Plant and algal lysophosphatidic acid acyltransferases increase docosahexaenoic acid accumulation at the sn-2 position of triacylglycerol in transgenic Arabidopsis seed oil

Although docosahexaenoic acid (DHA), an important dietary omega-3 polyunsaturated fatty acid (PUFA), is at present primarily sourced from marine fish, bioengineered crops producing DHA may offer a more sustainable and cost-effective source. DHA has been produced in transgenic oilseed crops, however, DHA in seed oil primarily occupies the sn-1/3 positions of triacylglycerol (TAG) with relatively low amounts of DHA in the sn-2 position. To increase the amount of DHA in the sn-2 position of TAG and in seed oil, putative lysophosphatidic acid acyltransferases (LPAATs) were identified and characterized from the DHA-producing alga Schizochytrium sp. and from soybean (Glycine max). The affinity-purified proteins were confirmed to have LPAAT activity. Expression of the Schizochytrium or soybean LPAATs in DHA-producing Arabidopsis expressing the Schizochytrium PUFA synthase system significantly increased the total amount of DHA in seed oil. A novel sensitive band-selective heteronuclear single quantum coherence (HSQC) NMR method was developed to quantify DHA at the sn-2 position of glycerolipids. More than two-fold increases in sn-2 DHA were observed for Arabidopsis lines expressing Schizochytrium or soybean LPAATs, with one Schizochytrium LPAAT driving DHA accumulation in the sn-2 position to 61% of the total DHA. Furthermore, expression of a soybean LPAAT led to a redistribution of DHA-containing TAG species, with two new TAG species identified. Our results demonstrate that transgenic expression of Schizochytrium or soybean LPAATs can increase the proportion of DHA at the sn-2 position of TAG and the total amount of DHA in the seed oil of a DHA-accumulating oilseed plant. Additionally, the band-selective HSQC NMR method that we developed provides a sensitive and robust method for determining the regiochemistry of DHA in glycerolipids. These findings will benefit the advancement of sustainable sources of DHA via transgenic crops such as canola and soybean.

Identification of iron-chelating phenolics contributing to seed coat coloration in soybeans (Glycine max (L.) Merr.) expressing aryloxyalkanoate dioxygenase-12

Soybeans (Glycine max (L.) Merr.) genetically modified to express aryloxyalkanoate dioxygenase-12 (AAD-12), an enzyme that confers resistance to the herbicide 2,4-D, can sometimes exhibit a darker seed coat coloration than equivalent unmodified soybeans. The biochemical basis for this coloration was investigated in a non-commercial transgenic event, DAS-411Ø4-7 that exhibited more pronounced AAD-12-associated seed coat coloration than the commercial event, DAS-444Ø6-6. Analysis of color-enriched seed coat fractions from DAS-411Ø4-7 showed that the color was due to localized accumulation of iron-chelating phenolics, particularly the isoflavone genistin, that are associated with seed coat pectic polysaccharide and produce a brown chromophore. The association between genistin, iron, and pectic polysaccharide was characterized using a variety of analytical methods. Darker seeds from commercial soybean event DAS-444Ø6-6 also show higher genistin content localized to the darker colored portions of the seed coat (with no increase in whole seed genistin levels).

Transgenic and Genome Editing Approaches for Modifying Plant Oils

Vegetable oils are important for human and animal nutrition and as renewable resources for chemical feedstocks. We provide an overview of transgenic and genome editing approaches for modifying plant oils, describing useful model and crop systems and different strategies for transgenic modifications. We also describe new genome editing approaches that are beginning to be applied to oilseed plants and crops. These approaches are illustrated with examples for modifying the nutritional quality of vegetable oils by altering fatty acid desaturation, producing non-native fatty acids in oilseeds, and enhancing the overall accumulation of oil in seeds and leaves.

Zinc finger nuclease-mediated precision genome editing of an endogenous gene in hexaploid bread wheat (Triticum aestivum) using a DNA repair template

Sequence-specific nucleases have been used to engineer targeted genome modifications in various plants. While targeted gene knockouts resulting in loss of function have been reported with relatively high rates of success, targeted gene editing using an exogenously supplied DNA repair template and site-specific transgene integration has been more challenging. Here, we report the first application of zinc finger nuclease (ZFN)-mediated, nonhomologous end-joining (NHEJ)-directed editing of a native gene in allohexaploid bread wheat to introduce, via a supplied DNA repair template, a specific single amino acid change into the coding sequence of acetohydroxyacid synthase (AHAS) to confer resistance to imidazolinone herbicides. We recovered edited wheat plants having the targeted amino acid modification in one or more AHAS homoalleles via direct selection for resistance to imazamox, an AHAS-inhibiting imidazolinone herbicide. Using a cotransformation strategy based on chemical selection for an exogenous marker, we achieved a 1.2% recovery rate of edited plants having the desired amino acid change and a 2.9% recovery of plants with targeted mutations at the AHAS locus resulting in a loss-of-function gene knockout. The latter results demonstrate a broadly applicable approach to introduce targeted modifications into native genes for nonselectable traits. All ZFN-mediated changes were faithfully transmitted to the next generation.

