Abstract:
Novel DNA and enzymes such as Plant Thioredoxin-Porphobilinogen Synthase (T-PPS) or Plant Porphobilinogen Synthase (PPS), together with novel compositions thereof and methods using such enyzmes are claimed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/171,785 filed Dec. 22, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to novel enzymes for converting a compound or substrate known as aminolevulinate (ALA) to another compound or product known as prophobilinogen (PBG). The present invention also relates to novel DNA that encodes for such enzymes. 
     BACKGROUND OF THE INVENTION 
     Enzymes are important laboratory tools for discovering new agricultural or related compounds such as insecticides, herbicides, fungicides, nematocides, antimicrobials and the like. In plants, there is a key biochemical pathway in which a pyrrole ring-containing compound known as porphobilinogen (PBG) is produced from two acyclic or open-chain molecules known as aminolevulinate (ALA). PBG is a precursor or substrate for both heme and chlorophyll biosynthesis in plants. Plants employ an enzyme that can catalyze or convert ALA to PBG. Unfortunately, this native enzyme cannot be expressed so that it is active in a non-plant host, such as bacteria, since such expressions have been found to yield an insoluble and inactive enzyme. Thus, it became desirable to develop active, plant-based enzymes that could convert aminolevulinate to porophobilinogen and yet still remain soluble under laboratory conditions. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides an active, plant-based protein or enzymes known as plant thioredoxin-porphobilinogen synthase (T-PPS) and plant porphobilinogen synthase (PPS) that can convert aminolevulinate to porophobilinogen and yet still remain soluble under laboratory conditions. The present invention has the further advantage of also providing DNA in a non-natural host such as bacteria, virus, yeast, etc. that will produce the desired protein or functional fragments thereof, outside of its native plant source. 
     Accordingly, in one embodiment, the present invention is directed toward DNA or polynucleotide characterized by 
     a) SEQ ID NOs: 1 or 7; 
     b) the complementary sequence thereof; or 
     c) the double stranded sequence of a) and b). 
     In a second embodiment, the present invention is directed towards DNA or polynucleotide characterized in that its homology to the sequence as shown in SEQ ID NOs: 1 or 7 is at least 80%. Preferably, the DNA or polynucleotide is characterized in that its homology to the sequence as shown in SEQ ID NOs: 1 or 7 is at least 90%. 
     In a third embodiment, the present invention is directed towards DNA or polynucleotide that encodes the total: or functional fragments of an amino acid sequence as shown in SEQ ID NOs: 2 or 8. 
     In a fourth embodiment, the present invention is directed towards RNA characterized in that it is complementary to the DNA of SEQ ID NOs: 1 or 7. 
     In a fifth embodiment, the present invention is directed towards an expression construct, characterized in that it encompasses DNA or polynucleotide described in the first, second or third embodiments and a sequence that is functionally linked thereto that allows the DNA or polynucleotide to be expressed. 
     In a sixth embodiment, the present invention is directed towards a plasmid characterized in that it contains DNA or polynucleotide described in the first, second or third embodiments. 
     In a seventh embodiment, the present invention is directed towards a protein or polypeptide represented by SEQ ID NO: 2, known herein as “Plant Thioredoxin-Porphobilinogen Synthase” (T-PPS) or a protein represented by SEQ ID NO: 8, known as “Plant Porphobilinogen Synthase” (PPS). Preferably, the protein is in a buffered composition. 
     In an eight embodiment, the present invention is directed towards a method of determining the enzymatic activity of the protein or polypeptide of SEQ ID NO: 2 (i.e. plant thioredoxin-porphobilinogen synthase) or SEQ ID NO: 8 (i.e. plant porphobilinogen synthase), characterized by contacting or converting δ-aminolevulinic acid (a substrate) with said protein and measuring the amount of porphobilinogen (a product) formed therefrom. Preferably, the protein is in a buffered composition. 
     In a ninth embodiment, the present invention is directed toward a method of identifying a compound which can modify the activity of the protein or polypeptide of SEQ ID NOs: 2, 8 or a functional fragment thereof comprising contacting or converting δ-aminolevulinic acid with said protein or polypeptide or a functional fragment thereof in the presence of a test compound and measuring the amount of porphobilinogen formed therefrom. Preferably, the protein or polypeptide is in a buffered composition. Also preferred is that said identified compound inhibits said protein or polypeptide or functional fragment thereof. 
     In a tenth embodiment, the present invention is directed towards a method of inhibiting plant growth comprising applying to a plant a compound which inhibits the enzymatic activity of plant thioredoxin-porphobilinogen synthase or plant porphobilinogen synthase. Preferably, the protein or polypeptide is in a buffered composition. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying figures or drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. 
     The invention also relates to the use of substances which are found with the aid of the above-described method for use as herbicides. 
