Ubiquitous Urease Research Articles

Structure-Function Insights of Jaburetox and Soyuretox: Novel Intrinsically Disordered Polypeptides Derived from Plant Ureases.

Intrinsically disordered proteins (IDPs) and intrinsically disordered regions (IDRs) do not have a stable 3D structure but still have important biological activities. Jaburetox is a recombinant peptide derived from the jack bean (Canavalia ensiformis) urease and presents entomotoxic and antimicrobial actions. The structure of Jaburetox was elucidated using nuclear magnetic resonance which reveals it is an IDP with small amounts of secondary structure. Different approaches have demonstrated that Jaburetox acquires certain folding upon interaction with lipid membranes, a characteristic commonly found in other IDPs and usually important for their biological functions. Soyuretox, a recombinant peptide derived from the soybean (Glycine max) ubiquitous urease and homologous to Jaburetox, was also characterized for its biological activities and structural properties. Soyuretox is also an IDP, presenting more secondary structure in comparison with Jaburetox and similar entomotoxic and fungitoxic effects. Moreover, Soyuretox was found to be nontoxic to zebra fish, while Jaburetox was innocuous to mice and rats. This profile of toxicity affecting detrimental species without damaging mammals or the environment qualified them to be used in biotechnological applications. Both peptides were employed to develop transgenic crops and these plants were active against insects and nematodes, unveiling their immense potentiality for field applications.

Soyuretox, an Intrinsically Disordered Polypeptide Derived from Soybean (Glycine Max) Ubiquitous Urease with Potential Use as a Biopesticide.

Ureases from different biological sources display non-ureolytic properties that contribute to plant defense, in addition to their classical enzymatic urea hydrolysis. Antifungal and entomotoxic effects were demonstrated for Jaburetox, an intrinsically disordered polypeptide derived from jack bean (Canavalia ensiformis) urease. Here we describe the properties of Soyuretox, a polypeptide derived from soybean (Glycine max) ubiquitous urease. Soyuretox was fungitoxic to Candida albicans, leading to the production of reactive oxygen species. Soyuretox further induced aggregation of Rhodnius prolixus hemocytes, indicating an interference on the insect immune response. No relevant toxicity of Soyuretox to zebrafish larvae was observed. These data suggest the presence of antifungal and entomotoxic portions of the amino acid sequences encompassing both Soyuretox and Jaburetox, despite their small sequence identity. Nuclear Magnetic Resonance (NMR) and circular dichroism (CD) spectroscopic data revealed that Soyuretox, in analogy with Jaburetox, possesses an intrinsic and largely disordered nature. Some folding is observed upon interaction of Soyuretox with sodium dodecyl sulfate (SDS) micelles, taken here as models for membranes. This observation suggests the possibility for this protein to modify its secondary structure upon interaction with the cells of the affected organisms, leading to alterations of membrane integrity. Altogether, Soyuretox can be considered a promising biopesticide for use in plant protection.

Analysis of Herbivore Stress- and Phytohormone-Mediated Urease Expression in Soybean (Glycine max)

Ureases catalyze the hydrolysis of urea into ammonia and carbon dioxide and, thus, are involved in the metabolism and bioavailability of nitrogen. Ureases occur in plants, fungi, and bacteria. In plants, besides their enzymatic activity, ureases as proteins play a role in defense against insect and phytopathogenic fungi. Little is known about the regulation of urease in plants under stress and whether or not phytohormones may be involved. In this study, we addressed the regulation of ubiquitous urease (ubSBU) gene expression after phytohormone applications, insect herbivory, and mechanical damage in soybean (Glycine max cv. Williams 82). Stress-related phytohormones were applied. In addition, Spodoptera littoralis feeding and mechanical damage by MecWorm were performed. Ureolytic activity and transcripts for ubSBU and UreG were quantified. Roots and leaves showed the highest levels of ubSBU transcripts. The results show a significant increase of ubSBU transcripts upon jasmonic acid application and after herbivory, but downregulation after MecWorm treatment. UreG transcripts were downregulated after MecWorm, S. littoralis, and application of gibberellic acid, but upregulated by jasmonic acid. However, the ureolytic activities in leaves were influenced neither by phytohormones nor by herbivory and MecWorm. We conclude that the enzymatic activity of ureases is constitutive and basal levels of the enzyme are sufficient to perform the ureolytic activities in defense against insects and fungi. The defense role of ureases, which does not require the ureolytic activity, may underlie their differential regulation in response to different stress stimuli.

