Cloning and characterization of Sdga gene encoding alpha-subunit of heterotrimeric guanosine 5'-triphosphate-binding protein complex in Scoparia dulcis

A homology-based cloning strategy yielded Sdga, a cDNA clone presumably encoding alpha-subunit of heterotrimeric guanosine 5'-triphosphate-binding protein complex, from leaf tissues of Scoparia dulcis. Phylogenetic tree analysis of G-protein alpha-subunits from various biological sources suggested that, unlike in animal cells, classification of Galpha-proteins into specific subfamilies could not be applicable to the proteins from higher plants. Restriction digests of genomic DNA of S. dulcis showed a single hybridized signal in Southern blot analysis, suggesting that Sdga is a sole gene encoding Galpha-subunit in this plant. The expression level of Sdga appeared to be maintained at almost constant level after exposure of the leaves to methyl jasmonate as analyzed by reverse-transcription polymerase chain reaction. These results suggest that Sdga plays roles in methyl jasmonate-induced responses of S. dulcis without a notable change in the transcriptional level.

Cloning and expression of Dcga gene encoding alpha subunit of GTP-binding protein in carrot seedlings

A homology-based cloning strategy yielded a cDNA clone designated Dcga, presumably encoding alpha subunit of GTP-binding protein, from carrot (Daucus carota) seedlings. Molecular phylogenetic tree analysis of G protein alpha subunits from various biological sources suggested that, unlike in animal cells, classification of Galpha proteins into specific subfamilies could not be applicable to the proteins from higher plants. The restriction digests prepared from genomic DNA of carrot showed one or two hybridized signals in Southern blot analyses, and the expression level of Dcga was appreciably decreased upon the exposure of carrot to high temperature or the prolonged treatment with salt. These results suggest that Dcga occurs as single or double copy genes in carrot genome, and its transcript might play specific roles in heat- and salt-induced responses of the plant.

Cloning and expression of calmodulin gene in Scoparia dulcis

A homology-based cloning strategy yielded a cDNA clone, designated Sd-cam, encoding calmodulin protein from Scoparia dulcis. The restriction digests of genomic DNA of S. dulcis showed a single hybridized signal when probed with the fragment of this gene in Southern blot analyses, suggesting that Sd-cam occurs as a sole gene encoding calmodulin in the plant. The reverse-transcription polymerase chain reaction analysis revealed that Sd-cam was appreciably expressed in leaf, root and stem tissues. It appeared that transcription of this gene increased transiently when the leaf cultures of S. dulcis were treated with methyl jasmonate and calcium ionophore A23187. These results suggest that transcriptional activation of Sd-cam is one of the early cellular events of the methyl jasmonate-induced responses of S. dulcis.

Expression of ASK1-like genes in arrested stamens of female Silene latifolia plants

Silene latifolia is a dioecious plant in which sex is determined by heteromorphic sex chromosomes. In female plants, stamen development is arrested before microspore mother cells are formed. In this study, we isolated four cDNAs (SlSKP1-1 to 4) encoding ASK1-like protein as expression markers to reveal when expression levels are reduced in arrested stamens of female flowers. Expression patterns of the SlSKP1 genes were analyzed by in-situ hybridization. We use the flower development classification of Grant et al. (in Plant J 6:471-480, 1994). SlSKP1 genes were highly expressed in primary parietal cells and primary sporogenous cells that develop into microspore mother cells in male flowers. Expression levels started to be reduced in the external stamens of the female flowers when stamen development was arrested at stage 7. Although microspore mother cells could not be developed in female flowers and SlSKP1 expression may be unnecessary in arrested stamens, SlSKP1 genes were still expressed in sporogenous cells of degenerated stamens at stage 8. Parietal cells stopped differentiating earlier than sporogenous cells in arrested stamens. These results suggest that not all types of cell are arrested simultaneously at a particular stage of stamen development during stamen suppression of S. latifolia.