GC-rich coding sequences reduce transposon-like, small RNA-mediated transgene silencing

The molecular basis of transgene susceptibility to silencing is poorly characterized in plants; thus, we evaluated several transgene design parameters as means to reduce heritable transgene silencing. Analyses of Arabidopsis plants with transgenes encoding a microalgal polyunsaturated fatty acid (PUFA) synthase revealed that small RNA (sRNA)-mediated silencing, combined with the use of repetitive regulatory elements, led to aggressive transposon-like silencing of canola-biased PUFA synthase transgenes. Diversifying regulatory sequences and using native microalgal coding sequences (CDSs) with higher GC content improved transgene expression and resulted in a remarkable trans-generational stability via reduced accumulation of sRNAs and DNA methylation. Further experiments in maize with transgenes individually expressing three crystal (Cry) proteins from Bacillus thuringiensis (Bt) tested the impact of CDS recoding using different codon bias tables. Transgenes with higher GC content exhibited increased transcript and protein accumulation. These results demonstrate that the sequence composition of transgene CDSs can directly impact silencing, providing design strategies for increasing transgene expression levels and reducing risks of heritable loss of transgene expression.

An Assessment of Surgical Experience among Obstetric and Gynaecology SpR Trainees

Changes in gynaecological practice have resulted in a significant reduction in surgical exposure for trainees. We have attempted to assess surgical experience among obstetric and gynaecology SpR's in Ireland using an anonymous on-line questionnaire. Trainees were asked to assess their own ability to perform a variety of general gynaecological procedures. There was a 97% response rate (29/33 trainees). There were 11 trainees who were in the final or penultimate year of the scheme. This group were analysed separately to assess competency rates in those approaching the end of the scheme. They were subdivided in to those who have completed one year in a general hospital doing pure gynaecology and those who have not. Approximately half of this group (6/11) had completed a pure gynaecology year. All of these trainees deemed themselves competent to perform all general gynaecological procedures listed, with the exception of trans-urethral tape procedures, for which 3/6 reported the requirement of direct supervision. Only 2/6 deemed themselves competent to perform a total laparoscopic hysterectomy. Year 4/5 trainees who had not completed a pure gynaecology year displayed significantly lower competency rates for most of the procedures. With the current changes in gynaecological practice, these results highlight the importance of dedicated gynaecological surgical training.

Early stage hit triage for plant chemical genetic screens and target site identification

The increasing use of plant biological screens of large compound libraries to discover informative chemical probes for plant chemical genetics requires efficient methods for hit selection and advancement. Downstream target identification and validation studies with selected chemistries can also be resource-intensive and have a significant failure rate. Several steps and considerations for early stage hit triage are outlined to increase the probability of success that downstream studies with the chemical probe will be robust and productive, especially for target site discovery. Conversely, problematic compounds can be shelved or avoided entirely, saving time and resources. These steps include assessment of compound availability, purity, stability and solubility; determination of the biological dose-response; early and iterative evaluation of analogs; avoidance of promiscuous "frequent-hitters"; consideration of physicochemical parameters affecting compound bioavailability and mobility, use of "low-barrier" biological testing systems; and assessing the potential for compound metabolism or bioconversion.

Metabolic engineering of seeds can achieve levels of omega-7 fatty acids comparable with the highest levels found in natural plant sources

Plant oils containing ω-7 fatty acids (FAs; palmitoleic 16:1Δ(9) and cis-vaccenic 18:1Δ(11)) have potential as sustainable feedstocks for producing industrially important octene via metathesis chemistry. Engineering plants to produce seeds that accumulate high levels of any unusual FA has been an elusive goal. We achieved high levels of ω-7 FA accumulation by systematic metabolic engineering of Arabidopsis (Arabidopsis thaliana). A plastidial 16:0-ACP desaturase has been engineered to convert 16:0 to 16:1Δ(9) with specificity >100-fold than that of naturally occurring paralogs, such as that from cat's claw vine (Doxantha unguis-cati). Expressing this engineered enzyme (Com25) in seeds increased ω-7 FA accumulation from <2% to 14%. Reducing competition for 16:0-ACP by down-regulating the β-ketoacyl-ACP synthase II 16:0 elongase further increased accumulation of ω-7 FA to 56%. The level of 16:0 exiting the plastid without desaturation also increased to 21%. Coexpression of a pair of fungal 16:0 desaturases in the cytosol reduced the 16:0 level to 11% and increased ω-7 FA to as much as 71%, equivalent to levels found in Doxantha seeds.

