CLA1, a novel gene required for chloroplast development, is highly conserved in evolution

Perspectives in anti-infective drug design. The late steps in the biosynthesis of the universal terpenoid precursors, isopentenyl diphosphate and dimethylallyl diphosphate

A mevalonate-independent pathway for the biosynthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) that has been elucidated during the last decade is essential in plants, many eubacteria and apicomplexan parasites, but is absent in Archaea and animals. The enzymes of the pathway are potential targets for the development of novel antibiotic, antimalarial and herbicidal agents. This review is focused on the late steps of this pathway. The intermediate 2C-methyl-D-erythritol 2,4-cyclodiphosphate is converted into IPP and DMAPP via 1-hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate by the consecutive action of the iron-sulfur proteins IspG and IspH. IPP and DMAPP can be interconverted by IPP isomerase which is essential in microorganisms using the mevalonate pathway, whereas its presence is optional in microorganisms using the non-mevalonate pathway. A hitherto unknown family of IPP isomerases using FMN as coenzyme has been discovered recently in Archaea and certain eubacteria.

Regulation of carotenoid biosynthesis in plants: evidence for a key role of hydroxymethylbutenyl diphosphate reductase in controlling the supply of plastidial isoprenoid precursors

Carotenoids are isoprenoid pigments that function as photoprotectors, precursors of the hormone abscisic acid (ABA), colorants and nutraceuticals. A major problem for the metabolic engineering of high carotenoid levels in plants is the limited supply of their isoprenoid precursor geranylgeranyl diphosphate (GGPP), formed by condensation of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) units usually synthesized by the methylerythritol phosphate (MEP) pathway in plastids. Our earlier work with three of the seven MEP pathway enzymes suggested that the first reaction of the pathway catalyzed by deoxyxylulose 5-phosphate synthase (DXS) is limiting for carotenoid biosynthesis during tomato (Lycopersicon esculentum) fruit ripening. Here we investigate the contribution of the enzyme hydroxymethylbutenyl diphosphate reductase (HDR), which simultaneously synthesizes IPP and DMAPP in the last step of the pathway. A strong upregulation of HDR gene expression was observed in correlation with carotenoid production during both tomato fruit ripening and Arabidopsis thaliana seedling deetiolation. Constitutive overexpression of the tomato cDNA encoding HDR in Arabidopsis did not increase carotenoid levels in etioplasts. By contrast, light-grown transgenic plants showed higher carotenoid levels and an enhanced seed dormancy phenotype suggestive of increased ABA levels. The analysis of double transgenic Arabidopsis plants overproducing both the enzyme taxadiene synthase (which catalyzes the production of the non-native isoprenoid taxadiene from GGPP) and either HDR or DXS showed a twofold stronger effect of HDR in increasing taxadiene levels. Together, the data support a major role for HDR in controlling the production of MEP-derived precursors for plastid isoprenoid biosynthesis.

CHLOROPLAST BIOGENESIS genes act cell and noncell autonomously in early chloroplast development

In order to identify nuclear genes required for early chloroplast development, a collection of photosynthetic pigment mutants of Arabidopsis was assembled and screened for lines with extremely low levels of chlorophyll. Nine chloroplast biogenesis (clb) mutants that affect proplastid growth and thylakoid membrane formation and result in an albino seedling phenotype were identified. These mutations identify six new genes as well as a novel allele of cla1. clb mutants have less than 2% of wild-type chlorophyll levels, and little or no expression of nuclear and plastid-encoded genes required for chloroplast development and function. In all but one mutant, proplastids do not differentiate enough to form elongated stroma thylakoid membranes. Analysis of mutants during embryogenesis allows differentiation between CLB genes that act noncell autonomously, where partial maternal complementation of chloroplast development is observed in embryos, and those that act cell autonomously, where complementation during embryogenesis is not observed. Molecular characterization of the noncell autonomous clb4 mutant established that the CLB4 gene encodes for hydroxy-2-methyl-2-(E)-butenyl 4-diphosphate synthase (HDS), the next to the last enzyme of the methylerythritol 4-phosphate (MEP) pathway for the synthesis of plastidic isoprenoids. The noncell autonomous nature of the clb4 mutant suggests that products of the MEP pathway can travel between tissues, and provides in vivo evidence that some movement of MEP intermediates exists from the cytoplasm to the plastid. The isolation and characterization of clb mutants represents the first systematic study of genes required for early chloroplast development in Arabidopsis.

