HERBICIDE RESISTANT PLANTS

Abstract: The invention provides a modified tubulin comprising a tubulin polypeptide which confers resistance to an anti-tubulin agent when expressed within a biological organism, said polypeptide having at least one amino acid substitution such that hydrophobicity is increased at one or more positions in the polypeptide. DNA sequences encoding such modified tubulins may be used to confer resistance on organisms which otherwise would be susceptible to one or more anti-tubulin agents. For example, a plant which is resistant to a herbicide that disrupts microtubule structure or function may be produced.

Description: HERBICIDE RESISTANT PLANTS This invention relates to modified tubulins, DNA sequences encoding such tubulins and organisms transformed with such DNA. Almost all eukaryotic cells contain microtubules which comprise a major component of the network of proteinaceous filaments known as the cytoskeleton. Microtubules thereby participate in the control of cell shape and intracellular transport. They are also the principal constituent of mitotic and meiotic spindles, cilia and flagella. In plants, microtubules have additional specialized roles in cell division and cell expansion during development. In terms of their composition, microtubules are proteinaceous hollow rods with a diameter of approximately 24nm and highly variable length. They are assembled from heterodimer subunits of an α-tubulin and a β-tubulin polypeptide, each with a molecular weight of approximately 50,000. Both polypeptides are highly flexible globular proteins (approximately 445 amino acids), each with a predicted 25% α helical and 40% β-pleated sheet content, in addition to the two major forms (α- and β-tubulin), there is a rare γ-tubulin form which does not appear to participate directly in the formation of microtubule structure, but rather it may function in the initiation of microtubule structure. In all organisms, the multiple α- and β-tubulin polypeptides are encoded by corresponding families of α- and β-tubulin genes, which are located in the nuclear genome. Many such genes (or corresponding cDNAs) have been isolated and sequenced. For example, maize has approximately 6 α-tubulin genes and approximately 8 β-tubulin genes dispersed over the genome (Villemur et al, 1992, 34th Maize Genetics Symposium) . Some of the α-tubulin genes from maize have been cloned and sequenced (Montoliu et al, 1989, Plant Mol Biol, 14, 1-15; Montoliu et al, 1990, Gene, 94, 201-207; Villemur et al, 1992, J Mol Biol, 227:81-96), as have some of the β-tubulin genes (Hussey et al, 1990, Plant Mol Biol, 15, 957-972). Comparison of amino acid sequences of the three documented maize α-tubulins indicates they have 93% homology. Maize β-tubulins exhibit 38% identity with these α-tubulins. In segments of divergence between the α- and β-tubulin amino acid sequences, homology ranges from 13% to 17%. Homology between the three α-tubulin amino acid sequences within these same α-/β-divergence regions ranges from 77% to 96%. Sequence information on the various tubulin forms shows that throughout evolution the protein domains involved in polymerization have been highly conserved, and interspecies amino acid sequence homology is generally high. For example, the four β-tubulin isotypes in human are identical with their counterparts in mouse. There is 82-90% homology between mammalian neuronal or constitutively expressed tubulins and algal, protozoan and slime mould tubulins. Considering plant sequences in more detail, there are long stretches in which the amino acid sequence of all the α- and β-tubulins are identical (Silflow et al, 1987, Developmental Genetics, 8, 435-460). For example, the 35 amino acids in positions 401-435 are identical in all plant α-tubulins, as are the 41 amino acids in the region between positions 240 and 281 in the plant β-tubulins. Conservation of amino acid residues is approximately 40% between the α- and β-tubulin families, and 85-90% within each of the α- and β-tubulin families. It should be noted that in general, most α-tubulins are 1 to 5 residues larger that the β-tubulins. The economic interest of tubulins lies in the effect of certain agents which interfere with tubulin structure and/or function. Such agents (including non-chemical stresses) are hereinafter referred to as "anti-tubulin agents" as they share a similar type of mode of action. Extreme conditions are known to destabilize the tubulins and/or microtubules. Such conditions include cold, pressure and certain chemicals. For example, Correia (1991, Phar ac Ther, 52:127-147) describes α- and β-tubulin interactions, microtubule assembly and drugs affecting their stability. Some anti-tubulin agents are often called "spindle poisons" or "antimitotic agents" because they cause disassembly of microtubules which constitute the mitotic spindle. For at least one hundred years, it has been known that certain chemical agents arrest mammalian cells in mitosis, and of these agents the best known is colchicine which was shown in the mid-1960s to inhibit mitosis by binding to tubulin. Many of these anti-tubulin agents have since found widespread use as cancer therapeutic agents (eg vincristine, vinblastine, podophyllotoxin) , estrogenic drugs, anti-fungal agents (eg griseofulvin) , antihelminthics (eg the benzimidazoles) and herbicides (eg the dinitroanilines) . Indeed some of the specific agents have uses against more than one class of organism. For example, the dinitroaniline herbicide trifluralin has recently been shown to inhibit the proliferation and differentiation of the parasitic protozoan Leishmania (Chan and Fong, 1990, Science, 249:924-926). The dinitroaniline herbicides may be considered as an example of one group of anti-tubulin agents. Dinitroaniline herbicides are widely used to control weeds in arable crops, primarily for grass control in dicotyledonous crops such as cotton and soya. Such herbicides include trifluralin, oryzalin, pendimethalin, ethalfluralin and others. The herbicidally active members of the dinitroaniline family exhibit a common mode of action on susceptible plants. For example, dinitroaniline herbicides disrupt the mitotic spindle in the meristems of susceptible plants, and thereby prevent shoot and root elongation (Vaughn KC and Lehnen LP, 1991, Weed Sci, 39:450-457). The molecular target for dinitroaniline herbicides is believed to be tubulin proteins which are the principle constituents of microtubules (Strachan and Hess, 1983, Pestic Bioche Physiology, 20, 141-150; Morejohn et al, 1987, Planta, 172, 252-264). The extensive interest in anti-tubulin agents in many branches of science has been accompanied by the identification of several mutants shown to resist the action of such agents (Oakley BR, 1985, Can J Biochem Cell Biol, 63:479-488). Several of these mutants have been shown to contain modified α- or β- tubulin genes, but to date the only resistant mutants to be fully characterised and sequenced are those in β-tubulin. For example, colchicine resistance in mammalian cell lines is closely associated with modified β-tubulin polypeptides (Cabral et al, 1980, Cell, 20, 29-36); resistance to benzimidazole fungicides has been attributed to a modified β-tubulin gene, for example in yeast (Thomas et al, 1985, Genetics, 112, 715-734) and Aspergillus (Jung et al, 1992, Cell Motility and the Cytoskeleton, 22:170-174); some benzimidazole resistant forms of nematode are known; and dinitroaniline-resistant