Organisation of the tomato polyphenol oxidase gene family

Development of polyphenol oxidase activity in the micropylar endosperm of tomato seeds

Polyphenol oxidase (PPO) activity increased markedly in the micropylar region of the endosperm of tomato (Lycopersicon esculentum) seeds after radicle protrusion. Tissue-printing analyses demonstrated that the majority of the activity is localized in the micropylar area. The increase in the activity was due to the increase in the amounts of enzyme. Within the micropylar endosperm region, two PPO isozymes were immunologically detected whose apparent molecular masses were estimated to be approximately 58 and 59 kDa, respectively, by SDS-PAGE. Although PPO activity also developed in the lateral portion of the endosperm, the level of this activity was much lower as compared with that in the micropylar region. Furthermore, the isozyme pattern in the lateral portion differed from that in the micropylar portion. The 58 kDa isozyme that was detected in the latter portion was absent, and only 59 kDa PPO was detectable in the former one. When the endosperm tissues were wounded, an enhancement of the enzyme activity was observed in the wounded region. The wound-induced development of the enzyme activity was associated with the induction of 58 kDa isozyme.

Isolation of a latent polyphenol oxidase from loquat fruit (Eriobotrya japonica Lindl.): kinetic characterization and comparison with the active form

Polyphenol oxidase (PPO) has been extracted from both soluble and particulate fractions of loquat fruit (Eriobotrya japonica Lindl. cv. Algerie). The soluble PPO (20% of total activity) was partially purified 3.3-fold after ammonium sulfate fractionation being in its active state. The particulate PPO fraction (80% of total activity) was purified to homogeneity in a latent form being activable by sodium dodecyl sulfate (SDS). The enzyme was purified 40.0-fold with a total yield of 15.3% after extraction by phase partitioning in Triton X-114 followed by three chromatographic steps. The molecular weight was estimated to be about 59.2 and 61.2 kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel filtration chromatography, respectively, indicating that latent PPO is a monomer. Latent PPO catalyzed the oxidation of chlorogenic acid (CA) at a rate 50-fold faster than that of 4-tert-butylcatechol (TBC) but the soluble active counterpart only twice. Both PPOs exhibited similar Km values for TBC but Km for CA was 5-fold higher for the latent than for the active soluble PPO. Other kinetic characteristics, including sensitivity to inhibitors, substrate specificity, thermal stability, temperature, and pH profiles, were quite different between both PPOs. These results provide strong evidences that the soluble active and the particulate latent are different forms of PPO in loquat fruit flesh. The results suggest that the major PPO form for the oxidation of CA, leading to enzymatic browning under physiological conditions, is the latent one.

Genetic and in silico comparative mapping of the polyphenol oxidase gene in bread wheat (Triticum aestivum L.)

Polyphenol oxidases (PPOs) are involved in the time-dependent darkening and discolouration of Asian noodles and other wheat end products. In this study, a doubled haploid (DH) population derived from Chara (moderately high PPO activity)/WW2449 (low PPO activity) was screened for PPO activity based on L-DOPA and L-tyrosine assays using whole seeds. Both these assays were significantly genetically correlated (r = 0.91) in measuring the PPO activity in this DH population. Quantitative trait loci (QTLs) analysis utilising a skeleton map enabled us to identify a major QTL controlling PPO activity based on L-DOPA and L-tyrosine on the long arm of chromosome 2A. The simple sequence repeat (SSR) marker GWM294b explained over 82% of the line mean phenotypic variation from samples collected in both 2000 and 2003. Four SSR markers were validated for PPO linkage in genetically diverse backgrounds and proven to correctly predict the PPO activity in more than 92% of wheat lines. Physical mapping using deletion lines of Chinese Spring has confirmed the location of the GWM294b, GWM312 and WMC170 on chromosome 2AL, between deletion breakpoints 2AL-C to 0.85. In order to identify functional gene markers, data searches for alignments between rice BAC/PAC clones assembled on chromosome 1 and 4, chromosome 7, and (1) the wheat expressed sequence tags mapped in deletion bin (2AL-C to 0.85) and (2) the coding sequence of a previously cloned wheat PPO gene were made and found significant sequence similarities with the PPO gene or common central domain of tyrosinase. Available PPO gene sequences in the National Centre for Biotechnology Information (NCBI) database have revealed that there is a significant molecular diversity at the nucleotide and amino acid level in the wheat PPO genes.