A mutation of the CRUMPLED LEAF gene that encodes a protein localized in the outer envelope membrane of plastids affects the pattern of cell division, cell differentiation, and plastid division in Arabidopsis

We identified a novel mutation of a nuclear-encoded gene, designated as CRUMPLED LEAF (CRL), of Arabidopsis thaliana that affects the morphogenesis of all plant organs and division of plastids. Histological analysis revealed that planes of cell division were distorted in shoot apical meristems (SAMs), root tips, and embryos in plants that possess the crl mutation. Furthermore, we observed that differentiation patterns of cortex and endodermis cells in inflorescence stems and root endodermis cells were disturbed in the crl mutant. These results suggest that morphological abnormalities observed in the crl mutant were because of aberrant cell division and differentiation. In addition, cells of the crl mutant contained a reduced number of enlarged plastids, indicating that the division of plastids was inhibited in the crl. The CRL gene encodes a novel protein with a molecular mass of 30 kDa that is localized in the plastid envelope. The CRL protein is conserved in various plant species, including a fern, and in cyanobacteria, but not in other organisms. These data suggest that the CRL protein is required for plastid division, and it also plays an important role in cell differentiation and the regulation of the cell division plane in plants. A possible function of the CRL protein is discussed.

Functional analysis of the final steps of the 1-deoxy-D-xylulose 5-phosphate (DXP) pathway to isoprenoids in plants using virus-induced gene silencing

Isoprenoid biosynthesis in plant plastids occurs via the 1-deoxy-d-xylulose 5-phosphate (DXP) pathway. We used tobacco rattle virus (TRV) to posttranscriptionally silence the expression of the last two enzymes of this pathway, the IspG-encoded (E)-4-hydroxy-3-methylbut-2-enyl diphosphate synthase (HDS) and the IspH-encoded isopentenyl/dimethylallyl diphosphate synthase (IDDS), as well as isopentenyl/dimethylallyl diphosphate isomerase (IDI), the enzyme that interconverts IPP and DMAPP. TRV-IspG and TRV-IspH infected Nicotiana benthamiana plants had albino leaves that contained less than 4% of the chlorophyll and carotenoid pigments of control leaves. We applied [(13)C]DXP and [(14)C]DXP to silenced leaves and found that 2-C-methyl-d-erythritol 2,4-cyclodiphosphate accumulated in plants blocked at HDS while DXP, (E)-4-hydroxy-3-methylbut-2-enyl phosphate and (E)-2-methylbut-2-ene-1,4-diol accumulated in IDDS-blocked plants. Albino leaves from IspG- and IspH-silenced plants displayed a disorganized palisade mesophyll, reduced cuticle, fewer plastids, and disrupted thylakoid membranes. These findings demonstrate the participation of HDS and IDDS in the DXP pathway in plants, and support the view that plastid isoprenoid biosynthesis is metabolically and physically segregated from the mevalonate pathway. IDI-silenced plants had mottled white-pale green leaves with disrupted tissue and plastid structure, and showed an 80% reduction in pigments compared to controls. IPP pyrophosphatase activity was higher in chloroplasts isolated from IDI-silenced plants than in control plant chloroplasts. We suggest that a low level of isoprenoid biosynthesis via the DXP pathway can occur without IDI but that this enzyme is required for full function of the DXP pathway.