Effect of soybean ureases on seed germination and plant development

Urease catalyzes the hydrolysis of urea to ammonia and carbon dioxide. The ammonia (nitrogen (N) product of urease activity) is incorporated into organic compounds. Thus, urease is involved in N remobilization, as well as in primary N assimilation. Two urease isoforms have been described for soybean: the embryo-specific, encoded by the Eu1 gene, and the ubiquitous urease, encoded by Eu4. A third urease-encoding gene was recently identified, designated Eu5, which encodes the putative protein product SBU-III. The present study aimed to evaluate the contribution of soybean ureases to seed germination and plant development. Analyses were performed using Eu1/Eu4/Eu5-co-suppressed transgenic plants and mutants of the Eu1 and Eu4 urease structural genes, as well as a urease-null mutant (eu3-a) that activates neither the ubiquitous nor embryo-specific ureases. The co-suppressed plants presented a developmental delay during the first month after germination; shoots and roots were significantly smaller and lighter. Slower development was observed for the double eu1-a/eu4-a mutant and the eu3-a single mutant. The N content in transgenic plants was significantly lower than in non-transgenic plants. Among the mutants, eu3-a presented the lowest and eu1-a the highest N content. Altogether, these results indicate that increased ureolytic activity plays an important role in plant development.

Soybean ubiquitous urease with purification facilitator: An addition to the moonlighting studies toolbox

Abstract Ureases are nickel-dependent enzymes that catalyze the hydrolysis of urea to ammonia and carbon dioxide. In soybean (Glycine max), the embryo-specific urease (eSBU), the ubiquitous urease (uSBU), and a third isoform (SBU-III) are synthesized. Our group has previously demonstrated that eSBU, purified from seeds, has antifungal properties against phytopathogenic fungi, entomotoxicity against Dysdercus peruvianus, the ability to induce blood platelet aggregation, and these properties are independent of its enzymatic activity. Here we describe the biological properties of apo-uSBU fused to glutathione S-transferase (GST) produced in Escherichia coli. Removal of GST affected apo-uSBU stability. We performed a Response Surface Methodology to optimize GST-uSBU production to 5 mg per liter and then bioassays were carried out. The recombinant protein exhibited inhibitory effects on filamentous fungi and affected fungal secondary metabolism. Candida albicans and C. tropicalis were also susceptible to GST-uSBU and formed pseudo-hyphae. The fusion protein was toxic against Rhodnius prolixus, with the toxicity being accompanied by in vivo and in vitro hemocyte aggregation. Rabbit platelet also aggregated in the presence of GST-uSBU. Thus, uSBU displayed similar biological properties as previously described for eSBU even when fused to GST, reinforcing the proposed role of ureases in plant defense.

Structural and transcriptional characterization of a novel member of the soybean urease gene family

In plants, ureases have been related to urea degradation, to defense against pathogenic fungi and phytophagous insects, and to the soybean−Bradyrhizobium japonicum symbiosis. Two urease isoforms have been described for soybean: the embryo-specific, encoded by Eu1 gene, and the ubiquitous urease, encoded by Eu4. A third urease-encoding locus exists in the completed soybean genome. The gene was designated Eu5 and the putative product of its ORF as SBU-III. Phylogenetic analysis shows that 41 plant, moss and algal ureases have diverged from a common ancestor protein, but ureases from monocots, eudicots and ancient species have evolved independently. Genomes of ancient organisms present a single urease-encoding gene and urease-encoding gene duplication has occurred independently along the evolution of some eudicot species. SBU-III has a shorter amino acid sequence, since many gaps are found when compared to other sequences. A mutation in a highly conserved amino acid residue suggests absence of ureolytic activity, but the overall protein architecture remains very similar to the other ureases. The expression profile of urease-encoding genes in different organs and developmental stages was determined by RT-qPCR. Eu5 transcripts were detected in seeds one day after dormancy break, roots of young plants and embryos of developing seeds. Eu1 and Eu4 transcripts were found in all analyzed organs, but Eu4 expression was more prominent in seeds one day after dormancy break whereas Eu1 predominated in developing seeds. The evidence suggests that SBU-III may not be involved in nitrogen availability to plants, but it could be involved in other biological role(s).

