Aspects related to mevalonate biosynthesis in plants

Targeted metabolome and transcriptome analyses reveal changes in gibberellin and related cell wall-acting enzyme-encoding genes during stipe elongation in Flammulina filiformis

Flammulina filiformis, a typical agaric fungus, is a widely cultivated and consumed edible mushroom. Elongation of its stipe (as the main edible part) is closely related to its yield and commercial traits; however, the endogenous hormones during stipe elongation and their regulatory mechanisms are not well understood. Gibberellin (GA) plays an important role in the regulation of plant growth, but little has been reported in macro fungi. In this study, we first treated F. filiformis stipes in the young stage with PBZ (an inhibitor of GA) and found that PBZ significantly inhibited elongation of the stipe. Then, we performed GA-targeted metabolome and transcriptome analyses of the stipe at both the young and elongation stages. A total of 13 types of GAs were detected in F. filiformis; the contents of ten of them, namely, GA3, GA4, GA8, GA14, GA19, GA20, GA24, GA34, GA44, and GA53, were significantly decreased, and the contents of three (GA5, GA9, and GA29) were significantly increased during stipe elongation. Transcriptome analysis showed that the genes in the terpenoid backbone biosynthesis pathway showed varying expression patterns: HMGS, HMGR, GPS, and FPPS were significantly upregulated, while CPS/KS had no significant difference in transcript level during stipe elongation. In total, 37 P450 genes were annotated to be involved in GA biosynthesis; eight of them were upregulated, twelve were downregulated, and the rest were not differentially expressed. In addition, four types of differentially expressed genes involved in stipe elongation were identified, including six signal transduction genes, five cell cycle-controlling genes, twelve cell wall-related enzymes and six transcription factors. The results identified the types and content of GAs and the expression patterns of their synthesis pathways during elongation in F. filiformis and revealed the molecular mechanisms by which GAs may affect the synthesis of cell wall components and the cell cycle of the stipe through the downstream action of cell wall-related enzymes, transcription factors, signal transduction and cell cycle control, thus regulating stipe elongation. This study is helpful for understanding the roles of GAs in stipe development in mushrooms and lays the foundation for the rational regulation of stipe length in agaric mushrooms during production.

LcWRKY17, a WRKY Transcription Factor from Litsea cubeba, Effectively Promotes Monoterpene Synthesis

The WRKY gene family is one of the most significant transcription factor (TF) families in higher plants and participates in many secondary metabolic processes in plants. Litsea cubeba (Lour.) Person is an important woody oil plant that is high in terpenoids. However, no studies have been conducted to investigate the WRKY TFs that regulate the synthesis of terpene in L. cubeba. This paper provides a comprehensive genomic analysis of the LcWRKYs. In the L. cubeba genome, 64 LcWRKY genes were discovered. According to a comparative phylogenetic study with Arabidopsis thaliana, these L. cubeba WRKYs were divided into three groups. Some LcWRKY genes may have arisen from gene duplication, but the majority of LcWRKY evolution has been driven by segmental duplication events. Based on transcriptome data, a consistent expression pattern of LcWRKY17 and terpene synthase LcTPS42 was found at different stages of L. cubeba fruit development. Furthermore, the function of LcWRKY17 was verified by subcellular localization and transient overexpression, and overexpression of LcWRKY17 promotes monoterpene synthesis. Meanwhile, dual-Luciferase and yeast one-hybrid (Y1H) experiments showed that the LcWRKY17 transcription factor binds to W-box motifs of LcTPS42 and enhances its transcription. In conclusion, this research provided a fundamental framework for future functional analysis of the WRKY gene families, as well as breeding improvement and the regulation of secondary metabolism in L. cubeba.

