Structure of the horseradish peroxidase isozyme C genes

Construction of the SHP-GLOX lignin regulation system and its application in rice straw

BACKGROUND

There is great productivity of rice(Oryza sativa L. spp. japonica) straw in China, which is a potential source of biomass for biofuel and forage. However, the high levels of lignins in rice straw limited its usage and induced the formation of agricultural waste. In order to modify the lignins contents to improve biofuel production and forage digestibility, we selected Soybean hull peroxidase (SHP) and Glyoxal oxidase (GLOX) as candidate genes to improve quality of rice straw. SHP, a class III plant peroxidase, is derived from multiple sources. It has several advantages, such as high resistance to heat, high stability under acidic and alkaline conditions, and a broad substrate range. SHP is speculated to be useful for lignin degradation. Glyoxal oxidase (GLOX) is an extracellular oxidase that can oxidize glyoxal and methylglyoxal in the extracellular medium to generate H₂O₂.

RESULTS

In the present study, the SHP and GLOX genes in pCAMBIA3301-glycine-rich protein (GRP)-SHP-GLOX, designated the K167 vector, were optimized and introduced into rice embryos using Agrobacterium-mediated transformation. Positive transgenic rice embryos were examined using molecular, physiological, biochemical and fermentation tests. The outcomes suggested that SHP degraded lignin effectively.

CONCLUSIONS

This research has created a rice breeding material with normal growth and yield but stalks that are more amenable to degradation in the later stage for use in breeding rice varieties whose stalks are easily used for energy. Our results will improve the industrial and commercial applications of rice straw.

A simple, real-time assay of horseradish peroxidase using biolayer interferometry

Horseradish peroxidase (HRP) isoenzyme C1a is one of the most widely used enzymes for various analytical methods in bioscience research and medical fields. In these fields, real-time monitoring of HRP activity is highly desirable because the utility of HRP as a reporter enzyme would be expanded. In this study, we developed a simple assay system enabling real-time monitoring of HRP activity by using biolayer interferometry (BLI). The HRP activity was quantitatively detected on a BLI sensor chip by tracing a binding response of tyramide, a substrate of HRP, onto an immobilized protein. This system could be applied to analyses related to oxidase activity, as well as to the functional analysis of recombinant HRP.

Rational metabolic-flow switching for the production of exogenous secondary metabolites in bamboo suspension cells

The synthetic biology-driven production of high-value plant secondary metabolites in microbial hosts has attracted extensive attention despite various challenges, including correct protein expression and limited supplies of starting materials. In contrast, plant cell cultures are rarely used for this purpose owing to their slow proliferation rates and laborious transformation processes. Here, we propose a "rational metabolic-flow switching" strategy to efficiently produce exogenous secondary metabolites using suspension-cultured bamboo (Phyllostachys nigra; Pn) cells as model production hosts. The Pn cells biosynthesise hydroxycinnamic acid amides (HCAAs) of putrescine as major secondary metabolites, which indicates that the phenylpropanoid and polyamine biosynthetic pathways are highly active and that the Pn cells may produce alternative secondary metabolites derived from those pathways. Stable transformants of Pn cells expressing agmatine coumaroyltransferase of barley (Hordeum vulgare) were generated with the expectation of metabolic-flow switching from HCAAs of putrescine to those of agmatine. In the recombinant Pn cells, the levels of HCAAs of putrescine decreased and the HCAAs of agmatine were produced instead. The production titre of the major product, p-coumaroylagmatine, reached approximately 360 mg/L, providing a proof-of-concept for the usefulness of "rational metabolic-flow switching" in synthetic biology using plant cell hosts.

Transient Expression and Purification of Horseradish Peroxidase C in Nicotiana benthamiana

Horseradish peroxidase (HRP) is a commercially important reagent enzyme used in molecular biology and in the diagnostic product industry. It is typically purified from the roots of the horseradish (Armoracia rusticana); however, this crop is only available seasonally, yields are variable and often low, and the product is a mixture of isoenzymes. Engineering high-level expression in transiently transformed tobacco may offer a solution to these problems. In this study, a synthetic Nicotiana benthamiana codon-adapted full-length HRP isoenzyme gene as well as C-terminally truncated and both N- and C-terminally truncated versions of the HRP C gene were synthesized, and their expression in N. benthamiana was evaluated using an Agrobacterium tumefaciens-mediated transient expression system. The influence on HRP C expression levels of co-infiltration with a silencing suppressor (NSs) construct was also evaluated. Highest HRP C levels were consistently obtained using either the full length or C-terminally truncated HRP C constructs. HRP C purification by ion exchange chromatography gave an overall yield of 54% with a Reinheitszahl value of >3 and a specific activity of 458 U/mg. The high level of HRP C production in N. benthamiana in just five days offers an alternative, viable, and scalable system for production of this commercially significant enzyme.

Cellulase from Trichoderma harzianum interacts with roots and triggers induced systemic resistance to foliar disease in maize

Trichoderma harzianum is well known to exhibit induced systemic resistance (ISR) to Curvularia leaf spot. We previously reported that a C6 zinc finger protein (Thc6) is responsible for a major contribution to the ISR to the leaf disease, but the types of effectors and the signals mediated by Thc6 from Trichoderma are unclear. In this work, we demonstrated that two hydrolases, Thph1 and Thph2, from T. harzianum were regulated by Thc6. Furthermore, an electrophoretic mobility shift assay (EMSA) study revealed that Thc6 regulated mRNA expression by binding to GGCTAA and GGCTAAA in the promoters of the Thph1 and Thph2 genes, respectively. Moreover, the Thph1 and Thph2 proteins triggered the transient production of reactive oxygen species (ROS) and elevated the free cytosolic calcium levels in maize leaf. Furthermore, the genes related to the jasmonate/ethylene signaling pathway were up-regulated in the wild-type maize strain. However, the ΔThph1- or ΔThph2-deletion mutants could not activate the immune defense-related genes in maize to protect against leaf disease. Therefore, we conclude that functional Thph1 and Thph2 may be required in T. harzianum to activate ISR in maize.