Identification and characterisation of a bryophyte polyphenol oxidase encoding gene from Physcomitrella patens

Polyphenol oxidases (PPO) are enzymes of secondary plant metabolism that catalyse the oxidation of polyphenols to quinones. Because of their ubiquitous appearance in the plant kingdom, an important role is assumed; however, the exact physiological function of PPOs remains unclear. In this work, the identification, cloning, and characterisation of a bryophyte PPO from the moss Physcomitrella patens is presented. PPO activity from protein extracts was determined polarographically after activation by SDS. Four Physcomitrella ESTs with homologies to known plant PPOs were selected from publicly accessible databases, and PCR experiments demonstrated that they belong to the same gene, named Pp_ppo1. The identified cDNA was found to be 2402 bp long, containing a single open reading frame of 1611 bp encoding for a 536 amino acid protein with a molecular mass of 60.1 kDa. Cloning and sequencing of a genomic part of Pp_ppo1 revealed the presence of a 94-bp intron. The time course of Pp_ppo1 gene expression in liquid culture was monitored by real time RT-PCR, revealing increasing transcription levels until the 4th day, a maximum between the 4th and the 8th day, and decreasing transcription until the 12th day. A comparison of the deduced amino acid sequence of Pp_ppo1 with seed plant PPOs revealed similarities such as the presence of two highly conserved copper-binding domains and a similar pattern of hydrophobic regions, but also differences such as a stronger membrane association and a shorter signal sequence, thus reflecting the phylogenetic distance of Physcomitrella from seed plants.

Antisense downregulation of polyphenol oxidase results in enhanced disease susceptibility

Polyphenol oxidases (PPOs; EC 1.14.18.1 or EC 1.10.3.2) catalyze the oxidation of phenolics to quinones, highly reactive intermediates whose secondary reactions are responsible for much of the oxidative browning that accompanies plant senescence, wounding, and responses to pathogens. To assess the impact of PPO expression on resistance to Pseudomonas syringae pv. tomato we introduced a chimeric antisense potato PPO cDNA into tomato (Lycopersicon esculentum L.). Oxidation of caffeic acid, the dominant o-diphenolic aglycone of tomato foliage, was decreased ca. 40-fold by antisense expression of PPO. All members of the PPO gene family were downregulated: neither immunoreactive PPO nor PPO-specific mRNA were detectable in the transgenic plants. In addition, the antisense PPO construct suppressed inducible increases in PPO activity. Downregulation of PPO in antisense plants did not affect growth, development, or reproduction of greenhouse-grown plants. However, antisense PPO expression dramatically increased susceptibility to P. syringae expressing the avirulence gene avrPto in both Pto and pto backgrounds. In a compatible (pto) interaction, plants constitutively expressing an antisense PPO construct exhibited a 55-fold increase in bacterial growth, three times larger lesion area, and ten times more lesions cm(-2) than nontransformed plants. In an incompatible (Pto) interaction, antisense PPO plants exhibited 100-fold increases in bacterial growth and ten times more lesions cm(-2) than nontransformed plants. Although it is not clear whether hypersusceptibility of antisense plants is due to low constitutive PPO levels or failure to induce PPO upon infection, these findings suggest a critical role for PPO-catalyzed phenolic oxidation in limiting disease development. As a preliminary effort to understand the role of induced PPO in limiting disease development, we also examined the response of PPO promoter::beta-glucuronidase constructs when plants are challenged with P. syringae in both Pto and pto backgrounds. While PPO B inducibility was the same in both compatible and incompatible interactions, PPO D, E and F were induced to higher levels and with different expression patterns in incompatible interactions.

Cloning and characterization of red clover polyphenol oxidase cDNAs and expression of active protein in Escherichia coli and transgenic alfalfa

Red clover (Trifolium pratense) leaves contain high levels of polyphenol oxidase (PPO) activity and o-diphenol substrates. Wounding of leaves during harvest and ensiling results in browning of leaf tissues from activity of PPO on the o-diphenols. In association with browning, leaf proteins remain undegraded during ensiling, presumably due to PPO-generated o-quinone inhibition of leaf proteases. We cloned three red clover PPO cDNAs, PPO1, PPO2, and PPO3, from a leaf cDNA library. Sequence comparisons among the three red clover PPO clones indicated they are 87% to 90% identical at the nucleotide level (80%-83% amino acid identity). All three encode proteins predicted to localize to the chloroplast thylakoid lumen. RNA-blotting and immunoblotting experiments indicated PPO1 is expressed primarily in young leaves, PPO2 in flowers and petioles, and PPO3 in leaves and possibly flowers. We expressed mature PPO1 in Escherichia coli. A portion of the expressed protein was soluble and functional in an assay for PPO activity. We also expressed the red clover PPO cDNAs under the control of a constitutive promoter in alfalfa (Medicago sativa). The expressed red clover PPO proteins were active in alfalfa extracts as evidenced by o-diphenol-dependant extract browning and quantitative assays of PPO activity. Proteolysis in leaf extracts of alfalfa expressing red clover PPO1 was dramatically reduced in the presence of an o-diphenol compared to controls. Transgenic alfalfa expressing red clover PPO should prove an excellent model system to further characterize the red clover PPO enzymes and PPO-mediated inhibition of postharvest proteolysis in forage plants.