Transcriptome-wide identification of WRKY transcription factors and their expression profiles in response to methyl jasmonate in Platycodon grandiflorus

Platycodon grandiflorus, a perennial flowering plant widely distributed in China and South Korea, is an excellent resource for both food and medicine. The main active compounds of P. grandiflorus are triterpenoid saponins. WRKY transcription factors (TFs) are among the largest gene families in plants and play an important role in regulating plant terpenoid accumulation, physiological metabolism, and stress response. Numerous studies have been reported on other medicinal plants; however, little is known about WRKY genes in P. grandiflorus. In this study, 27 PgWRKYs were identified in the P. grandiflorus transcriptome. Phylogenetic analysis showed that PgWRKY genes were clustered into three main groups and five subgroups. Transcriptome analysis showed that the PgWRKY gene expression patterns in different tissues differed between those in Tongcheng City (Southern Anhui) and Taihe County (Northern Anhui). Gene expression analysis based on RNA sequencing and qRT-PCR analysis showed that most PgWRKY genes were expressed after induction with methyl jasmonate (MeJA). Co-expressing PgWRKY genes with triterpenoid biosynthesis pathway genes revealed four PgWRKY genes that may have functions in triterpenoid biosynthesis. Additionally, functional annotation and protein-protein interaction analysis of PgWRKY proteins were performed to predict their roles in potential regulatory networks. Thus, we systematically analyzed the structure, evolution, and expression patterns of PgWRKY genes to provide an important theoretical basis for further exploring the molecular basis and regulatory mechanism of WRKY TFs in triterpenoid biosynthesis.

Transcriptome analysis identifies putative genes involved in triterpenoid biosynthesis in Platycodon grandiflorus

Comprehensive transcriptome analysis of different Platycodon grandiflorus tissues discovered genes related to triterpenoid saponin biosynthesis. Platycodon grandiflorus (Jacq.) A. DC. (P. grandiflorus), a traditional Chinese medicine, contains considerable triterpenoid saponins with broad pharmacological activities. Triterpenoid saponins are the major components of P. grandiflorus. Here, single-molecule real-time and next-generation sequencing technologies were combined to comprehensively analyse the transcriptome and identify genes involved in triterpenoid saponin biosynthesis in P. grandiflorus. We quantified four saponins in P. grandiflorus and found that their total content was highest in the roots and lowest in the stems and leaves. A total of 173,354 non-redundant transcripts were generated from the PacBio platform, and three full-length transcripts of β-amyrin synthase, the key synthase of β-amyrin, were identified. A total of 132,610 clean reads obtained from the DNBSEQ platform were utilised to explore key genes related to the triterpenoid saponin biosynthetic pathway in P. grandiflorus, and 96 differentially expressed genes were selected as candidates. The expression levels of these genes were verified by quantitative real-time PCR. Our reliable transcriptome data provide valuable information on the related biosynthesis pathway and may provide insights into the molecular mechanisms of triterpenoid saponin biosynthesis in P. grandiflorus.

The specific molecular architecture of plant 3-hydroxy-3-methylglutaryl-CoA lyase

3-Hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase (HMGL) is involved in branched-chain amino acid catabolism leading to acetyl-CoA production. Here, using bioinformatics analyses and protein sequence alignments, we found that in Arabidopsis thaliana a single gene encodes two HMGL isoforms differing in size (51 kDa, HMGL51 and 46 kDa, HMGL46). Similar to animal HMGLs, both isoforms comprised a C-terminal type 1 peroxisomal retention motif, and HMGL51 contained a mitochondrial leader peptide. We observed that only a shortened HMGL (35 kDa, HMGL35) is conserved across all kingdoms of life. Most notably, all plant HMGLs also contained a specific N-terminal extension (P100) that is located between the N-terminal mitochondrial targeting sequence TP35 and HMGL35 and is absent in bacteria and other eukaryotes. Interestingly, using HMGL enzyme assays, we found that rather than HMGL46, homodimeric recombinant HMGL35 is the active enzyme catalyzing acetyl-CoA and acetoacetate synthesis when incubated with (S)-HMG-CoA. This suggested that the plant-specific P100 peptide may inactivate HMGL according to specific physiological requirements. Therefore, we investigated whether the P100 peptide in HMGL46 alters its activity, possibly by modifying the HMGL46 structure. We found that induced expression of a cytosolic HMGL35 version in A. thaliana delays germination and leads to rapid wilting and chlorosis in mature plants. Our results suggest that in plants, P100-mediated HMGL inactivation outside of peroxisomes or mitochondria is crucial, protecting against potentially cytotoxic effects of HMGL activity while it transits to these organelles.