Cloning and expression of calmodulin genes regulating phytoalexin production in carrot cells

A homology-based cloning strategy yielded four cDNA clones encoding the open reading frame of carrot calmodulin, designated cam-4 and cam-8 from an oligogalacturonide elicitor-treated cell culture and cam-11 and cam-16 from cells exposed to ethylene, respectively. Reverse-transcription polymerase chain reaction analyses revealed that the expression of the cam-4 gene increased transiently when carrot seedlings were treated with oligogalacturonides, although, the cells incubated with fungal mycelial walls or ethylene did not show a significant change in the expression level. In contrast, marked and transient expression was observed for either cam-11 or cam-16 only when carrot cells were treated with ethylene. These results suggest that two classes of stimuli which are capable of triggering phytoalexin production in carrot cells, oligogalacturonides and ethylene, evoke the activation of the Ca2+ cascade in the cells by expressing distinct calmodulin genes to initiate the biosynthesis of the compound.

Multiple Forms of Calmodulin Genes in Carrot Treated with Fungal Mycelial Walls

Eleven cDNA clones encoding the open-reading frame of calmodulin and calmodulin-like proteins were isolated from carrot seedlings treated with fungal mycelial walls. These calmodulin genes were mainly expressed in the stems and leaves of carrot, although the transcriptional level was almost negligible in the seeds and root tissues. Based on nucleotide homology, these genes were divided into two classes, class I and class II, and reverse transcription-polymerase chain reaction analyses revealed that the expression level of class II genes transiently increased when carrot seedlings were treated with the mycelial walls. In contrast, the level of class I genes apparently did not show any significant change even after treatment with the fungal components. These results suggest that the defense responses induced in the fungi-infected carrot accompany the increase in the transcriptional levels of a certain calmodulin gene(s).

Activation of Phenylpropanoid Metabolism in Sesame by Over-expression of Carrot Calmodulin Gene

Transgenic sesame (Sesamum schinzianum ASCH.) was produced by Agrobacterium-mediated transfection of a carrot calmodulin gene, cam-4, which was specifically expressed upon the contact of carrot cells with oligogalacturonide elicitor. Coding region of cam-4 was ligated to the downstream of 35S promoter of cauliflower mosaic virus and subcloned into pMATGBO-DB3.1. A. tumefaciens 4404 was transformed with the constructed vector, and the crown gall tissues formed in the sesame seedlings were transferred onto appropriate media to obtain the re-differentiated plants. The reverse-transcription polymerase chain reaction followed by Southern blot analysis revealed that cam-4 gene was appreciably expressed in the transgenic plants. Activities of two key enzyme regulating phenylpropanoid metabolisms, phenylalanine ammonia-lyase and caffeic acid O-methyltransferase, and the contents of phenolic compounds in the transformed sesame were markedly elevated as compared with those of the control. These results suggest that the over-expression of cam-4 gene enhances the biosynthetic activities of phenylpropane derivatives in the transformed sesame plants.

Activation of acyl condensation reaction of monomeric 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme, by free coenzyme A

6-Hydroxymellein (6HM) synthase is a multifunctional polyketide enzyme induced in carrot cells, whose fully active homodimer catalyzes condensation of acyl-CoAs and the NADPH-dependent ketoreduction of the enzyme-bound intermediate. 6HM-forming activity of the synthase was markedly decreased when the reaction mixture pH was adjusted from 7.5 to 6.0. However, under these slightly acidic conditions, the acyl condensation catalyzed by the dissociated monomer enzyme was appreciably stimulated by addition of free coenzyme A (CoA). In contrast, the condensation reaction at pH 6.0 was significantly inhibited in the presence of CoA when the reaction was carried out with the NADPH-omitted dimer synthase. Among the kinetic parameters of the acyl condensation, velocity of the monomer-catalyzing reaction at the acidic pH was appreciably increased upon addition of CoA while K(m)s did not show any significant change in the presence and absence of the compound. These results suggest that CoA associates with a specific site in the dissociated monomeric form of 6HM synthase, and the velocity of the acyl condensation reaction catalyzed by the CoA-synthase complex appreciably increases in acidic conditions.

