter selected to drive transcription, and can be specifically modified so as to increase translation of the mRNA. Examples of transcriptional enhancers include, but are not restricted to, elements < from the CaMN 35S promoter and octopine synthase genes as for example described by Last et al. (U.S. Patent No. 5,290,924, which is incorporated herein by reference). It is proposed that the use of an enhancer element such as the ocs element, and particularly multiple copies of the element, will act to increase the level of transcription from adjacent promoters when applied in the context of plant transformation. Alternatively, the omega sequence derived from the coat protein gene of the tobacco mosaic virus (Gallie et al, 1987) may be used to enhance translation of the mRNA transcribed from a polynucleotide according to the invention. As the DNA sequence inserted between the transcription initiation site and the start of the coding sequence, i.e., the untranslated leader sequence, can influence gene expression, one can also employ a particular leader sequence. Preferred leader sequences include those that comprise sequences selected to direct optimum expression of the foreign or endogenous DNA sequence. For example, such leader sequences include a preferred consensus sequence which can increase or maintain mRNA stability and prevent inappropriate initiation of translation as for example described by Joshi (1987, Nucl. Acid Res., 15:6643), which is incorporated herein by reference. However, other leader sequences, e.g., the leader sequence of RTBN, have a high degree of secondary structure that is expected to decrease mRΝA stability and/or decrease translation of the mRΝA. Thus, leader sequences (i) that do not have a high degree of secondary structure, (ii) that have a high degree of secondary structure where the secondary structure does not inhibit mRΝA stability and/or decrease translation, or (iii) that are derived from genes that are highly expressed in plants, will be most preferred. Regulatory elements such as the sucrose synthase intron as, for example, described by Nasil et al. (1989, Plant Physiol, 91:5175), the Adh intron I as, for example, described by Callis et al. (1987, Genes Develop., H), or the TMN omega element as, for example, described by Gallie et al. (1989, The Plant Cell, 1:301) can also be included where desired. Other such regulatory elements useful in the practice of the invention are known to those of skill in the art. Additionally, targeting sequences may be employed to target a protein product of the foreign or endogenous DΝA sequence to an intracellular compartment within plant cells or to the extracellular environment. For example, a DΝA sequence encoding a transit or signal peptide sequence may be operably linked to a sequence encoding a desired protein such that, when translated, the transit or signal peptide can transport the protein to a particular intracellular or extracellular destination, and can then be post-translationally removed. Transit or signal peptides act by facilitating the transport of proteins through intracellular membranes, e.g., endoplasmic reticulum, vacuole, vesicle, plastid, mitochondrial and plasmalemma membranes. For example, the targeting sequence can direct a desired protein to a particular organelle such as a vacuole or a plastid (e.g., a chloroplast), rather than to the cytosol. Thus, the chimeric DΝA construct can further comprise a plastid transit peptide encoding DΝA sequence operably linked between a promoter region or promoter variant according to the invention and the foreign or endogenous DΝA sequence. For example, reference may be made to Heijne et al. (1989, Eur. J. Biochem., 180:535) and Keegstra et al. (1989, Ann. Rev. Plant Physiol. Plant Mol. Biol, 40:471), which are incorporated herein by reference. A chimeric DNA construct can also be introduced into a vector, such as a plasmid. Plasmid vectors include additional DNA sequences that provide for easy selection, amplification, and transformation of the expression cassette in prokaryotic and eukaryotic cells, e.g., pUC-derived vectors, pSK-derived vectors, pGEM-derived vectors, pSP-derived vectors, or pBS-derived vectors. Additional DNA sequences include origins of replication to provide for autonomous replication of the vector, selectable marker genes, preferably encoding antibiotic or herbicide resistance, unique multiple cloning sites providing for multiple sites to insert DNA sequences or genes encoded in the chimeric DNA construct, and sequences that enhance transformation of prokaryotic and eukaryotic cells. The vector preferably contains an element(s) that permits either stable integration of the vector into the host cell genome or autonomous replication of the vector in the cell independent of the genome of the cell. The vector may be integrated into the host cell genome when introduced into a host cell. For integration, the vector may rely on a foreign or endogenous DNA sequence present therein or any other element of the vector for stable integration of the vector into the genome by homologous recombination. Alternatively, the vector may contain additional nucleic acid sequences for directing integration by homologous recombination into the genome of the host cell. The additional nucleic acid sequences enable the vector to be integrated into the host cell genome at a precise location in the chromosome. To increase the likelihood of integration at a precise location, the integrational elements should preferably contain a sufficient number of nucleic acids, such as 100 to 1,500 base pairs, preferably 400 to 1,500 base pairs, and most preferably 800 to 1,500 base pairs, which are highly homologous with the corresponding target sequence to enhance the probability of homologous recombination. The integrational elements may be any sequence that is homologous with the target sequence in the genome of the host cell. Furthermore, the integrational elements may be non-encoding or encoding nucleic acid sequences. For cloning and subcloning purposes, the vector may further comprise an origin of replication enabling the vector to replicate autonomously in a host cell such as a bacterial cell. Examples of bacterial origins of replication are the origins of replication of plasmids pBR322, pUC19, ρACYC177, and pACYC184 permitting replication in E. coli, and pUBHO, pE194, pTA1060, and pAMβl permitting replication in Bacillus. The origin of replication may be one having a mutation to make its function temperature-sensitive in a Bacillus cell (see, e.g., Ehrlich, 1978, Proc. Natl. Acad. Sci. USA 75:1433). 6.3.4 Marker genes To facilitate identification of transformants, the chimeric DNA construct desirably comprises a selectable or screenable marker gene as, or in addition to, a polynucleotide sequence according to the invention. The actual choice of a marker is not crucial as long as it is functional (i.e., selective) in combination with the plant cells of choice. The marker gene and the foreign or endogenous DNA sequence of interest do not have to be linked, since co-transformation of unlinked genes as, for example, described in U.S. Pat. No. 4,399,216 is also an efficient process in plant transformation. Included within the terms selectable or screenable marker genes are genes that encode a "secretable marker" whose secretion can be detected as a means of identifying or selecting for transformed cells. Examples include markers that encode a secretable antigen that can be identified by antibody interaction, or secretable enzymes that can be detected by their catalytic activity. Secretable proteins include, but are not restricted to, proteins that are inserted or trapped in the cell wall (e.g., proteins that include a leader sequence such as that found in the expression unit of extensin or tobacco PR-S); small, diffusible proteins detectable, e.g. by ELISA; and small active enzymes detectable in extracellular solution (e.g., α-amylase, β-lactamase, phosphinothricin acetyltransf erase). 6.3.5 Selectable markers Examples of bacterial selectable markers are the dal genes from Bacillus subtilis or Bacillus licheniformis, or markers that confer antibiotic resistance such as ampicillin, kanamycin, erythromycin, chloramphenicol or tetracycline resistance. Exemplary selectable markers for selection of plant transformants include, but are not limited to, a hyg gene which encodes hygromycin B resistance; a neomycin phosphotransferase (neo) gene conferring resistance to kanamycin, paromomycin, G418 and the like as, for example, described by Potrykus et al. (1985, Mol. Gen. Genet. 