POLYPHENOL OXIDASE GENES

Abstract: A DNA sequence including a gene coding for a polypeptide having polyphenol oxidase (PPO) activity, or a fragment thereof.

Description: POLYPHENOL OXIDASE GENES The present invention relates to a method of modifying polyphenol oxidase (PPO) activity in fruit and vegetables and to DNA sequences for use therein. Browning of plant tissues often occurs following injury or damage and this generally results in spoilage of fruit and vegetables. Undesirable browning also occurs during processing of plant materials to produce food or other products. Steps are taken during transport/ storage, and processing to prevent these browning reactions. Often this involves the use of chemicals such as sulphur dioxide but the use of these substances is likely to be restricted in the future due to concerns about their safety and consumer acceptance. For example, the US Food and Drug Administration banned the use of sulphite for most fresh fruit and vegetables in 1986. The production of fruit and vegetable varieties with an inherently low susceptibility to brown would remove the need for these chemical treatments. Accordingly, it is an object of the present invention to overcome or at least alleviate one or more of the difficulties related to the prior art. It will be understood that browning in plants is predominantly catalysed by the enzyme PPO. PPO is localised in the plastids of plant cells whereas the phenolic substrates of the enzyme are stored in the plant cell vacuole. This compartmentation prevents the browning reaction from occurring unless the plant cells are damaged and the enzyme and its substrates are mixed. If the amount of this enzyme could be decreased the susceptibility of the tissue to brown would be reduced. PPO sequence information may be used to construct synthetic genes which genes may be transformed into plants to decrease expression of the normal PPO gene, thereby decreasing synthesis of the enzyme. It will also be understood that in certain instances the browning reactions in plants are desirable, such as in the production of black tea, cocoa, coffee, black pepper, black olives, etc. In these instances it may be desirable to increase the level of PPO to produce desired levels of browning or changes in flavour compounds. The role of PPO in normal plant growth and development is not understood at present. There are a number of instances where increased levels of this enzyme are correlated with increased resistance to plant pathogens. It follows that genetic manipulation of plants to increase the level of PPO activity may confer useful resistance against pathogens and pests. The grapevine PPO gene codes for an additional 103 amino acids upstream of the N-terminus of the mature protein. This region has the properties of a chloroplast transit peptide and is most likely responsible for targeting of the protein to be imported into the chloroplast and processed to produce the mature PPO protein. Transformation of plants with this gene may therefore result in correct targeting and maturation of the grapevine PPO in other species and result in accumulation of active grapevine PPO enzyme in the plastids of these tissues. The terms "gene encoding PPO", "gene coding for PPO" or "PPO gene" as used herein should be understood to refer to the PPO gene or a sequence substantially homologous therewith. In a first aspect of the present invention, there is provided a DNA sequence including a gene coding for a polypeptide having polyphenol oxidase (PPO) activity or a fragment thereof. The DNA sequence may include the pre-sequence of the PPO gene coding for a transit peptide. The DNA sequence may be modified. The DNA sequence may include a sequence coding for antisense mRNA to the normal PPO gene, or a fragment thereof. The DNA sequence may further include a catalytic cleavage site. Alternatively the presequence may be replaced by other targeting sequences to direct the PPO protein to other cellular compartments. The DNA sequence may include the putative _ _ chloroplast transit sequence and the mature grape vine PPO protein, as illustrated in Figure 1. The DNA sequence may include the gene coding for a polypeptide having broad bean leaf PPO activity, as illustrated in Figure 2. The DNA sequence may include the gene coding for polypeptides having apple fruit PPO activity, as illustrated in Figure 3. The DNA sequence may include the gene coding for polypeptides having potato tuber PPO activity, as illustrated in Figure 4. In a further aspect of the present invention there is provided a method for preparing a recombinant DNA plasmid including a DNA sequence coding for PPO or a fragment thereof, which method includes providing a DNA sequence including the gene coding for a polypeptide having PPO activity or a fragment thereof; and a plasmid expression vector; and reacting the DNA sequence and the plasmid expression vector to deploy the DNA sequence within the plasmid expression vector. The DNA sequence coding for PPO may be formed from polyadenylated RNA, for example isolated from a plant sample. The plant may be selected from apples, potatoes, grapes and beans. Preferably the plant sample is isolated from sultana grape berries, broad bean leaves, apple peel or cortex or potato tubers. In order to provide a DNA sequence coding for PPO, in a preferred aspect of the present invention the method for the preparation of a recombinant DNA plasmid may include the preliminary steps of providing a source of PPO polypeptide; isolating polyadenylated RNA coding for PPO therefrom; and treating the polyadenylated RNA to construct copy DNA (cDNA) . The isolation of the polyadenylated RNA may be conducted utilising an oligo-dT spun column. The step of treating the polyadenylated RNA to