Loss of function of 3-hydroxy-3-methylglutaryl coenzyme A reductase 1 (HMG1) in Arabidopsis leads to dwarfing, early senescence and male sterility, and reduced sterol levels

3-Hydroxy-3-methylglutaryl-CoA reductase (HMGR) catalyzes the first committed step in the cytosolic isoprenoid biosynthesis pathway in higher plants. To understand the contribution of HMGR to plant development, we isolated T-DNA insertion mutants for HMG1 and HMG2. The hmg1 and hmg2 mutants were both more sensitive than the wild type (WT) to lovastatin, an inhibitor of HMGR. The hmg2 mutant showed no visible phenotype under normal growth conditions. In contrast, the hmg1 mutant exhibited dwarfing, early senescence, and sterility. Expression of senescence-associated genes 12 (SAG12), a marker gene for senescence, was induced in the hmg1 mutant at an earlier stage than in the WT. Levels of trans-cytokinins--hormones known to inhibit senescence--were not lower in hmg1. The mutant did not have the typical appearance of brassinosteroid (BR)-deficient mutants, except for a dwarf phenotype, because of the suppression of cell elongation. The expression of several genes involved in cell elongation was suppressed in hmg1. WT plants treated exogenously with inhibitors of sterol biosynthesis had similar gene expression and sterility characteristics as the hmg1 mutants. Pleiotropic phenotypes were rescued by feeding with squalene, the precursor of sterols and triterpenoids. The sterol levels in hmg1 mutants were lower than in the WT. These findings suggest that HMG1 plays a critical role in triterpene biosynthesis, and that sterols and/or triterpenoids contribute to cell elongation, senescence, and fertility.

A defect in atToc159 of Arabidopsis thaliana causes severe defects in leaf development

Plastid protein import 2 (ppi2), a mutant of Arabidopsis thaliana, lacks a homologue of a component of the translocon at the outer envelope membrane of chloroplasts (Toc), designated Toc159 of the pea. Toc159 is thought to be essential for the import of photosynthetic proteins into chloroplasts. In order to investigate the effect of protein import on the plant development, we examined the morphologies of the developing leaves and the shoot apical meristems (SAM) in the ppi2 plants. Our histological analysis revealed that the development of leaves is severely affected in ppi2, while the structure of SAM is normal. Abnormalities in leaves became obvious in the later stages of leaf development, resulting in the generation of mature leaves with fewer mesophyll cells and more intercellular spaces as compared with the wild type. Palisade and spongy tissues of the mature leaves were indistinguishable in ppi2. Replication of chloroplast DNA was also suggested to be impaired in ppi2. Our results suggest that protein import into chloroplasts is important for the normal development of leaves.

Cross-talk between the cytosolic mevalonate and the plastidial methylerythritol phosphate pathways in tobacco bright yellow-2 cells

In plants, two pathways are utilized for the synthesis of isopentenyl diphosphate, the universal precursor for isoprenoid biosynthesis. The key enzyme of the cytoplasmic mevalonic acid (MVA) pathway is 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGR). Treatment of Tobacco Bright Yellow-2 (TBY-2) cells by the HMGR-specific inhibitor mevinolin led to growth reduction and induction of apparent HMGR activity, in parallel to an increase in protein representing two HMGR isozymes. Maximum induction was observed at 24 h. 1-Deoxy-d-xylulose (DX), the dephosphorylated first precursor of the plastidial 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway, complemented growth inhibition by mevinolin in the low millimolar concentration range. Furthermore, DX partially re-established feedback repression of mevinolin-induced HMGR activity. Incorporation studies with [1,1,1,4-2H4]DX showed that sterols, normally derived from MVA, in the presence of mevinolin are synthesized via the MEP pathway. Fosmidomycin, an inhibitor of 1-deoxy-d-xylulose-5-phosphate reductoisomerase, the second enzyme of the MEP pathway, was utilized to study the reverse complementation. Growth inhibition by fosmidomycin of TBY-2 cells could be partially overcome by MVA. Chemical complementation was further substantiated by incorporation of [2-13C]MVA into plastoquinone, representative of plastidial isoprenoids. Best rates of incorporation of exogenous stably labeled precursors were observed in the presence of both inhibitors, thereby avoiding internal isotope dilution.