Chlamydomonas mutants possess a modified β-tubulin gene (Lee and Huang, 1990, Plant Cell, 2, 1051-1057). Some of these mutants, although resistant to one anti- tubulin agent, also show increased susceptibility to other anti-tubulin agents (such as cold stress). Among certain weed species, some biotypes have evolved resistance to dinitroaniline herbicides. Three examples of species in which dinitroaniline resistant (R) biotypes have emerged are goosegrass, Eleusine indica (Mudge et al, 1984, Weed Sci, 32, 591-594); green foxtail, Setaria viridis (Morrison et al, 1989, Weed Technol, 3, 554-551); and Amaranthus palmeri (Gossett et al, 1992, Weed Technology, 6:587-591). These resistant (R) biotypes emerged following selective pressure exerted by repeated application of trifluralin. A range of resistant biotypes of each species exists but the nature and source of the resistance trait is unclear and the biotypes are genetically undefined. The R biotypes of these species exhibit cross-resistance to a wide range of dinitroaniline herbicides, including oryzalin, pendimethalin and ethalfluralin. All dinitroaniline herbicides have a similar mode of action and are therefore believed to share a common target site. Many of the R biotypes are also cross-resistant to other herbicide groups such as the phosphorothioamidates, which include amiprophos-methyl and butamifos, or chlorthal-dimethyl. The phenomenon of cross-resistance exhibited by resistant biotypes strongly indicates that the herbicide resistance trait is a consequence of a modified target site. In addition, the resistant biotypes appear to have no competitive disadvantage as they grow vigorously and can withstand various stresses (such as cold). It has not been previously shown which specific gene is modified in Eleusine indica or Setaria viridis to confer the dinitroaniline resistance trait. Research by K C Vaughn and M A Vaughn (American Chemical Society Symposium Series, 1989, 364-375) showed an apparent alteration in the electrophoretic properties of β-tubulin present in an R biotype of Eleusine indica, and suggested dinitroaniline resistance results from the presence of a modified β-tubulin polypeptide. The results of recent work by Waldin, Ellis and Hussey (1992, Planta, 188:258-264) provide no evidence that dinitroaniline herbicide resistance is associated with an electrophoretically modified β-tubulin polypeptide in the resistant biotypes of Eleusine indica or Setaria viridis which were studied. Recombinant DNA technology allows genes to be transferred between sexually incompatible organisms. Using these techniques, resistance genes can be introduced into organisms from unrelated species. For example, herbicide resistant crop plants may be produced. The purpose in providing crop plants which resist the action of a herbicide is to facilitate the destruction of weeds growing between the plants by the overall application of an effective concentration of a herbicide which would destroy the crop plant in its normal, that is herbicide sensitive, state. Such resistant plants are also useful for use in a locus of any short term carry-over of herbicide from a previous application. An object of the present invention is to provide genetic material for use in imparting resistance to anti-tubulin agents to a biological organism. In this context, a resistant organism is defined as one which displays enhanced tolerance to an anti-tubulin agent when compared to a standard organism. Resistance may vary from a slight increase in tolerance to the effects of the anti-tubulin agent to total resistance where the organism is unaffected by the presence of agent at several times the normal level. For example, pre-emergent or pre-plant use of dinitroaniline herbicides is precluded when maize (Zea mays) is grown because the seedlings and young plants of this crop are injured by these herbicides. The availability of maize hybrids which are resistant to dinitroaniline herbicides would allow the beneficial control of weeds in this crop by application of such herbicides. According to a first aspect of the present invention . , there is provided a modified tubulin comprising a tubulin polypeptide which confers resistance to an anti-tubulin agent when expressed within a biological organism, said polypeptide having at least one amino acid substitution such that hydrophobicity is increased at one or more positions in the polypeptide. Such modified tubulins may be used to confer resistance on organisms which otherwise would be susceptible to one or more anti-tubulin agents. In addition, the modified tubulins may be used to develop diagnostic kits to detect resistance in human or other cell lines (eg using specific antibodies derived from the tubulins). Such kits may be used for medical purposes (eg to detect drug resistance in ne atode parasites) or for agricultural purposes (eg enabling fast identification of herbicide resistant plants which are tubulin mutants). The modified tubulin protein may also be useful in separation columns or in screening methods relying on the binding of anti-tubulin agents. Modified tubulins according to the invention may be isolated from a biological organism which is naturally resistant to one or more anti-tubulin agents. Such organisms include yeast, fungi, algae, animals and plants (for example, weed biotypes of Eleusine indica, Setaria viridis, or Amaranthus palmeri which have evolved resistance to dinitroaniline herbicides). We have isolated and characterised a modified tubulin obtainable from the dinitroaniline resistant (R + ) biotype of Eleusine indica (goosegrass) , deposited as seed at The National Collections of Industrial and Marine Bacteria (23 St Machar Drive, Aberdeen, Scotland, AB2 1RY) under the terms of the Budapest Treaty on 11 May 1992 under the accession number NCIMB 40504. This modified tubulin belongs to the α-tubulin family and, being the most abundant tubulin, is designated as the αl-tubulin isotype. This specific αl-tubulin is a modified polypeptide which is present only in the R + biotype and confers dinitroaniline resistance. The distinctive electrophoretic properties of this αl-tubulin differentiate it from the αl isotype found in the susceptible (S) biotype (see Example 2). The modified αl-tubulin isolated from seedlings of the resistant (R + ) biotype of goosegrass is "shifted" towards an apparently higher molecular weight (slower migration in 2D gels) and more acidity (lower pi) compared to the equivalent αl-tubulin isolated from seedlings of the susceptible (S) biotype. According to a second aspect of the invention, there is provided a DNA construct comprising a DNA sequence encoding a modified tubulin polypeptide which confers resistance to an anti-tubulin agent when expressed within a biological organism, said polypeptide having at least one amino acid substitution such that hydrophobicity is increased at one or more positions in the polypeptide. Such DNA constructs may be used to confer resistance on organisms which otherwise would be susceptible to one or more anti-tubulin agents. For example, the modified tubulin sequence may be used as a selectable marker in transformation experiments involving any cell type (including bacterial, fungal, plant or animal cells). Standard techniques are employed (as described for β-tubulin mutations used as selectable markers in Aspergillus, Cercospora and Septoria: Cooley et al, J Gen Microbiol, 1991, 137:2085-2091; Upchurch et al, 1991, Appl Env Microbiol, 