A ubiquitin carrier protein from wheat germ is structurally and functionally similar to the yeast DNA repair enzyme encoded by RAD6

Interaction of the tail with the catalytic region of a class II E2 conjugating enzyme

Ubiquitination plays an important role in many biological processes, including DNA repair, cell cycle regulation, and protein degradation. In the latter pathway the ubiquitin-conjugating enzymes or E2 enzymes are important proteins forming a key E2-ubiquitin thiolester prior to substrate labelling. While the structure of the 150-residue catalytic domain has been well characterized, a subset of E2 enzymes (class II) carry a variable length C-terminal "tail" where structural detail is not available. The presence of this C-terminal extension plays an important role in target recognition, ubiquitin chain assembly and oligomerization. In this work NMR spectroscopy was used to determine the secondary structure of the 215-residue yeast E2 protein Ubc1 and the interactions of its C-terminus with the catalytic domain. The C-terminal tail of Ubc1 was found to contain three alpha-helices between residues D169-S176, K183-L193 and N203-L213 providing the first evidence for a well-defined secondary structure in this region. Chemical shift mapping indicated that residues in the L2 loop of the catalytic domain were most affected indicating the C-terminus of Ubc1 likely interacts with this region. This site of interaction is distinct from that observed in the E2-ubiquitin thiolester and may act to protect the catalytic C88 residue and direct the interaction of ubiquitin in the thiolester intermediate.

Crystal structure of murine/human Ubc9 provides insight into the variability of the ubiquitin-conjugating system

Murine/human ubiquitin-conjugating enzyme Ubc9 is a functional homolog of Saccharomyces cerevisiae Ubc9 that is essential for the viability of yeast cells with a specific role in the G2-M transition of the cell cycle. The structure of recombinant mammalian Ubc9 has been determined from two crystal forms at 2.0 A resolution. Like Arabidopsis thaliana Ubc1 and S. cerevisiae Ubc4, murine/human Ubc9 was crystallized as a monomer, suggesting that previously reported hetero- and homo-interactions among Ubcs may be relatively weak or indirect. Compared with the known crystal structures of Ubc1 and Ubc4, which regulate different cellular processes, Ubc9 has a 5-residue insertion that forms a very exposed tight beta-hairpin and a 2-residue insertion that forms a bulge in a loop close to the active site. Mammalian Ubc9 also possesses a distinct electrostatic potential distribution that may provide possible clues to its remarkable ability to interact with other proteins. The 2-residue insertion and other sequence and structural heterogeneity observed at the catalytic site suggest that different Ubcs may utilize catalytic mechanisms of varying efficiency and substrate specificity.

Proteolysis in plants: mechanisms and functions

Proteolysis is essential for many aspects of plant physiology and development. It is responsible for cellular housekeeping and the stress response by removing abnormal/misfolded proteins, for supplying amino acids needed to make new proteins, for assisting in the maturation of zymogens and peptide hormones by limited cleavages, for controlling metabolism, homeosis, and development by reducing the abundance of key enzymes and regulatory proteins, and for the programmed cell death of specific plant organs or cells. It also has potential biotechnological ramifications in attempts to improve crop plants by modifying protein levels. Accumulating evidence indicates that protein degradation in plants is a complex process involving a multitude of proteolytic pathways with each cellular compartment likely to have one or more. Many of these have homologous pathways in bacteria and animals. Examples include the chloroplast ClpAP protease, vacuolar cathepsins, the KEX2-like proteases of the secretory system, and the ubiquitin/26S proteasome system in the nucleus and cytoplasm. The ubiquitin-dependent pathway requires that proteins targeted for degradation become conjugated with chains of multiple ubiquitins; these chains then serve as recognition signals for selective degradation by the 26S proteasome, a 1.5 MDa multisubunit protease complex. The ubiquitin pathway is particularly important for developmental regulation by selectively removing various cell-cycle effectors, transcription factors, and cell receptors such as phytochrome A. From insights into this and other proteolytic pathways, the use of phosphorylation/dephosphorylation and/or the addition of amino acid tags to selectively mark proteins for degradation have become recurring themes.