Crosstalk between cytosolic and plastidial pathways of isoprenoid biosynthesis in Arabidopsis thaliana

In plants, the formation of isopentenyl diphosphate and dimethylallyl diphosphate, the central intermediates in the biosynthesis of isoprenoids, is compartmentalized: the mevalonate (MVA) pathway, which is localized to the cytosol, is responsible for the synthesis of sterols, certain sesquiterpenes, and the side chain of ubiquinone; in contrast, the recently discovered MVA-independent pathway, which operates in plastids, is involved in providing the precursors for monoterpenes, certain sesquiterpenes, diterpenes, carotenoids, and the side chains of chlorophylls and plastoquinone. Specific inhibitors of the MVA pathway (lovastatin) and the MVA-independent pathway (fosmidomycin) were used to perturb biosynthetic flux in Arabidopsis thaliana seedlings. The interaction between both pathways was studied at the transcriptional level by using GeneChip (Affymetrix) microarrays and at the metabolite level by assaying chlorophylls, carotenoids, and sterols. Treatment of seedlings with lovastatin resulted in a transient decrease in sterol levels and a transient increase in carotenoid as well as chlorophyll levels. After the initial drop, sterol amounts in lovastatin-treated seedlings recovered to levels above controls. As a response to fosmidomycin treatment, a transient increase in sterol levels was observed, whereas chlorophyll and carotenoid amounts decreased dramatically when compared with controls. At 96 h after fosmidomycin addition, the levels of all metabolites assayed (sterols, chlorophylls, and carotenoids) were substantially lower than in controls. Interestingly, these inhibitor-mediated changes were not reflected in altered gene expression levels of the genes involved in sterol, chlorophyll, and carotenoid metabolism. The lack of correlation between gene expression patterns and the accumulation of isoprenoid metabolites indicates that posttranscriptional processes may play an important role in regulating flux through isoprenoid metabolic pathways.

Cloning and heterologous expression of early genes in gibberellin and steviol biosynthesis via the methylerythritol phosphate pathway in Stevia rebaudiana

The ent-kaurene skeleton of chloroplast diterpene glycosides, which are produced in large quantities in the leaves of Stevia rebaudiana Bertoni, is formed via the recently discovered 2-C-methyl-D-erythritol 4-phosphate pathway. The enzymes catalyzing the first two steps of this pathway, 1-deoxy-D-xylulose-5-phosphate synthase (DXS) and 1-deoxy-D-xylulose-5-phosphate reductoisomerase (DXR) were characterized. Using reverse transcriptase-PCR, the dxs and dxr cDNAs were cloned, which comprise ORFs of 2148 and 1422 nucleotides, respectively. The cDNA-derived amino acid sequences for DXS and DXR contain 716 and 474 residues, encoding polypeptides of about 76.6 and 51 kDa, respectively. DXS and DXR from Stevia both contain an N-terminal plastid targeting sequence and show high homology to other known plant DXS and DXR enzymes. Furthermore, we demonstrated through heterologous expression in Escherichia coli that the cloned cDNAs encode functional proteins.Key words: Stevia rebaudiana Bertoni, Asteraceae, isoprenoids, 2-C-methyl-D-erythritol 4-phosphate pathway, 1- deoxy-D-xylulose-5-phosphate synthase, 1-deoxy-D-xylulose-5-phosphate reductoisomerase.