57(10) :2935-2939; Seip et al, 1990, App Env Microbiol, 56(12) :3686-3692) . In addition, the constructs may be used to develop diagnostic DNA probes (which may be sensitive enough to detect a single base change) for medical, agricultural or other purposes. The DNA sequence encoding the modified tubulin may be synthesised ab initio or may be isolated from a biological organism which is naturally resistant to one or more anti-tubulin agents. Such organisms include yeast, fungi, algae, animals and plants (for example, seeds or plants or cell cultures of a herbicide resistant biotype). In the Eleusine indica R + biotype, the DNA encoding the modified αl-tubulin (a modified polypeptide component of the target site for dinitroaniline herbicides) is the "herbicide resistance gene" responsible for the resistance trait. A cDNA clone (hereinafter called R + #2) encoding the modified αl-tubulin has been isolated and sequenced. The coding sequence differs from that of the equivalent cDNA clone (S#15) isolated from the susceptible (S) biotype to give a particular amino acid substitution (isoleucine for threonine) at position 239. This is the first α-tubulin mutant to have been sequenced. The single base change C to T in R + #2 is the most significant sequence difference between the αl-tubulin genes from the susceptible (S) and resistant (R + ) plants. It results in the amino acid substitution Thr 239 (threonine predicted by the codon ACA in the S biotype) to lie 239 (isoleucine predicted by the codon ATA in the R + biotype). A degree of nucleotide variation between the biotypes which is unrelated to the dinitroaniline resistance trait may be due to genetic polymorphism. However, the significance of the sequence modification is emphasised by the unusual codon used at the lie 239 position. The C to T mutation results in the codon ATA for isoleucine in R + #2. The ATA codon is not used to code for isoleucine in the wild type molecule (S#15 from the susceptible biotype) and hence is not a preferred codon. (The isoleucines at all other positions in the modified αl-tubulin from the R + biotype and in the unmodified αl-tubulin from the S biotype are predicted by other codons, as shown in Table 1) . TABLE 1 CODON USAGE IN THE E INDICA αl-TUBULIN CLONES Unmodified αl-tubulin cDNA clone (S#15) (Susceptible biotype) THR: act ace aCa acg 4 17 1 1 ILE: att ate ata 5 18 0 Modified αl-tubulin cDNA clone (R + #2) (Resistant biotype) THR: act ace aca acg 4 17 0 1 ILE: att ate aTa 5 18 1 In addition, there is remarkable conservation of amino acid sequence of tubulin polypeptides throughout the diversity of eukaryotic phylogenic groups. This suggests that selection pressure has acted against mutations in tubulin genes which cause amino acid substitutions that may disrupt tubulin function. In general, the sequences of tubulins from different organisms are relatively similar both within and between the different tubulin forms (α, β and Y): amino acid sequences of certain segments or single amino acids at certain positions of the polypeptide appear to be invariant (see Figure 5 in Silflow et al, 1987, Developmental Genetics, 8, 435-460). For example, Thr 239 is conserved in the five sequenced α-tubulin genes/cDNAs of maize (tual, 2, 3, 5 and 6) and in the six sequenced α-tubulins of Arabidopsis. Indeed, threonine is found at an equivalent position in each of the 99 tubulin protein sequences (α-, β- and γ-tubulins) which have been examined, as described below. The Swiss-Prot Version 23 database contains sequences covering all forms of life (including mammals, plants, insects, fungi and bacteria). A search for tubulin proteins using the IG-Suite program FINDSEQ retrieved 113 protein sequences. Of these, 41 α-tubulin, 53 β-tubulin and 5 γ-tubulin full length sequences were aligned using the IG-Suite program GENALIGN resulting in the α-, β- and γ-tubulin consensus sequences for the region shown in Table 2. The indicated amino acid positions (*) relating β- and γ-tubulins to α-tubulin Threonine 239 are based on the "average" position of the conserved threonine within each class of tubulin. The Thr 239-equivalent position (*) falls within a sequence designated as "compact domain 2" (residues 165-260) of tubulin polypeptides by Fosket and MoreJohn (1992, Ann Rev PI Phys, 43:201-240). Table 2 shows the overall consensus within the region. It also shows those amino acids with 100% identity in all the individual tubulin sequences used for the comparison (including the Thr 239-equivalent residues marked by *) . • TABLE 2 ALIGNMENT OF α-, β- AND γ-TUBULIN CONSENSUS SEQUENCES ,239 α-tubulin - RPTYTNLNRLIAQWSSITASLRFDGALNVDLTEFQT *237 β-tubulin - TPTYGDLNHLVSATMSGVTTCLRFPGQLNADLRKLAV *240 γ-tubulin - NPSFSQINQLVSTIMSASTTTLRYPGYMNNDLIGIIA * consensus - -PTY—LN-LVS—MS—TT-LRFPG-LN-DL identity - -P L T R N In summary, the wild type αl-tubulin from the E indica susceptible biotype contains the Thr 239 residue which is typical of known tubulin proteins. However, the modified αl-tubulin from the resistant (R + ) biotype contains a unique amino acid substitution: the lie 239 residue which is encoded by a unique codon within the protein. This suggests that the substitution of isoleucine in a tubulin at a position equivalent to the asterisk in Table 2 (usually 239 in a α-tubulin, 237 in a β-tubulin and 240 in a γ-tubulin) of any biological organism may produce a modified tubulin which confers resistance to an anti-tubulin agent when expressed within the organism. In fact, the substitution of any one or more amino acids which results in an equivalent alteration in physical properties of the polypeptide may produce a modified tubulin which confers such resistance. The modified αl-tubulin has altered physical properties which are detectable as an altered hydropathy profile. The hydropathy of proteins is known to differ between organisms; for example, Silflow et al (1987, Dev Genetics, 8:435-460) have compared the hydropathy profile of animal and plant tubulins. Using the IG-Suite program SOAP, the hydropathic index of a selected protein can be evaluated along its sequence according to the method of Kyte and Doolittle (1982, J Mol Biol, 157:105-132). Figure 1 shows the hydropathy profile of sequence ELS15 (the protein sequence of the unmodified αl-tubulin from the susceptible E_ indica biotype); Figure 2 shows the hydropathy profile of sequence ELR2 (the protein sequence of the modified αl-tubulin from the resistant R + biotype). Both the ELS15 and the ELR2 sequences' have a total of 451 amino acids. The hydropathic index of both sequences was computed using an interval of nine amino acids from amino acid 180 to amino acid 270. A comparison of Figures 1 and 2 reveals a significant change in the region covering amino acid positions 237-240 which increases in relative hydrophobicity from approximately 15 in the unmodified protein to over 20 in the modified protein. This shift in hydrophobicity is distinctive of the ELR2-type of modified tubulin: it is apparently not found in other modified tubulins known to be associated with resistance to anti-tubulin agents. For example, the hydropathic index of a yeast β-tubulin was evaluated using the method of Kyte and Doolittle as described above. The β-tubulin (total 457 amino acids) also occurs in a mutated form which confers resistance to colchicine in yeast and has an arginine to histidine substitution at position 241 (Thomas et al, 1985, Genetics, 112:715-734). Figure 3 shows the hydropathy profile of