construct cDNA according to this aspect of the present invention may include treating the polyadenylated RNA with reverse transcriptase and an adapter primer to form first strand cDNA; and amplifying the cDNA so formed using the polymerase chain reaction (PCR) . The step of reacting the polyadenylated RNA with reverse transcriptase may utilise an oligonucleotide adapter primer having the sequence 5'-GACTCGAGTCGACΑTCGATTTTTTTTTTTTTTTTT. The step of amplifying the cDNA may utilise an adapter primer having the sequence 5'-GACTCGAGTCGACATCG and a 5*-end primer. The 5'-end primer may have the sequence 5'-CCIATICAGGCICCIGATATIICIAAGTGTGG when utilized for the amplification of grape cDNA. The 5'-end primer may have the sequence 5'-GCGGATCCTT.CT]TA[CT]GA.CT]GA[GA]AA[CT]AA. when utilized for the amplification of bean cDNA. The 5'-end primer may have the sequence (5*-GCGAATTCGA[AG]GA[TC]ATGGGIAA[TC]TT[TC]TA) when utilised for the amplification of apple cDNA. The 5'-end primer may have the sequences GEN3: (5*-GCGAATTCTT.TC] [TC_TICCITT[TC]CA[TC] [AC]G) GEN7: (5'-GCGAATTCAA.TC]GTIGA.TC] [AC]GIATGTGG) when utilised for the amplification of potato cDNA. Alternatively, the step of treating the polyadenylated RNA to construct cDNA according to this aspect of the present invention may include treating the polyadenylated RNA with reverse transcriptase and a PPO specific primer to form first strand cDNA; treating the cDNA so formed with terminal d Transferase to attach a polyadenosine tail sequence at the 3' end of the cDNA; and —_>— amplifying the polyadenylated cDNA so formed by PCR. The step of treating the polyadenylated RNA with reverse transcriptase may utilise a PPO specific oligonucleotide primer having the sequence 5'-AATCTTTGTGGTGACTGGCG for grape PPO or the PPO-specific primer is an oligonucleotide primer having the sequence 5'GACGGTACATTAGTCTTAAAT for potato tuber PPO. The step of amplifying the cDNA may utilise an oligonucleotide adapter primer having the sequence 5'-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT and a PPO specific oligonucleotide primer having the sequence 5 ■ -ACCATCAGGCACGGTGGCGG for grape PPO or the sequence 5'-TGCTCATCAACTGGAGTTGAG for potato tuber PPO. The plasmid expression vector for the cloning of the double stranded cDNA may be of any suitable type. The plasmid vector Bluescript SK + has been found to be suitable. The cloning step may take any suitable form. A preferred form may include blunt-ending the cDNA with Klenow fragment; fractionating the cDNA so formed on an Agarose gel; isolating a fragment of the expected size from the gel; and ligating said fragment into the Hindlll or E_ς___RI site of a Bluescript SK + vector. In order to test the clones so formed, a suitable microorganism may be transformed with the plasmid expression vector, the microorganism cultured and the polypeptide encoded therein expressed. The microorganism Escherichia coli DH5 has been found to be suitable. In a further aspect of the present invention there is provided a recombinant DNA plasmid including a DNA sequence coding for a polypeptide having PPO activity, or a fragment thereof, which plasmid is capable of being replicated, transcribed and translated in a unicellular organism. The plasmid expression vector may be of any suitable type. The recombinant plasmid may contain a constitutive promoter element upstream of the DNA sequence coding for a polypeptide having PPO activity. In a further aspect of the present invention there is provided a method of decreasing the level of PPO activity in a plant tissue, which method includes providing a DNA construct including a modified PPO gene or fragment thereof; and a plant sample; and introducing said DNA construct into said plant sample to produce a transgenic plant. The DNA construct may include a sequence encoding antisense mRNA to the normal PPO gene or a fragment thereof. The DNA construct may include a PPO gene or fragment thereof incorporating a catalytic cleavage site. The plant may be of any suitable type. In a preferred aspect the plant may be selected from the group including grapevine, potato, apple and bean. In a further aspect of the present invention there is provided a method of increasing the level of PPO activity in a plant tissue, which method includes providing a DNA construct including a ' gene coding for a polypeptide having PPO activity or a fragment thereof; and a plant sample; and introducing said DNA construct into said plant sample to produce a transgenic plant. The DNA construct may include a DNA sequence encoding the PPO pre-sequence or a fragment thereof. The plant may be of any suitable type. In a preferred aspect the plant may be selected from the group comprising tobacco, broad bean, tomato, tea, coffee and cocoa. The pre-sequence coding for the transit peptide may be replaced with other targeting sequences to direct the PPO protein to other cellular compartments. Sequences are already known which direct foreign genes into the vacuole, mitochondrion or intercellular space of plant cells. In addition the transit sequence for grapevine PPO could be used to target other proteins into the plastids. The DNA construct may include a constitutive promoter which would result in expression of the introduced genes throughout the plant. It will be understood that in many plant tissues PPO is highly expressed in certain tissue types. For example, PPO activity is much higher in the skin of grape berries than in the pulp, and the peel of potato tubers has higher activity than the cortex. It may be desirable to alter levels of PPO activity only in certain plant tissues or at certain stages of plant development and this may be achieved by the use of specific promoter elements. For example, use of the patatin promoter alters