Identification of an allele of CLA1 associated with variegation in Arabidopsis thaliana

We have identified a mutant of Arabidopsis thaliana (lvr111) that exhibits a variegated phenotype, reduced isoprenoid pigmentation, and dwarfism in comparison with wild-type plants. Segregation analysis indicated that this phenotype was caused by a single, semi-dominant mutation and PCR-based marker mapping placed the mutation near position 56 on the RI map of chromosome IV. The lvr111 lesion was identified by genomic PCR and sequence analysis as a missense mutation (D306N) in the CLA1 gene (AT4g15560) and complementation analysis confirmed the allelic relationship between lvr111 and CLA1. CLA1 encodes 1-deoxy-d-xylulose 5-phosphate synthase, which catalyses the first step of the non-mevalonate isoprenoid biosynthetic pathway. These observations demonstrate that, unlike the albinism caused by severe alleles of CLA1, weaker alleles are associated with leaf variegation.

From Genes to Photosynthesis in Arabidopsis thaliana

Although photosynthesis in higher plants is of cyanobacterial descent, it differs strikingly in organization and regulation from the prokaryotic process. Genomics, proteomics, and comparative genome analysis are now providing powerful new tools for the molecular dissection of photosynthesis in higher plants. Mutant screens and reverse genetics identify an increasing number of gene-function relationships that have a bearing on photosynthesis, revealing a marked interdependency between photosynthesis and other cellular processes. Photosynthesis-related functions are mostly located in the chloroplast, but can also be located in other compartments of the plant cell. The analysis by DNA-array hybridization of mRNA expression patterns both in the chloroplast and the nucleus, under various environmental conditions and/or in different genetic backgrounds that affect the function of the plastid, is rapidly improving our understanding of how photosynthesis is regulated, and it reveals that plastid-to-nucleus signaling plays a central role in its control.

Chloroplast research in the genomic age

Chloroplast research takes significant advantage of genomics and genome sequencing, and a new picture is emerging of how the chloroplast functions and communicates with other cellular compartments. In terms of evolution, it is now known that only a fraction of the many proteins of cyanobacterial origin were rerouted to higher plant plastids. Reverse genetics and novel mutant screens are providing a growing catalogue of chloroplast protein-function relationships, and the characterization of plastid-to-nucleus signalling mutants reveals cell-organelle interactions. Recent advances in transcriptomics and proteomics of the chloroplast make this organelle one of the best understood of all plant cell compartments.

Contribution of the mevalonate and methylerythritol phosphate pathways to the biosynthesis of gibberellins in Arabidopsis

Gibberellins (GAs) are diterpene plant hormones essential for many developmental processes. Although the GA biosynthesis pathway has been well studied, our knowledge on its early stage is still limited. There are two possible routes for the biosynthesis of isoprenoids leading to GAs, the mevalonate (MVA) pathway in the cytosol and the methylerythritol phosphate (MEP) pathway in plastids. To distinguish these possibilities, metabolites from each isoprenoid pathway were selectively labeled with (13)C in Arabidopsis seedlings. Efficient (13)C-labeling was achieved by blocking the endogenous pathway chemically or genetically during the feed of a (13)C-labeled precursor specific to the MVA or MEP pathways. Gas chromatography-mass spectrometry analyses demonstrated that both MVA and MEP pathways can contribute to the biosyntheses of GAs and campesterol, a cytosolic sterol, in Arabidopsis seedlings. While GAs are predominantly synthesized through the MEP pathway, the MVA pathway plays a major role in the biosynthesis of campesterol. Consistent with some crossover between the two pathways, phenotypic defects caused by the block of the MVA and MEP pathways were partially rescued by exogenous application of the MEP and MVA precursors, respectively. We also provide evidence to suggest that the MVA pathway still contributes to GA biosynthesis when this pathway is limiting.