The potential of the mevalonate pathway for enhanced isoprenoid production

The cytosol-localised mevalonic acid (MVA) pathway delivers the basic isoprene unit isopentenyl diphosphate (IPP). In higher plants, this central metabolic intermediate is also synthesised by the plastid-localised methylerythritol phosphate (MEP) pathway. Both MVA and MEP pathways conspire through exchange of intermediates and regulatory interactions. Products downstream of IPP such as phytosterols, carotenoids, vitamin E, artemisinin, tanshinone and paclitaxel demonstrate antioxidant, cholesterol-reducing, anti-ageing, anticancer, antimalarial, anti-inflammatory and antibacterial activities. Other isoprenoid precursors including isoprene, isoprenol, geraniol, farnesene and farnesol are economically valuable. An update on the MVA pathway and its interaction with the MEP pathway is presented, including the improvement in the production of phytosterols and other isoprenoid derivatives. Such attempts are for instance based on the bioengineering of microbes such as Escherichia coli and Saccharomyces cerevisiae, as well as plants. The function of relevant genes in the MVA pathway that can be utilised in metabolic engineering is reviewed and future perspectives are presented.

Past achievements, current status and future perspectives of studies on 3-hydroxy-3-methylglutaryl-CoA synthase (HMGS) in the mevalonate (MVA) pathway

HMGS functions in phytosterol biosynthesis, development and stress responses. F-244 could specifically-inhibit HMGS in tobacco BY-2 cells and Brassica seedlings. An update on HMGS from higher plants is presented. 3-Hydroxy-3-methylglutaryl-coenzyme A synthase (HMGS) is the second enzyme in the mevalonate pathway of isoprenoid biosynthesis and catalyzes the condensation of acetoacetyl-CoA and acetyl-CoA to produce S-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA). Besides HMG-CoA reductase (HMGR), HMGS is another key enzyme in the regulation of cholesterol and ketone bodies in mammals. In plants, it plays an important role in phytosterol biosynthesis. Here, we summarize the past investigations on eukaryotic HMGS with particular focus on plant HMGS, its enzymatic properties, gene expression, protein structure, and its current status of research in China. An update of the findings on HMGS from animals (human, rat, avian) to plants (Brassica juncea, Hevea brasiliensis, Arabidopsis thaliana) will be discussed. Current studies on HMGS have been vastly promoted by developments in biochemistry and molecular biology. Nonetheless, several limitations have been encountered, thus some novel advances in HMGS-related research that have recently emerged will be touched on.

Brassica juncea 3-hydroxy-3-methylglutaryl (HMG)-CoA synthase 1: expression and characterization of recombinant wild-type and mutant enzymes

3-hydroxy-3-methylglutaryl (HMG)-CoA synthase (HMGS; EC 2.3.3.10) is the second enzyme in the cytoplasmic mevalonate pathway of isoprenoid biosynthesis, and catalyses the condensation of acetyl-CoA with acetoacetyl-CoA (AcAc-CoA) to yield S-HMG-CoA. In this study, we have first characterized in detail a plant HMGS, Brassica juncea HMGS1 (BjHMGS1), as a His6-tagged protein from Escherichia coli. Native gel electrophoresis analysis showed that the enzyme behaves as a homodimer with a calculated mass of 105.8 kDa. It is activated by 5 mM dithioerythreitol and is inhibited by F-244 which is specific for HMGS enzymes. It has a pH optimum of 8.5 and a temperature optimum of 35 degrees C, with an energy of activation of 62.5 J x mol(-1). Unlike cytosolic HMGS from chicken and cockroach, cations like Mg2+, Mn2+, Zn2+ and Co2+ did not stimulate His6-BjHMGS1 activity in vitro; instead all except Mg2+ were inhibitory. His6-BjHMGS1 has an apparent K(m-acetyl-CoA) of 43 microM and a V(max) of 0.47 micromol x mg(-1) x min(-1), and was inhibited by one of the substrates (AcAc-CoA) and by both products (HMG-CoA and HS-CoA). Site-directed mutagenesis of conserved amino acid residues in BjHMGS1 revealed that substitutions R157A, H188N and C212S resulted in a decreased V(max), indicating some involvement of these residues in catalytic capacity. Unlike His6-BjHMGS1 and its soluble purified mutant derivatives, the H188N mutant did not display substrate inhibition by AcAc-CoA. Substitution S359A resulted in a 10-fold increased specific activity. Based on these kinetic analyses, we generated a novel double mutation H188N/S359A, which resulted in a 10-fold increased specific activity, but still lacking inhibition by AcAc-CoA, strongly suggesting that His-188 is involved in conferring substrate inhibition on His6-BjHMGS1. Substitution of an aminoacyl residue resulting in loss of substrate inhibition has never been previously reported for any HMGS.