Ultra-high-throughput screening of an in vitro-synthesized horseradish peroxidase displayed on microbeads using cell sorter

The C1a isoenzyme of horseradish peroxidase (HRP) is an industrially important heme-containing enzyme that utilizes hydrogen peroxide to oxidize a wide variety of inorganic and organic compounds for practical applications, including synthesis of fine chemicals, medical diagnostics, and bioremediation. To develop a ultra-high-throughput screening system for HRP, we successfully produced active HRP in an Escherichia coli cell-free protein synthesis system, by adding disulfide bond isomerase DsbC and optimizing the concentrations of hemin and calcium ions and the temperature. The biosynthesized HRP was fused with a single-chain Cro (scCro) DNA-binding tag at its N-terminal and C-terminal sites. The addition of the scCro-tag at both ends increased the solubility of the protein. Next, HRP and its fusion proteins were successfully synthesized in a water droplet emulsion by using hexadecane as the oil phase and SunSoft No. 818SK as the surfactant. HRP fusion proteins were displayed on microbeads attached with double-stranded DNA (containing the scCro binding sequence) via scCro-DNA interactions. The activities of the immobilized HRP fusion proteins were detected with a tyramide-based fluorogenic assay using flow cytometry. Moreover, a model microbead library containing wild type hrp (WT) and inactive mutant (MUT) genes was screened using fluorescence-activated cell-sorting, thus efficiently enriching the WT gene from the 1:100 (WT:MUT) library. The technique described here could serve as a novel platform for the ultra-high-throughput discovery of more useful HRP mutants and other heme-containing peroxidases.

An updated view on horseradish peroxidases: recombinant production and biotechnological applications

Horseradish peroxidase has been the subject of scientific research for centuries. It has been used exhaustively as reporter enzyme in diagnostics and histochemistry and still plays a major role in these applications. Numerous studies have been conducted on the role of horseradish peroxidase in the plant and its catalytic mechanism. However, little progress has been made in its recombinant production. Until now, commercial preparations of horseradish peroxidase are still isolated from plant roots. These preparations are commonly mixtures of various isoenzymes of which only a small fraction has been described so far. The composition of isoenzymes in these mixed isolates is subjected to uncontrollable environmental conditions. Nowadays, horseradish peroxidase regains interest due to its broad applicability in the fields of medicine, life sciences, and biotechnology in cancer therapy, biosensor systems, bioremediation, and biocatalysis. These medically and commercially relevant applications, the recent discovery of new natural isoenzymes with different biochemical properties, as well as the challenges in recombinant production render this enzyme particularly interesting for future biotechnological solutions. Therefore, we reviewed previous studies as well as current developments with biotechnological emphasis on new applications and the major remaining biotechnological challenge—the efficient recombinant production of horseradish peroxidase enzymes.

Peroxidase gene discovery from the horseradish transcriptome

BACKGROUND

Horseradish peroxidases (HRPs) from Armoracia rusticana have long been utilized as reporters in various diagnostic assays and histochemical stainings. Regardless of their increasing importance in the field of life sciences and suggested uses in medical applications, chemical synthesis and other industrial applications, the HRP isoenzymes, their substrate specificities and enzymatic properties are poorly characterized. Due to lacking sequence information of natural isoenzymes and the low levels of HRP expression in heterologous hosts, commercially available HRP is still extracted as a mixture of isoenzymes from the roots of A. rusticana.

RESULTS

In this study, a normalized, size-selected A. rusticana transcriptome library was sequenced using 454 Titanium technology. The resulting reads were assembled into 14871 isotigs with an average length of 1133 bp. Sequence databases, ORF finding and ORF characterization were utilized to identify peroxidase genes from the 14871 isotigs generated by de novo assembly. The sequences were manually reviewed and verified with Sanger sequencing of PCR amplified genomic fragments, resulting in the discovery of 28 secretory peroxidases, 23 of them previously unknown. A total of 22 isoenzymes including allelic variants were successfully expressed in Pichia pastoris and showed peroxidase activity with at least one of the substrates tested, thus enabling their development into commercial pure isoenzymes.

CONCLUSIONS

This study demonstrates that transcriptome sequencing combined with sequence motif search is a powerful concept for the discovery and quick supply of new enzymes and isoenzymes from any plant or other eukaryotic organisms. Identification and manual verification of the sequences of 28 HRP isoenzymes do not only contribute a set of peroxidases for industrial, biological and biomedical applications, but also provide valuable information on the reliability of the approach in identifying and characterizing a large group of isoenzymes.

Purification and basic biochemical characterization of 19 recombinant plant peroxidase isoenzymes produced in Pichia pastoris

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A novel 2-step purification for recombinant HRP from P. pastrois was developed.

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Both purification steps are negative chromatography steps.

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The amount of N-glycosylation sites and the success in purification correlate.

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HRP isoenzymes differ significantly in substrate specificity and catalytic activity.

The plant enzyme horseradish peroxidase (HRP) is used in several important industrial and medical applications, of which especially biosensors and diagnostic kits describe an emerging field. Although there is an increasing demand for high amounts of pure enzyme preparations, HRP is still isolated from the plant as a mixture of different isoenzymes with different biochemical properties. Based on a recent next generation sequencing approach of the horseradish transcriptome, we produced 19 individual HRP isoenzymes recombinantly in the yeast Pichia pastoris. After optimizing a previously reported 2-step purification strategy for the recombinant isoenzyme HRP C1A by substituting an unfavorable size exclusion chromatography step with an anion exchange step using a monolithic column, we purified the 19 HRP isoenzymes with varying success. Subsequent basic biochemical characterization revealed differences in catalytic activity, substrate specificity and thermal stability of the purified HRP preparations. The preparations of the isoenzymes HRP A2A and HRP A2B were found to be highly interesting candidates for future applications in diagnostic kits with increased sensitivity.

Activity of the C-terminal-dependent vacuolar sorting signal of horseradish peroxidase C1a is enhanced by its secondary structure

Plant class III peroxidase (PRX) catalyzes the oxidation and oxidative polymerization of a variety of phenolic compounds while reducing hydrogen peroxide. PRX proteins are classified into apoplast type and vacuole type based on the absence or the presence of C-terminal propeptides, which probably function as vacuolar sorting signals (VSSs). In this study, in order to improve our understanding of vacuole-type PRX, we analyzed regulatory mechanisms of vacuolar sorting of a model vacuole-type PRX, the C1a isozyme of horseradish (Armoracia rusticana) (HRP C1a). Using cultured transgenic tobacco cells and protoplasts derived from horseradish leaves, we characterized HRP C1a's VSS, which is a 15 amino acid C-terminal propeptide (C15). We found that the C-terminal hexapeptide of C15 (C6), which is well conserved among vacuole-type PRX proteins, forms the core of the C-terminal-dependent VSS. We also found that the function of C6 is enhanced by the remaining N-terminal part of C15 which probably folds into an amphiphilic α-helix.