PPO levels only in the tuber tissue of potato plants. This decreases PPO activity in the tuber, and reduce browning, but PPO activity in other parts of the potato plant is not altered. Accordingly, the DNA construct may include a promoter which is specific to the peel or skin of fruit and vegetables to target foreign proteins specifically to the outer tissue layers. This may allow properties of the skin or peel, such as colour, flavour, resistance to pathogens, etc to be manipulated independently of the inner parts of the fruit or vegetable which are consumed. In a preferred aspect, the DNA construct may include a binary vector into which has been introduced a DNA sequence encoding PPO or a fragment thereof. In a further preferred aspect, the introduction of the DNA construct into the plant may be by infection of the plant with an Aαrobacterium containing the DNA construct. In a further aspect of the present invention there is provided a transgenic plant, which plant contains a synthetic gene capable of modifying expression of the normal PPO gene. The plant may be of any suitable type. In a preferred aspect the plant may be selected from the group comprising grapevine, potato, apple, tobacco, bean, peach, pear and apricot. In a still further aspect of the present invention there is provided a plant vaccine including a sequence encoding PPO or a fragment thereof. In a still further aspect of the present invention there is provided a DNA probe including a DNA sequence encoding PPO or a fragment thereof. The probe may be labelled in any suitable manner. A radioactive or non-radioactive labelling may be used. For convenience, the probe may be provided in the form of a cloned insert in a suitable plasmid vector. The grapevine PPO sequence may be used to design general purpose oligonucleotide primers for use in the PCR to obtain this gene from other species. Plant PPO proteins are known to contain copper as a prosthetic group and two regions of the grape protein sequence which show homology to sequences from hemocyanin and tyrosinase proteins, corresponding to the copper binding sites on these proteins have been identified. Since these regions are apparently conserved between widely diverse organisms they are suitable for design of probes and primers to obtain other plant PPO genes. Accordingly, in a still further aspect of the present invention there is provided a method of isolating a DNA sequence including a gene coding for a polypeptide having PPO activity or a fragment thereof from a plant species, which method includes providing a cDNA or genomic library; and a DNA probe including a DNA sequence including a gene for a polypeptide having polyphenol oxidase (PPO) activity or a fragment thereof; and hybridising the probe with the genomic library to _ _ identify clones containing said DNA sequence. The DNA probe may include a DNA sequence including a fragment of the apple, potato, grape or bean PPO gene which is highly conserved between different species. The DNA probe may be prepared by a method which includes providing total cDNA from a plant species; and two or more oligonucleotide primers which hybridise specifically with the PPO gene and which include sequences of the apple, potato, grape or bean PPO gene which are highly conserved between different species; and performing PCR to amplify a DNA sequence including the PPO gene or a fragment thereof. The oligonucleotide primers may include DNA sequences corresponding to the copper binding sites on the PPO protein. In a still further aspect of the present invention there is provided a method of isolating a DNA sequence including a gene coding for a polypeptide having PPO activity, or a fragment thereof, from a plant species, which method includes providing mRNA isolated from the plant; a poly-dT adapter primer; and two or more oligonucleotide primers; treating the mRNA with reverse transcriptase and an adapter primer to form first strand cDNA; and amplifying the cDNA so formed using the oligonucleotide primers and the polymerase chain reaction. In a preferred aspect the oligonucleotide primers may be based on the apple, potato, grape or bean PPO gene sequences. In a still further aspect of the present invention there is provided a method for purification of the PPO protein, which method includes providing a plant sample; a detergent; and one or more chromatography columns; extracting the plant sample with the detergent; treating the extract so formed with ammonium sulphate; and fractionating the extract so formed by passing it through the chromatography columns. The plant sample may be of any suitable type. The plant sample may be grapevine berries. This tissue contains high levels of PPO and in the juice of mature grape berries most of the PPO activity is bound to the solids and can be separated from the juice by centrifugation and then solubilised with detergents. The plant sample may be bean leaves. The detergent may be cationic. The detergent hexadecyltrimethylammonium bromide (CTAB) has been found to be suitable. The chromatography columns may be sepharose based. Three chromatography columns may be used. Q-sepharose followed by phenyl-sepharose followed by hydroxylapatite has been found to be suitable. In a further aspect of the present invention there is provided a PPO protein in a substantially pure form, having the N-terminal amino acid sequence - PIQAPDISKCGTATVPDGVTP. The present invention will now be more fully described with reference to the accompanying Examples and drawings. It should be understood, however, that the description following is illustrative only and should not be taken in any way as a restriction on the generality of the invention described above. In the figures: FIGURE 1: The composite full-length GPOl cDNA