A review of tobacco BY-2 cells as an excellent system to study the synthesis and function of sterols and other isoprenoids

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate (IPP), the universal precursor for isoprenoid biosynthesis. In this paper we review findings and observations made primarily with tobacco BY-2 cells (TBY-2), which have proven to be an excellent system in which to study the two biosynthetic pathways. A major advantage of these cells as an experimental system is their ability to readily take up specific inhibitors and stably- and/or radiolabeled precursors. This permits the functional elucidation of the role of isoprenoid end products and intermediates. Because TBY-2 cells undergo rapid cell division and can be synchronized within the cell cycle, they constitute a highly suitable test system for determination of those isoprenoids and intermediates that act as cell cycle inhibitors, thus giving an indication of which branches of the isoprenoid pathway are essential. Through chemical complementation; and use of precursors, intracellular compartmentation can be elucidated, as well as the extent to which the plastidial and cytosolic pathways contribute to the syntheses of specific groups of isoprenoids (e.g., sterols) via exchange of intermediates across membranes. These topics are discussed in the context of the pertinent literature.

Inhibition of squalene synthase and squalene epoxidase in tobacco cells triggers an up-regulation of 3-hydroxy-3-methylglutaryl coenzyme a reductase

To get some insight into the regulatory mechanisms controlling the sterol branch of the mevalonate pathway, tobacco (Nicotiana tabacum cv Bright Yellow-2) cell suspensions were treated with squalestatin-1 and terbinafine, two specific inhibitors of squalene synthase (SQS) and squalene epoxidase, respectively. These two enzymes catalyze the first two steps involved in sterol biosynthesis. In highly dividing cells, SQS was actively expressed concomitantly with 3-hydroxy-3-methylglutaryl coenzyme A reductase and both sterol methyltransferases. At nanomolar concentrations, squalestatin was found to inhibit efficiently sterol biosynthesis as attested by the rapid decrease in SQS activity and [(14)C]radioactivity from acetate incorporated into sterols. A parallel dose-dependent accumulation of farnesol, the dephosphorylated form of the SQS substrate, was observed without affecting farnesyl diphosphate synthase steady-state mRNA levels. Treatment of tobacco cells with terbinafine is also shown to inhibit sterol synthesis. In addition, this inhibitor induced an impressive accumulation of squalene and a dose-dependent stimulation of the triacylglycerol content and synthesis, suggesting the occurrence of regulatory relationships between sterol and triacylglycerol biosynthetic pathways. We demonstrate that squalene was stored in cytosolic lipid particles, but could be redirected toward sterol synthesis if required. Inhibition of either SQS or squalene epoxidase was found to trigger a severalfold increase in enzyme activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase, giving first evidence for a positive feedback regulation of this key enzyme in response to a selective depletion of endogenous sterols. At the same time, no compensatory responses mediated by SQS were observed, in sharp contrast to the situation in mammalian cells.

Zeaxanthin and menaquinone-7 biosynthesis in Sphingobacterium multivorum via the methylerythritol phosphate pathway

Feeding of [1-(13)C]glucose, [U-(13)C(6)]glucose, [3-(13)C]alanine and [1-(13)C]acetate to Sphingobacterium multivorum showed that this bacterium utilizes the methylerythritol phosphate pathway for the biosynthesis of menaquinone-7 and zeaxanthin, a carotenoid of industrial importance. Differential incorporation of the labeled precursors gave some insight into the preferred carbon sources involved in isoprenoid biosynthesis.