Post-translational modifications of the basic peroxidase isoenzyme from Zinnia elegans

The major basic peroxidase (ZePrx) from Zinnia elegans suspension cell cultures was purified and cloned. The purification resolved ZePrxs in two isoforms (ZePrx33.44 and ZePrx34.70), whose co-translational and post-translational modifications are characterized. Based on the N-terminal sequence obtained by Edman degradation of mature ZePxs, it may be expected that the immature polypeptides of ZePrxs contain a signal peptide (N-terminal pro-peptide) of 30 amino acids, which directs the polypeptide chains to the ER membrane. These immature polypeptides are co-translationally processed by proteolytic cleavage, and modeling studies of digestions suggested that the processing of the N-terminal pro-peptide of ZePrxs is performed by a peptidase from the SB clan (S8 family, subfamily A) of serine-type proteases. When the post-translational modifications of ZePrxs were characterized by trypsin digestion, and tryptic peptides were analyzed by reverse phase nano liquid chromatography (RP-nanoLC) coupled to MALDI-TOF MS, it was seen that, despite the presence in the primary structure of the protein of several (disulphide bridges, N-glycosylation, phosphorylation and N-myristoylation) potential post-translational modification sites, ZePrxs are only post-translationated modified by the formation of N-terminal pyroglutamate residues, disulphide bridges and N-glycosylation. Glycans of ZePrxs belong to three main types and conduce to the existence of at least ten different molecular isoforms. The first glycans belong to both low and high mannose-type glycans, with the growing structure Man(3-9)(GlcNAc)(2). Low mannose-type glycans, Man(3-4)(GlcNAc)(2), coexist with the truncated (paucimannosidic-type) glycan, Man(3)Xyl(1)Fuc(1)(GlcNAc)(2), in the G(3) and G(4 )sub-isoforms of ZePrx33.44. In ZePrx34.70, on the other hand, the complex-type biantennary glycan, Man(3)Xyl(1)Fuc(3)(GlcNAc)(5), and the truncated (paucimannosidic-type) glycan, Man(3)Xyl(1)Fuc(1)(GlcNAc)(2), appear to fill the two putative sites for N-glycosylation. Since the two N-glycosylation sites in ZePrxs are located in an immediately upstream loop region of helix F'' (close to the proximal histidine) and in helix F'' itself, and are flanked by positive-charged amino acids that produce an unusual positive-net surface electrostatic charge pattern, it may be expected that glycans not only affect reaction dynamics but may well participate in protein/cell wall interactions. These results emphasize the complexity of the ZePrx proteome and the difficulties involved in establishing any fine structure-function relationship.

High-efficiency secretory production of peroxidase C1a using vesicular transport engineering in transgenic tobacco

Horseradish peroxidase isozyme C1a (HRP C1a) is widely used as a reporter enzyme in a variety of detection procedures such as enzyme-linked immunosorbent assay (ELISA) and western blotting. We previously isolated the gene encoding HRP C1a and showed that HRP C1a is at first translated as a preproprotein containing propeptides at its N- and C-termini (N-terminal secretion signal peptide and C-terminal propeptide; CTPP). The signal peptide (sp) is necessary for endoplasmic reticulum (ER) translocation and the CTPP acts as a vacuolar sorting determinant. Furthermore, HRP C1a was secreted into the culture medium from cells expressing the HRP C1a gene without the CTPP region. We optimized the secretory production system of HRP C1a in tobacco plants. To determine a suitable signal peptide for high-efficient secretion of proteins, three types of sp derived from HRP C1a (C1Psp), beta-D-glucan exohydrolase (GEsp) and 38 kDa peroxidase (38Psp) were compared. GE and 38P are secretory proteins highly accumulated in the culture medium of BY2 cells. The secretion efficiency was increased by 34% and 53% when GEsp and 38Psp was used, respectively. Next, we used a translational enhancer, the 5'-untranslated region of Nicotiana tabacum alcohol dehydrogenase gene (NtADH 5'-UTR). The production of HRP C1a was increased by placing NtADH 5'UTR in front of the ORF in BY2 cells. These results indicate that the localization and expression level of recombinant proteins can be controlled by the use of propeptides and 5'UTR, respectively. Finally, high-efficiency secretory production of the HRP C1a was also achieved in transgenic tobacco.

Characterization of the last step of lignin biosynthesis in Zinnia elegans suspension cell cultures

The last step of lignin biosynthesis in Zinnia elegans suspension cell cultures (SCCs) catalyzed by peroxidase (ZePrx) has been characterized. The k(3) values shown by ZePrx for the three monolignols revealed that sinapyl alcohol was the best substrate, and were proportional to their oxido/reduction potentials, signifying that these reactions are driven exclusively by redox thermodynamic forces. Feeding experiments demonstrate that cell wall lignification in SCCs is controlled by the rate of supply of H(2)O(2). The results also showed that sites for monolignol beta-O-4 cross-coupling in cell walls may be saturated, suggesting that the growth of the lineal lignin macromolecule is not infinite.

Molecular cloning and tissue-specific transcriptional regulation of the first peroxidase family member, Udp1, in stinging nettle (Urtica dioica)

A full-length cDNA clone, designated Udp1, was isolated from Urtica dioica (stinging nettle), using a polymerase chain reaction based strategy. The putative Udp1 protein is characterized by a cleavable N-terminal signal sequence, likely responsible for the rough endoplasmic reticulum entry and a 310 amino acids mature protein, containing all the important residues, which are evolutionary conserved among different members of the plant peroxidase family. A unique structural feature of the Udp1 peroxidase is defined into the short carboxyl-terminal extension, which could be associated with the vacuolar targeting process. Udp1 peroxidase is differentially regulated at the transcriptional level and is specifically expressed in the roots. Interestingly, wounding and ultraviolet radiation stress cause an ectopic induction of the Udp1 gene expression in the aerial parts of the plant. A genomic DNA fragment encoding the Udp1 peroxidase was also cloned and fully sequenced, revealing a structural organization of three exons and two introns. The phylogenetic relationships of the Udp1 protein to the Arabidopsis thaliana peroxidase family members were also examined and, in combination with the homology modelling approach, dictated the presence of distinct structural elements, which could be specifically involved in the determination of substrate recognition and subcellular localization of the Udp1 peroxidase.