nucleotide sequence and derived protein sequence encoding both the putative chloroplast transit sequence and the mature grapevine PPO protein. The translation start site is shown in bold face and the N-terminal of the mature PPO protein is marked _ _ with an asterisk. The dashed line indicates the location of the N-terminal primer and the two solid lines indicate the regions used to construct the two antisense primers for cloning the transit peptide sequence. FIGURE 2: Nucleic acid and derived protein sequence of the BPOl clone of broad bean leaf polyphenol oxidase. The solid line indicates the region of the B15 primer used to amplify the cDNA by the polymerase chain reaction. ________________: Nucleic acid and derived protein sequences of the clones pSR7 and pSR8 encoding apple fruit PPO. The solid line indicates the region of the GEN4 primer used to amplify the cDNA by the polymerase chain reaction. FIGURE 4: Nucleic acid and derived protein sequences of the clones encoding potato tuber PPO. The solid line indicates the region of the GEN3 primer used to amplify the cDNA by the polymerase chain reaction. EXAMPLE 1 PURIFICATION OF THE PPO PROTEIN PPO was purified from grapevine berries. Initial experiments showed that this tissue contained high levels of the enzyme and that there appeared to be only one form of the enzyme as determined by electrophoresis in sodium dodecyl sulphate polyacrylamide (SDS-PAGE) gels. In the juice of mature grape berries most of the PPO activity was bound to the solids and could be separated from the juice by centrifugation and then solubilised with detergents. Enzyme activity during the purification was measured as oxygen uptake in the presence of the substrate 4-methyl catechol. All steps during the purification were carried out at 4°C. Thirty kilograms of Sultana grapes were crushed with a small scale wine press and 100ml of a solution of lOOmM ascorbate plus lOmM dithiothreitol was added to each 900ml of grape juice. The juice was centrifuged for 10 mins at 10,000xg and the supernatant discarded. The pellet fraction was resuspended in 25mM sodium phosphate, pH 7.2 plus lOmM ascorbate and ImM dithiothreitol to a final volume of 1.75L, then 250ml of a 4% (w/v) solution of the cationic detergent hexadecyltrimethylammonium bromide (CTAB) was added. After incubating for 20 mins the extract was centrifuged for 15 mins at 15,000xg. The supernatant was brought to 45% saturation with solid ammonium sulphate and the pH was adjusted to 7.0 then it was centrifuged for 15 mins at 15,000xg. This supernatant was brought to 95% saturation with solid ammonium sulphate and the pH was adjusted to 7.0 then it was centrifuged for 30 mins at 15,000xg. The pellet was resuspended in 20mM Bis-tris- propane, pH 7.5 plus lOmM ascorbate and 2mM dithiothreitol (Buffer A) in a final volume of lOOml. The extract was desalted on a 4x40cm column of Sephadex G25 equilibrated with Buffer A at a flow rate of lOml/min and the active fractions were pooled. The extract was applied to a 2.5x10cm column of Q-Sepharose Fast Flow equilibrated with Buffer A at a flow rate of 6ml/min and then the column was washed with 400ml of Buffer A. The PPO was eluted with a gradient of 0-500mM NaCl in Buffer A and the active fractions were pooled. Ammonium sulphate was added to a final concentration of 1M, and the pH was adjusted to 7.0. This fraction was loaded onto a 1x35cm column of Phenyl Sepharose Fast Flow equilib- rated with 50mM sodium phosphate, pH7.0, plus 1M ammonium sulphate, 1M KC1, and ImM dithiothreitol (Buffer B) at a flow rate of 1.5ml/min. The column was washed with 120ml Buffer B then the PPO was eluted with a gradient of 100-0% Buffer B. The active fractions were pooled and concentrated on an i_micon PM10 ultrafiltration membrane then diafiltered with the same membrane against three changes of 20mM potassium phosphate, pH7.0, plus ImM dithiothreitol (Buffer C) . This fraction was applied to a 1x30cm column of Hydroxylapatite equilibrated with Buffer C at a flow rate of lml/min. The column was washed with 50ml of Buffer C then PPO was eluted with a gradient of 0-500mM potassium phosphate in Buffer C. The pooled active fractions were made 20% (v/v) in glycerol and frozen at -80°C. This procedure resulted in a 180-fold purification of PPO and yielded 3.5mg of purified PPO protein, The purification is summarised below: PURIFICATION OF GRAPE BERRY PPO * From 30 Kg grapes The purity of the preparation was checked by denaturing SDS-PAGE. A single diffuse band of protein with an apparent molecular weight of 40kDa was present in the final preparation. EXAMPLE 2 AMINO ACID SEQUENCING Approximately lmg of purified PPO protein was desalted on a 2.5x20cm column of Sephadex G25 equilibrated with 20mM ammonium bicarbonate, pH7.6, at a flow rate of 5ml/min. The protein peak was collected and dried under nitrogen. The dried protein was carboxymethylated and the N-terminal amino acid sequence was determined with an automated amino acid sequenator by Edman degradation. The following sequence was obtained: APIQAPDISKCGTATVPDGVTP EXAMPLE 3 CLONING OF GRAPE PPO GENE Total RNA was isolated from Sultana grape berries according to the method of Rezaian and Krake (1). A poly(A) " " " -enriched RNA fraction was obtained by passing the total RNA through one oligo-dT spun column (Pharmacia LKB Biotechnology) . First strand cDNA was synthesised in a reaction mixture containing 50 mM Tris-HCl pH 8.3, 25 mM KCl, 10 mM 2195 —1 ,4.