Robust crop resistance to broadleaf and grass herbicides provided by aryloxyalkanoate dioxygenase transgenes

Engineered glyphosate resistance is the most widely adopted genetically modified trait in agriculture, gaining widespread acceptance by providing a simple robust weed control system. However, extensive and sustained use of glyphosate as a sole weed control mechanism has led to field selection for glyphosate-resistant weeds and has induced significant population shifts to weeds with inherent tolerance to glyphosate. Additional weed control mechanisms that can complement glyphosate-resistant crops are, therefore, urgently needed. 2,4-dichlorophenoxyacetic acid (2,4-D) is an effective low-cost, broad-spectrum herbicide that controls many of the weeds developing resistance to glyphosate. We investigated the substrate preferences of bacterial aryloxyalkanoate dioxygenase enzymes (AADs) that can effectively degrade 2,4-D and have found that some members of this class can act on other widely used herbicides in addition to their activity on 2,4-D. AAD-1 cleaves the aryloxyphenoxypropionate family of grass-active herbicides, and AAD-12 acts on pyridyloxyacetate auxin herbicides such as triclopyr and fluroxypyr. Maize plants transformed with an AAD-1 gene showed robust crop resistance to aryloxyphenoxypropionate herbicides over four generations and were also not injured by 2,4-D applications at any growth stage. Arabidopsis plants expressing AAD-12 were resistant to 2,4-D as well as triclopyr and fluroxypyr, and transgenic soybean plants expressing AAD-12 maintained field resistance to 2,4-D over five generations. These results show that single AAD transgenes can provide simultaneous resistance to a broad repertoire of agronomically important classes of herbicides, including 2,4-D, with utility in both monocot and dicot crops. These transgenes can help preserve the productivity and environmental benefits of herbicide-resistant crops.

The emerging field of chemical genetics: potential applications for pesticide discovery

The use of small molecules to probe biological systems, generally described as 'chemical genetics', has grown considerably in the past 7 years, especially in areas related to human biology and therapeutics. This review describes some aspects of chemical genetics technologies that can be usefully applied to pesticide target discovery and lead generation. The chemical genetics approach (consisting of a phenotype screen, a chemical library and a robust target identification methodology) is compared with conventional and target-based screening. The outcomes of a chemical genetics approach are novel protein targets coupled with in vivo-active chemical ligands. The 'chemistry-first' paradigm of the chemical genetics approach can circumvent some of the obstacles that have emerged for the exploitation of novel but chemically unvalidated targets identified from genetic or genomic screens. Some of the advantages and challenges in using chemical genetics approaches are reviewed.

Chemical genetic identification of glutamine phosphoribosylpyrophosphate amidotransferase as the target for a novel bleaching herbicide in Arabidopsis

A novel phenyltriazole acetic acid compound (DAS734) produced bleaching of new growth on a variety of dicotyledonous weeds and was a potent inhibitor of Arabidopsis (Arabidopsis thaliana) seedling growth. The phytotoxic effects of DAS734 on Arabidopsis were completely alleviated by addition of adenine to the growth media. A screen of ethylmethanesulfonate-mutagenized Arabidopsis seedlings recovered seven lines with resistance levels to DAS734 ranging from 5- to 125-fold. Genetic tests determined that all the resistance mutations were dominant and allelic. One mutation was mapped to an interval on chromosome 4 containing At4g34740, which encodes an isoform of glutamine phosphoribosylamidotransferase (AtGPRAT2), the first enzyme of the purine biosynthetic pathway. Sequencing of At4g34740 from the resistant lines showed that all seven contained mutations producing changes in the encoded polypeptide sequence. Two lines with the highest level of resistance (125-fold) contained the mutation R264K. The wild-type and mutant AtGPRAT2 enzymes were cloned and functionally overexpressed in Escherichia coli. Assays of the recombinant enzyme showed that DAS734 was a potent, slow-binding inhibitor of the wild-type enzyme (I(50) approximately 0.2 microm), whereas the mutant enzyme R264K was not significantly inhibited by 200 microm DAS734. Another GPRAT isoform in Arabidopsis, AtGPRAT3, was also inhibited by DAS734. This combination of chemical, genetic, and biochemical evidence indicates that the phytotoxicity of DAS734 arises from direct inhibition of GPRAT and establishes its utility as a new and specific chemical genetic probe of plant purine biosynthesis. The effects of this novel GPRAT inhibitor are compared to the phenotypes of known AtGPRAT genetic mutants.