Transgenic tobacco (Nicotiana tabacum L. cv. Samsun-NN) plants over-expressing a synthetic HRP-C gene are altered in growth, development and susceptibility to abiotic stress

The physiological role of class III peroxidases (EC 1.11.1.7) in controlling plant growth and development has been investigated by over-expression of both native and heterologous peroxidases. However, it has remained an enigma as to why the phenotypes of different peroxidase over-expressing transgenics vary. In order to resolve the conflicting information about the consequences of peroxidase over-expression, we have explored the role of the subcellular targeting of HRP-C in controlling stem growth, root development, axillary branching and abiotic stress tolerance in tobacco (Nicotiana tabacum L.). Altering the sub-cellular targeting of vacuolar HRP-C, such that over-expressed peroxidase accumulates in the cytoplasm and cell wall, induced phenotypic changes that are typically associated with altered auxin homeostasis, and over-expression of cell wall located peroxidases. We conclude that sub-cellular targeting is a determinant of the phenotype of peroxidase over-expressing plants.

Cloning and molecular characterization of the basic peroxidase isoenzyme from Zinnia elegans, an enzyme involved in lignin biosynthesis

The major basic peroxidase from Zinnia elegans (ZePrx) suspension cell cultures was purified and cloned, and its properties and organ expression were characterized. The ZePrx was composed of two isoforms with a M(r) (determined by matrix-assisted laser-desorption ionization time of flight) of 34,700 (ZePrx34.70) and a M(r) of 33,440 (ZePrx33.44). Both isoforms showed absorption maxima at 403 (Soret band), 500, and 640 nm, suggesting that both are high-spin ferric secretory class III peroxidases. M(r) differences between them were due to the glycan moieties, and were confirmed from the total similarity of the N-terminal sequences (LSTTFYDTT) and by the 99.9% similarity of the tryptic fragment fingerprints obtained by reverse-phase nano-liquid chromatography. Four full-length cDNAs coding for these peroxidases were cloned. They only differ in the 5'-untranslated region. These differences probably indicate different ways in mRNA transport, stability, and regulation. According to the k(cat) and apparent K(m)(RH) values shown by both peroxidases for the three monolignols, sinapyl alcohol was the best substrate, the endwise polymerization of sinapyl alcohol by both ZePrxs yielding highly polymerized lignins with polymerization degrees > or =87. Western blots using anti-ZePrx34.70 IgGs showed that ZePrx33.44 was expressed in tracheary elements, roots, and hypocotyls, while ZePrx34.70 was only expressed in roots and young hypocotyls. None of the ZePrx isoforms was significantly expressed in either leaves or cotyledons. A neighbor-joining tree constructed for the four full-length cDNAs suggests that the four putative paralogous genes encoding the four cDNAs result from duplication of a previously duplicated ancestral gene, as may be deduced from the conserved nature and conserved position of the introns.

Differential expression of 10 sweetpotato peroxidase genes in response to bacterial pathogen, Pectobacterium chrysanthemi

To understand the function of each peroxidase (POD, EC 1.11.1.7) in terms of biotic stress, changes in POD specific activity and expression of 10 POD genes were investigated in four cultivars of sweetpotato (Ipomoea batatas) after infection with Pectobacterium chrysanthemi. POD specific activity (units mg(-1) protein) increased from 16 h after inoculation (HAI) in three varieties. POD activities of two cultivars, Shinwhangmi and White Star, reached a maximum level at 24 HAI by about three times compared to mock treatment (MT), and then decreased, whereas those of Zami and Yulmi continuously increased until 36 HAI. Native gel analysis revealed that one POD isoenzyme with a high electrophoretic mobility significantly increased in response to pathogen infection in all cultivars. Additionally, 10 POD genes displayed differential expression patterns upon bacterial infection by northern analysis. Several POD genes such as swpa2, swpa3, swpa4, swpa5, swpb1 were induced upon bacterial infection, but other genes were not. Particularly, swpa4 gene was markedly expressed in response to bacterial infection in four different cultivars, suggesting that this gene has a stress-inducible promoter. These results indicate that some specific POD isoenzymes are involved in defense in relation to pathogenesis of P. chrysanthemi in sweetpotato plants.

Horseradish peroxidase: a modern view of a classic enzyme

Horseradish peroxidase is an important heme-containing enzyme that has been studied for more than a century. In recent years new information has become available on the three-dimensional structure of the enzyme and its catalytic intermediates, mechanisms of catalysis and the function of specific amino acid residues. Site-directed mutagenesis and directed evolution techniques are now used routinely to investigate the structure and function of horseradish peroxidase and offer the opportunity to develop engineered enzymes for practical applications in natural product and fine chemicals synthesis, medical diagnostics and bioremediation. A combination of horseradish peroxidase and indole-3-acetic acid or its derivatives is currently being evaluated as an agent for use in targeted cancer therapies. Physiological roles traditionally associated with the enzyme that include indole-3-acetic acid metabolism, cross-linking of biological polymers and lignification are becoming better understood at the molecular level, but the involvement of specific horseradish peroxidase isoenzymes in these processes is not yet clearly defined. Progress in this area should result from the identification of the entire peroxidase gene family of Arabidopsis thaliana, which has now been completed.

A biochemical and molecular characterization of LEP1, an extensin peroxidase from lupin

An analysis of apoplastic extensin cross-linking activity in vegetative organs of Lupinus albus indicated that leaves contained the highest specific activity. Assays of peroxidases fractionated from this material demonstrated that this activity could be largely attributed to a soluble and apoplastic 51-kDa peroxidase, denoted LEP1. Relative to other purified peroxidases, LEP1 demonstrates high extensin cross-linking activity and can be classified as an extensin peroxidase (EP). Optimal conditions for the in vitro oxidation of other phenolic substrates included 1.5-3.0 mm peroxide at pH 5.0. EP activity of LEP1 was low under these conditions but optimal and substantially higher with 100 microm peroxide and neutral pH, suggesting that physiological changes in pH and peroxide in muro could heavily influence the extensin cross-linking activity of LEP1 in vivo. Analysis of LEP1 glycans indicated 11-12 N-linked glycans, predominantly the heptasaccharide Man3XylFucGlcNAc2, but also larger structures showing substantial heterogeneity. Comparative assays with horseradish peroxidase isoform C and peanut peroxidases suggested the high level of glycosylation in LEP1 may be responsible for the high solubility of this EP in the apoplastic space. A full-length cDNA corresponding to LEP1 was cloned. Quantitative reverse transcriptase-PCR demonstrated LEP1 induction in apical portions of etiolated hypocotyls 30-60 min after exposure to white light, prior to the onset of growth inhibition. Comparative modeling of the translated sequence indicated an unusually unobstructed equatorial cleft across the substrate access channel, which might facilitate interaction with extensin and confer higher EP activity.