The N-terminal domain of tomato 3-hydroxy-3-methylglutaryl-CoA reductases. Sequence, microsomal targeting, and glycosylation

The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the conversion of 3-hydroxy-3-methylglutaryl-CoA to mevalonic acid, considered the rate-limiting step in isoprenoid biosynthesis. In plants, isoprenoid compounds play important roles in mediating plant growth and development, electron transport, photosynthesis, and disease resistance. Sequence comparisons of plant HMGR proteins with those from yeast and mammalian systems reveal high levels of sequence identity within the catalytic domain but significant divergence in the membrane domain. Mammalian HMGRs are integral membrane proteins of the endoplasmic reticulum with eight membrane-spanning regions. In contrast, the membrane domain of plant HMGRs is predicted to contain only one to two transmembrane spans. We have isolated and sequenced a clone (pCD4) encoding exon 1 of tomato hmg1. The membrane domain structures of two differentially regulated tomato HMGR isoforms, HMG1 and HMG2, were analyzed using in vitro transcription and translation systems. Microsomal membrane insertion of the tomato HMGRs is co-translational and does not involve cleavage of an N-terminal targeting peptide. HMGR membrane topography was established by protease protection studies of the HMG1 membrane domain and an analogous region of HMG2 engineered to contain a c-myc epitope tag. The data indicate that both tomato HMGRs span the membrane two times with both the C and N termini located in the cytosol. Lumenal localization of the short peptide predicted to lie within the endoplasmic reticulum was further confirmed by in vitro glycosylation of an asparagine-linked glycosylation site present in HMG2.

Arabidopsis thaliana contains two differentially expressed farnesyl-diphosphate synthase genes

The enzyme farnesyl-diphosphate synthase (FPS; EC 2.5.1.1/EC 2.5.1.10) catalyzes the synthesis of farnesyl diphosphate (FPP) from isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP). This reaction is considered to be a rate-limiting step in isoprenoid biosynthesis. Southern blot analysis indicates that Arabidopsis thaliana contains at least 2 genes (FPS1 and FPS2) encoding FPS. The FPS1 and FPS2 genes have been cloned and characterized. The two genes have a very similar organization with regard to intron positions and exon sizes and share a high level of sequence similarity, not only in the coding region but also in the intronic sequences. Northern blot analysis showed that FPS1 and FPS2 have a different pattern of expression. FPS1 mRNA accumulates preferentially in roots and inflorescences, whereas FPS2 mRNA is predominantly expressed in inflorescences. The cDNA corresponding to the FPS1 gene was isolated by functional complementation of a mutant yeast strain defective in FPS activity (Delourme, D., Lacroute, F., and Karst, F. (1994) Plant Mol. Biol. 26, 1867-1873). By using a reverse transcription-polymerase chain reaction strategy we have cloned the cDNA corresponding to the FPS2 gene. Analysis of the FPS2 cDNA sequence revealed an open reading frame encoding a protein of 342 amino acid residues with a predicted molecular mass of 39,825 Da. FPS1 and FPS2 isoforms share an overall amino acid identity of 90.6%. Arabidopsis FPS2 was able to rescue the lethal phenotype of an ERG20-disrupted yeast strain. We demonstrate that FPS2 catalyzes the two successive condensations of IPP with both DMAPP and geranyl diphosphate leading to FPP. The significance of the occurrence of different FPS isoforms in plants is discussed in the context of the complex organization of the plant isoprenoid pathway.

