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

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a method of modifying polyphenol ozidase (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 tb 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 follow 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. 
     SUMMARY OF THE INVENTION 
     In a first aspect of the present invention, there is provided a DNA sequence including a gene coding for a polypeptide having plant polyphenol ozidase (PPO) activity or a fragment thereof. 
     The DNA sequence may include a pre-sequence of a plant 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 a plant PPO gene, or a fragment thereof. 
     The DNA sequence may include a catalytic cleavage site. 
     Alternatively the presequence may be replaced by other targeting sequences to direct the polypeptide having plant PPO activity to other cellular compartments. 
     The DNA sequence may include a putative chloroplast transit sequence and a mature grape vine PPO protein, as illustrated in FIG.  1 . 
     The DNA sequence may include a gene coding for a polypeptide having a broad bean leaf PPO activity, as illustrated in FIG.  2 . 
     The DNA sequence may include a gene coding for a polypeptide having apple fruit PPO activity, as illustrated in FIG.  3 . 
     The DNA sequence may include a gene coding for a polypeptide having potato tuber PPO activity, as illustrated in FIG.  4 . 
     In a further aspect of the present invention there is provided a DNA sequence including a sequence coding for antisense mRNA to a plant PPO gene, or a fragment thereof. 
     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 a polypeptide having plant PPO activity or a fragment thereof, which method includes 
     providing
         a DNA sequence including a 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 a polypeptide having plant PPO activity; 
     isolating polyadenylated RNA coding for a polypeptide having plant PPO activity 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′-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT. (SEQ ID NO:15)       

     The step of amplifying the cDRA may utilise an adapter primer having the sequence
         5′-GACTCGAGTCGACATCG (SEQ ID NO:16)
 
and a 5′-end primer.
       

     The 5′-end primer may have the sequence
         5′-CCIATICAGGCICCIGATATIICIAAGTGTGG (SEQ ID NO:17)
 
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. (SEQ ID NO:18)
 
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) (SEQ ID NO:19)
 
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) (SEQ ID NO:20)   GEN7: (5′-GCGAATTCAA[TC]GTIGA[TC][AC]GIATGTGG) (SEQ ID NO:21)
 
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 (SEQ ID NO:22)
 
for grape PPO or the PPO-specific primer is an oligonucleotide primer having the sequence
   5′GACGGTACATTAGTCTTAAAT (SEQ ID NO:23)
 
for potato tuber PPO.
       

     The step of amplifying the cDNA may utilise an oligonucleotide adapter primer having the sequence
         5′-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT (SEQ ID NO:15)
 
and a PPO specific oligonucleotide primer having the sequence
   5′-ACCATCAGGCACGGTGGCGG (SEQ ID NO:24)
 
for grape PPO or the sequence
   5′-TGCTCATCAACTGGAGTTGAG (SEQ ID NO:25)
 
for potato tuber PPO.
       

     The plasmid expression vector for the cloning of th 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, for example with Klenow fragment; 
     fractionating the cDNA so formed, for example on an Agarose gel; 
     isolating a fragment of the expected size, for example from the gel; and 
     ligating said fragment into a suitable restriction enzyme site, for example the HindIII or EcoRI 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 plant 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 gene coding for a polypeptide having plant PPO activity 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 a plant PPO gene or a fragment thereof. The DNA construct may include a gene coding for a polypeptide having plant PPO activity 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 plant 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 a pre-sequence of a plant PPO gene 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  Agrobacterium  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 coding for a polypeptide having plant PPO activity 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 coding for a polypeptide having plant 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 a gene coding for a polypeptide having plant PPO activity and which include sequences of the apple, potato, grape or bean PPO gene which are highly conserved between different species; and       

     performing PCT to amplify a DNA sequence including a gene coding for a polypeptide having plant PPO activity or a fragment thereof. 
     The oligonucleotide primers may include DNA sequences corresponding to the copper binding sites on the polypeptide having plant PPO activity. 
     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 
     amplifyig 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 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
         an expression library; and   a polyclonal antibody which has been raised against a purified polypeptide having PPO activity; and       

     reacting the polyclonal antibody with the expression library to identify clones containing a DNA sequence including a gene coding for a polypeptide having PPO activity or fragments thereof. 
     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 substantially pure form, having the N-terminal amino acid sequence
         APIQAPDISKCGTATVPDGVTP. (SEQ ID NO:26)       