Ubiquitous urease affects soybean susceptibility to fungi

The soybean ubiquitous urease (encoded by GmEu4) is responsible for recycling metabolically derived urea. Additional biological roles have been demonstrated for plant ureases, notably in toxicity to other organisms. However, urease enzymatic activity is not related to its toxicity. The role of GmEu4 in soybean susceptibility to fungi was investigated in this study. A differential expression pattern of GmEu4 was observed in susceptible and resistant genotypes of soybeans over the course of a Phakopsora pachyrhizi infection, especially 24 h after infection. Twenty-nine adult, transgenic soybean plants, representing six independently transformed lines, were obtained. Although the initial aim of this study was to overexpress GmEu4, the transgenic plants exhibited GmEu4 co-suppression and decreased ureolytic activity. The growth of Rhizoctonia solani, Phomopsis sp., and Penicillium herguei in media containing a crude protein extract from either transgenic or non-transgenic leaves was evaluated. The fungal growth was higher in the protein extracts from transgenic urease-deprived plants than in extracts from non-transgenic controls. When infected by P. pachyrhizi uredospores, detached leaves of urease-deprived plants developed a significantly higher number of lesions, pustules and erupted pustules than leaves of non-transgenic plants containing normal levels of the enzyme. The results of the present work show that the soybean plants were more susceptible to fungi in the absence of urease. It was not possible to overexpress active GmEu4. For future work, overexpression of urease fungitoxic peptides could be attempted as an alternative approach.Electronic supplementary materialThe online version of this article (doi:10.1007/s11103-012-9894-1) contains supplementary material, which is available to authorized users.

Interallelic complementation at the ubiquitous urease coding locus of soybean.

Soybean (Glycine max [L.] Merrill) mutant aj6 carries a single recessive lesion, aj6, that eliminates ubiquitous urease activity in leaves and callus while retaining normal embryo-specific urease activity. Consistently, aj6/aj6 plants accumulated urea in leaves. In crosses of aj6/aj6 by urease mutants at the Eu1, Eu2, and Eu3 loci, F(1) individuals exhibited wild-type leaf urease activity, and the F(2) segregated urease-negative individuals, demonstrating that aj6 is not an allele at these loci. F(2) of aj6/aj6 crossed with a null mutant lacking the Eu1-encoded embryo-specific urease showed that ubiquitous urease was also inactive in seeds of aj6/aj6. The cross of aj6/aj6 to eu4/eu4, a mutant previously assigned to the ubiquitous urease structural gene (R.S. Torisky, J.D. Griffin, R.L. Yenofsky, J.C. Polacco [1994] Mol Gen Genet 242: 404-414), yielded an F(1) having 22% +/- 11% of wild-type leaf urease activity. Coding sequences for ubiquitous urease were cloned by reverse transcriptase-polymerase chain reaction from wild-type, aj6/aj6, and eu4/eu4 leaf RNA. The ubiquitous urease had an 837-amino acid open reading frame (ORF), 87% identical to the embryo-specific urease. The aj6/aj6 ORF showed an R201C change that cosegregated with the lack of leaf urease activity in a cross against a urease-positive line, whereas the eu4/eu4 ORF showed a G468E change. Heteroallelic interaction in F(2) progeny of aj6/aj6 x eu4/eu4 resulted in partially restored leaf urease activity. These results confirm that aj6/aj6 and eu4/eu4 are mutants affected in the ubiquitous urease structural gene. They also indicate that radical amino acid changes in distinct domains can be partially compensated in the urease heterotrimer.

The soybean Eu3 gene encodes an Ni-binding protein necessary for urease activity.