Mutations in an auxin receptor homolog AFB5 and in SGT1b confer resistance to synthetic picolinate auxins and not to 2,4-dichlorophenoxyacetic acid or indole-3-acetic acid in Arabidopsis

Although a wide range of structurally diverse small molecules can act as auxins, it is unclear whether all of these compounds act via the same mechanisms that have been characterized for 2,4-dichlorophenoxyacetic acid (2,4-D) and indole-3-acetic acid (IAA). To address this question, we used a novel member of the picolinate class of synthetic auxins that is structurally distinct from 2,4-D to screen for Arabidopsis (Arabidopsis thaliana) mutants that show chemically selective auxin resistance. We identified seven alleles at two distinct genetic loci that conferred significant resistance to picolinate auxins such as picloram, yet had minimal cross-resistance to 2,4-D or IAA. Double mutants had the same level and selectivity of resistance as single mutants. The sites of the mutations were identified by positional mapping as At4g11260 and At5g49980. At5g49980 is previously uncharacterized and encodes auxin signaling F-box protein 5, one of five homologs of TIR1 in the Arabidopsis genome. TIR1 is the recognition component of the Skp1-cullin-F-box complex associated with the ubiquitin-proteasome pathway involved in auxin signaling and has recently been shown to be a receptor for IAA and 2,4-D. At4g11260 encodes the tetratricopeptide protein SGT1b that has also been associated with Skp1-cullin-F-box-mediated ubiquitination in auxin signaling and other pathways. Complementation of mutant lines with their corresponding wild-type genes restored picolinate auxin sensitivity. These results show that chemical specificity in auxin signaling can be conferred by upstream components of the auxin response pathway. They also demonstrate the utility of genetic screens using structurally diverse chemistries to uncover novel pathway components.

Vulval cancer, 1992-2002 audit--multidisciplinary care and prognostic factors

Vulval carcinoma is becoming increasingly common. Thirty-four cases of vulval carcinoma were treated from 01/01/1992-31/12/2002. The mean age was 67, range (18-90). The presenting complaints were "a lump" (76%)(25/33), "itch" (49%)(16/33), "discomfort" (30%)(10/33) and postmenopausal bleeding (21%)(7/33). Most patients presented with stage 1 or 2 disease (73%) (n = 24/33). The majority (97%) (32/33) underwent surgical treatment. Five-year survival was 61% (17/28), (disease-free survival 76% (13/17)). There were 12 cases of local/regional recurrence. Survival rates deteriorated with stage of disease. Lymph-node results, lowered survival from 79% (11/14), if negative, to 17% (1/6) if positive. Age >70 reduced survival from 69% (11/16) to 50% (6/12). We conclude that age, the stage of disease, and lymph-node status were important prognostic factors. The favourable outcomes reflect muItidisciplinary care--combining clinical examinations with regular home contact with specialist nurses, by telephone.

Screening for Chlamydia trachomatis in asymptomatic women attending outpatient clinics in a large maternity hospital in Dublin, Ireland

BACKGROUND

Chlamydia trachomatis can cause a sexually transmitted infection, which, untreated, may result in considerable morbidity.

METHODS

A prevalence study was conducted for C trachomatis using nucleic acid amplification technology in asymptomatic women, and certain risk factors that may be used to direct future screening strategies were assessed.

RESULTS

The study population comprised 945 asymptomatic women, of whom 783 were attending antenatal clinics, 91 were attending infertility clinics and 71 were attending family planning clinics. An overall C trachomatis prevalence of 3.7% (35/945) was found, with the highest prevalence of 11.2% (22/196) in Irish single women aged <25 years. Logistic regression analysis showed that single status and age <25 years were independent, statistically significant predictors of C trachomatis infection.

CONCLUSION

These results support routine screening of asymptomatic women who are sexually active and aged <25 years. An opportunist active screening of all sexually active women independent of age should be additionally considered if resources permit.

Structural basis for herbicidal inhibitor selectivity revealed by comparison of crystal structures of plant and mammalian 4-hydroxyphenylpyruvate dioxygenases

A high degree of selectivity toward the target site of the pest organism is a desirable attribute for new safer agrochemicals. To assist in the design of novel herbicides, we determined the crystal structures of the herbicidal target enzyme 4-hydroxyphenylpyruvate dioxygenase (HPPD; EC 1.13.11.27) from the plant Arabidopsis thaliana with and without an herbicidal benzoylpyrazole inhibitor that potently inhibits both plant and mammalian HPPDs. We also determined the structure of a mammalian (rat) HPPD in complex with the same nonselective inhibitor. From a screening campaign of over 1000 HPPD inhibitors, six highly plant-selective inhibitors were found. One of these had remarkable (>1600-fold) selectivity toward the plant enzyme and was cocrystallized with Arabidopsis HPPD. Detailed comparisons of the plant and mammalian HPPD-ligand structures suggest a structural basis for the high degree of plant selectivity of certain HPPD inhibitors and point to design strategies to obtain potent and selective inhibitors of plant HPPD as agrochemical leads.