the wild-type β-tubulin, and Figure 4 shows the hydropathy profile of the mutant β-tubulin. The hydropathic index of both sequences was computed using an interval of nine amino acids from amino acid 180 to amino acid 270. Comparison of Figures 3 and 4 does not reveal any significant difference in the hydropathy profile: there is no increase in hydrophobicity at the mutation site. The distinctive shift in the hydropathy profile of the modified tubulin according to the invention affects the three-dimensional structure of the polypeptide. This change in conformation presumably alters the affinity and/or binding site(s) for anti-tubulin agents directly (by altering the αl-tubulin/agent interaction) or indirectly (by altering the α-/β-tubulin interaction or by altering the binding of microtubule associated protein), and results in the modified tubulin conferring resistance to such agents. In the modified αl-tubulin from the resistant R + E indica biotype, the substitution of Thr239 with lie 239 replaces a polar (hydrophilic) residue (-C 2 H 4 OH) with a non-polar (hydrophobic) residue (C.Hg). None of the known β-tubulin mutants show this distinctive polar to non-polar amino acid change. In a DNA construct according the invention. the DNA sequence encoding the modified tubulin preferably encodes an altered amino acid at a position equivalent to 239 of an α-tubulin, 237 of a β-tubulin, 240 of a γ-tubulin. Preferably the altered amino acid is isoleucine, but substitution with other amino acids which give the required effect on polypeptide structure/function is also possible (for example non-polar amino acids such as leucine or valine). For example, the DNA sequence encoding the modified tubulin may be a cDNA or genomic DNA or synthetic DNA sequence corresponding to the sequence of the cDNA clone R #2 which is given in Table 3 below. The related genomic DNA may be isolated by screening a genomic library with suitable probes derived from the cDNA sequence. Clone R + #2 has been deposited at The National Collections of Industrial and Marine Bacteria (23 St Machar Drive, Aberdeen, Scotland, AB2 IRY) under the terms of the Budapest Treaty on 24 May 1993 under the accession number NCIMB 40558. TABLE 3 DNA SEQUENCE OF CLONE R + #2 10 20 30 40 50 I I I I I 1 CTCGCCTCTCTCCGCGACTCCAAGCTTTCTCCCTCCTCCCATTTCCCGTC 51 GCCGCCGCCTCACCCGCCCGACACCATGAGGGAGTGCATCTCGATCCACA 101 TCGGCCAGGCCGGTATCCAGGTCGGAAACGCTTGCTGGGAGCTCTACTGC 151 CTCGAGCATGGCATCCAGGCTGACGGTCAGATGCCCGGTGACAAGACCAT 201 TGGAGGAGGTGATGATGCTTTCAACACCTTCTTCAGTGAGACTGGCGCCG 251 GCAAGCATGTGCCCCGTGCCGTGTTTGTTGACCTTGAGCCCACTGTGATC 301 GATGAGGTCAGGACTGGCACCTACCGCCAGCTGTTTCACCCTGAGCAGCT 351 CATCAGTGGCAAGGAGGATGCTGCCAACAACTTTGCCCGTGGTCACTACA 401 CCATTGGCAAGGAGATTGTTGACCTGTGCCTTGACCGCATCAGGAAGCTT 451 GCCGACAACTGTACTGGTCTCCAGGGCTTCCTTGTCTTCAACGCTGTCGG 501 TGGAGGAACGGGCTCTGGTCTTGGTTCCCTCCTCCTTGAGCGCCTGTCTG 551 TTGACTACGGCAAGAAGTCCAAGCTCGGGTTCACTGTCTACCCGTCTCCC 601 CAGGTCTCCACCTCGGTGGTTGAGCCATACAACAGTGTGCTGTCCACCCA 651 CTCCCTCCTTGAGCACACCGATGTGGCTGTGCTGCTTGACAACGAGGCCA 701 TCTACGACATCTGCCGCCGCTCCCTGGACATTGAGCGCCCAACCTACACC 751 AACCTGAACAGGCTTGTTTCTCAGGTCATTTCATCACTGATAGCCTCTCT 801 GAGGTTCGATGGTGCTCTGAACGTGGATGTGAACGAGTTCCAGACCAACT 851 TGGTGCCCTACCCGAGGATCCACTTCATGCTTTCATCCTACGCTCCAGTG 901 ATCTCCGCGGAGAAGGCCTACCACGAGCAGCTGTCCGTGGCTGAGATCAC 951 CAACAGCGCGTTCGAGCCTTCCTCCATGATGGCCAAGTGCGACCCCCGCC 1001 ACGGCAAGTACATGGCCTGCTGCCTCATGTACCGTGGTGATGTGGTGTCC 1051 AAGGACGTGAACGCCGCCGTTGCCACCATCAAGACCAAGCGCACCATCCA 1101 GTTCGTGGACTGGTGCCCCACTGGCTTCAAGTGCGGTATCAACTACCAGC 1151 CACCCAGCGTCGTCCCCGGCGGCGACCTGGCCAAGGTGCAGAGGGCCGTG 1201 TGCATGATCTCCAACTCCACCAGTGTCGTCGAGGTGTTCTCCCGCATCGA 1251 CCACAAGTTCGACCTCATGTACGCCAAGCGCGCCTTCGTCCACTGGTACG 1301 TGGGTGAGGGTATGGAGGAGGGTGAGTTCTCCGAGGCGCGTGAGGACCTT 1351 GCTGCCCTTGAGAAGGACTACGAGGAGGTCGGCGCTGAGTTCGACGAGGG 1401 TGAGGAAGGTGATGAGGGTGACGAGTACTAGATGAATCTACGCTTCCTGC 1451 TGTTGTGTCAGGCCTGTGTGCCGCTGCTATCCTGTGATCTGCCCGAGGGC 1501 GCTATCGTGTCGTGTCAGTTTGAACTATTTGTCATTGTGTGGTTACAACC 1551 CCTGAAGTTGTAGACATGTTTAATTCAAAAAAAAAAAAAAAAA In a DNA construct according to the invention, the DNA sequence is generally preceded by a transcriptional initiation region operative in a biological cell so that the construct can generate RNA in the cell. The transcriptional initiation region may be any suitable promoter which is operative in the particular cell type. Either a constitutive promoter or an inducible or develop entally regulated promoter may be used as circumstances require. Suitable plant-operative promoters include the constitutive 35S cauliflower mosaic virus promoter, the CAB promoter driving expression in green tissues, or the tomato polygalacturonase gene promoter sequence (Bird e_t al, 1988, Plant Molecular Biology, 11:651-662). According to a third aspect of the invention, there is provided a method to produce a biological organism resistant to an anti-tubulin agent by transformation of a cell of the organism with a DNA construct comprising a DNA sequence encoding a modified tubulin which confers resistance to an anti-tubulin agent when expressed within a biological organism. The invention further provides a transgenic cell containing said DNA construct, and organisms derived therefrom. For example, the modified tubulin may be used purely as a selectable marker in transformation experiments and an appropriate anti-tubulin agent is then used to select for the transgenic cells. Transgenic cells expressing the modified tubulin may also be used in mode of action and cross-resistance studies to aid the development of new drugs with an anti-tubulin mode of action. As another example, a herbicide resistant plant may be derived from a transgenic plant cell produced by transformation of the cell with a DNA construct including the R #2 DNA sequence encoding a modified α-tubulin. Expression of the modified tubulin within the plant confers increased herbicide resistance (including increased herbicide tolerance) to a herbicide with a mode of action involving disruption of microtubule structure or function. The herbicide resistance gene (encoding the modified α-tubulin) may be transferred into any monocotyledonous or dicotyledonous plant species using standard transformation techniques (cells or callus may be subsequently regenerated into whole plants). Preferably the recipient is maize or sugarbeet, and preferably the DNA construct is stably integrated within the plant genome. These plants may be inter-crossed or crossed with other plant lines to produce seeds and progeny. The transgenic plants and their progeny may be resistant to members of the dinitroaniline family of herbicides such as trifluralin, oryzalin, pendimethalin, ethalfluralin and others. The plants may also be resistant to other herbicides with a similar mode of action, such as the phosphorothioa idates or chlorthal-dimethyl. One option is to transfer the complete gene, including flanking sequences necessary for regulated gene expression, derived from "Ξ indica R + biotype into the recipient plant cell. Alternatively, the coding region of the gene or of the cDNA derived from E indica R + biotype, flanked by DNA regulatory sequences from another source, may be transferred into the recipient cell. The transgenic plants and their progeny may be tested for herbicide resistance by germination and growth in the presence of a discriminatory dose of dinitroaniline, phosphorothioamidate or chlorthal-dimethyl herbicide. It is possible to evaluate the R + #2 "resistance gene" in the transgenic cell