Cloning of a novel ubiquitin-conjugating enzyme (E2) gene from the ciliate Paramecium tetraurelia

We isolated a 1.7 kb gene (UbcP1) for a ubiquitin-conjugating enzyme from a P. tetraurelia cDNA library and sequenced it. Its deduced polypeptide sequence consists of 425 amino acid residues (48 kDa). The UbcP1 protein contains novel N- and C-terminal extensions in addition to a UBC domain, and within the UBC domain it shares low identity with sequences of other known E2s. A constructed phylogenetic tree suggests that the UbcP1 protein may represent a member of a distinct subfamily of E2s. Southern blot analysis showed that the N-terminal extension of the UbcP1 is conserved in P. multimicronucleatum.

Thi1, a thiamine biosynthetic gene in Arabidopsis thaliana, complements bacterial defects in DNA repair

An Arabidopsis thaliana cDNA was isolated by complementation of the Escherichia coli mutant strain BW535 (xth, nfo, nth), which is defective in DNA base excision repair pathways. This cDNA partially complements the methyl methane sulfonate (MMS) sensitive phenotype of BW535. It also partially corrects the UV-sensitive phenotype of E. coli AB1886 (uvrA) and restores its ability to reactivate UV-irradiated lambda phage. It has an insert of ca. 1.3 kb with an open reading frame of 1047 bp (predicting a protein with a molecular mass of 36 kDa). This cDNA presents a high homology to a stress related gene from two species of Fusarium (sti35) and to genes whose products participate in the thiamine biosynthesis pathway, THI4, from Saccharomyces cerevisiae and nmt2 from Schizosaccharomyces pombe. The Arabidopsis predicted polypeptide has homology to several protein motifs: amino-terminal chloroplast transit peptide, dinucleotide binding site, DNA binding and bacterial DNA polymerases. The auxotrophy for thiamine in the yeast thi4::URA3 disruption strain is complemented by the Arabidopsis gene. Thus, the cloned gene, named thi1, is likely to function in the biosynthesis of thiamine in plants. The data presented in this work indicate that thi1 may also be involved in DNA damage tolerance in plant cells.

Identification of a novel family of ubiquitin-conjugating enzymes with distinct amino-terminal extensions

The ubiquitin/proteasome system is the main eukaryotic nonlysosomal protein degradation system. Substrate selectivity of this pathway is thought to be mediated in part by members of a large family of ubiquitin-conjugating (E2) enzymes, which catalyze the covalent attachment of ubiquitin to proteolytic substrates. E2 enzymes have a conserved approximately 150-residue so-called UBC domain, which harbors the cysteine residue required for enzyme-ubiquitin thioester formation. Some E2 enzymes possess additional carboxyl-terminal extensions that are involved in substrate specificity and intracellular localization of the enzyme. Here we describe a novel family of E2 enzymes from higher eukaryotes (Drosophila, mouse, and man) that have amino-terminal extensions but lack carboxyl-terminal extensions. We have identified four different variants of these enzymes that have virtually identical UBC domains (94% identity) but differ in their amino-terminal extensions. In yeast, these enzymes can partially complement mutants deficient in the UBC4 E2 enzyme. This indicates that members of this novel E2 family may operate in UBC4-related proteolytic pathways.

Caenorhabditis elegans UBC-1, a ubiquitin-conjugating enzyme homologous to yeast RAD6/UBC2, contains a novel carboxy-terminal extension that is conserved in nematodes

The RAD6/UBC2 gene from Saccharomyces cerevisiae encodes a ubiquitin-conjugating enzyme involved in DNA repair, induced mutagenesis, and sporulation. Here we report the isolation and characterization of the Caenorhabditis elegans RAD6 homolog designated ubc-1. Ubc-1 encodes a 21.5-kD protein that shares considerable identity with RAD6 (66%) as well as with other RAD6 homologs, including Schizosaccharomyces pombe rhp6+ (70%), Drosophila melanogaster Dhr6 (83%), and the two human homologs HHR6A and HHR6B (84% and 83%, respectively). However, UBC-1 is distinct in being the only known RAD6 homolog, other than RAD6 itself, with a carboxy-terminal extension. Analysis of UBC-1 homologs from C. briggsae and Ascaris suum show that the presence of the carboxy-terminal extension is conserved in nematodes. When constitutively expressed from the yeast promoter ADH1, ubc-1 complements the DNA repair functions in a S. cerevisiae rad6 delta mutant. Surprisingly, ubc-1 fails to complement the sporulation function of RAD6, despite its possession of an acidic carboxy-terminal tail. C. elegans UBC-1 is capable of forming a thiol-ester bond with ubiquitin, but, unlike RAD6, is unable to transfer ubiquitin to histone H2B in vitro. Both cis and trans splicing are involved in the maturation of the ubc-1 transcript. The presence of the SL2 trans-splice leader in the ubc-1 transcript suggests that ubc-1 may be co-transcribed as part of a polycistronic message.