199:183); a glutathione-S-transf erase gene from rat liver conferring resistance to glutathione derived herbicides as, foi example, described in EP-A 256 223; a glutamine synthetase gene conferring, upon overexpression, resistance to glutamine synthetase inhibitors such as phosphinothricin as, for example, described WO87/05327, an acetyl transferase gene from Streptomyces viridochromo genes conferring resistance to the selective agent phosphinothricin as, for example, described in EP-A 275 957, a gene encoding a 5-enolshikimate-3-phosphate synthase (EPSPS) conferring tolerance to N-phosphonomethylglycine as, for example, described by Hinchee et al. (1988, Biotech., 6:915), a bar gene conferring resistance against bialaphos as, for SUBSTlfUTE SHEET (RULE 26) example, described in WO91/02071; a nitrilase gene such as bxn from Klebsiella ozaenae which confers resistance to bromoxynil (Stalker et al, 1988, Science, 242:419); a dihydrofolate reductase (DHFR) gene conferring resistance to methotrexate (Thillet et al, 1988, J. Biol. Chem., 263:12500); a mutant acetolactate synthase gene (ALS), which confers resistance to imidazolinone, sulfonylurea or other ALS -inhibiting chemicals (EP- A-154 204); a mutated anthranilate synthase gene that confers resistance to 5-methyl tryptophan; or a dalapon dehalogenase gene that confers resistance to the herbicide. 6.3.6 Screenable markers Preferred screenable markers include, but are not limited to, a uidA gene encoding a β-glucuronidase (GUS) enzyme for which various chromogenic substrates are known; a β-galactosidase gene encoding an enzyme for which chromogenic substrates are known; an aequorin gene (Prasher et al, 1985, Biochem. Biophys. Res. Comm., 126:1259), which may be employed in calcium-sensitive bioluminescence detection; a green fluorescent protein gene (Niedz et al, 1995 Plant Cell Reports, 14:403); a luciferase (luc) gene (Ow et al, 1986, Science, 234:856), which allows for bioluminescence detection; a β-lactamase gene (Sutcliffe, 1978, Proc. Natl. Acad. Sci. USA 75:3737), which encodes an enzyme for which various chromogenic substrates are known (e.g., PAD AC, a chromogenic cephalosporin); an R-locus gene, encoding a product that regulates the production of anthocyanin pigments (red colour) in plant tissues (Dellaporta et al, 1988, in Chromosome Structure and Function, pp. 263-282); an α-amylase gene (Ikuta et al, 1990, Biotech., 8:241); a tyrosinase gene (Katz et al, 1983, J. Gen. Microbiol, 129:2703) which encodes an enzyme capable of oxidising tyrosine to dopa and dopaquinone which in turn condenses to form the easily detectable compound melanin; or a xylE gene (Zukowsky et al, 1983, Proc. Natl. Acad. Sci. USA 80:1101), which encodes a catechol dioxygenase that can convert chromogenic catechols. 7. Introduction of chimeric construct into plant cells A number of techniques are available for the introduction of DNA into a plant host cell. There are many plant transformation techniques well known to workers in the art, and new techniques are continually becoming known. The particular choice of a transformation technology will be determined by its efficiency to transform certain plant species as well as the experience and preference of the person practising the invention with a particular methodology of choice. It will be apparent to the skilled person that the particular choice of a transformation system to introduce a chimeric DNA construct into plant cells is not essential to or a limitation of the invention, provided it achieves an acceptable level of nucleic acid transfer. Guidance in the practical implementation of transformation systems for plant improvement is provided by Birch (1997, Annu. Rev. Plant Physiol. Plant Molec. Biol 48: 297-326). In principle both dicotyledonous and monocotyledonous plants that are amenable to transformation, can be modified by introducing a chimeric DNA construct according to the invention into a recipient cell and growing a new plant that harbours and expresses a polynucleotide according to the invention. Introduction and expression of foreign or chimeric DNA sequences in dicotyledonous (broadleaved) plants such as tobacco, potato and alfalfa has been shown to be possible using the T-DNA of the tumour-inducing (Ti) plasmid of Agrobacterium tumefaciens (See, for example, U beck, U.S. Patent No. 5,004,863, and International application PCT/US93/02480). A construct of the invention may be introduced into a plant cell utilising A. tumefaciens containing the Ti plasmid. In using an A. tumefaciens culture as a transformation vehicle, it is most advantageous to use a non-oncogenic strain of the Agrobacterium as the vector carrier so that normal non-oncogenic differentiation of the transformed tissues is possible. It is preferred that the Agrobacterium harbours a binary Ti plasmid system. Such a binary system comprises (1) a first Ti plasmid having a virulence region essential for the introduction of transfer DNA (T-DNA) into plants, and (2) a chimeric plasmid. The chimeric plasmid contains at least one border region of the T-DNA region of a wild-type Ti plasmid flanking the nucleic acid to be transferred. Binary Ti plasmid systems have been shown effective to transform plant cells as, for example, described by De Framond (1983, Biotechnology, 1:262) and Hoekema et al. (1983, Nature, 303:179). Such a binary system is preferred inter alia because it does not require integration into the Ti plasmid in Agrobacterium. Methods involving the use of Agrobacterium include, but are not limited to: (a) co-cultivation of Agrobacterium with cultured isolated protoplasts; (b) transformation of plant cells or tissues with Agrobacterium; or (c) transformation of seeds, apices or meristems with Agrobacterium. Recently, rice and corn, which are monocots, have been shown to be susceptible to transformation by Agrobacterium as well. However, many other important monocot crop plants, including oats, sorghum, millet, and rye, have not yet been successfully transformed using Agrobacterium-mediated transformation. The Ti plasmid, however, may be manipulated in the future to act as a vector for these other monocot plants. Additionally, using the Ti plasmid as a model system, it may be possible to artificially construct transformation vectors for these plants. Ti plasmids might also be introduced into monocot plants by artificial methods such as microinjection, or fusion between monocot protoplasts and bacterial spheroplasts containing the T-region, which can then be integrated into the plant nuclear DNA. In addition, gene transfer can be accomplished by in situ transformation by Agrobacterium, as described by Bechtold et al. (1993, C.R. Acad. Sci. Paris, 316:1194). This approach is based on the vacuum infiltration of a suspension of Agrobacterium cells. Alternatively, the chimeric construct may be introduced using root-inducing (Ri) plasmids of Agrobacterium as vectors. Cauliflower mosaic virus (CaMN) may also be used as a vector for introducing of exogenous nucleic acids into plant cells (U.S. Pat. No. 4,407,956). CaMN DΝA genome is inserted into a parent bacterial plasmid creating a recombinant DΝA molecule that can be propagated in bacteria. After cloning, the recombinant plasmid again may be cloned and further modified by introduction of the desired nucleic acid sequence. The modified viral portion of the recombinant plasmid is then excised from the parent bacterial plasmid, and used to inoculate the plant cells or plants. The chimeric nucleic acid construct can also be introduced into plant cells by electroporation as, for example, described by Fromm et al. (1985, Proc. Natl. Acad. Sci, U.S.A, 82:5824) and Shimamoto et al. (1989, Nature 338:274-276). In this technique, plant protoplasts are electroporated in the presence of vectors or nucleic acids containing the relevant nucleic acid sequences. Electrical impulses of high field strength reversibly permeabilise membranes allowing the introduction of nucleic acids. Electroporated plant protoplasts reform the cell wall, divide and form a plant callus. Another method for introducing the chimeric nucleic acid construct into a plant cell is high velocity ballistic penetration by small particles (also known as particle bombardment or microprojectile bombardment) with the nucleic acid to be introduced contained either within the matrix of small beads or particles, or on the surface thereof as, for example described by Klein et al. (1981, Nature 327:70). Although typically only a single introduction of a new nucleic acid sequence is required, this method particularly provides for multiple introductions. Alternatively, the chimeric nucleic acid construct can be introduced into a plant cell by contacting the plant cell using mechanical or chemical means. For example, a nucleic acid can be mechanically transferred by miciOinjection directly into plant cells by use of micropipettes. Alternatively, a nucleic acid may be transferred into the plant cell by using polyethylene glycol which forms a precipitation complex with genetic material that is taken up by the cell. There are a variety of methods known currently for transformation of monocotyledonous plants. Presently, preferred methods for transformation of monocots are microprojectile bombardment of explants or suspension cells, and direct DNA uptake or electroporation as, for example, described by Shimamoto et al. (1989, supra). Transgenic maize plants have been obtained by introducing the Streptomyces hygroscopicus bar gene into embryogenic cells of a maize suspension culture by microprojectile bombardment (Gordon-Kamm, 1990, Plant Cell, 2:603-618). The introduction of genetic material into aleurone protoplasts of other monocotyledonous crops such as wheat and barley has been reported (Lee, 1989, Plant Mol. Biol. 13:21-30). Wheat plants have been regenerated from embryogenic suspension culture by selecting only the aged compact and nodular embryogenic callus tissues for the establishment of the embryogenic suspension cultures (Vasil, 1990, Bio/Technol. 