culture lines as dinitroaniline resistance can be expressed in undifferentiated culture tissue. (Expression of dinitroaniline resistance in undifferentiated culture tissue was demonstrated using amorphous callus of Setaria viridis: tissue from the R biotype proved 10-fold more tolerant to trifluralin than tissue from the S biotype) . The invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a plot of hydropathy profile for sequence ELS15. Figure 2 is a plot of hydropathy profile for sequence ELR2.. Figure 3 is a plot of hydropathy profile for a yeast wild-type β-tubulin. Figure 4 is a plot of hydropathy profile for a yeast mutant β-tubulin. Figure 5 shows the dose-response curves for trifluralin treatment of Eleusine indica seedlings. Figure 6 shows the Western blot for the α-tubulin polypeptides extracted from Eleusine indica seedlings. Figure 7 is a diagram showing the construction of vectors for maize transformation. Figure 8 is a diagram showing the structure of the vector pCaMVIlN. Figure 9 is a diagram showing the structure of the vector pPGl. Figure 10 is a diagram showing the structure of the vector p35SPACB. EXAMPLE 1 CHARACTERISATION OF A RESISTANT (R + ) BIOTYPE OF ELEUSINE INDICA: RESPONSE TO DINITROANILINE HERBICIDE. To evaluate the response of susceptible (S) and resistant (R) biotypes of Eleusine indica (goosegrass) to dinitroaniline herbicides, seedling growth was monitored following seed germination in darkness on gelled medium supplemented with herbicide. Seeds of a susceptible (S) and a resistant (R) biotype of E indica were surface sterilised in 20% (v/v) Domestos for 15 minutes, washed in sterile water and plated onto germination medium (MS salts, 20 g/1 sucrose, 4 g/1 Gelrite, pH 5.8). Trifluralin herbicide (2,6-dinitro-N,N-dipropyl-4- (trifluoromethyl)benzenamine) was dissolved in dimethylsulphoxide and added to cooled autoclaved medium. Plated seeds were incubated in darkness at 25°C and length of shoot (encompassing mesocotyl, coleoptile and leaf) was measured after 5 or 6 days. Trifluralin caused severe stunting of the shoots and roots of S biotype seedlings at concentrations which had no effect on seedlings of the R biotype. The dose-response curves are shown in Figure 5. Both the S and R biotypes exhibited a dose-dependent inhibition of shoot elongation on trifluralin. However, an approximately 40-fold higher concentration of trifluralin was required for equivalent injury of the R biotype relative to the S biotype, as judged by ID 5Q values. ( 1 ] - C . Q is the concentration required for 50% growth inhibition) . Seeds of this "40-fold" resistant E indica biotype, designated ICITRI40 (R + ), were deposited at The National Collections of industrial and Marine Bacteria on 11 May 1992 under the accession number NCIMB 40504. Cross-resistance has been demonstrated to dinitroanilines (pendimethalin, oryzalin), phosphorothioamidates (butamifos, amiprophos-methyl) and a terepthalate (chlorthal-dimethyl). No difference in sensitivity to propyzamide was found between the S and R + biotypes. Taxol is a mitotic inhibitor which has a unique mode of action: it promotes tubulin polymerisation such that the microtubules are hyper-stabilized. The response of goosegrass to taxol was examined. Taxol proved highly potent on shoots and roots of germinating seedlings (more inhibitory than trifluralin on a molar basis). There was no evidence for differential sensitivity between the S and R biotypes. Thus dinitroaniline resistance in goosegrass shows different characteristics from the colchicine/dinitroaniline resistance in Chlamydomonas documented by Lee and Huang (1990, Plant Cell, 2, 1051-1057). The two Chlamydomonas mutants, characterised by a point mutation in βl-tubulin (encoding a single amino acid substitution of the native lysine 350 (basic residue) to either glutamic acid (acidic residue) or methionine (non-polar residue)), exhibited cross-resistance to pronamide and super-sensitivity to taxol. Thus the molecular basis for dinitroaniline resistance in goosegrass appears to be different from that in Chlamydomonas. EXAMPLE 2 IDENTIFICATION OF A MODIFIED α-TUBULIN ISOTYPE IN THE RESISTANT (R + ) BIOTYPE OF ELEUSINE INDICA. The electrophoretic properties of α- and β-tubulin polypeptides extracted from seedlings of dinitroaniline susceptible and resistant biotypes of Eleusine indica were examined to determine which tubulin polypeptide has a modified amino acid composition. In both the S and R biotypes, four α-tubulin isotypes were identified. The ICITRI40 R + resistant biotype (NCIMB 40504) differed from the S biotype by one α-tubulin isotype which exhibited lower electrophoretic mobility and lower isoelectric point (pi) than its counterpart in the susceptible biotype. This "shifted" isotype is the most abundant α-tubulin in dark-grown seedlings, as judged by the size of the immuno-stain, and is therefore designated as αl-tubulin. The experiments are described below. The total protein from dark-grown seedlings was extracted as follows. Seedlings grown for 14 days were washed in double distilled water and crushed in liquid nitrogen. The resulting powder was solubilized by boiling in 6 ml of Laemmli sample buffer which contained 62.5 mM Tris-HCl, 10% (w/v) glycerol, 2.3% (w/v) SDS (Fisons, UK) and 5% (w/v) β-mercaptoethanol. Any insoluble material was removed by centrifugation and the proteins in the supernatant precipitated with 9 volumes of acetone at -20°C for 3 hours. The precipitate was dried and resuspended in 100 μl of 20 mM Tris-HCl (pH 7.6), 5 mM MgCl 2 , 1% (w/v) Nonidet P40 and 5 mg/ml leupeptin. To this thick slurry was added 4 μl of 1 mg/ml DNase and the mixture incubated on ice for 15 minutes. To a 50 μl aliquot of the mixture were added 60 mg of urea and 100 μl of 9.5 M urea, 2% (w/v) Nonidet P40, 5% (w/v) β-mercaptoethanol and 2% (w/v) ampholines (3:2 ratio of pH ranges 3.5-10 and 5-7). These samples were stored at -80°C prior to electrophoresis. The denatured polypeptides were separated on 2-D gels, using isoelectric focusing on the first dimension and SDS-polyacrylamide gel electrophoresis on the second dimension. Two-dimensional SDS-polyacrylamide gel electrophoresis (PAGE) was executed as described by O'Farrell (1975, J Biol Chem, 250:4007-4021). The volume of protein sample loaded onto the gels ranged from 1-15 μl . The polypeptides were blotted onto a nitrocellulose membrane, and α-tubulin or β-tubulin polypeptides were detected using specific monoclonal antibodies. Proteins were transferred from the polyacrylamide gels to nitrocellulose by the method of Towbin et al (1979, Proc Natl Acad Sci USA, 76, 4350-4354). Well-characterised IgG-type monoclonal antibodies specific to α-tubulin or β-tubulin were used to probe western blots. Bound anti-tubulin antibodies were visualised by immunoperoxidase staining (Birkett et al, 1985, FEBS Lett, 187:211-218). To detect α-tubulin TAT-1, an anti-α-tubulin antibody raised against the cytoskeleton of Trypanosoma brucei (Woods et al, 1989, J Cell Sci, 93, 491-500) and Amersham anti-α-tubulin antibody were used sequentially, the immunoblot being stained between and after reaction with the two antibody probes, β-tubulin was detected by probing the western blots sequentially with the antibody KMX-1 (Birkett et al, 1985) and Amersham anti-β-tubulin antibody, and with staining of the immunoblot between and after reaction with the two different antibody probes. A tubulin isotype is defined as an immunoreacting spot on a 2-D