Molecular cloning, expression and characterization of a ubiquitin conjugation enzyme (E2(17)kB) highly expressed in rat testis

Ubiquitin-conjugating enzymes (E2s) play a key role in ubiquitin-mediated proteolysis by catalysing the conjugation of ubiquitin to protein substrates. We have previously reported the cDNA cloning of a 14 kDa conjugating enzyme [E2(14)k; Wing, Dumas and Banville (1992) J. Biol. Chem. 267, 6495-6501] that efficiently supported ubiquitination and protein degradation in reticulocyte extracts. Surprisingly, the structure of this E2 was markedly more similar to the Saccharomyces cerevisiae DNA repair gene RAD6, than to the S. cerevisiae UBC4/UBC5 genes which are required for the degradation of short-lived proteins and support much of the ubiquitination of yeast proteins. This suggested that mammalian homologues of UBC4/UBC5 remained to be identified. Using oligonucleotides derived from the S. cerevisiae UBC4 sequence as primers in a PCR reaction with rat muscle cDNA as a template, a 390 bp DNA fragment was amplified which predicted an amino acid sequence that was 83% identical to yeast UBC4. Screening a rat testes cDNA library identified a family of cDNAs which predicted two very similar proteins with basic pIs and molecular masses of approx. 16,700 Da. Isoform 2E was expressed in Escherichia coli and purified to homogeneity. It supported ubiquitination to reticulocyte and testis proteins more rapidly in vitro and produced larger conjugates than E2(14)k. Examination of RNA from different tissues indicated that this type of E2 was expressed in a broad spectrum of tissues but at particularly high levels in the testis. Fractionation of a testis extract by anion-exchange chromatography identified several putative ubiquitin protein ligase activities with which this E2 could interact in promoting conjugation of ubiquitin to proteins. One of these activities supported conjugation of ubiquitin to histone H2A, a substrate degraded in the ubiquitin system by a non-N-end rule mechanism. This paper reports the first cloning of a apparent mammalian homologue of S. cerevisiae UBC4/UBC5. Its high expression in testis and ability to efficiently support conjugation to testis proteins suggest that this family of E2s may play a role in the proteolysis that occurs during spermatogenesis.

Floral expression of a gene encoding an E2-related ubiquitin-conjugating protein from Arabidopsis thaliana

An Arabidopsis thaliana gene (UBC6) encoding a homologue to ubiquitin-conjugating enzymes has been isolated which is capable of encoding a protein of 183 amino acids of ca. 21 kDa. Northern analysis indicates that the gene is expressed in flowers, seeds and, to a somewhat lesser extent, in 10-day seedlings but not in mature leaves, callus and pre-flowering plants. This pattern of expression is confirmed using transgenic Arabidopsis plants containing a UBC6 promoter-GUS gene fusion construct. These plants display GUS activity in mature anthers prior to dehiscence, in developing embryos, sepals and the style after pollination.

Differential expression of several E2-type ubiquitin carrier protein genes at different developmental stages in Arabidopsis thaliana and Nicotiana sylvestris

We characterized three genes encoding different E2-type ubiquitin carrier proteins involved in the ubiquitin-mediated proteolytic pathway: UbcAt3 shows homologies to the yeast CDC34 gene and Ub-cAt4a and UbcAt4b are two different genes homologous to the Ubc1/4/5 subfamily in yeast. Their accumulation was analysed and compared with that of the different families encoding polyubiquitins, as well as the monoubiquitin fusion protein, which is considered as a marker for cell division, during various developmental stages including G0/S transition and senescence of higher plant cells. Our results imply that these Ubc genes are under the control of complex mechanisms, and are differentially regulated, but not necessarily co-regulated with ubiquitin genes. Even the closely related UbcAt4a and UbcAt4b genes of the same multigene subfamily are controlled by distinct regulatory mechanisms.