Bacterial expression of the catalytic domain of 3-hydroxy-3-methylglutaryl-CoA reductase (isoform HMGR1) from Arabidopsis thaliana, and its inactivation by phosphorylation at Ser577 by Brassica oleracea 3-hydroxy-3-methylglutaryl-CoA reductase kinase

The catalytic domain of 3-hydroxy-3-methylglutaryl-CoA reductase isoform 1 (HMGR1cd) from Arabidopsis thaliana has been expressed in Escherichia coli in a catalytically active form and purified. The high efficiency of the bacterial expression system together with the simplicity of the purification procedure used in this study resulted in the attainment of large quantities of pure enzyme (about 5 mg/l culture) with a final specific activity of up to 17 U/mg. This specific activity is higher than that reported to date for any 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR) purified from a plant source. HMGR1cd activity was completely blocked by the HMGR inhibitor mevinolin (IC50 = 12.5 nM). No significant differences were observed between the Km values of HMGR1cd for NADPH (71 +/- 7 microM) and (S)-3-hydroxy-3-methylglutaryl-CoA (8.3 +/- 1.5 microM) and those of pure HMGR preparations obtained from different plant sources. The purified HMGR1cd was reversibly inactivated by phosphorylation at a single site by Brassica oleracea HMGR kinase A, which is functionally related to the mammalian AMP-activated protein kinase. The site of phosphorylation is Ser577 in the complete sequence of A. thaliana HMGR1. The results in this paper represent the first evidence that a higher plant HMGR is regulated by direct phosphorylation, at least in a cell-free system. Our results also reinforce the view that the AMP-activated protein kinase/SNF1 family is an ancient and highly conserved protein kinase system.

Some new aspects of isoprenoid biosynthesis in plants--a review

Plants are capable of synthesizing a myriad of isoprenoids and prenyl lipids. Much attention has been focused on 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the enzyme that synthesizes mevalonate and is generally considered responsible for the regulation of substrate flux to isoprenoids. In contrast to vertebrates, where there seems to exist only one HMGR gene, in plants a small family of isogenes appears differentially expressed in regard to location and time. Much less is known in plants about the preceding steps, viz. the conversion of acetyl-CoA to HMG-CoA. An enzyme system has been isolated from radish that can catalyze this transformation, and which shows some unusual properties in vitro. The intracellular localization of the early steps of isoprenoid biosynthesis in plant cells is still a matter of debate. The various observations and hypotheses derived from incorporation and inhibition studies are somewhat contradictory, and an attempt is being made to rationalize various findings that do not at first seem compatible. There are good arguments in favor of an exclusively cytoplasmic formation of isopentenyl pyrophosphate (IPP) via mevalonic acid, but other studies and observations suggest an independent formation in plastids. Other possibilities are being considered, such as the existence of independent (compartmentalized) biosynthetic pathways of IPP formation via the so-called Rohmer pathway. Substrate channeling through the formation of end product-specific multienzyme complexes (metabolons) with no release of substrate intermediates will also be discussed.

Conversion of acetyl-coenzyme A into 3-hydroxy-3-methylglutaryl-coenzyme A in radish seedlings. Evidence of a single monomeric protein catalyzing a FeII/quinone-stimulated double condensation reaction

We solubilized from radish membranes and purified to apparent homogeneity a monomeric protein (55.5 kDa) capable of catalyzing the two-step conversion of acetyl-CoA into 3-hydroxy-3-methylglutaryl(HMG)-CoA. Unlike the situation described for other eukaryotes (yeast, animals), both enzyme activities needed for HMG-CoA synthesis (acetoacetyl-CoA thiolase, AACT and HMG-CoA synthase, HMGS) appear to be localized on a single polypeptide. Thus, the enzyme system is further referred to as AACT/HMGS. The reaction as catalyzed by purified AACT/HMGS is strongly stimulated in vitro in presence of FeII-chelates (namely EDTA) and of quinone cofactors with pyrroloquinoline quinone (PQQ) being by far the most effective one studied so far. Whereas the FeII stimulation is apparently due to a Vmax effect, PQQ increases the affinity of the enzyme system towards acetyl-CoA (1.9 microM vs. 5.9 microM, at 50 microM FeII, 100 microM EDTA, 20 microM PQQ). Stimulation by naphthoquinone (NQ) can be overcome in the presence of halogenated NQ-derivatives, while activation by PQQ remains unaffected, possibly indicating a much more specific-binding of the latter cofactor. Gel filtration experiments of enzyme after preincubation in presence of PQQ indicate that there is no covalent-binding of the quinone cofactor to the enzyme. As is also shown with partially purified enzyme from maize membranes, phenylhydrazine, known to react with PQQ as the prosthetic group of quinoproteins (see van der Meer et al. (1987) FEBS Lett. 221, 299-304), efficiently inhibits the reaction. The data lead us to suggest a reaction mechanism that involves radical formation by the redox couple FeII/PQQ, thereby possibly facilitating the energetically unfavorable Claisen condensation as catalyzed during the first partial (AACT) reaction.