     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures: 
         FIGS. 1-1  to  1 - 4 : 
       The composite full-length GP01 cDNA nucleotide sequence and derived protein sequence encoding both the putative chloroplast transit sequence and he 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. 
         FIGS. 2-1  and  2 - 2 : 
       Nucleic acid and derived protein sequence of the BPO1 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. 
       FIG.  3 : 
       Nucleic acid and derived protein sequences of the clones pSR7 (SEQ ID NO:5 and 6, respectively) and pSR8 (SEQ ID NO:7 and 8 respectively) encoding apple fruit PPO. The solid line indicates the region of the GEN4 primer used to amplify the cDNA by the polymerase chain reaction. 
         FIGS. 4-1  to  4 - 8 : 
       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. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 100 ml of a solution of 100 mM ascorbate plus 10 mM dithiothreitol was added to each 900 ml of grape juice. The juice was centrifuged for 10 mins at 10,000×g and the supernatant discarded. The pellet fraction was resuspended in 25 mM sodium phosphate, pH 7.2 plus 10 mM ascorbate and 1 mM dithiothreitol to a final volume of 1.75 L, then 250 ml of a 4% (w/v) solution of the cationic detergent hezadecyltrimethylammonium bromide (CTAB) was added. After incubating for 20 mins the extract was centrifuged for 15 mins at 15,000×g. 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 15,000×g. 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 15,000×g. The pellet was resuspended in 20 mM Bis-tris-propane, pH 7.5 plus 10 mM ascorbate and 2 mM dithiothreitol (Buffer A) in a final volume of 100 ml. The extract was desalted on a 4×40 cm column of Sephadex G25 equilibrated with Buffer A at a flow rate of 10 ml/min and the active fractions were pooled. 
     The extract was applied to a 2.5×10 cm column of Q-Sepharose Fast Flow equilibrated with Buffer A at a flow rate of 6 ml/min and then the column was washed with 400 ml of Buffer A. The PPO was eluted with a gradient of 0-500 mM 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 1×35 cm column of Phenyl Sepharose Fast Flow equilibrated with 50 mM sodium phosphate, pH7.0, plus 1M ammonium sulphate, 1M KCl, and 1 mM dithiothreitol (Buffer B) at a flow rate of 1.5 ml/min. The column was washed with 120 ml Buffer B then the PPO was eluted with a gradient of 100-0% Buffer B. The active fractions were pooled and concentrated on an Amicon PM10 ultrafiltration membrane then diafiltered with the same membrane against three changes of 20 mM potassium phosphate, pH7.0, plus 1 mM dithiothreitol (Buffer C). This fraction was applied to a 1×30 cm column of Hydroxylapatite equilibrated with Buffer C at a flow rate of 1 ml/min. The column was washed with 50 ml of Buffer C then PPO was eluted with a gradient of 0-500 mM 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 3.5 mg of purified PPO protein. The purification is summarised below: 
     
       
         
               
             
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                   
               
               
                 PURIFICATION OF GRAPE BERRY PPO 
               
             
          
           
               
                   
                   
                   
                 Spec. 
                   
                   
               
               
                   
                 Protein 
                 Act. 
                 Act. 
                 Recov. 
                 Purif. 
               
               
                 Step 
                 (mg) 
                 (U) 
                 (U/mg) 
                 (%) 
                 (-fold) 
               
               
                   
               
             
          
           
               
                 Juice* 
                 19,360 
                 7,040 
                 0.4 
                 100 
                 1 
               
               
                 CTAB extract 
                 960 
                 2,070 
                 2.2 
                 29 
                 6 
               
               
                 Ammonium sulphate 
                 600 
                 1,760 
                 2.9 
                 25 
                 8 
               
               
                 Q-Sepharose 
                 130 
                 1,520 
                 11.8 
                 22 
                 33 
               
               
                 Phenyl Sepharose 
                 10.8 
                 400 
                 37 
                 6 
                 103 
               