Characterization of plant beta-ureidopropionase and functional overexpression in Escherichia coli

Pyrimidine bases are rapidly catabolized in growing plant tissues. The final enzyme of the catabolic pathway, beta-ureidopropionase (beta-UP; EC 3.5.1.6), was partially purified from the shoots of etiolated maize (Zea mays) seedlings. The enzyme had a K(m) for beta-ureidopropionate (the substrate derived from uracil) of 11 microM. Only one enantiomer of racemic beta-ureidoisobutyrate (derived from thymine) was processed with a K(m) of 6 microM. The enzyme was inactivated by dialysis against 1,10-phenanthroline and activity could be partially restored by addition of Zn(2+). Maize beta-UP was very sensitive to inactivation by iodoacetamide. This could be prevented by addition of substrate, indicating the presence of an active site Cys. The enzyme was strongly inhibited by short chain aliphatic acids and aryl propionates, the most potent inhibitor of which was 2-(2, 6-dinitrophenoxy)-propionate (I(50) = 0.5 microM). A gene for Arabidopsis beta-UP encodes a polypeptide of 405 amino acids and has about 55% homology with the enzymes from other eukaryotic organisms. Several highly conserved residues link the plant beta-UP with a larger class of prokaryotic and eukaryotic amidohydrolases. An Arabidopsis cDNA truncated at the N terminus by 14 residues was cloned and overexpressed in Escherichia coli. The recombinant enzyme (43.7 kD) was soluble, functional, and purified to homogeneity with yields of 15 to 20 mg per 30 g fresh weight of E. coli cells. The recombinant enzyme from Arabidopsis and the native enzyme from maize had molecular masses of approximately 440 kD, indicating the enzyme is a decamer at pH 7.

Maize ribosome-inactivating protein (b-32). Homologs in related species, effects on maize ribosomes, and modulation of activity by pro-peptide deletions

The ribosome-inactivating protein (RIP) from maize (Zea mays L.) is unusual in that it is produced in the endosperm as an inactive pro-form, also known as b-32, which can be converted by limited proteolysis to a two-chain active form, alpha beta RIP. Immunological analysis of seed extracts from a variety of species related to maize showed that pro/alpha beta forms of RIP are not unique to maize but are also found in other members of the Panicoideae, including Tripsacum and sorghum. Ribosomes isolated from maize were quite resistant to both purified pro- and alpha beta maize RIPs, whereas they were highly susceptible to the RIP from pokeweed. This suggests that the production of an inactive pro-RIP is not a mechanism to protect the plant's own ribosomes from deleterious action of the alpha beta RIP. RIP derivatives with various pro-segments removed were expressed at high levels in Escherichia coli. Measurement of their activity before and after treatment with subtilisin Carlsberg clearly identified the 25-amino acid intradomain insertion, rather than the N- or C-terminal extensions, as the major element responsible for suppression of enzymatic activity. A RIP with all three processed regions deleted had activity close to that of the native alpha beta form.

Proteolysis of the 85-kilodalton crystalline cysteine proteinase inhibitor from potato releases functional cystatin domains

The protein crystals found in potato (Solanum tuberosum L.) tuber cells consist of a single 85-kD polypeptide. This polypeptide is an inhibitor of papain and other cysteine proteinases and is capable of binding several proteinase molecules simultaneously (P. Rodis, J.E. Hoff [1984] Plant Physiol 74: 907-911). We have characterized this unusual inhibitor in more detail. Titrations of papain activity with the potato papain inhibitor showed that there are eight papain binding sites per inhibitor molecule. The inhibition constant (Ki) value for papain inhibition was 0.1 nM. Treatment of the inhibitor with trypsin resulted in fragmentation of the 85-kD polypeptide into a 32-kD polypeptide and five 10-kD polypeptides. The 32-kD and 10-kD fragments all retained the ability to potently inhibit papain (Ki values against papain were 0.5 and 0.7 nM, respectively) and the molar stoichiometries of papain binding were 2 to 3:1 and 1:1, respectively. Other nonspecific proteinases such as chymotrypsin, subtilisin Carlsberg, thermolysin, and proteinase K also cleaved the 85-kD inhibitor polypeptide into functional 22-kD and several 10-kD fragments. The fragments obtained by digestion of the potato papain inhibitor with trypsin were purified by reverse-phase high-performance liquid chromatography, and the N-terminal amino acid sequence was obtained for each fragment. Comparison of these sequences showed that the fragments shared a high degree of homology but were not identical. The sequences were homologous to the N termini of members of the cystatin superfamily of cysteine proteinase inhibitors. Therefore, the inhibitor appears to comprise eight tandem cystatin domains linked by preteolytically sensitive junctions. We have called the inhibitor potato multicystatin (PMC). By immunoblot analysis and measurement of papain inhibitory activity, PMC was found at high levels in potato leaves (up to 0.6 microgram/g fresh weight tissue), where it accumulated under conditions that induce the accumulation of other proteinase inhibitors linked to plant defense. PMC may have a similar defensive role, for example in protecting the plant from phytophagous insects that utilize cysteine proteinases for dietary protein digestion.