Mutation in Eu3 eliminates activity of both soybean ureases, the embryo-specific (encoded by Eu1) and the tissue-ubiquitous (encoded by Eu4). eu3-e1 is a completely recessive null allele. Eu3-e3 is a semi-dominant specifying 0.1% wild-type urease activity in the homozygous state and 5-10% as a heterozygote (Meyer-Bothling et al. 1987). Antibodies to plant UreG, a homologue of the bacterial urease accessory protein, revealed a 32 kDa protein (p32) in embryos of the Eu3/Eu3 precursor genotype. p32 is identical to UreG by the criteria of size, antigenicity, and its ability to bind Ni2+, a trait expected from the deduced histidine-rich N-terminus of UreG. UreG was absent in eu3-e1/eu3-e1, and lack of UreG co-segregated with eu3-e1. Eu3-e3 specified a UreG transcript which coded valine in place of alanine at residue 142 (A142V) confirming thatEu3 encodes UreG, which is renamed Eu3. Eu3 (A142V) retained Ni-binding ability. Eu3 is directly involved in urease activation, since anti-Eu3 (UreG) antibodies inhibited the in vitro activation of urease. Eu1 (embryo urease) and Eu3 accumulated in parallel in the developing embryo. The presence of Eu1 was not necessary for the high embryonic level of Eu3. However, the presence of Eu3 appeared to be important for accumulation of Eu1, perhaps by stabilizing it by Ni insertion. At the level of sensitivity employed Eu3 was detected in crude extracts of embryos but not non-embryonic tissues which have 1/500th the embryo urease activity. Functional Eu3, however, is necessary for activation of the ubiquitous urease in non-embryonic tissues.

Soybean genes involved in nickel insertion into urease

In soybean, mutation in the Eu2 or in the Eu3 gene eliminates the activities, but not the proteins, of the embryo-specific and the ubiquitous ureases, encoded by Eu1 and Eu4, respectively. This pleiotropic urease-negative phenotype is consistent with accessory gene functions encoded by Eu2 and Eu3, i.e. correct insertion of Ni into the metallocentre of each urease. To test an accessory gene function an examination was made of segregation of alleles at Eu2 and Eu3 with segregation of an RFLP revealed by a plant homologue of the bacterial urease accessory gene, ureG. The eu3-e1/eu3-e1 mutant, which has a urease activity-null phenotype, lacked a 1.4 kb EcoRV genomic fragment found in progenitor cultivar Williams, cv Williams 82 and in two mutants at the Eu2 locus, eu2/eu2 and EN24, the latter described here for the first time. The lack of the 1.4 kb band segregated with eu3-e1 in a cross of eu3-e1/eu3-e1 × EN24. The second approach was to attempt partial correction of the urease-negative trait by Ni supplementation in vitro. First, a small, reproducible stimulation of activity in mixed extracts of mutants which complement genetically, namely {eu2/eu2 plus eu3-e1/eu3-e1) and (EN24 plus eu3-e1/eu3-e1) was observed. Activation proceeded for several hours in these extracts containing endogenous Ni. In mixed extracts from Ni-free embryos, activation was dependent on added Ni; Ni had no effect on individual mutant extracts. By genetic and biochemical criteria the ubiquitous urease was the sole or major species activated, an activation which approached 10% normal activity.

A single gene (Eu4) encodes the tissue-ubiquitous urease of soybean.

We sought to determine the genetic basis of expression of the ubiquitous (metabolic) urease of soybean. This isozyme is termed the metabolic urease because its loss, in eu4/eu4 mutants, leads to accumulation of urea, whereas loss of the embryo-specific urease isozyme does not. The eu4 lesion eliminated the expression of the ubiquitous urease in vegetative and embryonic tissues. RFLP analysis placed urease clone LC4 near, or within, the Eu4 locus. Sequence comparison of urease proteins (ubiquitous and embryo-specific) and clones (LC4 and LS1) indicated that LC4 and LS1 encode ubiquitous and embryo-specific ureases, respectively. That LC4 is transcribed into poly(A)+ RNA in all tissues was indicated by the amplification of its transcript by an LC4-specific PCR primer. (The LS1-specific primer, on the other hand, amplified poly(A)+ RNA only from developing embryos expressing the embryo-specific urease.) These observations are consistent with Eu4 being the ubiquitous urease structural gene contained in the LC4 clone. In agreement with this notion, the mutant phenotype of eu4/eu4 callus was partially corrected by the LC4 urease gene introduced by particle bombardment.