Homologues of wheat ubiquitin-conjugating enzymes--TaUBC1 and TaUBC4 are encoded by small multigene families in Arabidopsis thaliana

Covalent attachment of ubiquitin to other cellular proteins has been implicated in a multitude of diverse physiological processes in eukaryotes including selective protein degradation. This attachment is carried out by a multi-enzyme pathway consisting of three classes of enzymes: ubiquitin-activating enzymes (E1s), ubiquitin-conjugating enzymes (E2s), and ubiquitin-protein ligases (E3s). E2s accept activated ubiquitin from E1 and conjugate it to target proteins with or without the participation of specific E3s. Previously, we have isolated wheat cDNAs encoding 16 and 23 kDa E2s, TaUBC1 and TaUBC4, respectively. TaUBC1 shows structural homology to the yeast RAD6 E2 that is essential for DNA repair whereas TaUBC4 is related to the yeast ScUBC8 E2, both of which effectively conjugate ubiquitin to histones in vitro but as yet are without a known in vivo function. Here, we report the isolation of genomic and cDNA homologues of these genes from Arabidopsis thaliana. In Arabidopsis, both of these E2s are encoded by three member gene families. Members of the AtUBC1 gene family, comprising AtUBC1, 2 and 3, encode 150-152 amino acid proteins that are 83-99% identical to each other and TaUBC1 and contain four introns that are conserved with respect to position. Members of the AtUBC4 gene family, comprising AtUBC4, 5 and 6, encode 187-191 amino acid proteins that are 73-88% identical to each other and TaUBC4 and contain five introns that are conserved with respect to position.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional expression and molecular characterization of AtUBC2-1, a novel ubiquitin-conjugating enzyme (E2) from Arabidopsis thaliana

The first member of a novel subfamily of ubiquitin-conjugating E2-proteins was cloned from a cDNA library of Arabidopsis thaliana. Genomic blots indicate that this gene family (AtUBC2) consists of two members and is distinct from AtUBC1, the only other E2 enzyme known from this species to date (M.L. Sullivan and R.D. Vierstra, Proc. Natl. Acad. Sci. USA 86 (1989) 9861-9865). The cDNA sequence of AtUBC2-1 extends over 794 bp which would encode a protein of 161 amino acids and a calculated molecular mass of 18.25 kDa. The protein encoded by AtUBC2-1 is shown to accept 125I-ubiquitin from wheat E1 enzymes, when expressed from Escherichia coli hosts as fusion protein carrying N-terminal extensions. It is deubiquitinated in the presence of lysine and, by these criteria, is considered a functional E2 enzyme.

Elimination of the yeast RAD6 ubiquitin conjugase enhances base-pair transitions and G.C----T.A transversions as well as transposition of the Ty element: implications for the control of spontaneous mutation

The RAD6 gene of the yeast Saccharomyces cerevisiae encodes an enzyme that conjugates ubiquitin to other proteins. Defects in RAD6 confer a mutator phenotype due, in part, to an increased rate of transposition of the yeast Ty element. To further delineate the role of protein ubiquitination in the control of spontaneous mutagenesis in yeast, we have characterized 202 mutations that arose spontaneously in the SUP4-o gene carried on a centromere vector in a RAD6 deletion strain. The resulting mutational spectrum was compared to that for 354 spontaneous SUP4-o mutations isolated in the isogenic wild-type parent. This comparison revealed that the rad6 mutator enhanced the rate of single base-pair substitution, as well as Ty insertion, but did not affect the rates of the other mutational classes detected. Relative to the wild-type parent, Ty inserted at considerably more SUP4-o positions in the rad6 strain with a significantly smaller fraction detected at a transposition hotspot. These findings suggest that, in addition to the rate of transposition, protein ubiquitination might influence the target site specificity of Ty insertion. The increase in the substitution rate accounted for approximately 90% of the rad6 mutator effect but only the two transitions and the G. C----T.A transversion were enhanced. Analysis of the distribution of these events within SUP4-o suggested that the site specificity of the substitutions was influenced by DNA sequence context. Transformation of heteroduplex plasmid DNAs into the two strains demonstrated that the rad6 mutator did not reduce the efficiency of correcting mismatches that could give rise to the transitions or transversion nor did it bias restoration of the mismatches to the incorrect base-pairs. These results are discussed in relation to possible mechanisms that might link ubiquitination of proteins to spontaneous mutation rates.