Import of polyphenol oxidase by chloroplasts is enhanced by methyl jasmonate

Polyphenol oxidase (PPO; EC 1.10.3.2 or EC 1.14.18.1) takes part in the response of tomato plants (Lycopersicon esculentum Mill.) to wounding and herbivore attack, mediated by the octadecanoid wound-signaling pathway. Wounding and methyl jasmonate (MeJA) induce expression of ppo genes and markedly increase the level of the enzyme. We report that pretreatment with MeJA also markedly increased the ability of isolated tomato chloroplasts to import and process PPO precursors (pPPO). Pea (Pisum sativum L.) chloroplasts showed no such response. Wounding or ethylene alone was ineffective but ethylene was synergistic with MeJA. Treatment with MeJA conferred a strong binding of pPPO, or its processing intermediate, to thylakoids and subsequent translocation into the lumen and processing to the mature protein. The effect on PPO import and translocation was evident after 8-16 h exposure to MeJA. Membrane-bound pPPO was cross-linked to a proteinaceous component of the thylakoid translocation apparatus, apparently induced by MeJA. The import and processing of other nuclear-encoded thylakoid proteins were not affected by MeJA in tomato. A 90-kDa protein that co-fractionated with thylakoids was induced along with the increase in competence for PPO import, and was identified as the proteinase-inhibitor multicystatin. It is concluded that the 90-kDa protein observed is part of the MeJA-induced defense response of tomato, not a component of the thylakoid translocation apparatus.

Transcriptional regulation of a pineapple polyphenol oxidase gene and its relationship to blackheart

Two genes encoding polyphenol oxidase (PPO) were isolated from pineapple (Ananas comosus[L.] Merr. cv. Smooth Cayenne). Sequence analyses showed that both contained a single intron and encoded typical chloroplast-localized PPO proteins, the sequences of which corresponded to two pineapple PPO cDNAs, PINPPO1 and PINPPO2, recently described by Stewart et al. (2001). Southern blot analyses suggested that pineapple contained only two PPO genes. Analysis of expression of PINPPO1 promoter GUS fusion constructs showed this promoter had a low basal activity and was cold- and wound-inducible, consistent with known mRNA expression profiles. Striking homologies to gibberellin response complexes (GARC) were observed in sequences of both the PINPPO1 and PINPPO2 promoters. Transient assays in mature pineapple fruit and stable expression in transgenic tobacco showed that PINPPO1 promoter-GUS fusions were indeed gibberellin (GA) responsive. A role for the element within the putative GARCs in mediating GA-responsiveness of the PINPPO1 promoter was confirmed by mutational analysis. PINPPO2 was also shown to be GA-responsive by RT-PCR analysis. Mutant PINPPO1 promoter-GUS fusion constructs, which were no longer GA-inducible, showed a delayed response to cold induction in pineapple fruit in transient assays, suggesting a role for GA in blackheart development. This was supported by observations that exogenous GA(3) treatment induced blackheart in the absence of chilling. Sequences showing homology to GARCs are also present in some PPO promoters in tomato, suggesting that GA regulates PPO expression in diverse species.

(+)-Larreatricin hydroxylase, an enantio-specific polyphenol oxidase from the creosote bush (Larrea tridentata)

An enantio-specific polyphenol oxidase (PPO) was purified approximately 1,700-fold to apparent homogeneity from the creosote bush (Larrea tridentata), and its encoding gene was cloned. The posttranslationally processed PPO ( approximately 43 kDa) has a central role in the biosynthesis of the creosote bush 8-8' linked lignans, whose representatives, such as nordihydroguaiaretic acid and its congeners, have potent antiviral, anticancer, and antioxidant properties. The PPO primarily engenders the enantio-specific conversion of (+)-larreatricin into (+)-3'-hydroxylarreatricin, with the regiochemistry of catalysis being unambiguously established by different NMR spectroscopic analyses; the corresponding (-)-enantiomer did not serve as a substrate. This enantio-specificity for a PPO, a representative of a widespread class of enzymes, provides additional insight into their actual physiological roles that hitherto have been difficult to determine.