     “Buffer” or “buffered composition” refers to a solution in which a buffering agent has been added and which tends to prevent or resist rapid changes in pH upon the addition of small quantities of acid or base. 
     “Chemically synthesized,” as related to a sequence of DNA, means that the component nucleotides are assembled in vitro. Manual chemical synthesis of DNA may be accomplished using well established techniques in the art. 
     “Complementary” relates to the capability of purine and pyrimidine nucleotides to form base pairs with each other via hydrogen bonds. Complementary base pairs are, inter alia, guanine and cytosine, adenine and thymine, and adenine and uracil. 
     “DNA” or “polynucleotide” refers to deoxyribonucleic acid. 
     “Expression” or “expressing” refers to the transcription and/or in the case of a protein gene product, translation, of a heterologous or homologous gene to yield the gene product encoded by the structural portion of the gene or DNA fragment. 
     “Expression construct” refers to the union of a functional fragment in a plasmid, resulting in a vector that is capable of expressing the functional fragment. 
     “Functional fragments” describes those DNA fragments which encode for plant porphobilinogen synthase or the fusion protein thereof, or a polypeptide portion therof that still maintains a substantial amount of the activity or function of the plant porphobilinogen synthase or the fusion protein thereof. 
     “Fusion protein” refers to a chimeric protein or polypeptide in which plant porphobilinogen synthase or functional fragment thereof, is joined to a second protein or polypeptide such as thioredoxin, maltose binding protein, or other proteins. The second protein or polypeptide serves the function of helping or enhancing the solubility and/or the post-translational modification of the plant porphobilinogen synthase or functional fragment thereof. 
     “Gene” refers to a unit composed of a promoter region, a structural gene region and a transcription termination region. 
     “Gene product” refers to a specific protein or RNA product derived from the structural portion of the gene. 
     “Heterologous” is used to indicate that a nucleic acid sequence (e.g., a gene) or a protean has a different natural origin or source with respect to its current host. Heterologous is also used to indicate that one or more of the domains present in a protein differ in their natural origin with respect to other domains present. In cases where a portion of a heterologous gene originates from a different organism the heterologous gene is also known as a chimera. 
     “Homologous” is used to indicate that a nucleic acid sequence (e.g. a gene) or a protein has a similar or the same natural origin or source with respect to its current host. 
     “Homology” in relation to DNA means that DNA segments which are at least 15 base pairs long or strands which are complementary to the DNA match the corresponding DNA in at least 80%, preferably in 90%, of the nucleotides. Such a homology is determined, inter alia, with the aid of computer programs such as the GCG program (Devereux et al. (1983), Nucleic Acids Res. 12, 3 87-395). Homology also exists when a DNA segment is capable of hybridizing with the DNA strand in question or with its complementary strand. 
     “Hybridization” or “to hybridize” describes the process in which a single stranded nucleic acid molecule undergoes base pairing with a complementary DNA strand, where the capability of a single-stranded nucleic acid molecule depends on the stringency of the hybridization conditions. 
     “Nucleic acid sequence” as used herein refers to a nucleotide, oligonucleotide, or polynucleotide, and fragments thereof, and to DNA or RNA of genomic or synthetic origin which may be single- or double-stranded, and which may represent a sense or antisense strand. 
     “Plasmid” refers to an circular, autonomous (i.e. self-replicating) extrachromosomal genetic element. The original plasmids used for the present invention are either commercially available or freely accessible or can be derived from such plasmids by known methods. 
     The terms “protein” or “polypeptide” are to be regarded as substantially equivalent. 
     “RNA” refers to ribonucleic acid. 
     “Stringency” relates to the hybridization conditions. “High stringency makes base pairing difficult. To do this, high temperatures of 42° C. or less are used, a formamide concentration of less than 20% and low salt (SSC) concentrations, Alternatively, temperatures of 65° C. or less can be used in combination with a low salt concentration (SSPE). “Low stringency” conditions favor the formation of base pairs. The temperatures used here are 37° C. or less, the formamide concentration is less than 50%, and the salt concentration (SSC) is moderate. Alternatively, temperatures of 50° C. or less in combination with a medium to high salt concentration (SSPE) are used. 
     “Vector” describes a DNA vehicle used for introducing exogenous DNA into host cells. A vector contains a nucleotide sequence which encodes one or more polypeptides or proteins. For example, a plasmid is an example of a circular vector. 
     One skilled in the art is aware of the fact that the degenerate genetic code (i.e. 64 codons encode 20 amino acids) allow a large number of “silent” substitutions of nucleotide base pairs to be introduced into the sequence shown here without changing the identity of the protein products encoded by it. The scope of the invention includes all such substitutions. 
     DNA Isolation 
     The DNA or nucleic acid mentioned here can exist in complete cells, in cell lysates, in partially purified or biologically pure form, i.e. when other cell components or chemical precursors and by-products, in the case of chemical DNA synthesis, have been removed. 