               
                 Hydroxylapatite 
                 3.5 
                 230 
                 65 
                 3 
                 180 
               
               
                   
               
               
                 *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 40 kDa was present in the final preparation. 
     EXAMPLE 2 
     Amino Acid Sequencing 
     Approximately 1 mg of purified PPO protein was desalted on a 2.5×20 cm column of Sephadex G25 equilibrated with 20 mM ammonium bicarbonate, pH7.6, at a flow rate of 5 ml/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 (SEQ ID NO:26)       

     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 MgCl 2 , 4 mM DTT, 1 mM NaPPi, 1 mM dNTPs, 1 U ribonuclease inhibitor, 1.4 pg grape berry poly(A) + -enriched RNA, 21 U AMV reverse transcriptase (Promega Corp) and 0.5 μg Hybrid dT17-adapter primer
         (5′-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT(SEQ ID NO:15)
 
at 42° C. for 1 h. 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) (SEQ ID NO:17)
 
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 2 , 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) (SEQ ID NO:16)
 
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° 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 HincII 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 GPO1 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 GPO1-specific primer 1
         (5′-AATCTTTGTGGTGACTGGCG) (SEQ ID NO:22)
 
complementary to a region 44 bases downstream of the N-terminal primer region (i.e. 416-435 nt; FIG.  1 ). The reaction mixture was diluted to 2 ml with 0.1× TE and centrifuged through a Centricon 30 spin filter (Amicon Corp) at 4000 g for 20 win 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 reaction mixture containing 11.5 μl cDNA, 4 μl 5× Tailing Buffer (Promega Corp), 4 μl ATP (1 MM) and 10 U Terminal d Transferase incubated at 37° C. for 5 win 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 2 , 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 rnM Hybrid dT17-adapter primer and 900 nM GPO1-specific primer 2
   (5′-ACCATCAGGCACGGTGGCGG) (SEQ ID NO:24)
 
complementary to a region immediately downstream to the N-terminal primer binding region (374-393 nt; FIG.  1 ). Amplification involved 25 cycles of 94° C. for 1 min, 55° C. for 1 min, and 72° C. for 3 min. The resulting 430 bp fragment was cloned into Bluescript SK +  vector, sequenced as described above and found to contain the predicted region of overlapping sequence with the GPO1 clone and confirming this cDNA clone contained the 5′ end of the GPO1 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, 10 mM MgCl 2 , 4 mM DTT, 1 mM NaPPi, 1 mM 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′-GACTCGAGTCGACATCGATTTTTTTTTTTTTTTTT) (SEQ ID NO:15)
 
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° C.
       

     A 25-mer oligonucleotide primer (B15):
         (5′-GCGGATCCTT[CT]TA[CT]GA[CT]GA[GA]AA[CT]AA) (SEQ ID NO:18)
 
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 2 , 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 Tag DNA polymerase (Promega Corp), 100 nM Adapter primer (5′-GACTCGAGTCGACATCG) (SEQ ID NO:16) and 1 μM 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 SR +  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 (GPO1) and a positive clone (designated BPO1) 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) 30  -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 MgCl2, 4 mM DTT, 1 mM NaPPi, 1 mM dNTPs, 40 U ribonuclease 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) (SEQ ID NO:15)
 
at 42° C. for 1 h. 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) (SEQ ID NO:19)
 
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 MgCl2, 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) (SEQ ID NO:16)
 
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 1050 bp 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 (GPO1) 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 MgCl2, 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) (SEQ ID NO:15)
 
at 42° C. for 1 h. 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: (5′-GCGAATTCTT[TC][TC]TICCITT[TC]CA[TC][AC]G) (SEQ ID NO:20)   GEN7: (5′-GCGAATTCAA[TC]GTIGA[TC][AC]GIATGTGG) (SEQ ID NO:21)
 
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 MgCl2, 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) (SEQ ID NO:16)
 
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° 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. Fragments of 1500 bp and 1000 bp 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) (SEQ ID NO:27)
 
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 0.1× 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 5× 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 MgCl2, 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) (SEQ ID NO:25)
 
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 RRA 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.