- CT AU92 35 MgCl,, 4 mM DTT, 1 mM NaPPi, 1 mM dNTPs, 1 U ribonuclease inhibitor, 1.4 μg grape berry poly(A) -enriched RNA, 21 U AMV reverse transcriptase (Promega Corp) and 0.5 μg Hybrid dT17-adapter primer (5*-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT) at 42°C for lh. The reaction mixture was then diluted to 800 μl with TE (10 mM Tris-HCl pH 8.0, 1 mM EDTA) and stored at -20°C. A 32-mer oligonucleotide primer (5'-CCIATICAGGCICCIGATATIICIAAGTGTGG) was designed to the N-terminal protein sequence (amino acids 2-12) of purified grape PPO. Inosine was utilised in positions in which more than 2 bases could be selected based on codon usage tables. This and all other oligonucleotide primers described were synthesised on an Applied Biosystems DNA synthesiser. cDNA was amplified by polymerase chain reaction (PCR) essentially according to the method of Frohman (2) in a 50 μl reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgCl-, 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 5 μl diluted 1st. strand cDNA reaction mixture, 1.25 U Taq DNA polymerase (Promega Corp), 100 nM Adapter primer (5'-GACTCGAGTCGACATCG) and 1 μM N-terminal primer (described above) . Amplification involved an initial program of 5 cycles of denaturation at 94°C for 1 min, annealing at 55°C for 1 min, a slow ramp to 72°C over 2 min and elongation at 72°C for 3 min followed by 25 cycles of 94°C for 1 min, 55 β C for 1 min, and 72°C for 3 min. Amplified DNA was extracted with phenol/chloroform, precipitated with ethanol and resuspended in TE. DNA was blunt-ended with the Klenow fragment and fractionated on a 2% Nusieve GTG agarose (FMC Bioproducts) gel. A 1700 bp fragment was isolated from the gel and ligated into the Hindi site of a Bluescript SK + vector (Stratagene Cloning Systems). Ligated DNA was introduced into E. coli DH5. Positive clones (designated GPO) were isolated and sequenced by the dideoxy sequencing method (3). — _ _>— This confirmed the presence of the N-terminal primer and comparison of the derived protein sequence downstream of the primer with the N-terminal protein sequence obtained for purified grape PPO enzyme above confirmed that this clone coded for grape PPO. EXAMPLE 4 CLONING THE TRANSIT PEPTIDE SEQUENCE Northern blots of grape mRNA probed with the 1700 bp clone described above identified a transcript of 2200 bp which hybridised with the clone. This suggested that there was further sequence upstream of the 5-prime end of the clone even though the clone did code for the N-terminal of the mature PPO protein. A cDNA clone containing the 5'-end of GPOl mRNA (encoding the putative transit peptide) was amplified from grape berry RNA essentially as described in (2), but with nested antisense primers. First strand cDNA was synthesised from grape berry poly(A) + -enriched RNA as described above, but with the Hybrid dT17-adapter primer replaced with GPOl-specific primer 1 (5*-AATCTTTGTGGTGACTGGCG) complementary to a region 44 bases downstream of the N-terminal primer region (i.e. 416-435 nt; Figure 1). The reaction mixture was diluted to 2 ml with O.lx TE and centrifuged through a Centricon 30 spin filter (Amicon Corp) at 4000 g for 20 min to remove excess primer. This step was repeated and the remaining liquid concentrated to 20 μl using Speed Vac centrifugation. A poly (dA)-tail sequence was attached at the 3'end of the cDNA strand with Terminal d Transferase (Promega Corp) in a 20 μl reactio mixture containing 11.5 μl cDNA, 4 μl 5x Tailing Buffer (Promega Corp), 4 μl ATP (1 mM) and 10 U Terminal d Transferase incubated at 37°C for 5 min followed by 65°C for 5 min and then diluted to 500 μl with TE. PCR amplification of poly(dA)-tailed cDNA was carried out in a reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgCl-, 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 5 μl diluted 1st. strand cDNA reaction mixture, 1.25 U Taq DNA polymerase (Promega Corp), 200 nM Hybrid dT17-adapter primer and 900 - _. —lb— nM GPOl-specific primer 2 (5'-ACCATCAGGCACGGTGGCGG) complementary to a region immediately downstream to the N-terminal primer binding region (374-393 nt; Figure 1). Amplification involved 25 cycles of 94°C for 1 min, 55°C for 1 min, and 72°C for 3 min. The resulting 430bp fragment was cloned into Bluescript SK + vector, sequenced as described above and found to contain the predicted region of overlapping sequence with the GPOl clone and confirming this cDNA clone contained the 5' end of the GPOl mRNA. EXAMPLE 5 CLONING OF THE BEAN LEAF PPO GENE Total RNA was isolated from leaves of broad bean according to the method of Rezaian and Krake (1). A poly(A)+-enriched RNA fraction was obtained by passing the total RNA through one oligo-dT spun column (Pharmacia LKB Biotechnology) . First strand cDNA was synthesised in a reaction mixture containing 50 mM Tris-HCl pH 8.3, 25 mM KCl, lOmM MgCl-, 4mM DTT, ImM NaPPi, ImM dNTPs, 1 U ribonuclease inhibitor, 3.1 μg broad bean poly(A) -enriched RNA, 21 U AMV reverse transcriptase (Promega Corp) and 0.81 μg Hybrid dT17-adapter primer: (5'-GACTCG_-GTCG_(^TCGAl M l'TT Tl ,, l r'l 1 l TT) at 42°C for 1 hour. The reaction mixture was then diluted to 840 μl with TE (10 mM Tris-HCl pH 8.0, 1 mM EDTA) and stored at -20 β C. A 25-mer oligonucleotide primer (B15): (5'-GCGGATCCTT[CT]TA[CT]GA[CT]GA[GA]AA[CT]AA) was designed based on the sequence of grape PPO. cDNA was amplified by polymerase chain reaction (PCR) essentially according to the method of Frohman (2) in a 100 μl reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgCl., 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 20 μl diluted 1st. strand cDNA reaction mixture, 2.5 U Taq DNA polymerase (Promega Corp), 100 nM Adapter primer (5'-GACTCGAGTCGACATCG) and lμM B15 primer (described above) . Amplification involved an initial program of 3 cycles of denaturation at 94°C for 1 min, annealing at 37°C for 2 min, a slow ramp to 72°C over 2 min and elongation at 72°C for 3 min followed by 25 cycles of 94°C for 1 min, 55°C for 1 min, and 72°C for 3 min. Amplified DNA was extracted with