     The DNA mentioned here can be obtained by a series of genetic and recombinant DNA techniques, for example by means of amplification with the aid of the polymerase chain reaction (PCR) or else by de novo DNA synthesis. The DNA mentioned here can be isolated by means of RT-PCR amplification of total RNA from suitable plant cells using oligonucleotide or polynucleotide primers which are directed at a suitable region of SEQ ID NOs: 1 or 7 (see, for example, J. Sambrook et al, (1989), Molecular Cloning, 2nd edition, chapter 14). 
     Expressing and purifying the protein 
     The invention also relates to proteins or functional fragments thereof which have plant porphobilinogen synthase activity and which are encoded by an above-described DNA. 
     The skilled worker knows that the proteins of the present invention can be obtained by various routes, for example by chemical methods such as the solid-phase method. To obtain larger quantities of protein, the use of recombinant methods is recommended. Expression of a cloned plant porphobilinogen synthase gene or fragments thereof can take place in a series of suitable host cells which are known to the skilled worker. To this end, a plant porphobilinogen synthase gene is introduced into a host cell with the aid of known methods. 
     The integration of the cloned plant porphobilinogen synthase gene in the chromosome of the host cell is within the scope of the present invention. Preferably, the gene or functional fragments thereof are inserted into a plasmid, and the encoding regions of the plant porphobilinogen synthase gene or fragments thereof are functionally linked to a constitutive or inducible promoter. 
     The basic steps for generating the recombinant plant porphobilinogen synthase are: 
     1. Obtaining a natural, synthetic or semi-synthetic DNA that can express plant porphobilinogen synthase. 
     2. Introducing this DNA into an expression vector which is suitable for expressing plant porphobilinogen synthase either alone or as a fused protein. 
     3. Transformation of a suitable host cell, preferably a prokaryotic host cell, with this expression vector. 
     4. Growing this transformed host cell in a manner which is suitable for expressing plant porphobilinogen synthase. 
     5. Harvesting the cells and purifying plant porphobilinogen synthase by suitable known methods. 
     The encoding regions of plant porphobilinogen synthase can be expressed by the customary methods in  E. coli , either separately or together. Suitable expression systems for  E. coli  are commercially available, for example, the plasmids of the pET series, for example pET3a, pET23a, pET28a with HIS-Tag or pET32a with HIS-Tag for the simple purification and thioredoxin fusion for improving the solubility of the expressed enzyme, and pGEX with glutathion synthetase fusion plasmids are transformed into XDE3-lysogenic  E. coli  strains, for example. BL21(DE3), HMS 174(DE3) or AD494(DE3). Expression is induced with IPTG under standard conditions known to the skilled worker. After cell induction, incubation is carried out for 3 to 24 hours at temperatures from about 18° C. to about 37° C. The cells are disrupted by sonication in disruption buffer (10 to 200 mM tricine, 100 to 500 mM NaCl, pH 5 to 8). The protein which has been expressed can be purified by chromatographic methods, in the case of protein which has been expressed with a his-Tag by means of chromatography on an Ni—NTA column. 
     Alternatively, the proteins may also be expressed in plants. 
    
    
     EXAMPLE 1 
     Preparation of DNA Coding Sequence for Recombinant Plant Thioredoxin-porphobilinogen Synthase 
     Total RNA is collected from 8-10 day old tomato ( Lycopersicon esculentum ) fruit using published protocols and reagents (Trizol) from Life Technologies, Inc. (Rockville, Md.). Polynucleotide primers are designed and employed such that their use in polymerase chain reaction (PCR) generates a truncated form of the plant porphobilinogen synthase gene from tomato ( Lycopersicon esculentum ) total RNA. One hundred nanograms each of custom polynucleotide primers, TTATTCTCGAGTTACCTCTTCTCTCCACACAGG (SEQ ID NO: 5) and TATTAGAATTCGCTAGCAAGGAAGGGCATGA (SEQ: ID NO: 6), are incubated with 1 microgram of total RNA in a reverse transcriptase polymerase chain reaction (RT-PCR) kit (Life Technologies) according to the manufacturer&#39;s recommendations. 
     The resulting PCR product (i.e. plant porphobilinogen synthase DNAs) and plasmid pET32b(+) (Novagen, Madison, Wis.), are digested with restriction endonucleases EcoR I and Xho I, as directed by the manufacturer (Life Technologies) to give two linear proteins. Plasmid pET32b(+) contains DNA (SEQ ID NO: 3) which encodes for the thioredoxin (trxA) protein fragment (SEQ ID NO: 4). Thioredoxin is a functional fragment of the pET32b(+),plasmid and has 501 nucleotide base pairs. This portion is joined to the PPS functional fragment such that the last base, i.e. 501, of trxA is attached to the first base, i.e. 1, of PPS. This union creates a novel DNA capable of encoding a thioredoxin-PPS fusion protein. Ligation of these two linear DNAs with DNA T4 ligase produces the recombinant clone plant porphobilinogen synthase pET32b(+) (Life Technologies). DNA sequence analysis verifies the integrity of the plant thioredoxin-porphobilinogensynthase/ pET32b(+) clone containing DNA that encodes for plant thioredoxin-porphobilinogen synthase (SEQ ID NOs: 1 and 2), a fisio n protein. 