Cloning and characterization of a 20-kDa ubiquitin carrier protein from wheat that catalyzes multiubiquitin chain formation in vitro

Recent evidence indicates that the commitment to degrade cellular proteins by the ubiquitin proteolytic pathway is dependent on the covalent attachment of multiubiquitin chains to the target protein [Chau, V., Tobias, J. W., Bachmair, A., Marriott, D., Ecker, D. J., Gonda, D. K. & Varshavsky, A. (1989) Science 243, 1576-1583]. We have isolated a 20-kDa ubiquitin carrier protein [E2(20 kDa)] from wheat by using ubiquitin covalent affinity chromatography and anion-exchange HPLC that catalyzes multiubiquitin chain formation in vitro. This reaction is blocked by the addition of a mutant ubiquitin in which arginine has been substituted for lysine at residue 48, demonstrating that the coupling of ubiquitin to ubiquitin is likely to be through an isopeptide linkage between the C-terminal glycine and Lys48 of ubiquitin. By immunoscreening a wheat cDNA expression library with anti-E2(20 kDa) antibodies, a cDNA encoding the complete protein was isolated. The clone (designated UBC7) was confirmed as encoding E2(20 kDa) by comparison of the derived amino acid sequence with peptide sequences of E2(20 kDa) tryptic fragments. The encoded protein contains a single cysteine at position 91, which is presumably the active site, and has regions of amino acid sequence similarity to other known E2s from plants and yeast. Expression of this cDNA in Escherichia coli produced an active E2 capable of catalyzing multiubiquitin chain formation in vitro. By virtue of its activity, E2(20 kDa) may have a pivotal role in protein degradation by the ubiquitin-dependent proteolytic pathway.

Structural and functional conservation of two human homologs of the yeast DNA repair gene RAD6

The RAD6 gene of Saccharomyces cerevisiae encodes a ubiquitin-conjugating enzyme (E2) that is required for DNA repair, damage-induced mutagenesis, and sporulation. We have cloned the two human RAD6 homologs, designated HHR6A and HHR6B. The two 152-amino acid human proteins share 95% sequence identity with each other and approximately 70% and approximately 85% overall identity with the homologs from yeasts (S. cerevisiae and Schizosaccharomyces pombe) and Drosophila melanogaster, respectively. Neither of the human RAD6 homologs possess the acidic C-terminal sequence present in the S. cerevisiae RAD6 protein. Genetic complementation experiments reveal that HHR6A as well as HHR6B can carry out the DNA repair and mutagenesis functions of RAD6 in S. cerevisiae rad6 delta mutants.

Uncoupling ubiquitin-protein conjugation from ubiquitin-dependent proteolysis by use of beta, gamma-nonhydrolyzable ATP analogues

Pathways of ubiquitin-dependent protein degradation have in common two requirements for ATP. Ubiquitin activation by the enzyme E1 is accompanied by ATP hydrolysis to yield AMP and PPi, and during conjugate breakdown, the ubiquitin-dependent protease hydrolyzes ATP to ADP and Pi. We show here that either of two beta, gamma-nonhydrolyzable ATP analogues, 5'-adenylyl imidodiphosphate or 5'-adenylyl methylenediphosphate, can support ubiquitin-protein conjugation. With the ubiquitin-dependent protease, however, neither analogue could substitute for ATP. Thus, the substitution of a beta, gamma-nonhydrolyzable analogue for ATP offers a simple method to uncouple ubiquitin conjugation from proteolysis in crude systems. On the basis of pyrophosphate exchange kinetics, E1 has apparent Km and Vmax values that are similar for ATP and the analogues, but substrate inhibition by 5'-adenylyl methylenediphosphate made use of the beta, gamma-imido analogue preferable. In one application, beta, gamma-imido-ATP was used in combination with ubiquitin aldehyde (an inhibitor of ubiquitin-protein isopeptidases) to establish that several unfolded RNase A derivatives are recognized equally as ubiquitination substrates. This result extends an earlier study [Dunten, R. L., & Cohen, R. E. (1989) J. Biol. Chem. 264, 16739-16747] to show that conjugate yields, upon which relative ubiquitination rates were based, were not influenced by differential ubiquitin-dependent proteolysis. In a second application, ATP and beta, gamma-imido-ATP were compared in a pulse-chase experiment to investigate the contributions of ATP-dependent proteolysis and isopeptidase activities to conjugate stability.