8:429-434). The combination with transformation systems for these crops enables the application of the present invention to monocots. These methods may also be applied for the transformation and regeneration of dicots. Transgenic sugarcane plants have been regenerated from embryogenic callus as, for example, described by Bower et al. (1996, Molecular Breeding 2:239-249). Alternatively, a combination of different techniques may be employed to enhance the efficiency of the transformation process, e.g., bombardment with Agrobacterium coated microparticles (EP-A-486234) or microprojectile bombardment to induce wounding followed by co-cultivation with Agrobacterium (EP-A-486233). 8. Production and characterisation of differentiated transgenic plants 8 J Regeneration The methods used to regenerate transformed cells into differentiated plants are not critical to this invention, and any method suitable for a target plant can be employed. Normally, a plant cell is regenerated to obtain a whole plant following a transformation process. Regeneration from protoplasts varies from species to species of plants, but generally a suspension of protoplasts is made first. In certain species, embryo formation can then be induced from the protoplast suspension, to the stage of ripening and germination as natural embryos. The culture media will generally contain various amino acids and hormones, necessary for growth and regeneration. Examples of hormones utilised include auxins and cytokinins. It is sometimes advantageous to add glutamic acid and proline to the medium, especially for such species as corn and alfalfa. Efficient regeneration will depend on the medium, on the genotype, and on the history of the culture. If these variables are controlled, regeneration is reproducible. Regeneration also occurs from plant callus, explants, organs or parts. Transformation can be performed in the context of organ or plant part regeneration as, for example, described in Methods in Enzymology, Vol. 118 and Klee et al (1987, Annual Review of Plant Physiology, 38:467), which are incorporated herein by reference. Utilising the leaf disk-transformation- regeneration method of Horsch et al. (1985, Science, 227:1229, incorporated herein by reference), disks are cultured on selective media, followed by shoot formation in about 2-4 weeks. Shoots that develop are excised from calli and transplanted to appropriate root- inducing selective medium. Rooted plantlets are transplanted to soil as soon as possible after roots appear. The plantlets can be repotted as required, until reaching maturity. In vegetatively propagated crops, the mature transgenic plants are propagated by the taking of cuttings or by tissue culture techniques to produce multiple identical plants. Selection of desirable transgenotes is made and new varieties are obtained and propagated vegetatively for commercial use. In seed propagated crops, the mature transgenic plants can be self-crossed to produce a homozygous inbred plant. The inbred plant produces seed containing the newly introduced foreign gene(s). These seeds can be grown to produce plants that would produce the selected phenotype, e.g., early flowering. Parts obtained from the regenerated plant, such as flowers, seeds, leaves, branches, fruit, and the like are included in the invention, provided that these parts comprise cells that have been transformed as described. Progeny and variants, and mutants of the regenerated plants are also included within the scope of the invention, provided that these parts comprise the introduced nucleic acid sequences. It will be appreciated that the literature describes numerous techniques for regenerating specific plant types and more are continually becoming known. Those of ordinary skill in the art can refer to the literature for details and select suitable techniques without undue experimentation. 8.2 Characterisation To confirm the presence of the polynucleotide of the invention in the regenerating plants, a variety of assays may be performed. Such assays include, for example, "molecular biological" assays well known to those of skill in the art, such as Southern and Northern blotting and PCR; a protein expressed by the polynucleotide of the invention may be assayed for sucrose isomerase activity as for example described herein. 9. Production of Isomaltulose The present invention further relates to a process for the production of isomaltulose, using the polynucleotide or polypeptide sequences described herein or using variants or fragments thereof. The process involves contacting sucrose or a sucrose- containing medium or substrate with at least one member selected from (a) an organism which is transformed with a DNA sequence encoding a protein with sucrose isomerase activity, for example a genetically modified bacterium or plant; (b) an extracellular product or cellular extract from such a cell or organism; and (c) a protein with sucrose isomerase activity in isolated form, under conditions such that the sucrose is at least partly converted by the sucrose isomerase into isomaltulose. Subsequently, the isomaltulose is obtained from the medium or the organism and purified as is known in the art. Methods for the industrial production of isomaltulose, for example using immobilised cells or sucrose isomerase contacted with a medium-containing sucrose, are well known (Cheetham et al. 1985, Biotech. Bioeng. 27: 471-481; Takazoe, 1989, Palatinose - an isomeric alternative to sucrose, hi Progress in Sweeteners (Grenby, T.H., ed) Barking: Elsevier, pp. 143-167; and references respectively therein). The present invention improves these methods by providing novel sucrose isomerases with beneficial properties including a higher efficiency of isomaltulose production. Furthermore, the present invention reveals for the first time the capacity to produce isomaltulose directly in plants. This is highly advantageous because it avoids the expense of extracting sucrose from plants and providing this as a substrate for conversion to isomaltulose by other organisms, extracts, or isolated enzymes through industrial fermentation. Instead, the sucrose produced by photosynthesis in plants genetically modified as described herein is converted to isomaltulose by sucrose isomerase activity in the plant tissue. The resulting isomaltulose is then harvested using procedures well established for the harvesting of other sugars, particularly sucrose, from plants. The plant materials with stored isomaltulose are first harvested, then crushed to expel the juice containing isomaltulose and/or passed through diffusion apparatus to extract the soluble isomaltulose from the insoluble plant materials. The isomaltulose is then purified by treatments to remove impurities and concentrated by evaporation and crystallisation stages well known to those skilled in the art (Cooke and Scott, 1993, The Sugar Beet Crop: science into practice. London: Chapman & Hall; Meade, 1977, Cane Sugar Handbook. New York: Wiley, and references respectively therein). In order that the invention may be readily understood and put into practical effect, particular preferred embodiments will now be described by way of the following non- limiting examples. EXAMPLES EXAMPLE 1 Isolation of sucrose isomerase-encoding polynucleotides using oligonucleotide primers based on regions specified by Mattes et al. This strategy was tested on a known sucrose isomerase expressing bacterium (Erwinia rhapontici Accession Number WAC2928), and 30 additional independent bacterial isolates. Degenerate PCR primers were designed based on regions specified by Mattes et al. (supra) as conserved regions from their analysis of sucrose isomerase genes known to them. Forward primer consisted of the sequence extending from nucleotides 139-155 of SEQ ID NO: 1, 5'Jgg tgg aa(a,g) ga(g,a) get gt-3' [SEQ ID NO: 38]. Reverse primer consisted of the sequence extending from nucleotides 625-644 of SEQ ID NO: 1, 5'-toc cag tta g(g,a)t ccg get g-3' [SEQ ID NO: 39]. Bacterial genomic DNAs were used as templates for PCR. The genomic DNAs were extracted according to Ausubel et al (1989, supra). The PCR reaction was carried out in a final volume of 50 μl comprising 100 ng DNA, 5 μL of 10 X PCR buffer (Promega), 2 μL dNTPs (5mM each NTP), forward primer and reverse primer 250 ng each, Taq polymerase 1 μL (Promega). Three parallel PCRs were run by using three different annealing temperatures: 46° C, 50° C or 53° C. After an initial 1 min at 94° C, 35 cycles were performed consisting of 1 min at 94° C, lmin at an annealing temperature and 1 min at 72° C. After running the PCR products on a 1% agarose gel, the bands within the size range from 0.3 to 1.0 kb were recovered and cloned into