phenol/chloroform, precipitated with ethanol and resuspended in TE. DNA was blunt-ended with the Klenow fragment and fractionated on a 2% Nusieve GTG agarose (FMC Bioproducts) gel. A 700 bp fragment was isolated from the gel and ligated into the EcoRV site of a Bluescript SK + vector (Stratagene Cloning Systems). Ligated DNA was introduced into E. coli DH5. Recombinant clones were screened using a radioactively labelled fragment of the grape PPO clone (GPOl) and a positive clone (designated BPOl) was isolated and sequenced by the dideoxy sequencing method (3). EXAMPLE 6 CLONING OF APPLE PPO GENES Total RNA was isolated from immature apple fruit according to the method of Rezaian and Krake (1). A poly(A) + -enriched RNA fraction was obtained using a PolyATtract mRNA kit (Promega corporation) . First strand cDNA was synthesised in a 25 μl reaction mixture containing 50 mM Tris-HCl pH 8.3, 25 mM KCl, 10 mM MgC12, 4 mM DTT, 1 mM NaPPi, 1 mM dNTPs, 40 U ribonuciease inhibitor, 1 μg apple poly(A) + -enriched RNA, 24 U AMV reverse transcriptase (Promega Corp) and 0.54 μg Hybrid dT17-adapter primer: (5'-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT) at 42°C for lh. The reaction mixture was then diluted to 525 μl with TE (10 mM Tris-HCl pH 8.0, 1 mM EDTA) and stored at -20°C. A 28-mer oligonucleotide primer (GEN4) : (5'-GCGAATTCGA.AG.GA.TC.ATGGGIAA.TC.TT.TC.TA) was designed based on the sequence of grape PPO. cDNA was amplified by polymerase chain reaction (PCR) essentially according to the method of Frohman (2) in a 100 μl reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgC12, 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 20 μl diluted 1st. strand cDNA reaction mixture, 2.5 U Taq DNA polymerase (Promega Corp), 100 nM Adapter primer (5'- GACTCGAGTCGACATCG) and 1 μM GEN4 primer (described above) . Amplification involved an initial program of 3 cycles of denaturation at 94°C for 1 min, annealing at 37°C for 2 min, a slow ramp to 72°C over 2 min and elongation at 72°C for 3 min followed by 25 cycles of 94°C for 1 min, 55°C for 1 min, and 72°C for 3 min. Amplified DNA was extracted with phenol/chloroform, precipitated with ethanol and resuspended in TE. DNA was blunt-ended with the Klenow fragment and fractionated on a 2% Nusieve GTG agarose (FMC Bioproducts) gel. A fragment of 1050bp was isolated from the gel and ligated into the Eco RV site of a Bluescript SK+ vector (Stratagene Cloning Systems). Ligated DNA was introduced into E. coli DH5. Recombinant clones were screened using a radioactively labelled fragment of the grape PPO clone (GPOl) and two positive clones (designated pSR7 and pSR8) were isolated and sequenced by the dideoxy sequencing method (3). EXAMPLE 7 CLONING OF POTATO PPO GENES Total RNA was isolated from immature potato tubers according to the method of Logemann et al (4) . A poly(A) + -enriched RNA fraction was obtained using a PolyATtract mRNA kit (Promega corporation). First strand cDNA was synthesised in a 25 μl reaction mixture containing 50 mM Tris-HCl pH 8.3, 25 mM KCl, 10 mM MgC12, 4 mM DTT, 1 mM NaPPi, 1 mM dNTPs, 40 U ribonuclease inhibitor, 1.8 μg potato poly(A) + -enriched RNA, 24 U AMV reverse transcriptase (Promega Corp) and 0.54 μg Hybrid dT17-adapter primer: (5"-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT) at 42°C for lh. The reaction mixture was then diluted to 525 μl with TE (10 mM Tris-HCl pH 8.0, 1 mM EDTA) and stored at -20°C. Two oligonucleotide primers were designed from regions within the sequences of grape and apple PPO: GEN3: (δ'-GCGAATTCTT.TC] [TC_TICCITT_TC]CA[TC] [AC]G) GEN7: (5'-GCGAATTCAA[TC]GTIGA[TC] [AC]GIATGTGG) cDNA was amplified by the polymerase chain reaction (PCR) essentially according to the method of Frohman (2) in a 100 μl reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgC12, 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 20 μl diluted 1st. strand cDNA reaction mixture, 2.5 U Taq DNA polymerase (Promega Corp), 100 nM Adapter primer (5'-GACTCGAGTCGACATCG) and 1 μM GEN primer (described above) . Amplification involved an initial program of 3 cycles of denaturation at 94°C for 1 min, annealing at 37°C for 2 min, a slow ramp to 72 β C over 2 min and elongation at 72 β C for 3 min followed by 25 cycles of 94°C for 1 min, 55°C for 1 min, and 72°C for 3 min. Amplified DNA was extracted with phenol/chloroform, precipitated with ethanol and resuspended in TE. DNA was blunt-ended with the Klenow fragment and fractionated on a 2% Nusieve GTG agarose (FMC Bioproducts) gel. Fragments of 1500bp and lOOObp were isolated from the gel and ligated into the Eco RV site of a Bluescript SK+ vector (Stratagene Cloning Systems). Ligated DNA was introduced into E. coli DH5. Recombinant clones were selected and three clones (designated pSRP32, pSRP33, and pSRP72) were isolated and sequenced by the dideoxy sequencing method (3). cDNA clones containing the 5'-end of potato tuber PPO mRNA were amplified from potato tuber RNA essentially as described in (2), but with nested antisense primers. First strand cDNA was synthesised from potato tuber poly(A) + -enriched RNA as described above, but with the Hybrid dT17-adapter primer replaced with potato tuber PPO-specific primer 1: (5'-GACGGTACATTAGTGTTAAAT) complementary to a region 257-278 bases downstream of the 5'-end of pSRP32 and pSRP33. The reaction mixture was diluted to 2 ml with O.lx TE and centrifuged through a Centricon 30 spin filter (Amicon Corp) at 4000 g for 20 min to remove excess primer. This step was repeated and the remaining liquid concentrated to 12 μl using Speed Vac centrifugation. A poly (dA)-tail sequence was attached at the 3'end of the cDNA strand with Terminal d Transferase (Promega Corp) in a 20 μl reaction mixture containing 11.5 μl cDNA, 4 μl 5x