Ectopic expression of a horseradish peroxidase enhances growth rate and increases oxidative stress resistance in hybrid aspen

We previously demonstrated that overexpression of the horseradish (Armoracia rusticana) peroxidase prxC1a gene stimulated the growth rate of tobacco (Nicotiana tabacum) plants. Here, the cauliflower mosaic virus 35S::prxC1a construct was introduced into hybrid aspen (Populus sieboldii x Populus grandidentata). The growth rate of these transformed hybrid aspen plants was substantially increased under greenhouse conditions. The average stem length of transformed plants was 25% greater than that of control plants. There was no other obvious phenotypic difference between the transformed and control plants. Fast-growing transformed hybrid aspen showed high levels of expression of prxC1a and had elevated peroxidase activities toward guaiacol and ascorbate. However, there was no increase of the endogenous class I ascorbate peroxidase activities in the transformed plants by separate assay and activity staining of native polyacrylamide gel electrophoresis. Furthermore, calli derived from the transformed hybrid aspen grew faster than those from control plants and were resistant to the oxidative stress imposed by hydrogen peroxide. Therefore, enhanced peroxidase activity affects plant growth rate and oxidative stress resistance.

A novel cold-inducible gene from Arabidopsis, RCI3, encodes a peroxidase that constitutes a component for stress tolerance

A cDNA from Arabidopsis corresponding to a new cold-inducible gene, RCI3 (for Rare Cold Inducible gene 3), was isolated. Isoelectric focusing electrophoresis and staining of peroxidase activity demonstrated that RCI3 encodes an active cationic peroxidase. RNA-blot analysis revealed that RCI3 expression in response to low temperature is negatively regulated by light, as RCI3 transcripts were exclusively detected in etiolated seedlings and roots of adult plants. RCI3 expression was also induced in etiolated seedlings, but not in roots, exposed to dehydration, salt stress or ABA, indicating that it is subjected to a complex regulation through different signaling pathways. Analysis of transgenic plants containing RCI3::GUS fusions established that this regulation occurs at the transcriptional level during plant development, and that cold-induced RCI3 expression in roots is mainly restricted to the endodermis. Plants overexpressing RCI3 showed an increase in dehydration and salt tolerance, while antisense suppression of RCI3 expression gave dehydration- and salt-sensitive phenotypes. These results indicate that RCI3 is involved in the tolerance to both stresses in Arabidopsis, and illustrate that manipulation of RCI3 has a potential with regard to plant improvement of stress tolerance.

In vivo conversion of a glycan to human compatible type by transformed tobacco cells

Horseradish peroxidase isozyme C (HRP; EC 1.11.1.7) was used as a model protein to evaluate the capacity of tobacco cells transformed with human beta 1,4-galactosyltransferase (GT6) to modify and galactosylate a foreign glycoprotein. Cells transformed with the HRP gene are designated as BY2-HRP and GT6-HRP, for wild type BY2 and GT6 transformed cells, respectively. Expression of HRP cells was confirmed by isoelectric focusing, peroxidase activity staining, Western blotting, and enzymatic assays. The presence of HRP galactosylated N-glycans in GT6-HRP cells was analyzed by lectin staining, affinity chromatography, and structural analyses of pyridylamino-labeled RCA(120)-bound sugar chains. The structure of Gal(1)GlcNAc(1)Man(5)GlcNAc(2) was proposed based from the results of exoglycosidase digestions and two-dimensional sugar chain mapping. Unlike the HRP produced in BY2-HRP cells, the HRP from GT6-HRP cells has galactosylated glycoproteins that did not bind to the xylose-specific antiserum, suggesting the absence of the beta 1,2-xylose residue in the sugar chain.

C-terminal propeptide of the Caldariomyces fumago chloroperoxidase: an intramolecular chaperone?

The Caldariomyces fumago chloroperoxidase (CPO) is synthesised as a 372-aa precursor which undergoes two proteolytic processing events: removal of a 21-aa N-terminal signal peptide and of a 52-aa C-terminal propeptide. The Aspergillus niger expression system developed for CPO was used to get insight into the function of this C-terminal propeptide. A. niger transformants expressing a CPO protein from which the C-terminal propeptide was deleted failed in producing any extracellular CPO activity, although the CPO polypeptide was synthesised. Expression of the full-length gene in an A. niger strain lacking the KEX2-like protease PclA also resulted in the production of CPO cross-reactive material into the culture medium, but no CPO activity. Based on these results, a function of the C-terminal propeptide in CPO maturation is indicated.

Overexpression of protein disulfide isomerase DsbC stabilizes multiple-disulfide-bonded recombinant protein produced and transported to the periplasm in Escherichia coli

Dsb proteins (DsbA, DsbB, DsbC, and DsbD) catalyze formation and isomerization of protein disulfide bonds in the periplasm of Escherichia coli. By using a set of Dsb coexpression plasmids constructed recently, we analyzed the effects of Dsb overexpression on production of horseradish peroxidase (HRP) isozyme C that contains complex disulfide bonds and tends to aggregate when produced in E. coli. When transported to the periplasm, HRP was unstable but was markedly stabilized upon simultaneous overexpression of the set of Dsb proteins (DsbABCD). Whereas total HRP production increased severalfold upon overexpression of at least disulfide-bonded isomerase DsbC, maximum transport of HRP to the periplasm seemed to require overexpression of all DsbABCD proteins, suggesting that excess Dsb proteins exert synergistic effects in assisting folding and transport of HRP. Periplasmic production of HRP also increased when calcium, thought to play an essential role in folding of nascent HRP polypeptide, was added to the medium with or without Dsb overexpression. These results suggest that Dsb proteins and calcium play distinct roles in periplasmic production of HRP, presumably through facilitating correct folding. The present Dsb expression plasmids should be useful in assessing and dissecting periplasmic production of proteins that contain multiple disulfide bonds in E. coli.