Genetic Tests of the Roles of the Embryonic Ureases of Soybean

We assayed the in vivo activity of the ureases of soybean (Glycine max) embryos by genetically eliminating the abundant embryo-specific urease, the ubiquitous urease, or a background urease. Mutant embryos accumulated urea (250-fold over progenitor) only when lacking all three ureases and only when developed on plants lacking the ubiquitous urease. Thus, embryo urea is generated in maternal tissue where its accumulation is not mitigated by the background urease. However, the background urease can hydrolyze virtually all urea delivered to the developing embryo. Radicles of 2-day-old germinants accumulated urea in the presence or absence of the embryo-specific urease (2 micromoles per gram dry weight radicle). However, mutants lacking the ubiquitous urease exhibited increased accumulation of urea (to 4-5 micromoles urea per gram dry weight radicle). Thus, the ubiquitous and not the embryo-specific urease hydrolyzes urea generated during germination. In the absence of both of these ureases, the background urease activity (4% of ubiquitous urease) may hydrolyze most of the urea generated. A pleiotropic mutant lacking all urease accumulated 34 micromoles urea per gram dry weight radicle (increasing 2.5-fold at 3 days after germination). Urea (20 millimolar) was toxic to in vitro-cultured cotyledons which contained active embryo-specific urease. Cotyledons lacking the embryo-specific urease accumulated more protein when grown with urea than with no nitrogen source. Among cotyledons lacking the embryo-specific urease, fresh weight increases were virtually unchanged whether grown on urea or on no nitrogen and whether in the presence or absence of the ubiquitous urease. However, elimination of the ubiquitous urease reduced protein deposition on urea-N, and elimination of both the ubiquitous and background ureases further reduced urea-derived protein. The evidence is consistent with the lack of a role in urea hydrolysis for the embryo-specific urease in developing embryos or germinating seeds. Because the embryo-specific urease is deleterious to cotyledons cultured in vitro on urea-N, its role may be to hydrolyze urea in wounded or infected embryos, creating a hostile environment for pest or pathogen. While the ubiquitous urease is operative in leaves and in seedlings, all or most of its function can be assumed by the background urease in embryos and in seedlings.

Soybean Roots Retain the Seed Urease Isozyme Synthesized during Embryo Development

Roots of young soybean (Glycine max [L.] Merr.) plants (up to 25 days old) contain two distinct urease isozymes, which are separable by hydroxyapatite chromatography. These two urease species (URE1 and URE2) differ in: (a) electrophoretic mobility in native gels, (b) pH dependence, and (c) recognition by a monoclonal antibody specific for the seed ("embryo-specific") urease. By these parameters root URE1 urease is similar to the abundant embryo-specific urease isozyme, while root URE2 resembles the "ubiquitous" urease which has previously been found in all soybean tissues examined (leaf, embryo, seed coat, and cultured cells). The embryo-specific and ubiquitous urease isozymes are products of the Eu1 and Eu4 structural genes, respectively. Roots of the eu1-sun/eu1-sun genotype, which lacks the embryo-specific urease (i.e. ;seed urease-null'), contain no URE1 urease activity. Roots of eu4/eu4, which lacks ubiquitous urease, lack the URE2 (leaflike) urease activity. From these genetic and biochemical criteria, then, we conclude that URE1 and URE2 are the embryo-specific and ubiquitous ureases, respectively. Adventitious roots generated from cuttings of any urease genotype lack URE1 activity. In seedling roots the seedlike (URE1) activity declines during development. Roots of 3-week-old plants contain 5% of the total URE1 activity of the radicle of 4-day-old seedlings, which, in turn, has approximately the same urease activity level as the dormant embryonic axis. The embryo-specific urease incorporates label from [(35)S]methionine during embryo development but not during germination, indicating that there is no de novo synthesis of the embryo-specific (URE1) urease in the germinating root. We conclude that the seedlike urease (URE1) found in roots of young soybean plants is a remnant of the Eu1-encoded, abundant, embryo-specific urease which accumulates in the embryonic root axis during seed development.