Biochemical characterization of two differentially expressed polyphenol oxidases from hybrid poplar

Two polyphenol oxidase isoforms with distinct expression patterns were identified in hybrid poplar (Populus trichocarpaxP. deltoides). PPO-1, corresponding to the previously cloned PtdPPO (Constabel et al., Plant Physiol. 124: 285-295) was primarily leaf tissue-specific and detected only after wounding. PPO-2 was expressed constitutively in all tissue types tested except mature leaves, with highest expression in very young leaves and conducting tissues such as roots, stems and petioles. These two PPO isoforms were partially purified from hybrid poplar by ammonium sulfate fractionation followed by hydrophobic interaction chromatography. They were found to differ in stability, pH optimum, and activation by SDS. Tests with common phenolic substrates showed that PPO-1 had a broader substrate specificity than PPO-2. The distinct enzymatic properties and expression patterns of these two PPO isoforms suggest that they may have different physiological functions in hybrid poplar.

Assaying gene content in Arabidopsis

Arabidopsis has been popular as a model plant system for decades. Completion of the Arabidopsis genome and the availability of large expressed sequence-tag collections from other dicot species provides an opportunity to assess gene content in Arabidopsis, specifically by identifying genes from dicot test species that are absent from Arabidopsis. I report here results from these sorts of comparisons, carried out in part to assess the extent to which Arabidopsis is representative of dicot genomes and also the degree to which gene loss and novel gene acquisition has accompanied angiosperm speciation. More than 10% of the contigs from each of three dicot test species have no detectable homologue in Arabidopsis. By means of cross comparison among the test species, 154 specific cases of gene loss in the lineage leading to Arabidopsis were identified, including several well characterized enzymes and a group of proteins with strong homologs in the photosynthetic bacterium Synechocystis. These results show that although Arabidopsis is broadly representative of the other dicot genomes, there seems to be substantial variation even among relatively closely related genera. Further, although we cannot yet draw a causative link, variation in actual gene content seems appears to be a feature of angiosperm speciation.

Cloning, characterization, and transcription of three laccase genes from Gaeumannomyces graminis var. tritici, the take-all fungus

Gaeumannomyces graminis var. tritici, a filamentous ascomycete, is an important root pathogen of cereals that causes take-all disease and results in severe crop losses worldwide. Previously we identified a polyphenol oxidase (laccase) secreted by the fungus when induced with copper. Here we report cloning and partial characterization of three laccase genes (LAC1, LAC2, and LAC3) from G. graminis var. tritici. Predicted polypeptides encoded by these genes had 38 to 42% amino acid sequence identity and had conserved copper-binding sites characteristic of laccases. The sequence of the LAC2 predicted polypeptide matched the N-terminal sequence of the secreted laccase that we purified in earlier studies. We also characterized expression patterns of these genes by reverse transcription-PCR. LAC1 was transcribed constitutively, and transcription of LAC2 was Cu inducible. All three genes were transcribed in planta; however, transcription of LAC3 was observed only in planta or in the presence of host (wheat) plant homogenate.

Isozymes of Ipomoea batatas catechol oxidase differ in catalase-like activity

The amino acid sequences of two isozymes of catechol oxidase from sweet potatoes (Ipomoea batatas) were determined by Edman degradation of BrCN cleavage fragments of the native protein and by sequencing of amplified cDNA fragments. Sequence alignment and phylogenetic analysis of plant catechol oxidases revealed about 80% equidistance between the two I. batatas catechol oxidases and approximately 40--60% to catechol oxidases of other plants. When H(2)O(2) was applied as substrate the 39 kDa isozyme, but not the 40 kDa isozyme, showed catalase-like activity. The structure of the 40 kDa isozyme was modeled on the basis of the published crystal structure of the 39 kDa isozyme [T. Klabunde et al., Nat. Struct. Biol. 5 (1998) 1084]. The active site model closely resembled that of the 39 kDa isozyme determined by crystallography, except for a mutation of Thr243 (40 kDa isozyme) to Ile241 (39 kDa isozyme) close to the dimetal center. This residue difference affects the orientation of the Glu238/236 residue, which is thought to be responsible for the catalase-like activity of the 39 kDa isozyme for which a catalytic mechanism is proposed.