pCR ® 2J vector using TOPO™TA Cloning ® Kit (Invitrogen) following the instructions from the kit. Plasmid inserts were sequenced at the Australian Genomic Research Facility, using ABI PRISM Big Dye Terminator Cycle Sequencing Ready Reaction Kit, using universal primers of M13 Reverse or M13 Forward available on the vector. The GenBank database was searched by the FASTA program through ANGIS, using the sequenced DNAs as queries. Using the primers from 'conserved regions' specified by Mattes et al. (supra), PCR products were amplified from Erwinia rhapontici and also from bacteria subsequently found to be negative for sucrose isomerase activity. Patterns of PCR products revealed by agarose gel electrophoresis included: no band from 2 isolates, one band from 3 isolates, and multiple bands from all other bacteria including Erwinia rhapontici. The DNAs in 12 bands, including six bands amplified from Erwinia rhapontici, were cloned and sequenced. None of the sequenced bands showed significant homology to the sucrose isomerases, including the region of the gene from Erwinia rhapontici taught by Mattes et al. Most of the sequenced bands showed high similarities to known glucosidase genes. Accordingly, it was concluded that the conserved sequences specified by Mattes et al. were not specific to sucrose isomerases, but were common to other classes of enzymes including glucosidases. As a consequence, these conserved sequences are not of direct use for the cloning of sucrose isomerases without onerous experimentation with PCR conditions and screening by other means to distinguish isomerase clones. EXAMPLE ! Functional Screening for Bacteria that Convert Sucrose to Isomaltulose Bacteria collection and isolation Bacterial samples were collected from a range of environmental sites selected for their potential to yield novel, sucrose metabolising bacteria. In particular, sites were chosen subject to periodic sucrose availability, which might favour organisms able to convert sucrose to storage isomers such as isomaltulose. Around 100 samples from sites in SouthEast Queensland were collected into MBVI liquid culture. MEvI is 0.2% isomaltulose (6-O-α-D-Glucopyranosyl-D-fructofuranose) plus MM (minimum medium containing 0.5% Na 2 PO 4> 0.45% KH 2 PO 4 , 0.1% NH CI, 0.05% MgSO 4 .7H 2 0, 0.005% Ferric Ammonium Citrate and 0.0005% CaCl 2 ). Following growth on an orbital shaker at 200 rpm for 2 hours at room temperature, 100 μL samples were streaked onto MSM (MM plus 4% sucrose) agar plates and grown overnight at 28° C. Following this two-stage enrichment, morphologically different colonies were isolated onto separate fresh plates of LB or MSM for further growth (578 colonies in total). After streaking to ensure purity of single-colony isolates, they were transferred in duplicate to both a replica patch plate and a 30 mL universal tube containing 5 L SLB (LB containing 4% sucrose) for further functional screening in an assay that preferentially reveals organisms with higher capacity for isomaltulose production. EXAMPLE 3 Sample preparation for aniline/diphenylamine assay The cultures grown overnight in 5 mL SLB were centrifuged at a speed of 10,000 x g for 10 minutes at room temperature. The supernatant was carefully poured off and 2 mL of a 50% sucrose solution in citrate/phosphate buffer (pH6) was added. Cells were gently resuspended and incubated at 28°C in a shaker for 48 hours. Following incubation, 1.5 mL culture was transferred to a fresh Eppendorf tube, boiled for 15 minutes at 100°C and centrifuged at 16,000 x g for 20 minutes at room temperature. Without touching the pellet, the supernatant was saved to a fresh tube for aniline/diphenylamine assay and capillary electrophoresis. EXAMPLE 4 Aniline/diphenylamine assay Samples were spotted evenly around the outside edge of a Whatman #1 filter paper with a positive control (from Erwinia rhapontici) and a negative control (from Escherichia coli) placed in the center. After the samples were spotted onto the filter paper, they were left to dry for 15 minutes while the color-developing reagent was prepared. The reagent was prepared as follows: a. 4mL Aniline made up to lOOmL using A.R. acetone; b. 4g Diphenylamine made up to lOOmL using A.R. acetone; c. 20mL of 85% Orthophosphoric acid. Components (a) and (b) were prepared separately in a fume cabinet ensuring complete mixing / dissolving of the aniline/diphenylamine respectively in acetone before they were combined in a glass beaker, after which the acid was added. After initial addition of the acid a cloudy white precipitate forms, which dissolves after vigorous swirling to yield a clear brown solution. The prepared filters were passed through the "developer", ensuring that each filter received even and equal exposure. The filters were then allowed to dry on paper toweling in the fume-hood for 15 minutes, then heated in an 80°C drying oven for 10 minutes. The results (color of spots) were recorded or photographed using a digital camera. If isomaltulose was present, the reaction yielded a yellow to brownish yellow spot due to the 1,6- linked glucosaccharide; whereas glucose yielded a dark grey spot, fructose yielded a silver-grey spot, and sucrose yielded a purple - brown spot due to the 1,2- linkage. The intensity of the color depends on the concentration of the sugars present. Twelve candidates were selected from the 578 colonies as indicated by the aniline/diphenylamine assay test. The identity of the isomaltulose product from the selected isolates was then verified by quantitative analysis using capillary electrophoresis to resolve and identify related metabolites. EXAMPLE S Sample preparation for capillary electrophoresis The ionic materials in the supernatant used for aniline/diphenylamine assay need to be removed before loading to the capillary for further analysis. This was done by passing through a Strong Cation Exchange (Bond Elut-SCX, 1210-2013) and a Strong Anion Exchange (Bond Elut-SAX, 1210-2017) column purchased from Varian. The columns were preconditioned by rinsing with one volume of methanol, followed by one volume of water, with the rinses being forced through the columns with the aid of a syringe. The bacterial supernatant was diluted 150-fold using sterile Milli-Q (SMQ) water before processing first through the SCX and then the SAX column. One mL of the diluted supernatant was placed in the SCX column. The sample was forced through the column with the aid of a 50-mL syringe. The eluate was collected directly into the SAX column. The sample was similarly forced through with the final eluate collected in a 1.5-mL Eppendorf tube. EXAMPLE 6 Capillary electrophoresis Separation by high performance capillary electrophoresis (HPCE), was performed using a Beckman P/ACE 5000 Series C.E. System utilising a 190 to 380 nm light source from a deuterium lamp along with and a Beckman P/ACE UV Absorbance Detector (254 nm [+ 10 nm] filter wheel) for sample detection. Capillaries were bare, fused silica capillaries, ID. 50 μ , O.D. 363μm (Supelco Cat. # 70550-U). Total capillary length was 77 cm, and length inlet to detector window was 69cm. The capillary detector window was made by burning the coating off the capillary using a match, and wiping with methanol. To achieve maximum reproducibility of migration times, the capillary was reconditioned every morning and evening using the following rinsing procedure: 2 min with SMQ, 10 min 0.1 M HC1, 2 min SMQ, 10 min 0.1 M NaOH, 2 min SMQ, 15 min 0.5 M ammonia and 2 min SMQ. All solutions were dissolved / diluted in SMQ and filtered through a 0.45 μm Micropore filter. An alkaline copper sulphate electrolyte with direct detection based on UV absorbance was employed to resolve and detect low concentrations of sucrose and its isomer isomaltulose, in addition to other sugars including glucose and fructose that are expected in cell extracts. Using an electrolyte consisting of 6mM copper (H) sulphate and 500 mM ammonia, pH 11.6, both the separation and the direct UV detection of neutral sugars is achieved based on the chelation reaction of the sugar with copper (H " ) under alkaline conditions. The electrolyte buffer (EB) was made fresh at the beginning of each day and degassed for 15min before use. After conditioning, the capillary was rinsed with EB for 15 min. The capillary was also rinsed with EB for 10 minutes between sample separations. Programmed parameters for batch runs are listed in Table 1. A positive and a negative control as described above were included in each sample. In addition, standards (consisting of sucrose and isomaltulose) were run before the first, and after the last samples, so that differences in migration time due to factors such as EB depletion, capillary heating etc. could be measured and corrected. TABLE 1. Parameters for batch run of capillary electrophoresis Three isolates named as 349J, 14s and 68J were confirmed as having the ability to convert sucrose into isomaltulose. The diluted supernatants from these three positive isolates were retested after being spiked separately with either 5mM sucrose, 0.5mM isomaltulose, 0.5mM fructose or 