Characterization of a genomic sequence coding for potato multicystatin, an eight-domain cysteine proteinase inhibitor

A gene coding for potato multicystatin (PMC), the crystalline inhibitor of cysteine proteases which is found in tubers, was isolated and characterized. The deduced polypeptide product of this genomic sequence is 757 amino acids long and has a molecular mass of 86,778 Da. It consists exclusively of eight closely related domains, with 53-89% identity of residues. Each repeated unit is homologous to the cystatin superfamily of cysteine protease inhibitors. To date, no other member of this family has been found to contain so many inhibitor domains in one polypeptide. Eight introns are proposed in the 3.5 kb of genomic DNA coding for PMC, one in each cystatin unit. There is a family of 4 to 6 such large genes in potato, while in pea and maize the homologues are much smaller, and probably code for single-domain cystatins. PMC transcripts are abundant in tubers, but scarce in undamaged leaves or stems of field-grown potatoes. The tuber messages are derived from at least four genes (including the cloned example). The pattern of gene expression, as well as the properties of the protein, suggest that PMC has a role in the plant's defense system.

Characterization and molecular cloning of a proenzyme form of a ribosome-inactivating protein from maize. Novel mechanism of proenzyme activation by proteolytic removal of a 2.8-kilodalton internal peptide segment

Ribosome-inactivating proteins (RIPs) are a widely distributed family of plant enzymes that are remarkably potent catalytic inactivators of eukaryotic protein synthesis. All RIPs described to date, including the A-chain of the plant cytotoxin ricin, are polypeptides of 25-32 kDa and share significant amino acid sequence homologies. We have characterized and cloned an RIP from maize (Zea mays). In contrast to previously described RIPs, we have found that maize RIP is synthesized and stored in the kernel as a 34-kDa inactive precursor (isoelectric point = 6.5). During germination, this neutral precursor is converted into a basic, active form (isoelectric point greater than 9) by limited proteolysis, which removes 25 amino acids (2.8 kDa) of net charge -6 from the center of the polypeptide chain. Additional processing also occurs at the amino and carboxyl termini of the polypeptide. The sequence of the internal processed region is unique and it is equivalent to an insertion centered around Thr-156 in the amino acid sequence of ricin toxin A-chain, i.e. in the center of the enzymatically active domain. The generation of an active enzyme by removal of a large amino acid segment from the middle of a precursor polypeptide chain represents a novel mechanism of proenzyme activation that is distinct from more conventional activation mechanisms involving NH2-terminal proteolytic processing. A two-chain active RIP (comprised of 16.5- and 8.5-kDa fragments that remain tightly associated) is produced from this processing event.

Two kunitz-type proteinase inhibitors from potato tubers

Two proteinase inhibitors have been isolated from tubers of potato (Solanum tuberosum). Based on N-terminal amino acid sequence homologies, they are members of the Kunitz family of proteinase inhibitors. Potato Kunitz inhibitor-1 (molecular weight 19,500, isoelectric point 6.9) is a potent inhibitor of the animal pancreatic proteinase trypsin, and its amino terminus has significant homology to a recently characterized cathepsin D Kunitz inhibitor from potato tubers (Mares et al. [1989] FEBS Lett 251:94-98). Potato Kunitz inhibitor-2 (molecular weight 20,500, isoelectric point 8.6) is an inhibitor of the microbial proteinase subtilisin Carlsberg; its amino terminus is almost identical to an abundant 22 kilodalton protein from potato tubers (Suh et al. [1990] Plant Physiol 94:40-45) and has significant homology to other Kunitz-type subtilisin inhibitors from small grains. Both Kunitz inhibitors are abundant proteins of the cortex of potato tubers.

Rapid reaction studies on the oxygenation reactions of catechol dioxygenase

The reaction of oxygen with catechol 1,2-dioxygenase from Pseudomonas arvilla ATCC 23974 in complex with catechol, 4-methylcatechol, and 4-fluorocatechol has been studied using single turnover stopped flow spectrophotometry. Two sequential enzyme intermediates have been resolved and their visible spectra characterized by computer-assisted methods. These intermediates are spectrally similar to those observed in a similar study with protocatechuate dioxygenase (Bull, C., Ballou, D. P., and Otsuka, S. J. Biol. Chem. 256, 12681-12686 (1981), although the first intermediate seen with the latter enzyme was not observed in this study. The rate of formation of intermediate I is oxygen-dependent and also accelerated by electron-donating substituents on the C-4 of the substrate. This is consistent with the proposed substrate reduction of dioxygen to form a hydroperoxide. Intermediate I is thus suggested to be a 6-hydroperoxycyclohexa-3,5-diene-1-one. The decay of intermediate I is also accelerated by electron donors and is consistent with the rearrangement of intermediate hydroperoxide via an acyl migration mechanism. It is inconsistent with mechanisms involving nucleophilic attack at the carbonyl carbon. Intermediate II is proposed to be an enzyme-product complex based on the resemblance of its visible spectra to those of the benzoate complex of catechol 1,2-dioxygenase and enzyme-product complexes of protocatechuate dioxygenase. Careful 18O2-labeling experiments have shown that no label is lost to the solvent, implying that no free hydroxide forms during catalysis.