Control of enzymatic browning in potato (Solanum tuberosum L.) by sense and antisense RNA from tomato polyphenol oxidase

Polyphenol oxidase (PPO) activity of Russet Burbank potato was inhibited by sense and antisense PPO RNAs expressed from a tomato PPO cDNA under the control of the 35S promoter from the cauliflower mosaic virus. Transgenic Russet Burbank potato plants from 37 different lines were grown in the field. PPO activity and the level of enzymatic browning were measured in the tubers harvested from the field. Of the tubers from 28 transgenic lines that were sampled, tubers from 5 lines exhibited reduced browning. The level of PPO activity correlated with the reduction in enzymatic browning in these lines. These results indicate that expression of tomato PPO RNA in sense or antisense orientation inhibits PPO activity and enzymatic browning in the major commercial potato cultivar. Expression of tomato PPO RNA in sense orientation led to the greatest decrease in PPO activity and enzymatic browning, possibly due to cosuppression. These results suggest that expression of closely related heterologous genes can be used to prevent enzymatic browning in a wide variety of food crops without the application of various food additives.

Evidence that a family of miniature inverted-repeat transposable elements (MITEs) from the Arabidopsis thaliana genome has arisen from a pogo-like DNA transposon

Sequence similarities exist between terminal inverted repeats (TIRs) of some miniature inverted-repeat transposable element (MITE) families isolated from a wide range of organisms, including plants, insects, and humans, and TIRs of DNA transposons from the pogo family. We present here evidence that one of these MITE families, previously described for Arabidopsis thaliana, is derived from a larger element encoding a putative transposase. We have named this novel class II transposon Lemi1. We show that its putative product is related to transposases of the Tc1/mariner superfamily, being closer to the pogo family. A similar truncated element was found in a tomato DNA sequence, indicating an ancient origin and/or horizontal transfer for this family of elements. These results are reminiscent of those recently reported for the human genome, where other members of the pogo family, named Tiggers, are believed to be responsible for the generation of abundant MITE-like elements in an early primate ancestor. These results further suggest that some MITE families, which are highly reiterated in plant, insect, and human genomes, could have arisen from a similar mechanism, implicating pogo-like elements.

Molecular cloning and characterization of apricot fruit polyphenol oxidase

A reverse transcriptase-polymerase chain reaction experiment was done to synthesize a homologous polyphenol oxidase (PPO) probe from apricot (Prunus armeniaca var Bergeron) fruit. This probe was further used to isolate a full-length PPO cDNA, PA-PPO (accession no. AF020786), from an immature-green fruit cDNA library. PA-PPO is 2070 bp long and contains a single open reading frame encoding a PPO precursor peptide of 597 amino acids with a calculated molecular mass of 67.1 kD and an isoelectric point of 6.84. The mature protein has a predicted molecular mass of 56.2 kD and an isoelectric point of 5.84. PA-PPO belongs to a multigene family. The gene is highly expressed in young, immature-green fruit and is turned off early in the ripening process. The ratio of PPO protein to total proteins per fruit apparently remains stable regardless of the stage of development, whereas PPO specific activity peaks at the breaker stage. These results suggest that, in addition to a transcriptional control of PPO expression, other regulation factors such as translational and posttranslational controls also occur.

Immunological and molecular comparison of polyphenol oxidase in Rosaceae fruit trees

An antibody raised against apple polyphenol oxidase (PPO) cross-reacted with PPOs from Japanese pear (Pyrus pyrifolia), pear (Pyrus communis), peach (Prunus persica), Chinese quince (Pseudocydonia sinensis) and Japanese loquat (Eriobotrya japonica). Core fragments (681 bp) of the corresponding PPO genes were amplified and characterized. The deduced protein sequences showed identities of 85.3 to 97.5%. Chlorogenic acid oxidase activity of these PPOs showed higher activities when assayed at pH 4 than at pH 6. These results indicate that PPOs in Rosaceae plants are structurally and enzymatically similar.

Purification and properties of a novel chloroplast stromal peptidase. Processing of polyphenol oxidase and other imported precursors

Polyphenol oxidases (PPOs) are nuclear-encoded chloroplast proteins that are targeted to the thylakoid lumen by a bipartite presequence. The N-terminal part of this sequence is removed by a stromal processing peptidase (SPP), and the resulting intermediate is translocated across the thylakoid and processed to the mature protein. A 4800-fold-purified SPP processed a PPO precursor (pPPO) at a site identical to that occurring in organelle. The in vitro product of SPP action on pPPO was further processed and translocated by thylakoids. This SPP processed other precursors but was inactive toward those of light-harvesting chlorophyll binding proteins. The enzyme appeared to be a metalloendopeptidase, like previously reported SPPs. However, it differed in substrate specificity, apparent size, and, most significantly, cleavage site of pPPO. Whereas the processing sites of lumen proteins determined so far were relatively distant from the hydrophobic core of the thylakoid targeting domain, pPPO was cleaved immediately before this domain. Cleavage removed the twin arginine motif characteristic of thylakoid targeting domains of lumen proteins, which are translocated by the DeltapH-dependent pathway. The possible significance of these observations to PPO translocation mechanism is discussed. It is suggested that several SPPs may exist in chloroplasts with preferences for different subsets of precursors.