0.5mM glucose to verify the identity of peaks in the sample based on comigration with a known sugar. EXAMPLE 7 Bacterial Genomic Library Construction Cosmid vector SuperCos 1 (Stratagene) was used for genomic library construction from an Australian isolate of Erwinia rhapontici (Accession Number WAC2928), and bacterial isolates 14S, 68 J and 349 J. The vector accommodates genomic DNA fragments ranging from 30 to 45 kb. EXAMPLE 8 Preparation of genomic DNA insert Because large fragments are required for cloning in the SuperCos 1 vector, the genomic DNA was extracted essentially by method of Priefer et al. (1984, Cloning with cosmids. In Advanced Molecular Genetics (Pϋhler, A. and Timmis, K.N., eds) Berlin: Springer- Verlag, pp. 190-201) to obtain high molecular weight (-150 kb) DNA before digestion. The hooked DNA was dissolved in TE buffer at 65° C for 3 hours or at 4° C for 2 days without shaking. The molecular size was estimated by checking on a 0.4% agarose gel. In order to clone into the BamΗ. I site of the SuperCos 1 vector, the chromosomal DNA was partially digested with restriction endonuclease Sau 3A. A series of test partial digests was conducted to determine the ideal conditions for obtaining the desired insert size range. Ten μg of genomic DNA in a 135 μL volume reaction using IX Sau 3A buffer was pre-equilibrated at 37° C for 5 minutes. Then, 0.5 units of Sau 3A was added, and after 0, 5, 10, 15, 20, 25, 30, 40 minutes, aliquots (15 μL) were removed and the reaction was immediately stopped at 68° C for 20 minutes. The aliquots were loaded on 0.5% agarose gel for electrophoresis. The optimal digestion period was determined for an average fragment size of 50 kb. The reaction was scaled up to 50 μg of genomic DNA in a 675 μL total volume. After digestion, 13 μL of 0.5 M EDTA, pH 8.0 was added to the sample. After a phenol/chloroform extraction, the DNA was precipitated by addition of 1/10 volume of sodium acetate (3M, pH 5.2) and 2.5 volume of ethanol according to Sambrook et al. (1989). The pellet was resuspended in 450 μL IX CIAP buffer and the DNA was CIAP treated for 60 minutes at 37° C. Another phenol/chloroform extraction was repeated to the CIAP treated DNA. The DNA was finally dissolved in 30 μL TE buffer for ligation. EXAMPLE 9 Preparation of vector DNA After 20 μg SuperCos 1 vector was digested by Xba I at 37° C for 3 hours, one unit CIAP per μg DNA was added to the reaction and incubated another hour at 37° C. Phenol/chloroform extraction and ethanol precipitation of the treated DNA using the method described above were performed. The Xba I/CIAP treated SuperCos 1 DNA was resuspended in TE buffer and checked on 0.8% agarose gel to see the single linear band with size of 7.6 kb. The vector DNA was further digested with BanϊH I, extracted with phenol/chloroform, ethanol precipitated, resuspended in TE buffer at 1 μg / μL for ligation. EXAMPLE 10 Ligation and packaging of DNA In a 15 μL volume, 2.5 μg Sau 3A partially digested bacterial genomic DNA and 1.0 μg SuperCos 1 vector DNA treated with Xba I /CIAP/5αmH I were heated at 70° C for 5 minutes. Then 2 μL 10 mM ATP, 2 μL 10X ligation buffer and 1 μL T4 DNA ligase (Invitrogen) were added to make up to 20 μL in total volume. After 4 hours incubation at room temperature, the ligation was put at 4° C overnight. Ligation efficiency was viewed by running 2 μL reaction against unligated mixture of vector and insert DNAs on a 0.8% agarose gel. One fourth of the ligation was in tro-packaged according to the manufacturer's instruction (Gigapack IH Gold Packaging Extract, Stratagene). Host cells of E.coli NM554 (Stratagene) were grown in LB medium with 0.2% maltose and lOmM MgSO 4 at 37° C with shaking from a single colony to an OD 60 o value of 1.0. The cells were harvested by centrifugation at 2,000 x g at 4° C for 10 minutes, then gently resuspended in 10 mM MgSO 4 to OD 60 o value of 0.5. After 10 μL packaged cosmid library was mixed with 50 μL NM554 cells in a 1.5 mL tube, they were incubated at room temperature for 30 minutes, then 400 μL LB was added to the tube. To allow expression of antibiotic resistance, the cells were incubated at 37° C for another hour with gentle shaking once every 15 minutes. The cells were centrifuged for 30 seconds and gently resuspend in 100 μL fresh LB broth. Fifty μL was spread on a LB plate with 50 μg/mL ampicillin. EXAMPLE 11 Functional Screening of Cosmid Libraries After functional screening of 600 colonies from each of the four cosmid libraries, aniline/diphenylamine assay and CE as described above, 4 clones from Erwinia rhapontici, 4 clones from 14S, 3 clones from 349J and 3 clones from 68J showed ability of conversion from sucrose to isomaltulose. EXAMPLE 12 Subcloning and sequencing Cosmid DNAs from positive colonies were prepared following the method of Sambrook et al (1989). To find the smallest functional fragment containing sucrose isomerase, the subclone insert of cosmid DNA was prepared through partial digestion by EcoR I, BamH I or Hind HI separately. Freshly digested pZerO™-2 vector (Invitrogen) by EcoR I, BamΗ. I or Hind HI were used for ligation with the inserts. All cloning procedures such as ligation and transformation into Top 10 E.coli strain followed the instructions provided by Invitrogen. Two hundred transformants of each ligation were picked, patched and grown for functional screening by aniline/diphenylamine assay as described above. The functionally positive subclones were further confirmed by CE analysis. Plasmid DNAs were isolated from the CE confirmed positives to check digest pattern on EcoR I, BamΗ. I or Hind JH. The digested fragments from cosmid insert were further subcloned into pZerO™-2 vector, assayed and sized as described above to obtain the functional clones with the smallest inserts for sequencing. Plasmid inserts were sequenced at the Australian Genomic Research Facility, using ABI PRISM Big Dye Terminator Cycle Sequencing Ready Reaction Kit. For the first round sequencing, universal primers (Sp6, T7, M13 Reverse or M13 Forward) starting the sites available on the pZerO™-2 vector were used, then custom primers were used for sequence extension. Sequences were conducted and confirmed from both strands of the DNA. EXAMPLE 13 Expression of the Three Sucrose Isomerase Genes in Ε.coli Based on the sequences of the genes cloned by functional screening as described above, three pairs of primers were designed for subcloning the three sucrose isomerase genes into expression vector pΕT 24b. By PCR, non-coding regions and leader sequences were deleted and an artificial start codon was incorporated. Each forward primer: 1) includes a start codon, 2) creates a plant-like context for translation start, 3) incorporates a Bam I restriction site for easily cloning and matching open reading frame of the gene. Each reverse primer incorporates a Kpn I restriction site and includes a stop codon. The primer base pairs are as follows: Erwinia rhapontici forward: 5'-gga tec aac aat ggc aac cgt tea gca ate aaa tg-3' [SEQ ID NO: 15] 14S forward: 5'-gga tec aac aat ggc aac cgt tea caa gga aag tg-3' [SEQ ID NO: 17] 68 J forward: 5 '-gga tec aac aat ggc aac gaa tat aca aaa gtc c-3' [SEQ ID NO: 13] Erwinia rhapontici reverse: 5'-ata ggt ace tta ctt aaa cgc gtg gat g-3' [SEQ ID NO: 16] 14S reverse: 5'-ata ggt ace tta ccg cag ctt ata cac acc-3' [SEQ ID NO: 18] 68J reverse: 5'-ata ggt ace tea gtt cag ctt ata gat ccc-3' [SEQ ID NO: 14] High fidelity DNA polymerase pfu (Stratagene) was used for PCR. The PCR products were directly cloned into pCR ® 2.1 vector using TOPO™TA Cloning ® Kit (Invitrogen) following the instructions from the kit. The three sucrose isomerase genes in the pCR ® 2.1 vector were cut and cloned into pGEM®-3Zf(+) then into pET 24b vector (Novagen) for expression in E.coli BL21(DE3) strain. Five mL LB medium with 50 μg /mL kanamycin was used for the BL21(DE3) cell culture. Fifteen cultures per construct were set up initially. Cells were grown at 37° C at 225 rpm shaking. Six to ten cultures per construct, with OD 60 o 1.000 ± 0.005, were selected for further induction. After 0.5 mL was sampled from each culture, IPTG was added to the culture to a final concentration of 1.0 mM. Incubation of the cultures was continued for another 3 hours. The induced cultures only with OD 600 1.750±0.005 were further selected for sucrose conversion analysis and protein measurement, allowing analysis of three replicate cultures per construct. From each of the selected IPTG-induced cultures, 1.5 mL was sampled for protein quantification, 0.5 mL for protein SDS-PAGE, 1.0 mL for quantification of conversion efficiency from sucrose into isomaltulose. EXAMPLE 14 Protein assay The cells were harvested by centrifugation (3,000 x g, 4° C, 10 min). The cell pellet was resuspended in 50 μL of 50 mM Tris-HCl pH 8.0, and 2 mM EDTA, then recentrifuged. The cell pellet was immediately frozen in liquid nitrogen and stored at -70° C. Cells were suspended in 0.5 mL extraction buffer (20 mM Tris-HCl, pH 7.4, 200 mM NaCl, 1 mM EDTA, 1 mM azide, 10 mM β-mercaptoethanol), then lysed by sonication (9 x 15 s pulse at 50 watts from a Branson Sonifier 450 microprobe), and centrifuged (10,000 x g, 4° C, 10 min). The supernatant was filtered through an Acrodisc ® 32 Super ® 0.45 μm membrane filter unit (GelmanScience). Protein was assayed according to Bradford (1976, Anal. Biochem. 