Xylem-specific expression of wound-inducible rice peroxidase genes in transgenic plants

A peroxidase gene, poxA, was isolated from a rice (Oryza sativa L.) genomic library. The gene consists of four exons whose combined sequences were identical to that of the prxRPA mRNA whose levels were dramatically stimulated by wounding as well as by treatment of rice shoots with ethephon or UV irradiation [H. Ito, F. Kimizuka, A. Ohbayashi, H. Matsui, M. Honma, A. Shinmyo, Y. Ohashi, A.B. Caplan, R.L. Rodriguez, Molecular cloning and characterization of two complementary DNAs encoding putative peroxidases from rice (Oryza sativa L.) shoots, Plant Cell Rep. 13 (1994) 361-366]. The temporal and spatial expression properties of the poxA gene promoter as well as that from a second related peroxidase gene, poxN, were analyzed in transgenic tobacco and rice plants using the uidA gene as a reporter. In transgenic tobacco, UV- and wound-responsive cis-elements were located within 144 bp from the translational start codon of the poxA gene. The poxN promoter, however, was inactive in the heterologous host as no significant GUS activity was evident. On the other hand, chimeric uidA genes containing 2.2 kb of the poxA promoter or 1.4 kb of poxN promoter were active in transgenic rice plants. Both peroxidase promoters directed GUS activities in a spatial and tissue specific manner coincident with the expression patterns exhibited by their mRNAs. Histochemical analysis of transgenic rice plants showed that both peroxidase genes are expressed in the vascular bundles of the shoot apex and lamina joint, and in xylem-parenchyma cells of the leaf blade and sheath.

Isolation of tobacco isoperoxidases accumulated in cell-suspension culture medium and characterization of activities related to cell wall metabolism

All of the most important guaiacol-type peroxidase (POX) isoforms accumulated in the culture medium of BY-2 tobacco (Nicotiana tabacum L. cv Bright Yellow 2) cells have been isolated. Five basic and two acidic isoforms were found. The four major isoforms (B2, B3, P1, and P2), all strongly basic, have been purified to homogeneity and partially sequenced. B2 and B3 are new isoforms showing high homology to only one POX isolated so far. Amino acid sequencing and specific activities indicated that basic isoPOXs constitute two pairs of strictly related isoforms (P1/P2 and B2/B3). Their specific activities measured in the presence of different substrates, as monolignols and NAD(P)H, indicated possible specialized functions in cell wall metabolism. Only P-type POXs were able to oxidize indoleacetic acid. Variations in pH could play a regulatory role by changing the relative contribution of different isoforms to total POX activity. Apart from cell culture medium, polyclonal antibodies obtained against P1 and P2 detected P1 in roots and in lower parts of stems. Immunocytochemical labeling indicated that P-type POXs were expressed in stem phloem and in phloem and epidermal cells of roots.

Barley coleoptile peroxidases. Purification, molecular cloning, and induction by pathogens

A cDNA clone encoding the Prx7 peroxidase from barley (Hordeum vulgare L.) predicted a 341-amino acid protein with a molecular weight of 36,515. N- and C-terminal putative signal peptides were present, suggesting a vacuolar location of the peroxidase. Immunoblotting and reverse-transcriptase polymerase chain reaction showed that the Prx7 protein and mRNA accumulated abundantly in barley coleoptiles and in leaf epidermis inoculated with powdery mildew fungus (Blumeria graminis). Two isoperoxidases with isoelectric points of 9.3 and 7.3 (P9.3 and P7.3, respectively) were purified to homogeneity from barley coleoptiles. P9.3 and P7.3 had Reinheitszahl values of 3.31 and 2.85 and specific activities (with 2,2'-azino-di-[3-ethyl-benzothiazoline-6-sulfonic acid], pH 5.5, as the substrate) of 11 and 79 units/mg, respectively. N-terminal amino acid sequencing and matrix-assisted laser desorption/ionization time-of-flight mass-spectrometry peptide analysis identified the P9. 3 peroxidase activity as due to Prx7. Tissue and subcellular accumulation of Prx7 was studied using activity-stained isoelectric focusing gels and immunoblotting. The peroxidase activity due to Prx7 accumulated in barley leaves 24 h after inoculation with powdery mildew spores or by wounding of epidermal cells. Prx7 accumulated predominantly in the epidermis, apparently in the vacuole, and appeared to be the only pathogen-induced vacuolar peroxidase expressed in barley tissues. The data presented here suggest that Prx7 is responsible for the biosynthesis of antifungal compounds known as hordatines, which accumulate abundantly in barley coleoptiles.

Analysis of two incompletely spliced Arabidopsis cDNAs encoding novel types of peroxidase

Most known class III peroxidase genes contain three introns at conserved positions. Two Arabidopsis cDNAs (ESTs), encoding novel type peroxidases ATP9a and ATP15a were sequenced, and found to contain inserts for intron 2. PCR and sequence analysis of genomic DNA revealed that the atp9a gene contains all three introns, whereas atp15a contains only introns 2 and 3. The ATP15a cDNA intron contained a single base substitution reducing the splicing potential significantly as compared with the genomic sequence. The putative enzymes share essential catalytic and structural features with horseradish peroxidase, despite a pair-wise sequence identity of only 40-45% among the three.

On becoming a parasite: evaluating the role of wall oxidases in parasitic plant development

BACKGROUND

The temporal and spatial control of the transition from vegetative to parasitic growth is critical to any parasite, but is essential to the sessile parasitic plants. It has been proposed that this transition in Striga spp. is controlled simply by an exuded oxidase that converts host cell-surface phenols into benzoquinones which act as developmental signals that mediate the transition. An understanding of this mechanism may identify the critical molecular events that made possible the evolution of parasitism in plants.

RESULTS

PoxA and PoxB are identified as the only apoplastic phenol oxidases in Striga asiatica seedlings, and the genes encoding them have been cloned and sequenced. These peroxidase enzymes are capable of oxidizing the 60 known inducing phenols into a small set of benzoquinones, and it is these quinones that induce parasitic development. Analysis of the reaction requirements and comparisons to host enzymes, however, lead us to argue that PoxA and PoxB are not necessary for host recognition.

CONCLUSIONS

A new model is proposed where constitutive production of an activated oxygen species (in the case of Striga, H2O2) mediates host recognition. This strategy would allow a parasite to exploit abundant host enzymes to produce the diffusible recognition signals by converting a standard host defense into a parasitic offense.