Cancer chemopreventive effects of a Brazilian folk medicine, Juca, on in vivo two-stage skin carcinogenesis

Gallic acid (1) and methyl gallate (2) were isolated from Juca, a Brazilian folk medicine, fruits of Caesalpinia ferrea MART (Leguminosae), decreased significantly the average number of papillomas per mouse in the experiment of the promoting effects of 12-O-tetra- decanoylphorbol-13-acetate (TPA) on skin tumor formation in mice initiated with 7,12-dimethylbenz[a]anthracene (DMBA).

Cancer chemopreventive effects of constituents of Caesalpinia ferrea and related compounds

The anti-tumor promoting effects of fruits of Caesalpinia ferrea MART. (Leguminosae) were tested by the in vitro Epstein-Barr virus early antigen (EBV-EA) activation assay, and its active constituents were identified as gallic acid (1) and methyl gallate (2). A total of 49 related compounds of 1 and 2 were analysed for the effects by this assay, and the structure activity relationships have been proposed. Three acetophenone derivatives, 2,6-dihydroxyacetophenone (48), 2,3,4-trihydroxyacetophenone (50) and 2,4,6-trihydroxy- acetophenone (51) were found to show potent inhibitory activity.

Regulation of catalytic activity of a multifunctional polyketide biosynthetic enzyme, 6-hydroxymellein synthase, by interaction between NADPH and phenylglyoxal-sensitive amino acid residue at the reaction center

Treatment of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme in carrot cells, with phenylglyoxal yielded a chemically modified protein in which approximately two moles of the reagent were covalently attached to each subunit of the enzyme. Only NADH- but not NADPH-associated form of native 6-hydroxymellein synthase was inhibited by cerulenin; however, the NADPH-synthase complex lost the insensitivity by the chemical modification of the enzyme protein with phenylglyoxal. Appreciable differences in K(m) values observed between the NADPH- and NADH-associated enzymes were greatly reduced by the treatment with phenylglyoxal. Although the catalytic activity of the NADPH-associated synthase was enhanced by the addition of free CoA, the compound exhibited a significant inhibitory activity to the phenylglyoxal-modified enzyme. A marked deuterium isotope effect in the catalytic reaction of the native synthase-NADPH complex was appreciably decreased in the chemically modified enzyme. These results strongly suggest that an electrostatic interaction between the phosphate group attached to the 2'-position of adenosyl moiety of NADPH and the phenylglyoxal-sensitive amino acid residue, probably arginine, at the reaction center of 6-hydroxymellein synthase regulates several biochemical properties of this multifunctional enzyme.

Involvement of GTP-binding protein in the induction of phytoalexin biosynthesis in cultured carrot cells

Biosynthetic activity of carrot phytoalexin 6-methoxymellen was induced in cell suspension culture by the treatment with oligogalacturonide elicitor; however, the elicitor-induced activity appreciably reduced in the presence of suramin, a potent inhibitor of GTP-binding proteins. In contrast, addition of G-protein activators, such as mastoparan or GTP-gamma-S, to carrot cell culture triggered 6-methoxymellein production even in the absence of uronide elicitor. An appreciable GTPase activity was found in purified plasma membrane of cultured carrot cells, and the hydrolytic activity was significantly increased by the addition of elicitor. Carrot plasma membrane was capable of associating with GTP-gamma-S, and the binding ability was markedly increased in the presence of elicitor. However, the binding activity markedly decreased when the membrane preparation was pre-incubated with GTP but not with ATP. These observations strongly suggest that a certain GTP-binding protein located at plasma membrane of cultured carrot cells plays an important role in the oligogalacturonide elicitor-induced 6-methoxymellein production.