4-Coumaroyl coenzyme A 3-hydroxylase activity from cell cultures of Lithospermum erythrorhizon and its relationship to polyphenol oxidase

A 4-coumaroyl-CoA 3-hydroxylase activity was purified 4600-fold from cell cultures of Lithospermum erythrorhizon. The enzyme showed a molecular mass of 42,400 +/- 1700 Da in gel chromatography and required ascorbate, NADH, or NADPH as cofactors. 4-Coumaroyl-CoA, 4-coumarate, p-cresol, and several other phenolic substances, but not tyrosine, were accepted as substrates for the hydroxylation. Besides hydroxylase activity, the enzyme showed diphenol oxidase activity. Both activities were inhibited by diethyldithiocarbamate or beta-mercaptoethanol, although at different concentrations. The enzyme showed striking similarity to a 4-coumaroyl-glucose 3-hydroxylase from sweet potato (Ipomoe batatas) roots, which has reportedly been purified to homogeneity and identified as a specific enzyme of chlorogenic acid biosynthesis. Close examination and comparison to a commercially available polyphenol oxidase, however, suggest that the enzyme activities purified from both Lithospermum and sweet potato are polyphenol oxidases rather than specific enzymes of secondary metabolism.

Mutational analysis of copper binding by human tyrosinase

Tyrosinase (EC 1.14.18.1) is a copper-containing enzyme that catalyzes several reactions in the biosynthesis of melanin pigments and is deficient in patients with type I oculocutaneous albinism (OCA1). Tyrosinase is thought to bind two copper ions, one at each of two conserved sequence motifs, termed CuA and CuB, but to date this has been directly proved only for the Neurospora and mushroom enzyme. Here, we demonstrate that mammalian tyrosinase directly binds copper, and that the CuA and CuB sites are both required for copper binding and for catalytic activity. We show that in human tyrosinase, copper binding by the CuB site is most likely coordinated by residues His363, His367, and His389, and that copper binding may be cooperative, with copper binding at one site facilitating copper binding by the other site. Furthermore, correct folding of the tyrosinase polypeptide appears to be necessary for copper binding, and a number of human OCA1 mutations disrupt copper binding and thus catalytic function of tyrosinase.

Sequence and structural features of plant and fungal tyrosinases

Tyrosinases from various organisms are compared with respect to enzymatic structure, primary, secondary and tertiary structure, domain structure, Cu binding sites, maturation mechanism and activation mechanism. On the basis of these comparisons, and by using hemocyanin structure as a template, a structure model for the active site of tyrosinases is proposed.

Isolation of a full-length cDNA encoding polyphenol oxidase from sugarcane, a C4 grass

Polyphenol oxidase (PPO) activity in sugarcane (a C4 grass) was highest in the growing point and declined down the stalk. Sugarcane PPO with an apparent molecular mass of 45 kDa was purified to homogeneity from immature stem tissue. Western analysis of sugarcane extracts with a polyclonal antibody raised to this protein suggested it resulted from cleavage of a 60 kDa protein during purification. The antibody was used to screen a sugarcane stem cDNA library. A full-length PPO clone (sugppo 1) was characterised and shown to encode a 67 kDa precursor protein comprising a plastid transit sequence of 8 kDa and a mature PPO protein of 59 kDa. High levels of expression of sugppo 1 were detected in the growing point of the stalk and in the immature tissue immediately below it, but no message was detected in RNA from mature stem or leaf. Comparison with other PPO sequences indicated that sugppo 1 was significantly different to PPO genes in C3 dicotyledonous plants.

Polyphenol oxidase and photosynthesis research

Very briefly, the present state of knowledge on the latent, lumen oriented polyphenol oxidase (PPO) of the chloroplast is reviewed. The location of PPO in the thylakoid membrane was described by D. Arnon 46 years ago. The N-terminus sequence of the spinach enzyme is reported. A historical sketch is given of the discovery of photophosphorylation and Arnon's visit to the admired O. Warburg.