A new mutant class of soybean lacks urease in leaves but not in leaf-derived callus or in roots

We reported earlier the recovery of two classes of soybean urease mutants in soybean (Glycine max L. Merr. cv. Williams). Class I mutants lack the embryo-specific urease while class II mutants lack the activities of both urease isozymes, the embryo-specific and the ubiquitous urease, the latter found in all tissues examined. We report here the recovery of a true-breeding mutant, aj3, which represents the third phenotypic class: normal levels of embryo-specific urease and little or no ubiquitous urease. Unlike class II mutant plants which lack urease in all tissue, aj3 lacks urease activity only in leaves (ca. 2% normal activity); its roots have near normal urease activity. Callus derived from leaves of aj3 has 14% to 40% the urease activity of Williams 82 callus. This partial reduction in urease activity in aj3 callus is sufficient to reduce growth with urea as sole nitrogen source and to confer resistance to 50 mM urea added to callus maintenance medium. Leaves of aj3 produce more than 40 times the urease antigen expected from their urease activity. The aj3 trait is due to a single recessive lesion which is not allelic with lesions at theEu2, Eu3 (class II) orEu1 (class I) loci. We designate the aj3 genotype aseu4/eu4.

Pleiotropic soybean mutants defective in both urease isozymes

Two new soybean [Glycine max (L.) Merr. cv. Williams] loci, designated Eu2 and Eu3, were identified in which ethyl methanesulfonate (EMS)-induced mutation eliminated urease activity. These loci showed no linkage to each other or to the "Sun-Eul" locus described in the accompanying paper (Meyer-Bothling and Polacco 1987). Unlike sun (seed urease-null) mutations those at Eu2 and Eu3 affected both urease isozymes: the embryo-specific (seed) and the ubiquitous (leaf) urease. The eu2/eu2 mutant had no leaf activity and 0.6% normal seed activity. Two mutant Eu3 alleles were recovered, eu3-e1 and Eu3-e3. The eu3-e1/eu3-e1 genotype lacked both activities while Eu3-e3/Eu3-e3 had coordinately reduced leaf (0.1%) and seed (0.1%) activities. Only the Eu3-e3 mutation showed partial dominance, yielding about 5%-10% normal activity for each urease in the heterozygous state. Each homozygous mutant contained normal levels of embryo-specific urease mRNA and protein subunit, both of normal size. However, urease polymerization was aberrant in all three mutants. In all cases where urease could be measured, it was found to be temperature sensitive and, in addition, the embryo-specific urease of Eu3-e3/Eu3-e3 had an altered pH dependence. These mutants may be defective in a urease maturation function common to both isozymes as suggested by the normal levels of urease gene product, coordinately (or nearly so) reduced urease isozyme activities, temperature sensitivity in both ureases (Eu3-e3) and the non-linkage of Eu2 and Eu3 to the locus encoding embryo-specific urease (Sun-Eul). Ubiquitous urease activity is reduced in mutant seed coat and callus culture as well as in leaf and cotyledon tissue. No mutant callus utilized urea (5 to 10 nM0 as sole nitrogen source. However, all mutant cell lines tolerated normally toxic levels of urea (25 to 250 mM) added to medium containing KNO3/NH4No3 as nitrogen source. Urea thus may be used in cell culture as a selection agent for phenotypes either lacking or regaining an active ubiquitous urease.