Halogenated protocatechuates as substrates for protocatechuate dioxygenase from Pseudomonas cepacia

Substrates containing electron-withdrawing groups were reacted with protocatechuate 3,4-dioxygenase and oxygen. Haloprotocatechuates (5-fluoro-, 5-chloro-, 5-bromo-, 2-chloro-, and 6-chloroprotocatechuates) are oxygenated by the enzyme at rates 28- to 3000-fold lower than that with the native substrate. These lower rates are due to both deactivation of substrate to O2 attack, and to the formation of abortive enzyme-substrate (ES) complexes. Such ES complexes with haloprotocatechuates are spectrally distinct from the normal ES complex. 6-Chloroprotocatechuate produces changes more like those due to protocatechuate. The abortive ES complexes, when rapidly mixed with oxygen, decay to free enzyme and product monophasically, and the dependence of the rates on O2 concentration shows that a rate-limiting step precedes reaction with O2. Thus these complexes are rather unreactive toward O2, and the rate-limiting step in oxygenation is their conversion to active complexes. In contrast, the reaction of O2 with the enzyme and 6-chloroprotocatechuate is biphasic, the first phase being dependent on O2 concentration (2 X 10(4) M-1 S-1) and the second not (7 S-1). The intermediate formed after the first phase strongly resembles the second intermediate seen in the reaction of enzyme with protocatechuate and O2 (Bull, C., Ballou, D. P., and Otsuka, S., (1981) J. Biol. Chem. 256, 12681-12686), implying that the electron-withdrawing effect of the chlorine slows the O2 addition step considerably while the conversion to the second intermediate is hardly affected. When the enzyme cycles through several turnovers with 6-chloroprotocatechuate, an enzyme species is formed that resembles the unreactive ES complexes seen with the other haloprotocatechuates, indicating that a small amount of the unreactive complex is in equilibrium with the reactive complex and that during successive turnovers the enzyme is slowly converted into the unreactive form. The formation of this form correlates with the observation that in assays the rate of product formation gradually decreases with time.

The characterization and magnetic properties of the azide and imidazole derivatives of Pseudomonas nitrite reductase

Optical absorption, mcd, and epr spectroscopy have been used to characterize the azide and imidazole derivatives of oxidized Pseudomonas nitrite reductase. At pH 7.0 azide binds solely to heme d1 with an affinity constant, Kaff = 360 M-1, whereas imidazole binds to both hemes c and d1 with kaff = 35 and 55 M-1, respectively. Low-temperature mcd and epr spectroscopy indicate that c and d1 are low-spin ferrihemes in both derivatives, although the epr of the heme d1-azide component is very weak and requires explanation. Attempts to obtain a high-spin heme d1 in the intact enzyme using the weak field ligands fluoride and thiocyanate have proved unsuccessful. Electron paramagnetic resonance experiments involving an oxidized enzyme derivatives in which heme d1 is complexed by NO, and hence epr silent, have enabled unambiguous assignment of the epr spectrum of Pseudomonas nitrite reductase.

Studies on heme d1 extracted from Pseudomonas aeruginosa nitrite reductase

Heme d1 has been extracted from Pseudomonas nitrite reductase. Imidazole, cyanide, and chloride-ferroheme, and CO, NO, cyanide, imidazole, and pyridine-ferroheme complexes have been prepared for study by UV/vis spectroscopy, and in some cass by epr and low-temperature mcd as well. Iron determinations have been carried out to assess extinction coefficients. Absorption spectra were used to monitor the transition of chloride-ferriheme d1 to an alkaline form of ferriheme d1 and a pka of 6.5 was determined for the process. The epr spectrum of chloride-ferriheme possessed the characteristic g = 6 signal of high spin (S = 5/2) iron, but the alkaline-ferriheme form gave no detectable epr signals. Electron paramagnetic resonance spectra were also obtained for cyanide and imidazole-ferriheme d1 and for NO-ferroheme d1. The imidazole complex gave signals that were very weak in comparison with the cyanide complex, but mcd measurements of imidazole-ferriheme d1 were consistent with it being a low-spin (S = 1/2) system. The epr signals of NO-ferroheme d1 were similar to those of the corresponding holo-enzyme complex. Reduction of alkaline-ferriheme d1 was found to be affected by the presence of oxygen, but under N2 give the same result with ascorbate and dithionite. Autoreduction of alkaline-ferriheme d1 was observed when placed under CO, and NO, atmospheres, or when treated with pyridine.