The 64 kDa polypeptide of spinach may not be the LHCII kinase, but a lumen-located polyphenol oxidase

Phosphorylation of chlorophyll alb-binding proteins of the of photosystem II light-harvesting assembly controls the energy distribution between the two photosystems as well as the turnover of thylakoid membrane proteins. The LHCII kinase, suggested to be a 64 kDa protein, is light-regulated by a mechanism involving reduction of plastoquinone and the participation of the cytochrome b6lf complex. A cDNA encoding that protein has been isolated from a lambda gt11-based library made from spinach polyadenylated RNA using a two-step strategy involving screening by polyclonal monospecific antisera and plaque hybridization. The protein of 73.1 kDa molecular mass represents a precursor which contains a bipartite transit peptide of 101 amino acid residues (11.0 kDa) that directs the protein into the thylakoid lumen. It can be phosphorylated in vitro, and exhibits significant homology to plant polyphenol oxidases not to kinases. The gene was therefore designated PpoA. Reinvestigation of components in the molecular mass range of 50-70 kDa disclosed five additional proteins which can accompany kinase-active cytochrome b6lf, photosystem II and AMS [1] preparations. Four of them can be phosphorylated in vitro; two with apparent molecular masses of 53 and 66 kDa are capable of phosphorylation and represent new, yet unidentified proteins.

The 60-kDa precursor to the dithiothreitol-sensitive tetrameric protease of spinach thylakoids: structural similarities between the protease and polyphenol oxidase

The 60-kDa precursor to the 39-kDa dithiothreitol-sensitive protease was purified from photosystem II membranes of spinach. When partially purified 60-kDa protein was stored at 4 degrees C, the protein was degraded to fragments of 39 and 21 kDa. The 39-kDa fragment was suggested to be identical to the 39-kDa protease from effects of dithiothreitol on these polypeptides. The N-terminal amino acid sequences of the 60-kDa protein and the 39-kDa protease were the same, APILPDVEK-, suggesting that the latter was derived from the N-terminal portion of the former. Immunostaining with polyclonal antibodies against the 60-kDa protein indicated that the 60-kDa protein represents the species that occurs in the native thylakoids. These and other structural properties suggest that the protein might be identical to polyphenol oxidase.

Molecular cloning of insect pro-phenol oxidase: a copper-containing protein homologous to arthropod hemocyanin

Pro-phenol oxidase [pro-PO; zymogen of phenol oxidase (monophenol, L-dopa:oxygen oxidoreductase, EC 1.14.18.1)] is present in the hemolymph plasma of the silkworm Bombyx mori. Pro-PO is a heterodimeric protein synthesized by hemocytes. A specific serine proteinase activates both subunits through a limited proteolysis. The amino acid sequences of both subunits were deduced from their respective cDNAs; amino acid sequence homology between the subunits was 51%. The deduced amino acid sequences revealed domains highly homologous to the copper-binding site sequences (copper-binding sites A and B) of arthropod hemocyanins. The overall sequence homology between silkworm pro-PO and arthropod hemocyanins ranged from 29 to 39%. Phenol oxidases from prokaryotes, fungi, and vertebrates have sequences homologous to only the copper-binding site B of arthropod hemocyanins. Thus, silkworm pro-PO DNA described here appears distinctive and more closely related to arthropod hemocyanins. The pro-PO-activating serine proteinase was shown to hydrolyze peptide bonds at the carboxyl side of arginine in the sequence-Asn-49-Arg-50-Phe-51-Gly-52- of both subunits. Amino groups of N termini of both subunits were indicated to be N-acetylated. The cDNAs of both pro-PO subunits lacked signal peptide sequences. This result supports our contention that mature pro-PO accumulates in the cytoplasm of hemocytes and is released by cell rupture, as for arthropod hemocyanins.

Systemin activates synthesis of wound-inducible tomato leaf polyphenol oxidase via the octadecanoid defense signaling pathway

Tomato plants overexpressing a prosystemin gene that encodes the precursor of a mobile wound signal called systemin have been shown previously to constitutively synthesize extraordinarily high levels of two defensive proteinase inhibitor proteins in leaves in the absence of wounding. We herein report that leaves of these transgenic plants possess enhanced levels of another defensive protein, polyphenol oxidase (PPO) at levels that are up to 70-fold higher than levels found in leaves of wild-type plants. Supplying young wild-type tomato plants with systemin through cut stems induced PPO activity in leaves, and wounding lower leaves of young tomato plants induced PPO activity in both wounded and unwounded leaves to levels equal to those induced by systemin. Exposing young tomato plants to methyl jasmonate vapor caused an increase in PPO activity equivalent to levels found in plants overexpressing the prosystemin gene. The data indicate that PPO and proteinase inhibitor genes are coactivated systemically by wounding via the octadecanoid signal transduction pathway and that systemin has a much broader role in signaling plant defensive genes than was previously known.

An apple polyphenol oxidase cDNA is up-regulated in wounded tissues

A full-length cDNA clone encoding apple (Malus domesticus) polyphenol oxidase (PPO) was isolated from a fruit peel cDNA library. Southern analysis indicated that apple PPO is encoded by a divergent multigene family. By northern analysis, PPO mRNA was only detected in a fruit sample taken one week after full bloom. PPO mRNA accumulated in wounded tissues, and also in peel tissue showing the symptoms of superficial scald, a post-harvest disorder. The induction of PPO mRNA provides the first evidence for transcriptional control of PPO expression after wounding or the manifestation of a physiological disorder.