gel which is reproducibly electrophoretically separable. In protein extracts from both S biotype and R + biotype seedlings, four α-tubulin isotypes and four β-tubulin isotypes were detected. Figure 6 shows the Western blot for α-tubulins from the R + resistant (ER) and susceptible (ES) biotype. The relative positions of the tubulin isotypes were closely similar in the S and R + biotypes, except for that of the most abundant α-tubulin isotype (the αl-tubulin isotype). The αl-tubulin from the R biotype was shifted towards lower electrophoretic mobility (higher molecular weight) and lower isoelectric point (more acidic) relative to that of the S biotype. To subject the electrophoretic data to statistical analysis, the position of the αl-tubulin isotype was measured in three pair-wise comparisons (S and R processed in parallel on each of three dates). The position was scored as the angle between a line drawn from αl to α2 isotypes and a line drawn from α2 to α3/α4 isotypes. Each measurement was replicated three or four times. Analysis of variance confirmed that the position of the αl-tubulin isotype was statistically different between the S and R + biotypes (p<0.0001). TABLE 4 ANGLE DEFINING THE POSITION OF THE αl-TUBULIN ISOTYPE RELATIVE TO THE α2, α3 AND α4 ISOTYPES biotype S_ R^ S l £ l x 110.8 131.0 127.5 133.1 73.8 120.7 n 5 5 4 4 5 5 SD 1.92 6.19 2.04 1.89 2.28 2.02 SE 0.86 2.77 1.02 0.94 1.02 0.90 (x= angle in degrees) EXAMPLE 3 IDENTIFICATION OF THE GENE CONFERRING DINITROANILINE RESISTANCE. The difference in electrophoretic property of αl-tubulin between S and R + biotypes may directly or indirectly reflect a difference in the amino aci-d composition of the αl-tubulin polypeptide. It was concluded that the gene encoding αl-tubulin polypeptide (hereinafter referred to as the αl-tubulin gene) in the R biotype has a mutated coding region, relative to that of the S biotype. It was also concluded that dinitroaniline resistance in the R + biotype iε attributable to expression of a modified αl-tubulin gene. The genetic analysis of dinitroaniline resistance in goosegrass was investigated by the study of Fl plants and F2 seed using pendimethalin as the selective agent. Putative Fl hybrids (susceptible S biotype x "40 fold" R + biotype) showed partial resistance to pendimethalin. Preliminary evaluation of F2 progeny indicated inheritance of dinitroaniline resistance from some of the putative Fl hybrids. A range of seedling phenotypes was obtained form fully sensitive to fully resistant. Initial observations suggest that resistance is not fully dominant, but the segregation data are difficult to interpret. Further F2 families may be evaluated to clarify the inheritance pattern of the dinitroaniline resistance trait. A range of F2 seedlings, classified as susceptible or resistant phenotypes, may be grown to provide F3 families for analysis of genetic linkage between the "shifted" αl-tubulin isotype and the resistance trait. (Analysis of the segregation pattern in Setaria viridis suggests that trifluralin resistance is a recessive (or possibly semi-dominant) trait at one locus). EXAMPLE 4 CLONING AND SEQUENCING OF THE DNA ENCODING αl-TUBULIN. The cDNA sequence encoding the modified αl-tubulin isotype in the R biotype was cloned by routine reco binant DNA techniques. (a) Construction of cDNA libraries cDNA libraries were constructed in λUniZAP from poly[A] RNA isolated from dark-grown seedlings of susceptible (S) and and "40 fold" resistant (R + ) E indica biotypes. Seedlings of the S biotype of goosegrass were germinated on gelled medium for 14 days in darkness. 458 μg of total RNA was obtained from 2.3 g of harvested shoots, and 366 μg of this RNA yielded 3.6 μg poly[A] + RNA. cDNA synthesised using the Stratagene λZAP kit was size-fractionated on a Sephacryl column, and the two fractions containing the largest cDNAs were used to construct libraries in λUniZAP. cDNA inserts cloned into λUniZAP were directionally orientated by the inclusion of an Xhol site in the poly-T primer used in 1st strand synthesis, and by the addition of EcoRl adaptors to the cDNA. Of a total of 5 x 10 phage plaques, 14 were picked at random to estimate the average insert size of this library. 10 of the 14 plaques gave PCR products, of which the average insert size was 1.2 kb (insert size ranged from 0.6 kb to 2.0 kb). A library of 2 x 10 phage plaques were produced from the second fraction obtained from the Sephacryl column. A cDNA library was similarly constructed from dark-grown seedlings of the R biotype of goosegrass. 3.04 μg of poly[A] + RNA was isolated from 532 μg of total RNA and was cloned aε cDNA into λUniZAP. cDNA eluted in the first fraction from a Sephacryl spin column was used to produce a ς library consisting of approximately 4 x 10 λ phage. Of 14 plaques analysed by PCR, insert sizes ranged from 0.5 kb to 3.15 kb, with an average 4 insert size of 1.43 kb. 6 x 10 pfu were plated and duplicate filter-lifts were prepared. (b) Library screening Approximately 0.5% of plaques in each library hybridised to a maize α-tubulin probe, consistent with the anticipated abundance of α-tubulin RNA in goosegrass seedlingε. 6 x 10 pfu (from the 5 x 10 S library) were plated and probed using a radiolabelled maize α-tubulin cDNA probe, pMRl9 (corresponding to the Z tual gene). Of approximately 300 plaques which hybridised, 25 were picked for purification to obtain single hybridising plaques. 14 purified plaques retained hybridisation to pMRl9. PCR analysis of the phage clones showed insert sizes ranging form 1.5 to 2.0 kb. Direct DNA sequencing of the PCR products was attempted but gave limited and unclear resultε; 4 εpecieε of cDNA appeared to be repreεented among the phage cloneε. One εet of filterε from the R library waε alεo probed with the generic α-tubulin probe (maize pMRl9) and thiε indicated that the frequency of α-tubulin cloneε observed in the R + biotype library was εimilar to that in the S biotype library (ie 0.5%) . (c) Isolation of clones corresponding to the most abundant α-tubulin mRNA species Clones were grouped into α-tubulin family members, based on divergence in the 3' non-coding region. Nucleotide sequence information from the 3' and/or 5' untranslated regions of selected cDNA clones from the S and R + librarieε indicated one highly prevalent α-tubulin species and at least two low abundance α-tubulin εpecieε. Inserts from 11 of the 14 S clones were in vivo excised to allow plaεmid εequencing of the cDNA. During thiε proceεε one of the cloneε appeared to lose the cDNA insert. DNA sequencing demonstrated that 11 of the original 13 clones have identical 3' regions, indicating theεe to be the moεt prevalent α-tubulin cDNA εpecieε: thiε group includeε the full-length cDNA clone S#15. The remaining two plaque-purified clones appear to be diεtinct from the prevalent cDNA εpecieε and from each other. One of theεe low abundance cloneε, S8, haε a nucleotide sequence which resembleε that of the maize εubfamily III α-tubulin gene (Zmtua3) . In order to isolate a full-length homologue of clone S#15 (ie the prevalent cDNA specieε in the S biotype library) from the R + biotype library, the εecond set of filters was probed with a 94-bp PCR fragment derived from the 5' non-coding region of clone S#15. Clones hybridising to this probe accounted for approximately 80% of all plaques which hybridised to the pMRl9 probe. Thiε indicated that the prevalence of the cDNA εpecieε observed in the S biotype library was