Tailing Buffer (Promega Corp), 4 μl ATP (1 mM) and 10 U Terminal d Transferase incubated at 37°C for 5 min followed by 65°C for 5 min and then diluted to 500 μl with TE. PCR amplification of poly(dA)-tailed cDNA was carried out in a reaction mixture containing 10 mM Tris-HCl (pH 9.0 at 25°C), 50 mM KCl, 1.5 mM MgC12, 0.2 mM dNTPs, 0.01% gelatin (w/v), 0.1% Triton X-100, 5 μl diluted 1st. strand cDNA reaction mixture, 1.25 U Taq DNA polymerase (Promega Corp), 200 nM Hybrid dT17-adapter primer and 900 nM potato tuber PPO-specific primer 2 (5 ■ -TGCTCATCAACTGGAGTTGAG) complementary to a region 233-254 bases downstream of the 5'-end of pSRP32 and pSRP33. Amplification involved 25 cycles of 94°C for 1 min, 50°C for 1 min, and 72°C for 3 min. The resulting fragment was cloned into Bluescript SK+ vector, sequenced as described above and found to contain the predicted region of overlapping sequence with the pSRP32 clone confirming this cDNA clone contained the 5'-end of the potato tuber mRNA. REFERENCES 1. Rezaian, M.A. and Krake, L.R. (1987). Nucleic acid extraction and vine detection in grapevine. J. Vir. Methods 17: 277-285. 2. Frohman, M.A. (1990) in PCR Protocols: A Guide to Methods and Applications (eds. M.A. Innis, Gelfand, D.H., Sninsky, J.J., White, T.J.) Academic Press, New York pp28-38. 3. Sanger, F., Nicklen, S. and Coulson, A.R. (1977). DNA sequencing with chain-terminating inhibitors. Proc. Natl. Acad. Sci. USA 74: 5463-5467. 4. Logemann, J. , Schell, J. and Willmitzer, L. (1987). Improved method for the isolation of RNA from plant tissues. Analytical Biochemistry 163:16-20. Finally, it is to be understood that various other modifications and/or alterations may be made without departing from the spirit of the present invention as outlined herein.

Claims: — "^_>__> — " Claims 1. A DNA sequence including a gene coding for a polypeptide having polyphenol oxidase (PPO) activity, or a fragment thereof. 2. A DNA sequence including the pre-sequence of the PPO gene coding for a transit peptide. 3. A DNA sequence according to claim 1, including a sequence coding for antisense mRNA to the normal PPO gene, or a fragment thereof. 4. A DNA sequence according to claim 3, incorporating a catalytic cleavage site. 5. A DNA sequence according to claim 2, wherein the pre-sequence is replaced by other targeting sequences to direct the PPO protein to other cellular compartments. 6. A DNA sequence according to claim 2, including a sequence coding for a putative chloroplast transit peptide sequence and the mature grape vine PPO polypeptide, as illustrated in Figure 1. 7. A DNA sequence according to claim 1, including the gene coding for a polypeptide having broad bean leaf PPO activity, as illustrated in Figure 2. 8. A DNA sequence according to claim 1, including the genes coding for polypeptides having apple fruit PPO activity, as illustrated in Figure 3. 9. A DNA sequence according to claim 1, including the genes coding for polypeptides having potato tuber PPO activity, as illustrated in Figure 4. 10. A method for preparing a recombinant DNA plasmid including a DNA sequence coding for a polypeptide having PPO activity or a fragment thereof, which method includes providing a DNA sequence including the gene coding for a polypeptide having PPO activity or a fragment thereof; and a plasmid expression vector; and reacting the DNA sequence and the plasmid expression vector to deploy the DNA sequence within the plasmid expression vector. 11. A method according to claim 10, wherein the DNA sequence is formed from polyadenylated RNA isolated from a plant sample. 12. A method according to claim 11, wherein the plant sample is selected from apple, potato, grape and bean. 13. A method according to claim 11, including the preliminary steps of providing a source of PPO polypeptide; isolating polyadenylated RNA coding for PPO therefrom; and treating the polyadenylated RNA to construct copy DNA (CDNA) . 14. A method according to claim 13, wherein the step of treating polyadenylated RNA includes treating the polyadenylated RNA with reverse transcriptase and an adapter primer to form first strand cDNA; and amplifying the cDNA so formed using the polymerase chain reaction (PCR) . 15. A method according to claim 14, wherein the adapter primer is an oligonucleotide adapter having the sequence 5 • -GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT. 16. A method according to claim 15, wherein the step of amplifying the cDNA utilises an adapter primer having the sequence 5*-GACTCGAGTCGACATCG and a 5'-end primer. 17. A method according to claim 16, wherein the 5*-end primer has the sequence 5'-CCIATICAGGCICCIGATATIICIAAGTGTGG when utilised for the amplification of grape cDNA; the sequence S'-GCGGATCCTTECT.TA.CT.GA.CT.GAtGA . AAliCT.AA. when utilized for the amplification of bean cDNA; the sequence (5'-GCGAATTCGA[AG]GA[TC]ATGGGIAA[TC]TT[TC]TA) when utilised for the amplification of apple cDNA; and the sequences GEN3: (5'-GCGAATTCTT[TC] _TC_TICCITT[TC_CA_TC] [AC]G) GEN7: (5'-GCGAATTCAA.TC]GTIGA.TC] [AC]GIATGTGG) when utilised for the amplification of potato cDNA. 18. A method according to claim 13, wherein the step of treating the polyadenylated RNA includes treating the polyadenylated RNA with reverse transcriptase and a PPO specific primer to form first strand cDNA; treating the cDNA so formed with terminal d Transferase to attach a polyadenosine tail sequence at the 3' end of the cDNA; and amplifying the polyadenylated cDNA so formed using PCR. 19. A method according to claim 18, wherein the PPO specific primer is an oligonucleotide primer having the sequence 5'-AATCTTTGTGGTGACTGGCG for grape PPO or the PPO-specific primer is an oligonucleotide primer having the sequence 5'GACGGTACATTAGTCTTAAAT for potato tuber PPO. 20. A method according to claim 19, wherein the step of amplifying the cDNA utilises an oligonucleotide oligonucleotide adapter primer having the sequence 5"-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT and a PPO specific oligonucleotide primer having the sequence 5'-ACCATCAGGCACGGTGGCGG for grape PPO or the sequence 5 • -TGCTCATCAACTGGAGTTGAG for potato tuber PPO. 21. A method according to claim 10, wherein the plasmid expression vector is the plasmid vector Bluescript SK + and the DNA sequence is a cDNA sequence; the step of deploying the DNA sequence within the plasmid expression vector includes blunt-ending the cDNA with Klenow fragment; fractionating the cDNA so formed on an Agarose gel; isolating a fragment of the expected size from the gel; and ligating said fragment into the Hind_III or ]j_____RI site of the Bluescript SK vector. 