Arabidopsis thaliana contains two differentially expressed 3-hydroxy-3-methylglutaryl-CoA reductase genes, which encode microsomal forms of the enzyme

The enzyme 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR; EC 1.1.1.34) catalyzes the first rate-limiting step in plant isoprenoid biosynthesis. Arabidopsis thaliana contains two genes, HMG1 and HMG2, that encode HMGR. We have cloned these two genes and analyzed their structure and expression. HMG1 and HMG2 consist of four exons and three small introns that interrupt the coding sequence at equivalent positions. The two genes share sequence similarity in the coding regions but not in the 5'- or 3'-flanking regions. HMG1 mRNA is detected in all tissues, whereas the presence of HMG2 mRNA is restricted to young seedlings, roots, and inflorescences. The similarity between the two encoded proteins (HMGR1 and HMGR2) is restricted to the regions corresponding to the membrane and the catalytic domains. Arabidopsis HMGR2 represents a divergent form of the enzyme that has no counterpart among plant HMGRs characterized so far. By using a coupled in vitro transcription-translation assay, we show that both HMGR1 and HMGR2 are cotranslationally inserted into endoplasmic reticulum-derived microsomal membranes. Our results suggest that the endoplasmic reticulum is the only cell compartment for the targeting of HMGR in Arabidopsis and support the hypothesis that in higher plants the formation of mevalonate occurs solely in the cytosol.

Enhanced Carotenoid Biosynthesis by Oxidative Stress in Acetate-Induced Cyst Cells of a Green Unicellular Alga, Haematococcus pluvialis

In a green alga, Haematococcus pluvialis, a morphological change of vegetative cells into cyst cells was rapidly induced by the addition of acetate or acetate plus Fe 2+ to the vegetative growth phase. Accompanied by cyst formation, algal astaxanthin formation was more enhanced by the addition of acetate plus Fe 2+ than by the addition of acetate alone. Encystment and enhanced carotenoid biosynthesis were inhibited by either actinomycin D or cycloheximide. However, after cyst formation was induced by the addition of acetate alone, carotenoid formation could be enhanced with the subsequent addition of Fe 2+ even in the presence of the inhibitors. The Fe 2+ -enhanced carotenogenesis was inhibited by potassium iodide, a scavenger for hydroxyl radical, suggesting that hydroxyl radical formed by an iron-catalyzed Fenton reaction may be required for enhanced carotenoid biosynthesis. Moreover, it was demonstrated that four active oxygen species, singlet oxygen, superoxide anion radical, hydrogen peroxide, and peroxy radical, were capable of replacing Fe 2+ in its role in the enhanced carotenoid formation in the acetate-induced cyst. From these results, it was concluded that oxidative stress is involved in the posttranslational activation of carotenoid biosynthesis in acetate-induced cyst cells.

Regulatory role of microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase in a tobacco mutant that overproduces sterols

In a tobacco mutant callus, containing up to tenfold more sterols than the wild-type genotype, HMG-CoA reductase activity is increased by a factor of approximately three, as is the case in mutant seedlings and plants. The rate of HMG-CoA synthesis from acetyl-CoA by the coupled enzyme system acetoacetyl-CoA thiolase/HMG-CoA synthase, as well as its conversion to acetyl-CoA plus acetoacetate by action of HMG-CoA lyase are not affected. These results confirm the key-regulating role of HMG-CoA reductase in sterol biosynthesis, which seems not to be confined only to the animal kingdom, but can also be extended to plants.