Isolation of a Vicia faba metallothionein-like gene: expression in foliar trichomes

Animal metallothioneins (MTs) are cysteine-rich, low-molecular-weight proteins that bind to heavy metals and are believed to play a role in their metabolism and detoxification. Genes encoding MT-like proteins have been isolated in a number of plants although their function remains to be elucidated. We describe the isolation and characterization of a bean cDNA encoding an MT-like protein. The bean gene, called MT, was isolated as a result of a differential screen for genes that are expressed in leaves but not in the most common cell type, the mesophyll cell. MT contained two regions with abundant cysteines and sequence comparison found that MT had greatest homology to MT-like subtype 2 from other plant species. Northern blot analysis demonstrated that MT was expressed in the left, stem and flower, at very low levels in roots and was not detectable in mesophyll protoplasts. MT transcript levels were not significantly affected by treatment with Cu, Zn or Cd. In the left, in situ hybridization studies demonstrated striking cell specificity with MT expression confined predominantly to trichomes. Possible explanations for the pronounced expression of MT in leaf trichomes are discussed.

Molecular cloning and characterisation of grape berry polyphenol oxidase

Polyphenol oxidase (PPO) was purified to homogeneity from Sultana grapes yielding a single protein with an apparent molecular mass of 40 kDa as determined by SDS-PAGE. A degenerate oligonucleotide primer based on the N-terminal amino acid sequence of this purified 40 kDa grape PPO protein was used to amplify a 1650 bp cDNA clone (GPO1M) by 3' rapid amplification of cDNA ends (3'-RACE). GPO1M hybridized to a single 2.2 kb transcript from grape berry mRNA indicating the presence of further upstream sequence which was cloned using 5'-RACE PCR. The complete 1990 bp cDNA (GPO1) encodes a 67 kDa protein consisting of a 10.6 kDa chloroplast transit peptide, a 40.5 kDa catalytic unit containing two copper-binding regions and a 16.2 kDa C-terminal extension. Southern analysis suggested the presence of only one PPO gene in grapevine. High levels of gene expression were found in young developing berries, leaves and roots, but there was little expression in mature tissues. Biogenesis of PPO in grapevine tissues, appears to involve synthesis of a 67 kDa precursor protein which is imported into the chloroplast and processed to remove a 10.6 kDa chloroplast transit peptide from the N-terminus and a 16.2 kDa peptide of unknown function from the C-terminus.

QTL analysis of trichome-mediated insect resistance in potato

Genetic mapping of several components of a complex type of insect resistance has been undertaken as a means toward more efficient use of the valuable characteristics of a wild relative of potato. RFLP maps constructed on interspecific diploid progenies of Solanum tuberosum × S. berthaultii were used in conjunction with morphological, biochemical and biological phenotyping to identify quantitative trait loci (QTLs) contributing to trichome-mediated insect resistance. By superimposing QTL data for a wide range of phenotypes including biochemical assays, correlative and direct screens for insect resistance, and adaptation to the target environment on the genetic maps, we have addressed the organization, action and interaction of genes controlling the resistance mechanism. The outcome contributes to an understanding of the association between component traits and between desirable and undesirable features of the donor species generated in an applied breeding program. Research is proceeding toward the development of selectable markers for the introgression and transfer of this resistance among potato gene pools.

cDNA cloning and expression of potato polyphenol oxidase

Polyphenol oxidases (PPOs) of plants are copper metalloproteins which catalyze the oxidation of mono- and o-diphenols to o-diquinones. Although PPOs are believed to be primarily responsible for the deleterious browning of many fruit and vegetable crops and are thought to be involved in plant-pest interactions, direct evidence for these roles is lacking. We report the cloning of two PPO cDNAs from Solanum tuberosum leaves. These cDNAs exhibit 97% and 98% sequence similarity at the DNA and deduced amino acid levels, respectively. Putative copper-binding regions of both cDNAs are very similar to those of mammalian, bacterial and Neurospora tyrosinases. Both leaf PPO cDNAs appear to encode polypeptides which are processed to a mature molecular weight of 57,000. In potato leaves, petioles, roots, and flowers, PPO is encoded by ca. 2 kb transcripts. Leaf PPO mRNA is developmentally regulated and only detectable in young foliage. In contrast, the protein profile of immunologically detectable PPO remains constant from the apical node through the eleventh leaf node.