22. A recombinant DNA plasmid including a DNA sequence coding for a polypeptide having PPO activity or a fragment thereof, which plasmid is capable of being replicated, transcribed and translated in a unicellular organism. 23. A recombinant plasmid according to claim 22, including a constitutive promoter element upstream of the DNA sequence. 24. A recombinant plasmid according to claim 23, wherein the DNA sequence codes for a polypeptide having grape, bean, apple or potato tuber PPO activity. 25. A method of decreasing the level of PPO activity in a plant tissue, which method includes providing a DNA construct including a modified PPO gene or fragment thereof; and a plant sample; and introducing said DNA construct into said plant sample to produce a transgenic plant. 26. A method according to claim 25, wherein the DNA construct includes a sequence coding for antisense mRNA to the normal PPO gene or a fragment thereof. 27. A method according to claim 26, wherein the DNA construct includes a PPO gene or fragment thereof, incorporating a catalytic cleavage site. 28. A method according to claim 25, wherein the plant sample is taken from a plant selected from grape vine, potato, apple and bean. 29. A method according to claim 25, wherein the DNA construct includes a binary vector into which has been introduced the DNA sequence; and a promoter which is specific to the skin or peel of a fruit or vegetable. 30. A method of increasing the level of PPO activity in a plant tissue, which method includes providing a DNA construct including a gene coding for a polypeptide having PPO activity or a fragment —__o— thereof; and a plant sample; and introducing said DNA construct into said plant sample to produce a transgenic plant. 31. A method according to claim 30, wherein the DNA construct includes a DNA sequence encoding the PPO pre-sequence coding for a transit peptide or a fragment thereof. 32. A method according to claim 31, wherein the PPO pre-sequence is replaced with other targeting sequences to direct the PPO protein to other cellular compartments. 33. A method according to claim 30, wherein the plant sample is taken from a plant selected from tobacco, broad bean, tomato, tea, coffee and cocoa. 34. A method according to claim 30, wherein the DNA construct includes a binary vector into which has been introduced the DNA sequence; and a promoter which is specific to the skin or peel of a fruit or vegetable. 35. A DNA probe including a DNA sequence including a gene coding for a polypeptide having PPO activity, or a fragment thereof. 36. A method of isolating a DNA sequence coding for a polypeptide having PPO activity, or a fragment thereof, from a plant species, which method includes providing a cDNA or genomic library; and a DNA probe including a DNA sequence coding for a polypeptide having PPO activity or a fragment thereof; and hybridising the probe with the genomic library to identify clones containing said DNA sequence. 37. A method according to claim 36, wherein the DNA probe includes a DNA sequence including a fragment of the apple, potato, grape or bean PPO gene which is highly conserved between different species. 38. A method according to claim 37, wherein the DNA probe is prepared by a method which includes providing total cDNA from a plant species; and two or more oligonucleotide primers which hybridise specifically with the PPO gene and which include sequences of the apple, potato, grape or bean PPO gene which are highly conserved between different species; and performing PCR to amplify a DNA sequence including the PPO gene or a fragment thereof. 39. A method according to claim 38, wherein the oligonucleotide primers include DNA sequences corresponding to the copper binding sites on the PPO protein. 40. A method of isolating a DNA sequence including a gene coding for a polypeptide having PPO activity, or a fragment thereof, from a plant species, which method includes providing mRNA isolated from the plant; a poly-dT adapter primer; and two or more oligonucleotide primers; treating the mRNA with reverse transcriptase and an adapter primer to form first strand cDNA; and amplifying the cDNA so formed using the oligonucleotide primers and the polymerase chain reaction. 41. A method of isolating the PPO gene or a fragment thereof from a plant species, which method includes providing an expression library; and a polyclonal antibody which has been raised against a purified PPO protein: and reacting the polyclonal antibody with the expression library to identify clones containing a DNA sequence including the PPO gene or fragments thereof. 42. A PPO protein in a substantially pure form, having the N-terminal amino acid sequence APIQAPDISKCGTATVPDGVTP.

Inventors: DRY IAN BARRY, ROBINSON SIMON PIERS, DRY, IAN, BARRY, ROBINSON, SIMON, PIERS
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1992
Main classification: C12N15
IPC codes: A01H1, C12N9, C12N15