Sea urchin ovoperoxidase: oocyte-specific member of a heme-dependent peroxidase superfamily that functions in the block to polyspermy

Ovoperoxidase is one of several oocyte-specific proteins that are stored within sea urchin cortical granules, released during the cortical reaction, and incorporated into the newly formed fertilization envelope. Ovoperoxidase plays a particularly important role in this process, crosslinking the envelope into a hardened matrix that is insensitive to biochemical and mechanical challenges and thus providing a permanent block to polyspermy. Here we present the primary structures of two ovoperoxidases as predicted from cDNAs cloned from the sea urchins Strongylocentrotus purpuratus (AF035380) and Lytechinus variegatus (AF035381). We also present a proposed scheme for the post-translational processing of ovoperoxidase based upon comparisons between the cDNA and protein structures and taking into account previously published reports. The sea urchin ovoperoxidase sequences conform to a profile shared by members of a heme-dependent animal peroxidase family, including the mammalian myelo-, lacto-, eosinophil, and thyroid peroxidases. Using in situ RNA hybridizations, we showed that the mRNA of S. purpuratus ovoperoxidase (4 kb) is present exclusively in oocytes, and is turned over rapidly following germinal vesicle breakdown. Taking into account our immunoblot and N-terminal sequencing data along with reports from similar peroxidases, we propose that ovoperoxidases are synthesized in a pre-pro form and proteolytically processed to result in the 70 and 50 kDa forms that are found in the fertilization envelope. The sequence and structural data presented here will facilitate our continuing studies of the biogenesis of cortical granules and the fertilization envelope. Additionally, since ovoperoxidase activities have been reported in a wide range of animals, these cDNAs will be useful in uncovering similar peroxidases used in the fertilization reactions of other metazoan eggs.

Differential induction of a peroxidase gene family during infection of rice by Xanthomonas oryzae pv. oryzae

Induction of peroxidase has been correlated with resistant interactions between rice and Xanthomonas oryzae pv. oryzae. To assist in analysis of the role of rice peroxidases in plant defense against the bacterial pathogen, three peroxidase genes, POX22.3, POX8.1, and POX5.1, were identified from a rice cDNA library that was constructed from leaves of plants undergoing a resistant reaction. These genes were highly similar in nucleic acid and amino acid sequences and belonged to a gene family. The three genes showed differential expression in infiltrated rice leaves during pathogen interactions and mechanical stress. Only two peroxidase genes, POX8.1 and POX22.3, were predominantly expressed during resistant interactions. These two genes also were expressed during susceptible interactions, but induction was delayed compared with resistant interactions. POXgX9, a fourth peroxidase gene that was isolated from a genomic library, is adjacent to POX22.3 in the rice genome and has greater than 90% similarity in nucleotide and amino acid sequence identity to POX22.3. Interestingly, POXgX9 was expressed only in the roots of rice plants. While POX22.3 was expressed in both leaves and roots, POX8.1 and POX5.1 were not detected in roots but were induced in leaves by mechanical wounding at different times after treatment. POX22.3, POX8.1, and POX5.1 were estimated to be present in single copies in rice haploid genome. These results indicate that different members of the rice peroxidase gene family are distinctly regulated in response to various environmental cues.

Annexin-like protein from Arabidopsis thaliana rescues delta oxyR mutant of Escherichia coli from H2O2 stress

Reactive oxygen species are common causes of cellular damages in all aerobic organisms. In Escherichia coli, the oxyR gene product is a positive regulator of the oxyR regulon that is induced in response to H2O2 stress. To identify genes involved in counteracting oxidative stress in plants, we transformed a delta oxyR mutant of E. coli with an Arabidopsis thaliana cDNA library and selected for clones that restored the ability of the delta oxyR mutant to grow in the presence of H2O2. Using this approach, we isolated a cDNA that has strong homology with the annexin super-gene family. The complemented mutant showed higher catalase activity. mRNA expression of the annexin gene in A. thaliana was higher in roots as compared with other organs and was also increased when the plants were exposed to H2O2 stress or salicylic acid. Based on the results presented in this study, we propose a novel physiological role for annexin in counteracting H2O2 stress.

Construction of gene expression system in cultured tobacco cells

To construct a gene expression system in cultured tobacco cells, useful regulatory elements of plant genes were studied. The promoter of the horseradish peroxidase gene, prxC2, showed high activity in tobacco cells, and it contained enhancer sequences and a cis element for wound induction. The heat shock promoter of the HSP18.2 gene from A. thaliana had strong activity of transcription when the incubation temperature of tobacco cells was shifted from 25 degrees C to 37 degrees C. These elements could be good candidates for foreign gene expression in tobacco cells.

Nucleotide sequences of four pathogen-induced alfalfa peroxidase-encoding cDNAs

We constructed an alfalfa cDNA library from mRNA extracted from leaves after infection with Pseudomonas syringae (incompatible interaction). Screening with oligodeoxyribonucleotides designed from regions conserved in all known peroxidases allowed the isolation of four cDNAs (Msprx1A, 1B, 1C and 2). Sequence analysis revealed the presence of open reading frames of 351, 355, 358 and 323 amino acids, respectively, with the characteristic consensus sequences of plant peroxidases. Sequence comparison showed that the Msprx2 product is significantly different from the others and, particularly, lacks a C-terminal propeptide which might be required for vacuolar targeting.

Molecular cloning and expression analysis of peroxidase genes from wheat

A PCR-based screening approach was used to isolate genomic clones from wheat encoding peroxidase isozymes. Three complete genes (pox1, pox2 and pox4) and one truncated gene (pox3) were characterized. The nucleotide sequences predicted mature proteins of 31 kDa, in which all the highly conserved motifs of secreted plant peroxidases were preserved. The coding regions showed 73-83% DNA sequence identity, with the highest level of similarity noted for the tandemly oriented pox2 and pox3. Expression of respective pox genes in various tissues of wheat was assessed by the RT-PCR technique, which showed that all four genes are active. The primary pox1 mRNA was spliced to remove three introns, whereas processing of the other pox transcripts involved only two intervening sequences. Splicing occurred at consensus GU/AG splice sites except for the first introns of pox1, pox2 and pox4 transcripts, where processing took place at unusual GC donor sites. The RNA analysis suggested that the pox1, pox2 and pox4 genes are predominantly expressed in roots. Lower levels of expression were found for pox4 and pox3 in leaves. Infection of wheat by the powdery mildew fungus selectively induced expression of pox2 in leaves.