72: 248-254) using bovine serum albumin as a standard. Ten μl protein extraction described above was mixed with 90 μl 0J5 M NaCl and 1 mL Coomassie brilliant blue solution (100 mg Coomassie Brilliant Blue G-250 in 50 mL 95% ethanol + 100 mL of 85% phosphoric acid + 850 mL SMQ). A 595 was determined and the protein content was calculated from the standard curve. EXAMPLE 15 SDS-PAGE SDS polyacrylamide gels were polymerised and run as described by Laemmli (1970, Nature 227: 680-685). Protein samples were heated at 100° C for 5 min in lx SDS- PAGE sample buffer (25 mM Tris-HCl pH 6.8, 1% (w/v) SDS, 5% (v/v) β-mercaptoethanol, 10% (v/v) glycerol, 0.005% (v/v) bromophenol blue), centrifuged at 12,000 x g for 1 min and the supernatants were applied to the gels. Each sample was loaded into two adjacent lanes. After running, one lane from the gel was stained in 0.025 % (w/v) Coomassie Blue R-250, destained in 30% (v/v) methanol, 10% (v/v) acetic acid, then expressed sucrose isomerase was cut from the unstained lane corresponding to the relative migration position of the stained gel lane. The sucrose isomerase protein was eluted from the gel slice by immersion into extraction buffer overnight at 4°C with gentle shaking. The eluted sucrose isomerase was quantified using the protein quantification method described above. EXAMPLE 16 Conversion ratio from sucrose into isomaltulose by sucrose isomerase expressed in E.coli The l.OmL culture was centrifuged, then resuspended in citrate/phosphate (pH 6.0) buffered 50% sucrose solution and assayed for isomaltulose conversion by CE analysis as described above. Conversion ratio was calculated by sucrose peak area and isomaltulose peak area normalised against standards of known concentration, using the software of Beckman P/ACE 5000 Series CE. System. EXAMPLE 17 Construct DNA preparation The sucrose isomerase (SI) gene insert in the pET 24b vector was further cloned between the Ubi promoter from the maize ubi-1 gene (Christensen and Quail, 1996, Transgen. Res. 5: 215-218) and the Agrobacterium nos terminator (Bevan et al., 1983, Nature 304: 183-187) to drive expression in sugarcane cells. Plasmids with the sucrose isomerase genes (pU3ZErw, pU3Z14s or pU3Z68J) and the aph A construct plasmid pEmuKN (as a selectable marker) were isolated by alkaline extraction (Sambrook et al, 1989, supra), and dissolved in TE buffer. Plasmid intactiiess and absence of genomic DNA or RNA were checked by gel electrophoresis and concentration was measured by spectrophotometry. The sucrose isomerase (UbiSI) gene construct and selectable marker construct were co-precipitated onto tungsten microprojectiles and introduced into sugarcane callus, followed by selection for transformed callus, and regeneration of transgenic plants, essentially described by Bower et al. (1996, Molec. Breed. 2: 239-249). EXAMPLE 18 Particle bombardment Precipitation reactions were conducted by adding the following at 4° C in turn to a 1.5 mL microfuge tube: 5 μL pEmuKN plasmid DNA (1 mg/mL), 5 μL UbiSI plasmid DNA (1 μg/μL), 50 μL tungsten (Bio-Rad M10, 100 μg/μL), 50 μL CaCl 2 (2.5M), 20 μL spermidine (100 mM free base). The preparation was mixed immediately after addition of each reagent, with minimal delay between addition of CaCl 2 and spermidine. The tungsten was then allowed to settle for 5 minutes on ice, before removal of 100 μL of supernatant and resuspension of the tungsten by running the tube base across a tube rack. Suspensions were used within 15 minutes, at a load of 4 μL/bombardment, with resuspension of the particles immediately before removal of each aliquot. Assuming the entire DNA is precipitated during the reaction, this is equivalent to 1.3 μg DNA/bombardment, on 667 μg tungsten/bombardment. Embryogenic callus from sugarcane cultivar Q117 was used for bombardment. Particles were accelerated by direct entrainment in a helium gas pulse, through the constriction of a syringe filter holder into the target callus in a vacuum chamber as described by Bower et al. (1996, supra). The tissue was osmotically conditioned for four hours before and after bombardment. After 48 hours recovery on solid medium without antibiotics, the bombarded callus was transferred to medium with 45 mg/L Geneticin for selection, callus development and plant regeneration. EXAMPLE 19 Functionality of the transformants in conversion of sucrose Samples were collected from independent transgenic callus and ground under liquid nitrogen. Also, untransformed Q117 callus and callus transformed with Ubi-Zuc were used as negative controls. The ground tissue was centrifuged at 16,000 x g at 4° C to pellet cell debris. The supernatant was diluted 10 folds in SMQ, then boiled for 20 minutes. After another centrifugation to remove denatured proteins, the supernatant was passed through Bond Elut™ SCX and SAX. CE analysis was performed as described above. RESULTS AND DISCUSSION RELATING TO THE EXAMPLES Three bacterial strains with sucrose isomerase activity were isolated An Australian isolate of Erwinia rhapontici (Accession Number: WAC 2928) was used as a positive control for isomaltulose production, because this species has previously been shown to produce a sucrose isomerase enzyme that converts sucrose to isomaltulose (Cheetham, 1985, supra). From a total of 578 bacteria isolated through the enrichment procedure, three strains yielded yellow colour reaction distinctive for isomaltulose in the aniline/diphenylamine assay, and a novel peak in the CE assay corresponding to the isomaltulose standard and to that of Erwinia rhapontici (Figure 1). These strains, designated 14S, 68J and 349J are all Gram-negative bacteria able to use either sucrose or isomaltulose as sole carbon source. All three strains grow well at 22- 30° C, and 68J also grows slowly at 4° C. Three sucrose isomerase genes were functionally cloned and sequenced Functional screening of genomic cosmid libraries of Erwinia rhapontici, 14S, 349J and 68J in E.coli yielded clones able to convert sucrose to isomaltulose (Figure 2). After several cycles of subcloning into pZerO™-2 vector and functional screening, the smallest functional inserts in pZerO™-2 vector ranged from 3 to 5 kb. Sequence from Erwinia rhapontici (Figure 3) showed a 1899 bp ORF encoding 632 amino acids (Figure 5). First strand sequencing revealed a gene in the 349J subclone with 99% identity to this Erwinia rhapontici ORF, so sequencing of 349J was stopped. Sequence from 14S revealed a 1797 bp ORF encoding 598 amino acids. Database searching by FASTA showed that 1305 bp of the SI gene from Erwinia rhapontici, and the full length of the SI gene from 14S had been disclosed by Mattes et al. (supra). Sequence from 68J (Figure 4) indicated a novel SI gene with an ORF of 1797 bp. At the nucleotide level, it has less than 70% identity to known sucrose isomerases, either with or without leader fragment (Table 2). At the amino acid level, the identity to other sucrose isomerases is between 63.4% to 70.6% with leader, or 64.6% to 73.7% without leader. The 68J predicated SI gene product is a protein with 598 amino acids (Figure 6), Mr of 69291and isoelectric point 7.5 due to 78 basic and 69 acidic amino acid residues. Phylogenic analysis of amino acid sequences shows the relatedness between 68J SI gene and known genes. All sucrose isomerase genes and glucosidases share conserved products of the domains for sugar binding. As a result the conserved sequences and corresponding primers described by Mattes et al. (supra) are not specific for sucrose isomerases and would yield many non- Si genes from different organisms. The SI gene of 68J shows nearly the same level of nucleotide identity to various glucosidases as it does to known SI genes of Pseudomonas mesoacidophila. TABLE 2. Comparison between characteristics of 68], other sucrose isomerases, sucrose isomerase fragments, and a glucosidase 0 0 *Comparison between 68J and nonfunctional fragments from incomplete sucrose isomerase genes. Sudz # sequences are disclosed in patent to Sudzucker (Mattes et al). Sucrose isomerase from 68J showed the highest conversion efficiency among the tested isomerases When the SI genes from Erwinia rhapontici, 14S and 68J were arranged for expression using the same vector (same promoter, start codon and termination sequences), there was no significant difference in total protein content or in expression level of sucrose isomerases, at around 10% of total protein (Table 3). However, the conversion efficiency from sucrose to isomaltulose by the cloned 68J gene product is 10 times that of the Erwinia rhapontici and 18 times that of the 14S gene products (Figure 7). In addition, the sucrose isomerase of 68 J generated relatively smaller proportions of glucose and fructose than that of 14S and Erwinia rhapontici. All other factors during gene expression and enzyme activity quantification