     EXAMPLE 2 
     Preparation of Plant Thioredoxin-Porphobilinogen Synthase (T-PPS) 
     Cloned plant thioredoxin-porphobilinogen synthase/pET32b(+) is transformed into a proprietary bacterial strain,  E. coli  AD)494(DE3)lysS (Novagen Inc., Madison, Wis.), according to the manufacturer&#39;s instructions. Transcription and translation of plant thioredoxin-porphobilinogen synthase/pET32b(+) in this host requires the sugar, isopropylthio-beta-galactoside (IPTG). Transformed bacteria are grown in LB liquid media (10 grams each tryptone and NaCl; 5 grams yeast extract; H 2 O to one liter) at 37° C. to an optical density of 0.6 at 600 nm. At that point, IPTG is added to a final concentration of 1 millimolar and the culture is incubated at 37° C. for 4 additional hours. Bacteria are pelleted via centrifugation, the supernatant discarded, and the pellet is frozen to −80° C. Pellets are resuspended in 100 millimolar tricine buffer, pH 7.9; 300 millimolar NaCl or other suitable biochemical buffer, mechanically disrupted, and centrifuged. Four hours post IPTG induction, soluble plant thioredoxin-porphobilinogen synthase protein (SEQ ID NO: 2) is detectable in the collected supernatant, as determined by western blot analysis, which is targeted to the attached HIS sequence, thirodoxin, or the S-tag portion of the protein product. 
     EXAMPLE 3 
     Testing Enzyme Activity of Plant Thioredoxin-porphobilinogen Synthase 
     Enzyme activity is tested in accordance with EC 4.2.1.24 using the following assay principle:                           
     The enzyme activity of plant thioredoxin-porphobilinogen synthase (T-PPS) is measured as porphobilinogen (PBG) formation from δ-aminolevulinic acid (Aminolevulinate or ALA). A typical reaction is as follows: the reaction is carried out at 37° C. for 1 hour in Tricine buffer pH 7.9 or other suitable biochemical buffer, 2.5 mM ALA, 5 mM MgCl 2  and 1.25 μg of purified recombinant protein from  E. coli  expression system in a final assay volume of 100 μl. PBG is quantified spectrophotometrically after reaction with the 100 μl Ehrlich&#39;s reagent solution. Optical density is determined after 60 minutes at 555 nm. This assay indicates that plant thioredoxin-porphobilinogen synthase is active, i.e. plant thioredoxin-porphobilinogen synthase can convert ALA to PBG. 
     EXAMPLE 4 
     Using Assay to Identify Inhibitory or Herbicidal Compounds 
     Essentially the same procedure as described in Example 3 is performed, except that a test compound is added to or mixed with the plant thioredoxin-porphobilinogen synthase prior to addition of δ-aminolevulinic acid. A decrease in product (i.e. PBG) compared to the control would indicate that the test compound is an inhibitor of plant thioredoxin-porphobilinogen synthase. 
     EXAMPLE 5 
     Preparation of DNA Encoding Sequence for Plant Porphobilinogen Synthase (PPS) 
     Essentially the same procedures are employed as described in Example 1, except that the plasmid pET-30b(+) (Novagen Inc., Madison, Wis.) is used in place of plasmid pET32b(+) to give a plant porphobilinogen synthase/pET-30b(+) clone that contains the DNA coding sequence for plant porphobilinogen synthase (PPS) (SEQ ID NO: 7). 
     EXAMPLE 6 
     Preparation of Plant Porphobilinogen Synthase (PPS) 
     The same procedures are employed as described in Example 2, except that the plant porphobilinogen synthase/pET-30b(+) of Example 5 is used in place of plant thioredoxin-porphobilinogen synthase/pET32b(+), to give the desired plant porphobilinogen synthase (SEQ ID NO: 8). 
     EXAMPLE 7 
     Testing Enzyme Activity of Plant Porphobilinogen Synthase 
     The same procedures are employed as described in Example 3, except that the plant porphobilinogen synthase of Example 5 is used in place of plant thioredoxin-porphobilinogen synthase. 
     EXAMPLE 8 
     Using Assay to Identify Inhibitory or Herbicidal Compounds 
     The same procedures are employed as described in Example 4, except that the plant porphobilinogen synthase of Example 5 is used in place of plant thioredoxin-porphobilinogen synthase.