Mutational analysis of the embryo-specific urease locus of soybean

By a non-destructive urease screen of M2 soybean (Glycine max [L.] Merr. cv. Williams) seeds, four true-breeding mutants (n4, n6, n7 and n8) were recovered which lack most (n6, n8) or all (n4, n7) embryo-specific urease activity. This trait was due to a single, recessive lesion at the Sun (seed urease-null) locus identified earlier in an exotic germplasm (PI 229324, Itachi). All sun mutants produced normal ubiquitous urease, the low abundance isozyme found in all soybean tissues examined. Tight mutants n4 and n7 accumulated no detectable embryo-specific urease protein or mRNA; n6 and n8 accumulated normal or near normal levels of urease mRNA but had seed urease protein levels approximately 5% and .05%, respectively, of the progenitor. Mutant n8 appeared to produce a low level of fully active urease (approximately 0.7% activity level, approximately 0.5% protein level) while n6 produced a higher level of an altered, nearly inactive urease (.0.09% activity level, approximately 5% protein level). Urease alterations in n6 were manifested by its increased temperature sensitivity and variation in aggregation state and pH preference. Thus, mutations in the Sun locus affected both the level and the nature of the embryo-specific urease gene products indicating that Sun encodes the embryo-specific urease. We reported earlier that the Eul locus, which controls the aggregation state of the embryo-specific urease, is on mep unit from Sun and that the Eul allele is cis to sun is not expressed (Kloth et al. 1987). That the level of urease gene product, its aggregation state and other enzyme properties can be affected by induced sun mutations, suggests that the Eul and sun alleles are at the same locus.

Developmental genetics of the soybean urease isozymes

AbstractThe soybean (Glycine max [L.] Merr.) contains two urease isozymes whose expression is regulated in a tissue‐specific and temporal manner. The ubiquitous urease is expressed in all tissues examined (leaf, embryo, seed coat, cell culture); the embryo‐specific urease is synthesized exclusively in the developing embryo. The embryo‐specific urease accumulates during seed development while the ubiquitous urease is found in highest levels during early development of both leaves and seeds. We have isolated mutants which fall in three phenotypic classes lacking one or both urease isozyme activities. Genetic analysis has thus far identified three unlinked loci which control the expression of urease(s). Genomic and cDNA clones of urease structural genes have also been recovered and we are working to assign these to genetic loci by sequence and RFLP analyses. That the ubiquitous urease isozyme is expressed in cell culture makes it possible to include cell culture in physiological and developmental studies. Additionally, we have developed direct selections for urease‐negative mutants, and their revertants, in cell culture. These selections will facilitate the study of the expression of cloned urease genes in genetically transformed tissue.

Structure and Possible Ureide Degrading Function of the Ubiquitous Urease of Soybean

Ubiquitous soybean urease, as opposed to the seed-specific urease, designates the seemingly identical ureolytic activities of suspension cultures and leaves. It also appears to be the basal urease in developing seeds of a variety, Itachi, which lacks the seed-specific urease (Polacco, Winkler 1984 Plant Physiol 74: 800-804). On native polyacrylamide gels the ureolytic activities in crude extracts of these three tissues comigrate as determined by assays of gel slices. At this level of resolution the ubiquitous urease also migrates with or close to the fast (trimeric) form of the seed-specific urease.The ubiquitous urease was purified approximately 100-fold from suspension cultures of two cultivars (Itachi and Prize) as well as from developing seeds of Itachi. These partially purified preparations allowed visualization of native urease on polyacrylamide gels by activity staining and of urease subunits on denaturing lithium dodecyl sulfate gels by electrophoretic transfer to nitrocellulose and immunological detection ("Western Blot"). The ubiquitous urease holoenzyme migrates slightly less rapidly than the fast seed urease in native gels; its subunit migrates slightly less rapidly than the 93.5 kilodaltons subunit of either the fast or slow (hexameric) seed enzyme. The ubiquitous urease elutes from an agarose A-0.5 meter column with the fast form of the seed urease species suggesting that the ubiquitous urease, like the fast seed urease, exists as a trimeric holoenzyme. The soybean cultivar, Prize, produces the hexameric seed urease; yet its ubiquitous urease (from leaf and suspension culture) is trimeric.The pH dependence of the ureolytic activity of seed coats of both seed urease-negative (Itachi) and seed urease-positive (Williams) cultivars suggests that this activity is exclusively the ubiquitous urease. Its relatively higher levels in seed coats than in embryos of Itachi suggests that the ubiquitous urease is involved in degradation of urea derived from ureides. Consistent with a ureide origin for urea is the observation that addition of a urease inhibitor, phenylphosphordiamidate, to extracts of developing Itachi seeds (seed coat plus embryo) results in accumulation of urea from allantoic acid.