were identical: the same ATG start codon context for gene constructs, the same vector pET 24b, the same host cell strain BL21(DE3), the same culture conditions, the same cell density before and after IPTG induction, the same amount of cells used for sucrose conversion, the same amount of total protein loaded on to SDS- PAGE and the same volume of supernatant with the same total protein content loaded on to CE. The experiment was performed three times with the same outcomes. The experimental results show high potential of the sucrose isomerase from 68J in industrial applications for isomaltulose production. TABLE 3. Total protein contents and assumed sucrose isomerase protein contents in E.coli cells with a SI gene of Erwinia rhapontici, 14S or 68 . # Results are means ± standard errors derived from 3 replications. *Including background of approximately 2% proteins that migrated with the sucrose isomerase. Sugarcane transgenic callus with 68J sucrose isomerase also showed the highest conversion ratio among the tested sucrose isomerase gene constructs Isomaltulose could be found in the cell extracts of transgenic sugarcane callus expressing the sucrose isomerase genes. Three out of three tested 68J transgenic lines showed the isomaltulose peak higher than the sucrose peak on the CE electrograph (Figure 8A). In contrast, three out of seven tested 14S transgenic lines showed the isomaltulose peak lower than the sucrose peak (Figure 8B). Isomaltulose could not be detected in the caUi of the other four tested 14S transgenic lines. The transgenic callus with the Erwinia rhapontici gene showed even lower isomaltulose levels than the 14S lines (Figure 8C). These results show for the first time the feasibility of production of isomaltulose by expression of sucrose isomerase in plants, and the high potential of sucrose isomerase 68J for this purpose. The disclosure of every patent, patent application, and publication cited herein is hereby incorporated herein by reference in its entirety. The citation of any reference herein should not be construed as an admission that such reference is available as "Prior Art" to the instant application Throughout the specification the aim has been to describe the preferred embodiments of the invention without limiting the invention to any one embodiment or specific collection of features. Those of skill in the art will therefore appreciate that, in light of the instant disclosure, various modifications and changes can be made in the particular embodiments exemplified without departing from the scope of the present invention. All such modifications and changes are intended to be included within the scope of the appended claims.

Claims: CLAIMS 1. A method for isolating novel polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes, said method comprising: (a) obtaining an environmental sample from a location in which organisms, capable of converting sucrose to isomaltulose, have a selective advantage; (b) screening for organisms producing isomaltulose from sucrose; and (c) isolating polynucleotides encoding isomaltulose-producing sucrose isomerase enzymes from isomaltulose-producing organisms. 2. The method of claim 1, further comprising selecting or otherwise enriching for dual sucrose- and isomaltulose-metabolising organisms which are capable of using both sucrose and isomaltulose as carbon sources for growth. 3. The method of claim 2, wherein said selection or enrichment comprises growing organisms of said environmental sample on an isomaltulose-containing medium for a time and under conditions sufficient to select or enrich for isomaltulose-metabolising organisms and growing said isomaltulose-metabolising organisms on a sucrose-containing medium for a time and under conditions sufficient to select or enrich for said dual sucrose- and isomaltulose-metabolising organisms. 4. The method of claim 2, wherein said selection or enrichment comprises growing organisms of said environmental sample on a sucrose-containing medium for a time and under conditions sufficient to select or enrich for sucrose-metabolising organisms and growing said sucrose-metabolising organisms on an isomaltulose-containing medium for a time and under conditions sufficient to select or enrich for said dual isomaltulose- and sucrose-metabolising organisms. 5. The method of claim 1, wherein said screening utilises an assay that quantifies isomaltulose production by an organism. 6. The method of claim 5, wherein said assay is an aniline/diphenylamine assay. 7. The method of claim 1, wherein said environmental sample comprises soil or plant matter. 8. The method of claim 1, wherein said environmental sample is obtained from a location that is subject to periodic or constant availability of substantial sucrose concentrations. 9. The method of claim 8, wherein said location is selected from a factory involved in processing or storage of sugar-containing plants or plant parts or a field containing remnants of harvested sugar-containing plants. 10. The method of claim 9, wherein said sugar-containing plant is sugar beet or sugarcane. 11. The method of claim 9, wherein said sugar-containing plant is sugarcane. 12. The method of claim 1, wherein said polynucleotides are isolated using a probe specific for sucrose isomerase-encoding polynucleotides. 13. The method of claim 1, wherein said polynucleotides are isolated using a probe that is capable of hybridising to a nucleotide sequence encoding a sucrose isomerase consensus sequence set forth in any one of SEQ ID NO: 19, 20, 21, 22, 23 and 24, or variant thereof. 14. The method of claim 13, wherein said variant has at least 80%, sequence identity to any one of the amino acid sequences set forth in SEQ JD NO: 19, 20, 21, 22, 23 and 24. 15. The method of claim 13, wherein said nucleotide sequence comprises the sequence set forth in any one of SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 or nucleotide sequence variant thereof. 16. The method of claim 15, wherein said nucleotide sequence variant has at least 60%, sequence identity to any one of the sequences set forth in SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36. 17. The method of claim 15, wherein said nucleotide sequence variant is capable of hybridising to any one of the sequences identified by SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 under at least low stringency conditions. 18. An isolated polypeptide, or a biologically active fragment thereof, or a variant or derivative of these, said polypeptide comprising an amino acid sequence selected from the group consisting of SEQ HD NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof. 19. The polypeptide of claim 18, wherein said biologically active fragment is at least 6 amino acids in length. 20. The polypeptide of claim 18, wherein said biologically active fragment comprises at least one consensus sequence selected from SEQ ID NO: 19, 20, 21, 22, 23 or 24. 21. The polypeptide of claim 18, wherein said variant has at least 75% sequence identity to any one of the sequences set forth in SEQ ID NO: 2, 4, 8, 10, and 26. 22. The polypeptide of claim 18, wherein said variant comprises the consensus sequence set forth in any one or more of SEQ ID NO: 19, 20, 21, 22, 23 and 24 or variant thereof. 23. The polypeptide of claim 22, wherein said consensus sequence variant has at least 80% sequence identity to any one of the sequences set forth in SEQ ID NO: 19, 20, 21, 22, 23 and 24. 24. An isolated polynucleotide encoding the polypeptide, fragment, variant or derivative of claim 18. 25. An isolated polynucleotide comprising the sequence set forth in any one of SEQ ID NO: 1, 3, 7 and 9, or a biologically active fragment thereof, or a polynucleotide variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 1, or 3 comprises a contiguous sequence of nucleotides contained within SEQ ID NO: 25 or polynucleotide variant thereof. 26. The polynucleotide of claim 25, wherein said biologically active fragment comprises at least 18 nucleotides. 27. The polynucleotide of claim 25, wherein said polynucleotide variant has at least 60% sequence identity to any one of the polynucleotides set forth in SEQ ID NO: 1, 3, 7 and 9. 28. The polynucleotide of claim 25, wherein said polynucleotide variant is capable of hybridising to any one of the polynucleotides identified by SEQ ID NO: 1, 3, 7 and 9 under at least low stringency conditions. 29. The polynucleotide of claim 25, wherein said polynucleotide variant comprises a nucleotide sequence encoding the consensus sequence set forth in any one or more of SEQ ID NO: 19, 20, 21, 22, 23 and 24 or variant thereof. 30. The polynucleotide of claim 29, wherein said nucleotide sequence is selected from SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 or 36 or nucleotide sequence variant thereof. 31. The polynucleotide of claim 30, wherein said nucleotide sequence variant has at least 60% sequence identity the nucleotide sequence selected from SEQ ID NO: 27, 28, 29, 30, 31. 32, 33, 34, 35 or 36. 