Molecular cloning of two tandemly arranged peroxidase genes from Populus kitakamiensis and their differential regulation in the stem

A genomic library was prepared from Populus kitakamiensis and screened with the cDNA for an anionic peroxidase from P. kitakamiensis. One genomic clone was isolated that contained two tandemly oriented genes for anionic peroxidases, prxA3a and prxA4a. Both genes consisted of four exons and three introns; the introns had consensus nucleotides, namely, GT and AG, at their 5' and 3' ends, respectively. The prxA3a and prxA4a genes encoded 347 and 343 amino acid residues, respectively, including putative signal sequences at the amino-termini. Putative promoters and polyadenylation signals were found in the flanking regions of both genes. The sequence of the coding region of prxA3a was completely identical to that of the cDNA clone pA3, whereas the sequence of the coding region of prxA4a was only 73% identical to that of the cDNA clone pA3. Northern blot analysis showed that the patterns of expression of the mRNAs that corresponded to prxA3a and prxA4a differed in stems of P. kitakamiensis.

Two cationic peroxidases from cell walls of Araucaria araucana seeds

We have previously reported the purification and partial characterization of two cationic peroxidases from the cell walls of seeds and seedlings of the South American conifer, Araucaria araucana. In this work, we have studied the amino acid composition and NH2-terminal sequences of both enzymes. We also compare the data obtained from these analyses with those reported for other plant peroxidases. The two peroxidases are similar in their amino acid compositions. Both are particularly rich in glycine, which comprises more than 30% of the amino acid residues. The content of serine is also high, ca 17%. The two enzymes are different in their content of arginine, alanine, valine, phenylalanine and threonine. Both peroxidases have identical NH2-terminal sequences, indicating that the two proteins are genetically related and probably are isoforms of the same kind of peroxidase. The amino acid composition and NH2-terminal sequence analyses showed marked differences from the cationic peroxidases from turnip and horseradish.

Thermostabilized chemical derivatives of horseradish peroxidase

Horseradish peroxidase finds a variety of uses in analysis, immunology, organic synthesis, and biosensors. Although moderately stable, its applicability to biosensors and other fields would be greatly enhanced if it could be made yet more stable. Appropriate chemical modification can substantially stabilize enzymes. Here we describe the use of bis-imidates and of bis-succinimides to modify free amino groups of commercial horseradish peroxidase under mild conditions of pH and temperature. Imidates yielded a marginal stabilization. Some of the succinimide derivatives, however, are much more thermostable than the native enzyme. Apparent half-lives indicate stabilizations of 6- to 23-fold, depending on the bis-succinimide used. These modifications preserve the carbohydrate side chains for subsequent reaction or immobilization.

Characterization and in situ localization of a salt-induced tomato peroxidase mRNA

NaCl treatment of tomato plants in hydroponic culture at concentrations as low as 50 mM resulted in enhanced accumulation of transcripts of TPX1, a full-length cDNA clone that we had isolated from a library of NaCl-treated tomato plants using a peroxidase-specific oligonucleotide probe. Although the overall amino acid sequence identity of TPX1 to other peroxidase genes was less than 45%, there was a very high degree of identity in all of the conserved domains. The deduced amino acid sequence included the presence of a N-terminal signal peptide but not the C-terminal extension present in peroxidases targeted to the vacuole. The mature protein has a theoretical pI value of 7.5. Transcripts that hybridized to TPX1 were detected only in the roots with higher levels of mRNA in epidermal and subepidermal cell layers. Isoelectric focusing of root extracts showed two major bands of peroxidase activity at pI 5.9 and 6.2. Both activities increased with salt treatment. Southern analysis indicated the presence of only a single TPX1 gene in tomato.

Quantitation of epsilon-amino group using amino acids as reference standards by trinitrobenzene sulfonic acid. A simple spectrophotometric method for the estimation of hapten to carrier protein ratio

A simple, sensitive and direct spectrophotometric method is presented for the determination of epsilon-amino groups of L-lysine present in carrier proteins, using the free amino acids L-lysine and L-glutamic acid as reference standards, and 2,4,6-trinitrobenzene 1-sulfonic acid (TNBS) reagent. The amount of epsilon-amino group present in the carrier protein after coupling with hapten is directly quantitated using the standard curve generated by the difference in absorbance observed with L-lysine and L-glutamic acid after their reaction with TNBS reagent. Spectral analysis of L-lysine and L-glutamic acid (27.36 nmol) derivatives of TNBS at 335 nm showed that TNP-L-lysine had twice the absorbance of TNP-L-glutamic acid, since TNBS reagent interacts equally with the alpha-amino and epsilon-amino groups present in L-lysine and the alpha-amino group of L-glutamic acid, respectively. The relationship between absorbance and concentration of epsilon-amino groups (up to 16 micrograms/ml) was found to be linear. The number of epsilon-amino groups of lysine present in carrier proteins such as BSA, HSA, thyroglobulin and the enzyme, horseradish peroxidase, were analyzed by the present method and were found to be similar to the reported values. Various carrier protein-hapten conjugates (protein-mycotoxin/vitamin/steroid hormone conjugates) were made and analyzed by the method developed in order to determine their mole to mole ratio.

A step towards understanding the folding mechanism of horseradish peroxidase. Tryptophan fluorescence and circular dichroism equilibrium studies

The guanidinium chloride denaturation/renaturation of the holo- and apo-horseradish peroxidase isoenzyme c (HRP) has been studied by fluorescence and circular dichroism spectroscopies. A distinct equilibrium intermediate of the apoprotein could be detected at low concentrations of guanidinium chloride (0.5 M). This intermediate has a secondary structure content like that of the native protein but a poorly defined tertiary structure. Renaturation of the apo-HRP is reversible and 100% activity could be obtained after addition of a twofold excess of free haem. The denaturation of the holo-HRP is more complex and occurs in two distinct steps; unfolding of the protein backbone and loss of the haem. The denatured protein folds back to its native conformation but the incorporation of the haem occurs only after the secondary structure is formed. Ca2+ appears to be important for the stability of the protein as the apo-HRP is more resistant to denaturation in the presence of Ca2+. The free-energy change during unfolding of the apo-HRP was determined in the absence and presence of Ca2+ and found to be 9.2 kJ/mol and 16.7 kJ/mol, respectively. The importance of Ca2+ to the protein stability was also supported by studies on the loss of the haem from the protoporphyrin-IX-apo-HRP complex.

Genomic DNA structure of a gene encoding cytosolic ascorbate peroxidase from Arabidopsis thaliana

A genomic DNA clone encoding cytosolic ascorbate peroxidase was isolated from a genomic library of Arabidopsis thaliana, using a cDNA for the enzyme as a probe. Nucleotide sequence and primer extension analyses of this gene (APX1) revealed nine exons split by eight introns, one of which is inserted in the 5'-untranslated region. The exon/intron organization of the APX1 gene differs from that of the guaiacol peroxidase genes.