UBA 1: an essential yeast gene encoding ubiquitin-activating enzyme

All known functions of ubiquitin are mediated through its covalent attachment to other proteins. The post-translational formation of ubiquitin--protein conjugates is preceded by an ATP-requiring step in which the carboxyl terminus of ubiquitin is adenylated and subsequently joined, through a thiolester bond, to a cysteine residue in the ubiquitin-activating enzyme, also known as E1. We report the isolation and functional analysis of the gene (UBA1) for the ubiquitin-activating enzyme of the yeast Saccharomyces cerevisiae. UBA1 encodes a 114 kd protein whose amino acid sequence contains motifs characteristic of nucleotide-binding sites. Expression of catalytically active UBA1 protein in E. coli, which lacks the ubiquitin system, confirmed that the yeast UBA1 gene encodes a ubiquitin-activating enzyme. Deletion of the UBA1 gene is lethal, demonstrating that the formation of ubiquitin--protein conjugates is essential for cell viability.

High performance liquid chromatography resolution of ubiquitin pathway enzymes from wheat germ

The highly conserved protein ubiquitin is involved in several cellular processes in eukaryotes as a result of its covalent ligation to a variety of target proteins. Here, we describe the purification of several enzymatic activities involved in ubiquitin-protein conjugate formation and disassembly from wheat germ (Triticum vulgare) by a combination of ubiquitin affinity chromatography and anion-exchange high performance liquid chromatography. Using this procedure, ubiquitin activating enzyme (E1), several distinct ubiquitin carrier proteins (E2s) with molecular masses of 16, 20, 23, 23.5, and 25 kilodaltons, and a ubiquitin-protein hydrolase (isopeptidase) were isolated. Purified E1 formed a thiol ester linkage with (125)I-ubiquitin in an ATP-dependent manner and transferred bound ubiquitin to the various purified E2s. The ubiquitin protein hydrolase fraction was sensitive to hemin, and in an ATP-independent reaction, was capable of removing the ubiquitin moiety from both ubiquitin (125)I-lysozyme conjugates (epsilon-amino or isopeptide linkage) and the ubiquitin 52-amino acid extension protein fusion (alpha-amino or peptide linkage). Using this procedure, wheat germ represents an inexpensive source from which enzymes involved in the ubiquitin pathway may be isolated.

Proteolytic activity during senescence of plants

Although information has rapidly developed concerning the intracellular localization of plant proteins, relatively few reports concern the intracellular location of endo- and exo-proteolytic activities. Relatively few proteases have been purified, characterized, and associated with a specific cellular location. With the exception of the processing proteases involved in transport of proteins across membranes, little progress has yet been made concerning determination of in vivo products of specific proteases. Information on the turnover of individual proteins and the assessment of rate-limiting steps in pathways as proteins are turned over is steadily appearing. Since chloroplasts are the major site of both protein synthesis and, during senescence, degradation, it was important to show unambiguously that chloroplasts can degrade their own constituents. Another important contribution was to obtain evidence that the chloroplasts contain proteases capable of degrading their constituents. This work has been more tenuous because of the low activities found and the possibility of contamination by vacuolar enzymes during the isolation of organelles. The possible targeting of cytoplasmic proteins for degradation by facilitating their transport into vacuoles is a field which hopefully will develop more rapidly in the future. Information on targeting of proteins for degradation via the ubiquitin (Ub) degradation pathway is developing rapidly. Future research must determine how much unity exists across the different eukaryotic systems. At present, it has important implications for protein turnover in plants, since apparently Ub is involved in the degradation of phytochrome. Little information has been developed regarding what triggers increased proteolysis with the onset of senescence, although it appears to involve protein synthesis. Thus far, the evidence indicates that the complement of proteases prior to senescence is sufficient to carry out the observed protein degradation. This field of study has great practical implications, e.g. maintaining photosynthesis during seed-fill in order to obtain greater crop yields. The current use of stay green' variants in the populations of several crop plants to produce increased yields shows the potential for future development. The near future should see exciting discoveries in these areas of research that will have far reaching effects on the construction of transgenic plants for future research accomplishments and agricultural use.