Role of reducing co-factors in catalytic reactions of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme in carrot cells

6-Hydroxymellein (6HM) synthase, a multifunctional polyketide biosynthetic enzyme in carrot cells, is capable of catalyzing the acyl-CoA condensation and the ketoreduction in the presence of the nucleotide reducing co-factors. Although free CoA at high concentrations functioned as the activator of the NADPH-dependent 6HM formation, the compound exhibited an appreciable inhibitory activity toward the reaction mediated by NADH. CoA showed a potent inhibitory activity against substrate entry into the reaction center of the NADH-associated enzyme while, in the presence of NADPH, the compound slightly inhibited the formation of the acylated enzyme. The catalytic rate of the synthase was appreciably decreased when NADPH was replaced by the deuterium-labeled compound, however, the kH/kD value was markedly reduced if NADH and [D]NADH were employed as the reducing co-factors. These results suggest that the phosphate group attached to 2'-position of the adenosyl moiety of NADPH associated with the ketoreducing domain of 6HM synthase plays an important role in the regulation of the enzyme activity.

Unusual arrangement of catalytic domains in head-to-tail associated homodimer of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme

6-Hydroxymellein synthase, a multifunctional polyketide synthetic enzyme in carrot, is organized as a homodimer, and the activity of the synthase was appreciably inhibited upon the specific alkylation of cysteine- and cysteamine-SHs at the reaction center with iodoacetoamide and chloroacetyl-CoA, respectively. Dissociation and stoichiometric recombination of the unmodified and the SH-modified enzyme subunits yielded a combination of unmodified-unmodified, unmodified-modified and modified-modified hybrid dimers that together exhibit 50% activity. In contrast, hybrid dimers obtained by reconstruction of the two modified enzymes showed essentially no catalytic activity. These results suggest that the two subunits of 6-hydroxymellein synthase are aligned in head-to-tail orientation to organize two reaction centers which are comprised of a cysteine and a complementary cysteamine SH group, belonging to and contributed from the same subunit in the homodimer structure.

Cross-reaction of chalcone synthase and stilbene synthase overexpressed in Escherichia coli

Chalcone synthase (CHS) and stilbene synthase (STS) are related plant polyketide synthases belonging to the CHS superfamily. CHS and STS catalyze common condensation reactions of p-coumaroyl-CoA and three C(2)-units from malonyl-CoA but different cyclization reactions to produce naringenin chalcone and resveratrol, respectively. Using purified Pueraria lobata CHS and Arachis hypogaea STS overexpressed in Escherichia coli, bisnoryangonin (BNY, the derailed lactone after two condensations) and p-coumaroyltriacetic acid lactone (the derailed lactone after three condensations) were detected from the reaction products. More importantly, we found a cross-reaction between CHS and STS, i.e. resveratrol production by CHS (2.7-4.2% of naringenin) and naringenin production by STS (1.4-2.3% of resveratrol), possibly due to the conformational flexibility of their active sites.

Role of reducing co-factor in cerulenin-insensitivity of 6-hydroxymellein synthase in carrot cell extract

The activity of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme of carrot, was not inhibited by cerulenin in the presence of NADPH. However, cerulenin showed a marked inhibitory activity to the synthase if the reducing co-factor was omitted from the assay mixture. The synthase was also sensitive to the antibiotic even in the presence of NADPH when the acyl condensation site and the reducing domain at the reaction center of the enzyme were dissociated under the high ionic strength condition. In addition, the synthase activity was appreciably inhibited when NADH was employed instead of NADPH. These observations strongly suggest that a phosphate group attached to 2'-position of adenosyl moiety of NADPH molecule plays an important role in the apparent insensitivity of 6-hydroxymellein synthase toward cerulenin.