reproduced in the R + biotype library. Of 3 clones picked for plaque purification, one failed at ^n vivo excision and one proved to be partial length. Clone R + #2 contained a full-length insert and sequence resultε from both the 5' and 3' untranslated regions indicated that this cDNA is broadly equivalent to clone S#15. (d) Characterisation of modified α-tubulin cDNA The coding regionε of the prevalent cDNA εpecieε (both strands of clones S#15 and R + #2) have been sequenced. Compariεon of S and R biotypeε revealed an amino acid substitution encoded by the prevalent cDNA species: threonine at position 239 is subεtituted by iεoleucine. The clone S#15 haε been fully sequenced and shows close similarity to maize subfamily I α-tubulin genes (Zmtual, Zmtua2 , Zmtua4) aε 95% of the N-terminal amino acids are identical. This subfamily is believed to code for the maize α4-tubulin isotype, which is the most abundant of the α-tubulin gene products in developing tissues. Like maize subfamily I α-tubulins, clone S#15 has a lysine residue at position 40 which is the site of post-translational acetylation (aεsociated with microtubule stability in algae, but not yet observed in plants). Table 5 below showε the DNA εequence of clone S#15 (1652 total baεes; composition: 323 A, 494 C, 448 G, 387 T, 0 OTHER). The coding region of clone R #2 has also been sequenced, and the nucleotide sequence and encoded amino acid sequence compared with that from S#15. Table 3 above εhows the DNA sequence of clone R + #2 (1593 total baseε; composition: 312 A, 484 C, 433 G, 364 T, 0 OTHER). Several baseε differ between S#15 and R + #2. The oεt εignificant εequence difference iε the εingle baεe change C to T in R + #2 reεulting in the amino acid εubεtitution Thr 239 (threonine predicted by the codon ACA in the S biotype) to lie 239 (iεoleucine predicted by the codon ATA in the R + biotype). Sequencing in both directionε of the cDNA from either clone confirmed thiε difference between the biotypeε. Thiε mutation haε been obεerved in two independent cloneε iεolated from the R cDNA library. Table 6 iε a compariεon of the predicted protein εequenceε from the E_ indica cDNA cloneε S#15 (sequence ELS15, top row) and R + #2 (sequence ELR2, bottom row). TABLE 5 DNA SEQUENCE OF CLONE S#15 10 20 30 40 50 I I I I I 1 GGAGACAGGCGTCTTCGTACTCGCCTCTCTCCGCGACTCCAAGCTTTCTC 51 CCTCCTCCCATTTCCCGTCGCCGCCGCCTCACCCGTACGACACCATGAGG 101 GAGTGCATCTCGATCCACATCGGCCAGGCCGGTATCCAGGTCGGAAACGC 151 TTGCTGGGAGCTCTACTGCCTCGAGCATGGCATCCAGGCTGACGGTCAGA 201 TGCCCGGTGACAAGACCATTGGAGGAGGTGATGATGCTTTCAACACCTTC 251 TTCAGTGAGACTGGCGCCGGCAAGCATGTGCCCCGTGCCGTGTTTGTTGA 301 CCTTGAGCCCACTGTGATCGATGAGGTCAGGACTGGCACCTACCGCCAGC 351 TGTTTCACCCTGAGCAGCTCATCAGTGGCAAGGAGGATGCTGCCAACAAC 401 TTTGCCCGTGGTCACTACACCATTGGCAAGGAGATTGTTGACCTGTGCCT 451 TGACCGCATCAGGAAGCTTGCCGACAACTGTACTGGTCTCCAGGGCTTCC 501 TTGTCTTCAACGCTGTCGGTGGAGGAACGGGCTCTGGTCTTGGTTCCCTC 551 CTCCTTGAGCGCCTGTCTGTTGACTACGGCAAGAAGTCCAAGCTCGGGTT 601 CACTGTCTACCCGTCTCCTCAGGTCTCCACCTCGGTGGTTGAGCCATACA 651 ACAGTGTGCTGTCCACCCACTCCCTCCTTGAGCACACCGATGTGGCTGTG 701 CTGCTTGACAACGAGGCCATCTACGACATCTGCCGCCGCTCCCTGGACAT 751 TGAGCGCCCAACCTACACCAACCTGAACAGGCTTGTTTCTCAGGTCATTT 801 CATCACTGACAGCCTCTCTGAGGTTCGATGGTGCTCTGAACGTGGATGTG 851 AACGAGTTCCAGACCAACTTGGTGCCCTACCCGAGGATCCACTTCATGCT 901 TTCATCCTACGCTCCAGTGATCTCCGCGGAGAAGGCCTACCACGAGCAGC 951 TGTCCGTGGCTGAGATCACCAACAGCGCGTTCGAGCCTTCCTCCATGATG 1001 GCCAAGTGCGACCCCCGCCACGGCAAGTACATGGCCTGCTGCCTCATGTA 1051 CCGTGGTGATGTGGTGTCCAAGGACGTGAACGCTGCCGTTGCCACCATCA 1101 AGACCAAGCGCACCATCCAGTTCGTGGACTGGTGCCCCACTGGCTTCAAG 1151 TGCGGTATCAACTACCAGCCACCCAGCGTCGTCCCCGGCGGCGACCTGGC 1201 CAAGGTGCAGAGGGCCGTGTGCATGATCTCCAACTCCACCAGTGTCGTCG 1251 AGGTGTTCTCCCGCATCGACCACAAGTTCGACCTCATGTACGCCAAGCGC 1301 GCCTTCGTCCACTGGTACGTGGGTGAGGGTATGGAGGAGGGTGAGTTCTC 1351 CGAGGCGCGTGAGGACCTTGCTGCCCTTGAGAAGGACTACGAGGAGGTCG 1 01 GCGCTGAGTTCGACGAGGGTGAGGAAGGTGATGAGGGTGACGAGTACTAG 1451 ATGAATCTACGCTTCCTGCTGTTGTGTCAGGCCTGTGTGCCGCTGCTATC 1501 CTGTGATCTGCCCGAGGGCGCTATCGTGTCGTGTCAGTTTGAACTATTTG 1551 TCATTGTGTGGTTACAACCCCTGAAGTTGTAGACATGTTTAATTCCCGCT 1601 TTGCTACTGGGTTATCAACATCGTTATGTTTGTCTAAAAAAAAAAAAAAA 1651 AA TABLE 6 ELS15 - MRECISIHIGQAGIQVGNACWELYCLEHGIQADGQMPGDKTIGGGDDAFNTFFSE -55 ******************************************************* ELR2 - MRECISIHIGQAGIQVGNACWELYCLEHGIQADGQMPGDKTIGGGDDAFNTFFSE -55 ELS15 - TGAGKHVPRAVFVDLEPTVIDEVRTGTYRQLFHPEQLISGKEDAANNFARGHYTI -110 ******************************************************* ELR2 - TGAGKHVPRAVFVDLEPTVIDEVRTGTYRQLFHPEQLISGKEDAANNFARGHYTI -110 ELS15 - GKEIVDLCLDRIRKLADNCTGLQGFLVFNAVGGGTGSGLGSLLLERLSVDYGKKS -165 ******************************************************* ELR2 - GKEIVDLCLDRIRKLADNCTGLQGFLVFNAVGGGTGSGLGSLLLERLSVDYGKKS -165 ELS15 - KLGFTVYPSPQVSTSWEPYNSVLSTHSLLEHTDVAILLDNEAIYDICRRSLDIE -220 ******************************************************* ELR2 - KLGFTVYPSPQVSTSWEPYNSVLSTHSLLEHTDVAILLDNEAIYDICRRSLDIE -220 ELS15 - RPTYTNLNRLVSQVISSLTASLRFDGALKVDVNEFQDNLVPYPRIHFMLSSYAPV -275 ****************** ************************************ ELR2 - RPTYTNLNRLVSQVISSLIASLRFDGALKVDVNEFQDNLVPYPRIHFMLSSYAPV -275 ELS15 - ISAEKAYHEQLSVAEITNSAFEPSSMMAKCDPRHGKYMACCLMYRGDWPKDVNA -330 ******************************************************* ELR2 - ISAEKAYHEQLSVAEITNSAFEPSSMMAKCDPRHGKYMACCLMYRGDVVPKDVNA -330 ELS15 - AVATIKTKRTIQFVDWCPTGFKCGINYQPPSWPGGDLAKVQRAVCMISNSTSW -385 ******************************************************* ELR2 - AVATIKTKRTIQFVDWCPTGFKCGINYQPPSVVPGGDLAKVQRAVCMISNSTSW -385 ELS15 - EVFSRIDHKFDLMYAKRAFVHWYVGEGMEEGEFSEAREDLAALEKDYEEVGAEFD -440 ******************************************************* ELR2 - EVFSRIDHKFDLMYAKRAFVHWYVGEGMEEGEFSEAREDLAALEKDYEEVGAEFD -440 ELS15 - EGEEGDEGDEY -451 *********** ELR2 - EGEEGDEGDEY -451 The DNA εequence difference in the coding region predictε a modified amino acid εequence of αl-tubulin between the two biotypeε. The substitution of Thr with lie replaces a polar reεidue (-C 2 H 4 OH) with a non-polar reεidue (C 4 H q ). Thiε non-conεervative εubstitution may directly change the electrophoretic properties of the tubulin polypeptide (as resolved by 2-D gel analysis) and/or may subtly alter the conformation of the α-tubulin polypeptide such that it becomes a target for post-tranεlational acetylation (aεεociated with microtubule stability, and detectable aε an electrophoretic εhift). Thus the R + #2 cDNA apparently correspondε to the electrophoretic variant αl-tubulin εhown to be preεent in the R biotype. Confirmation that the cDNA clone R + #2 encodeε the αl-tubulin polypeptide of the R + biotype may be obtained in two wayε. One approach iε to raiεe iεotype-εpecific antibodies to synthetic oligopeptides correεponding to variable coding regions of the α-tubulin cDNA clones. The identification of an antibody which specifically reacts with αl-tubulin iεotype pin-pointε the corresponding cDNA. A second approach iε to uεe the different cDNA cloneε aε gene-εpecific hybridiεation probeε on genomic DNA extracted from F2 progeny derived from εelfing a S x R + Fl hybrid. The genomic DNA iε cut with reεtriction enzymeε to diεplay the α-tubulin geneε aε restriction fragment length polymorphismε (RFLPε) within each biotype and between the S and R + biotypeε. Examination of the F2 population allowε genetic linkage to be aεεigned between the resistance trait, the modified αl-tubulin isotype, and a specific α-tubulin genomic reεtriction fragment. The R + biotype cDNA clone εpecifically hybridiεing with this resistance-linked restriction fragment