32. The polynucleotide of claim 30, wherein said nucleotide sequence variant is capable of hybridising the nucleotide sequence selected from SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 or 36 under at least low stringency conditions. 33. An expression vector comprising said polynucleotide of claim 24 operably linked to a regulatory polynucleotide. 34. An expression vector including a polynucleotide comprising the sequence set forth in any one of SEQ ID NO: 1, 3, 7 and 9, or a biologically active fragment thereof, or a polynucleotide variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 1, or 3 comprises a contiguous sequence of nucleotides contained within SEQ ID NO: 25 or polynucleotide variant thereof, wherein the polynucleotide is operably linked to a regulatory polynucleotide. 35. A host cell containing said expression vector of claim 33 or claim 34. 36. The host cell of claim 35, which is a bacterium or other prokaryote. 37. The host cell of claim 35, which is a plant cell or other eukaryote. 38. A plant cell plant containing the expression vector of claim 33 or claim 34, wherein said plant is a species capable of synthesising and/or accumulating sucrose. 39. The plant cell of claim 38, wherein said plant is selected from sugarcane or sugar beet. 40. The plant cell of claim 38, wherein said plant is sugarcane. 41. A method of producing a polypeptide, or a biologically active fragment thereof, or a variant or derivative of these, in recombinant form, said polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof, said method comprising: - culturing a host cell containing the expression vector of claim 33 such that said recombinant polypeptide, fragment, variant or derivative is expressed from said polynucleotide; and - isolating the said recombinant polypeptide, fragment, variant or derivative. 42. A method of producing a biologically active fragment of a polypeptide or of a variant or derivative thereof, said polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof, said method comprising: - detecting sucrose isomerase activity associated with a fragment of a polypeptide according to any one of SEQ ID NO: 2, 4, 8 and 10, or of a variant or derivative thereof, which indicates that said fragment is a said biologically active fragment. 43. A method of producing a biologically active fragment of a polypeptide or of a variant or derivative thereof, said polypeptide comprising an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof, said method comprising: - introducing into a cell a polynucleotide from which a fragment of a polypeptide according to any one of SEQ ID NO: 2, 4, 8 and 10, or of a variant or derivative thereof, can be produced; and - detecting sucrose isomerase activity, which indicates that said fragment is a said biologically active fragment. 44. A method of producing a polypeptide variant of a parent polypeptide comprising the sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof, said method comprising: - producing a modified polypeptide whose sequence is distinguished from the parent polypeptide by substitution, deletion or addition of at least one amino acid; and - detecting sucrose isomerase activity associated with the modified polypeptide, which indicates that that said modified polypeptide is a said polypeptide variant. 45. A method of producing a polypeptide variant of a parent polypeptide comprising the sequence set forth in any one of SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof, said method comprising: - producing a polynucleotide from which a modified polypeptide can be produced, said modified polypeptide having sequence that is distinguished from the parent polypeptide by substitution, deletion or addition of at least one amino acid; - introducing said polynucleotide into a cell; and - detecting sucrose isomerase activity, which indicates that said modified polypeptide is a said polypeptide variant. 46. A method for producing isomaltulose from sucrose, said method comprising contacting sucrose or a sucrose-containing substrate with the polypeptide, fragment, variant or derivative of claim 18, or with the host cell of claim 33 or claim 34, for a time and under conditions sufficient to produce isomaltulose. 47. An antigen-binding molecule that is immuno-interactive with the polypeptide, fragment, variant or derivative of claim 18. 48. The antigen-binding molecule of claim 47, which is immuno-interactive with an amino acid sequence selected from SEQ ID NO: 19, 20, 21, 22, 23 or 24 or a variant of said sequence having at least 80% sequence identity thereto. 49. A method for detecting a specific polypeptide or polynucleotide, comprising detecting the sequence of: (c) SEQ ID NO: 2, 4, 8 and 10, or biologically active fragment thereof at least 6 amino acids in length, or variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof; or (d) a polynucleotide encoding (a). 50. The method of claim 49, wherein the sequence of (b) is selected from SEQ ID NO: 1, 3, 7 and 9, or a biologically active fragment thereof at least 18 nucleotides in length, or a polynucleotide variant of these, with the proviso that said biologically active fragment of SEQ ID NO: 1, or 3 comprises a contiguous sequence of nucleotides contained within SEQ ID NO: 25 or polynucleotide variant thereof. 51. A method of detecting a sucrose isomerase in a sample, comprising: - contacting the sample with the antigen-binding molecule of claim 47; and - detecting the presence of a complex comprising said antigen-binding molecule and a polypeptide, or a biologically active fragment thereof, or a variant or derivative of these, in said contacted sample, wherein said polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10. 52. A method for detecting a sucrose isomerase in a sample, comprising: - detecting expression in a cell of a polynucleotide encoding a polypeptide, or a biologically active fragment thereof, or a variant or derivative of these, wherein said polypeptide comprises an amino acid sequence selected from the group consisting of SEQ ID NO: 2, 4, 8 and 10, with the proviso that said biologically active fragment of SEQ ID NO: 2 or 4 comprises a contiguous sequence of amino acids contained within SEQ ID NO: 26 or variant thereof. 53. The method of claim 52, wherein expression is detected using a probe specific for sucrose isomerase-encoding polynucleotides. 54. The method of claim 52, wherein expression is detected using a probe that is capable of hybridising to a nucleotide sequence encoding a sucrose isomerase consensus sequence set forth in any one of SEQ ID NO: 19, 20, 21, 22, 23 and 24, or variant thereof. 55. The method of claim 54, wherein said variant has at least 80%, sequence identity to any one of the amino acid sequences set forth in SEQ ID NO: 19, 20, 21, 22, 23 and 24. 56. The method of claim 54, wherein said nucleotide sequence comprises the sequence set forth in any one of SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 or nucleotide sequence variant thereof. 57. The method of claim 56, wherein said nucleotide sequence variant has at least 60%, sequence identity to any one of the sequences set forth in SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36. 58. The method of claim 56, wherein said nucleotide sequence variant is capable of hybridising to any one of the sequences identified by SEQ ID NO: 27, 28, 29, 30, 31, 32, 33, 34, 35 and 36 under at least low stringency conditions. 59. A probe comprising a nucleotide sequence which is capable of hybridising to at least a portion of a nucleotide sequence encoding SEQ ID NO: 2, 4, 8 and 10 under at least low stringency conditions. 60. A probe comprising a nucleotide sequence which is capable of hybridising to at least a portion of SEQ ID NO: 1, 3, 7 and 9 under at least low stringency conditions. 61. A method of isolating a sucrose isomerase from a sample, comprising: - contacting the sample with the antigen-binding molecule of claim 47 to form a complex comprising the sucrose isomerase and the antigen-binding molecule; and - separating the sucrose isomerase from the complex. 62. A transformed plant cell containing the expression vector of claim 33 or claim 34. 63. The plant cell of claim 62, wherein said plant is a species capable of synthesising and/or accumulating sucrose. 64. The plant cell of claim 62, wherein said plant is selected from sugarcane or sugar beet. 65. The plant cell of claim 62, wherein said plant is sugarcane. 66. .A differentiated plant comprising plant cells containing the expression vector of claim 33 or claim 34. 67. A method of producing isomaltulose, comprising - cultivating a differentiated plant comprising plant cells containing the expression vector of claim 33 or claim 34; and - harvesting isomaltulose from said cultivated plant. 68. Isomaltulose harvested from a differentiated plant comprising plant cells containing the expression vector of claim 33 or claim 34.

Inventors: BIRCH ROBERT GEORGE, BIRCH, ROBERT, GEORGE
Applicants: BIRCH ROBERT GEORGE, UNIV QUEENSLAND, BIRCH, ROBERT, GEORGE, THE UNIVERSITY OF QUEENSLAND
Application year: 2001
Main classification: C12N15
IPC codes: A01H5, C07H3, C07K16, C12N1, C12N5, C12N9, C12N15, C12P19, C12Q1, C13K13, G01N31, G01N33