Role of inward K+ channel located at carrot plasma membrane in signal cross-talking of cAMP with Ca2+ cascade

Treatment of cultured carrot cells with dibutyryl cAMP or forskolin resulted in the appreciable decrease in extracellular K+ concentration. This decrease was found to be transient and the concentration of the ion in the culture medium restored to the original level within few minutes. The cAMP-induced decrease in K+ level in the medium was almost completely inhibited when carrot cells were incubated in the presence of K+ channel blockers, CsCl and tetraethylammonium chloride. Appreciable amounts of 45Ca2+ were discharged from 45Ca2+-loaded inside-out vesicles of carrot plasma membrane by the stimulation with cAMP, however, the release of the ion was significantly inhibited in the presence of the K+ channel blockers. The release of 45Ca2+ from the vesicles was also observed when K+ current was evoked with an ionophore, valinomycin, even in the absence of cAMP. These results suggest that the gating of some of the inward K+ channels located at plasma membrane of cultured carrot cells is controlled by cytoplasmic concentration of cAMP and the inward K+ current across the plasma membrane induced by the nucleotide elicits Ca2+ influx into the cells possibly by the activation of voltage-dependent Ca2+ channels.

Effect of NADPH-associated keto-reducing domain on substrate entry into 6-hydroxymellein synthase, a multifunctional polyketide synthetic enzyme involved in phytoalexin biosynthesis in carrot

6-Hydroxymellein synthase is a polyketide biosynthetic enzyme induced in carrot cells which is organized as a homodimer composed of multifunctional subunits. The synthase liberates triacetic acid lactone, instead of 6-hydroxymellein, as a derailment product when the keto-reducing reaction at the triketide intermediate stage is interrupted. However, the efficiency of the triacetic acid lactone-forming reactions is markedly lower than that of the normal reaction, and the kinetic analyses have revealed that the affinity of the enzyme protein for acetyl-CoA is appreciably reduced in the abnormal reactions. It is assumed that the interaction of the NADPH-associated keto-reducing domain with a putative primary binding site(s) of the acyl-CoA in the enzyme structure affects the entry of the starter unit into the protein. The present finding should provide an example of the novel class of "subunit communication" of multimer enzymes.

Transacylase-like structure and its role in substrate channeling of 6-hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme in carrot cell extracts

6-Hydroxymellein synthase, a multifunctional polyketide biosynthetic enzyme of carrot, lost the binding ability toward its co-substrates, acetyl- and malonyl-CoAs, by the treatment with the blocking reagents for serine-OH. In contrast, the enzyme retained the binding ability even when the two SH groups at the reaction center (cysteine-SH of the condensation enzyme and cysteamine-SH of acyl carrier protein) were blocked, and one substrate bound to the SH-blocked enzyme was readily replaced by the other. It appeared that the cysteine-SH accepted only acetyl moiety while cysteamine-SH was preferentially malonylated in the presence of both of the substrates. These results suggest that transacylase-like domain is involved in the structure of 6-hydroxymellein synthase as a common primary binding site of its co-substrates, and acetyl and malonyl moieties are properly channeled from their CoA esters to cysteine-SH and acyl carrier protein-SH via this domain, respectively.

Dissociation of dimeric 6-hydroxymellein synthase, a polyketide biosynthetic enzyme in carrot cell extracts, with loss of keto-reducing activity

6-Hydroxymellein synthase, an inducible polyketide biosynthetic enzyme in carrot cell extracts, is composed of two identical subunits, and the homodimer is dissociated to monomeric peptides under high-ionic-strength conditions with loss of the synthase activity. Appreciable radioactivities were associated with the synthase proteins when the monomer enzyme was incubated with the radiolabeled substrates, acetyl-coenzyme A (CoA) and malonyl-CoA. Therefore, it appeared that the synthase does not lose the ability of binding the substrate even after the dissociation to monomers. The monomeric synthase liberated triacetic acid lactone as the derailment product instead of 6-hydroxymellein from the enzyme-attached triketomethylene chain which is the immediate precursor of an NADPH-dependent keto-reducing reaction involved in 6-hydroxymellein biosynthesis. These observations strongly suggest that the monomeric synthase retains the ability of ketomethylene chain elongation by the condensation of acyl-CoAs, but is lacking in an NADPH-dependent keto-reducing activity toward the triketide intermediate. Results obtained in the present experiments imply that the catalytic domain of acyl-CoA condensation is able to associate with that of keto reduction, possibly belonging to another subunit, only in the homodimeric structure to organize the multicatalytic reaction center.