corresponds to the αl-tubulin isotype. EXAMPLE 5 IDENTIFICATION OF THE GENE ENCODING THE αl-TUBULIN ISOTYPE A gene-specific probe may be constructed from the clone S#15 and from the clone R #2 (the αl-tubulin cDNA) and used to identify the homologous gene from a genomic DNA library of the corresponding biotype. PCR productε obtained from both S and R + genomic DNA have been cloned and εequenced. Compariεon of the genomic and cDNA sequences has confirmed that the baεe change iε not an artefact of the cDNA cloning procedure and is present within the gene from which the mRNA for clone R + #2 was transcribed. The genomic sequenceε contain an intron of approximately 110 bp in a position identical to that observed in two tande ly arranged maize α-tubulin genes. EXAMPLE 6 Construction of maize expression vectors containing αl-tubulin cDNA The αl-tubulin cDNA with the He 239 subεtitution (R + #2) haε been cloned into an expression vector for transformation into a maize cell line. The following two pairε of vectorε have been conεtructed. To demonεtrate linkage of the herbicide resistance trait to the αl-tubulin cDNA, the vectors are used to transform BMS maize cells (using any εuitable DNA delivery technique εuch as bombardment or electroporation) and dinitroaniline resistance is measured in the undifferentiated culture tissue. The vectorε allow two regimeε of selection according to their structure: (i) Vectors for transformation and initial selection on bialaphos followed by dinitroaniline: VECTOR pBTS: CaMV35S-Adhil/S#15 ORF/Noε - CaMV35S-Adhil/bar/Noε -(pUC) VECTOR pBTR + : CaMV35S-Adhil/R + #2 ORF/Noε - CaMV35S-Adhil/bar/Noε -(pUC) (ii) Vectorε for tranεformation and direct εelection uεing a dinitroaniline (pendimethalin or trifluralin): VECTOR pGTS: CaMV35S-Adhil/GUS/CaMV35S - CaMV35S-Adhil/S#15 ORF /Nos -(pUC) VECTOR pGTR + : CaMV35S-Adhil/GUS/CaMV35S - CaMV35S-Adhil/R + #2 ORF /Nos -(pUC) Figure 7 is a diagram showing the construction of these vectors for maize cell line transformation. The S#15 and R + #2 αl-tubulin expresεion cassettes were constructed using a 1.4 kb Hinf I fragment containing the complete coding region predicted by the cDNA sequence. The fragments were isolated, blunt ended and cloned between the CaMV35S-Adhil promoter and Nos 3' end in pCaMVIlN/Bam(blunt) . The resulting plasmids (pCSN and pCR + N) were εequenced and reεtriction mapped to confirm the orientation and integrity of the ORFε. The pCSN and pCR + N expression casεetteε were partially digeεted to yield a 2.7 kb Hind III fragment. The fragment waε iεolated and cloned into pPGl(minus GUS) resulting in vectors of type (i) and into p35S-PACB resulting in vectors of type (ϋ). Figures 8, 9 and 10 show the detailed structure of the vectors pCaMVUN, pPGl and p35SPACB respectively.

Claims: CLAIMS 1. A modified tubulin comprising a tubulin which confers resiεtance to an anti-tubulin agent when expreεsed within a biological organism, said polypeptide having at least one amino acid subεtitution such that hydrophobicity is increased at one or more poεitionε in the polypeptide. 2. A modified tubulin as claimed in claim 1 in which a polar amino acid is substituted by a non-polar amino acid. . 3. A modified tubulin as claimed in claim 1 which is an α-tubulin. 4. A modified tubulin as claimed in claim 3 which has an amino acid subεtitution at poεition 239 of the α-tubulin conεenεuε εequence εhown in Table 2. 5. A modified tubulin aε claimed in claim 1 which iε a β-tubulin. 6. A modified tubulin aε claimed in claim 5 which haε an amino acid εubεtitution at poεition 237 of the β-tubulin conεenεus sequence shown in Table 2. 7. A modified tubulin as claimed in claim 1 which iε a γ-tubulin. 8. A modified tubulin aε claimed in claim 7 which haε an amino acid εubεtitution at position 240 of the γ-tubulin consenεuε εequence εhown in Table 2. 9. A modified tubulin aε claimed in any of claimε 4, 6 or 8 in which the amino acid εubεtitution iε iεoleucine. 10. A modified tubulin aε claimed in claim 1 which iε capable of being iεolated from a plant. 11. A modified tubulin aε claimed in claim 10 which is capable of being isolated from an Eleusine indica biotype deposited at The National Collectionε of Industrial and Marine Bacteria on 11 May 1992 under the accesεion number NCIMB 40504. 12. A microtubule compriεing one or more modified tubulinε as claimed in claim 1. 13. A DNA construct comprising a DNA sequence which encodes a modified tubulin as claimed in any of claimε 1 to 9. 14. A DNA conεtruct aε claimed in claim 13 in which the DNA εequence correεpondε to the εequence εhown in Table 3. 15. A DNA construct as claimed in claim 14 which is capable of isolation from the clone R + #2 deposited at The National Collections of Industrial and Marine Bacteria on 24 May 1993 under the accesεion number NCIMB 40558. 16. A DNA construct as claimed in claim 13 in which the DNA sequence is capable of being iεolated from a plant. 17. A DNA conεtruct as claimed in claim 16 in which the DNA sequence is capable of being isolated from an Eleusine indica biotype deposited at The National Collections of industrial and Marine Bacteria on 11 May 1992 under the accession number NCIMB 40504. 18. A DNA construct as claimed in claim 13 in which the DNA sequence is preceded by a transcriptional initiation region operative in a biological cell so that the construct can generate mRNA in the cell. 19. A method to produce a biological organiεm resistant to an anti-tubulin agent comprising transformation of a cell of the organism with a DNA construct as claimed in claim 13. 20. A method as claimed in claim 19 in which a herbicide resiεtant plant is produced by transformation of a plant cell with the DNA construct. 21. A transgenic cell containing a DNA construct as claimed in claim 13. 22. A transgenic cell as claimed in claim 21 which iε a plant cell. 23. A plant derived from a cell as claimed in claim 22. 24. A plant as claimed in claim 23 which is resiεtant to a herbicide that diεrupts microtubule structure or function. 25. A plant as claimed in claim 24 which is resistant to a dinitroaniline herbicide. 26. A plant as claimed in claim 24 which is reεiεtant to a phoεphorothioamidate herbicide. 27. A plant aε claimed in claim 24 which iε reεiεtant to chlorthal-dimethyl. 28. A plant aε claimed in claim 23 which is Zea mays. 29. A plant as claimed in claim 23 which is sugarbeet. 30. Seeds of a plant as claimed in claim 23. 31. Progeny of a plant as claimed in claim 23. 32. A transgenic organism having improved resistance to an anti-tubulin agent due to a modified tubulin gene. 33. A transgenic organism as claimed in claim 32 which has a modified α-tubulin gene. 34. A transgenic organism as claimed in claim 32 which has improved resistance to a dinitroaniline. 35. A diagnostic probe for a modified tubulin aε claimed in claim 1. 36. A diagnoεtic probe aε claimed in claim 35 which iε an antibody to the modified tubulin. 37. A dignoεtic probe for a DNA conεtruct as claimed in claim 13.

Inventors: CRONIN KATHRYN ELIZABETH, ELLIS JOHN RAYMOND DI, HUSSEY PATRICK JOSEPH, RAY JOHN ANTHONY, WALDIN TERESA RUTH, CRONIN, KATHRYN, ELIZABETH, ELLIS, JOHN, RAYMOND +DI, HUSSEY, PATRICK, JOSEPH, RAY, JOHN, ANTHONY, WALDIN, TERESA, RUTH
Applicants: CRONIN KATHRYN ELIZABETH, ELLIS ROLAND RICHARD & LM, HUSSEY PATRICK JOSEPH, RAY JOHN ANTHONY, WALDIN TERESA RUTH, ZENECA LTD, ELLIS, ROLAND, RICHARD +LM, ZENECA LIMITED
Application year: 1993
Main classification: C12N15
IPC codes: C07K14