Electron paramagnetic resonance studies on Pseudomonas nitrosyl nitrite reductase. Evidence for multiple species in the electron paramagnetic resonance spectra of nitrosyl haemoproteins

The e.p.r. spectra of reduced 14NO- and 15NO-bound Pseudomonas nitrite reductase have been investigated at pH 5.8 and 8.0 in four buffer systems. At pH 8.0, absorption spectra indicated that only the haem d1 was NO-bound, but, although quantification of the e.p.r. signals in all cases accounted for NO bound the the haem d1 in both subunits of the enzyme, the precise form of the signals varied with buffer and temperature. A rhombic species, with gx = 2.07, gz = 2.01 and gy = 1.96, represented in the low-temperature spectra seen in all the buffers was converted at high temperatures (approx. 200K) into a form showing a reduced anisotropy. Hyperfine splitting on the gz component of this rhombic signal indicated a nitrogen atom trans to NO and it is proposed that histidine provides the endogenous axial ligand for haem d1. At pH 5.8, absorption spectra indicated NO binding to both haems c and d1 and e.p.r. quantifications accounted for NO-bound haems c and d1 in both enzyme subunits. The e.p.r. spectra at pH 5.8 were generally similar to those at pH 8.0 with respect to g-values and hyperfine coupling constants, but were broader with less well defined hyperfine splittings. As at pH 8, rhombic signals present in spectra at low temperatures were converted to less anisotropic forms at high temperatures. The results are discussed in relation to work on model nitrosyl-protohaem complexes [Yoshimura, Ozaki, Shintani & Watanabe (1979) Arch. Biochem, Biophys. 193, 301-313]. No. e.p.r. signal was observed from oxidized NO-bound Pseudomonas nitrite reductase at pH 6.0, over the temperature range 6-100K.

A re-evaluation of some basic structural and functional properties of Pseudomonas cytochrome oxidase

Determinations of iron content and dry-weight measurements on samples of Pseudomonas cytochrome oxidase were coupled with sodium dodecyl sulphate/polyacrylamide-gel-electrophoresis studies of both the native protein and covalently cross-linked oligomers in order to estimate the enzyme's molecular weight and spectral absorption coefficients. A value of epsilon(ox.) (410)=282x10(3) litre.mol(-1).cm(-1) was calculated for a dimeric protein molecule having a total molecular weight of 122000 (based on iron analysis). Steady-state kinetic observations of the enzyme-catalysed oxidation of reduced azurin by nitrite indicated a marked increase in enzyme inactivation as the pH was raised from 5.7 to 7.2. Since NO, a product of the nitrite reductase activity of Pseudomonas cytochrome oxidase, is known to bind to the enzyme, a study was undertaken to try to assess the potential of NO as a product inhibitor. Investigations showed that samples of the oxidized protein at pH values 4, 5 and 6 bound NO to both haem c and d(1) components, but oxidized enzyme samples at pH7 and above formed their reduced ligand-bound forms when placed under an atmosphere of the gas. Ascorbate-reduced enzyme samples at pH4, 5, 6 and 7 were also found to bind NO at both haem components, although at pH7 the rate of haem c binding was very slow. At pH8 and 9 only the ferrohaem d(1) bound NO. Titration experiments on the reduced protein over the pH range 5-7, with nitrite as a precursor of NO, showed that the haem d(1) had a much higher affinity than the haem c: experiments at pH5.2 and 5.9 with NO-equilibrated solutions revealed the same pattern of behaviour with the oxidized enzyme.

Some magnetic properties of Pseudomonas cytochrome oxidase

The magnetic properties of the haem groups of Pseudomonas cytochrome oxidase and its cyanide-bound derivatives were studied in both the oxidized and reduced states by means of m.c.d. (magnetic circular dichroism) at low temperatures. In addition, the oxidized forms of the enzyme were also investigated by e.p.r. (electron-paramagnetic-resonance) spectroscopy, and a parallel study, using both e.p.r. and m.c.d., was made on Pseudomonas cytochrome c-551 to aid spectral assignments. For ascorbate-reduced Pseudomonas cytochrome oxidase, the temperature-independence of those features in the m.c.d. spectrum corresponding to the haem c, and the temperature-dependence of those signals corresponding to the haem d1, showed the former to be low-spin and the latter to be high-spin (s = 2). However, addition of cyanide to the reduced enzyme gave a form of the protein that was completely low-spin. The e.p.r. and m.c.d. sectra of oxidized Pseudomonas cytochrome oxidase and its cyanide derivative were consistent with the haem c and d1 components being low-spin in both cases. Pseudomonas cytochrome c-551 was found to be low-spin in both its oxidized and reduced redox states.