Two distantly related genes encoding 1-deoxy-d-xylulose 5-phosphate synthases: differential regulation in shoots and apocarotenoid-accumulating mycorrhizal roots

Isopentenyl diphosphate, the universal precursor of isoprenoids, is synthesized by two separate routes, one in the cytosol and the other in plastids. The initial step of the plastidial pathway is catalysed by 1-deoxy-d-xylulose 5-phosphate synthase (DXS), which was previously thought to be encoded by a single-copy gene. We have identified two distinct classes of DXS-like cDNAs from the model legume Medicago truncatula. The deduced mature MtDXS1 and MtDXS2 proteins, excluding the predicted plastid-targeting peptides, are similar in size (72.7 and 71.2 kDa) yet share only 70% identity in their amino acid sequences, and both encode functional DXS proteins as shown by heterologous expression in Escherichia coli. Available DXS sequences from other plants can easily be assigned to either class 1 or class 2. Partial sequences of multiple DXS genes in a single genome may be found in the databases of several monocot and dicot plants. Blot analyses of RNA from M. truncatula, maize, tomato and tobacco demonstrate preferential expression of DXS1 genes in many developing plant tissues except roots. By contrast, DXS2 transcript levels are low in most tissues but are strongly stimulated in roots upon colonization by mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids. Monoterpene-synthesizing gland cells of leaf trichomes appear to be another site of DXS2 gene activity. The potential importance of DXS1 in many housekeeping functions and a still hypothetical role of DXS2 in the biosynthesis of secondary isoprenoids is discussed.

Expression and molecular analysis of the Arabidopsis DXR gene encoding 1-deoxy-D-xylulose 5-phosphate reductoisomerase, the first committed enzyme of the 2-C-methyl-D-erythritol 4-phosphate pathway

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) catalyzes the first committed step of the 2-C-methyl-D-erythritol 4-phosphate pathway for isoprenoid biosynthesis. In Arabidopsis, DXR is encoded by a single-copy gene. We have cloned a full-length cDNA corresponding to this gene. A comparative analysis of all plant DXR sequences known to date predicted an N-terminal transit peptide for plastids, with a conserved cleavage site, and a conserved proline-rich region at the N terminus of the mature protein, which is not present in the prokaryotic DXR homologs. We demonstrate that Arabidopsis DXR is targeted to plastids and localizes into chloroplasts of leaf cells. The presence of the proline-rich region in the mature Arabidopsis DXR was confirmed by detection with a specific antibody. A proof of the enzymatic function of this protein was obtained by complementation of an Escherichia coli mutant defective in DXR activity. The expression pattern of beta-glucuronidase, driven by the DXR promoter in Arabidopsis transgenic plants, together with the tissue distribution of DXR transcript and protein, revealed developmental and environmental regulation of the DXR gene. The expression pattern of the DXR gene parallels that of the Arabidopsis 1-deoxy-D-xylulose 5-phosphate synthase gene, but the former is slightly more restricted. These genes are expressed in most organs of the plant including roots, with higher levels in seedlings and inflorescences. The block of the 2-C-methyl-D-erythritol 4-phosphate pathway in Arabidopsis seedlings with fosmidomycin led to a rapid accumulation of DXR protein, whereas the 1-deoxy-D-xylulose 5-phosphate synthase protein level was not altered. Our results are consistent with the participation of the Arabidopsis DXR gene in the control of the 2-C-methyl-D-erythritol 4-phosphate pathway.

RASPBERRY3 gene encodes a novel protein important for embryo development

We identified a new gene that is interrupted by T-DNA in an Arabidopsis embryo mutant called raspberry3. raspberry3 has "raspberry-like" cellular protuberances with an enlarged suspensor characteristic of other raspberry embryo mutants, and is arrested morphologically at the globular stage of embryo development. The predicted RASPBERRY3 protein has domains found in proteins present in prokaryotes and algae chloroplasts. Computer prediction analysis suggests that the RASPBERRY3protein may be localized in the chloroplast. Complementation analysis supports the possibility that the RASPBERRY3 protein may be involved in chloroplast development. Our experiments demonstrate the important role of the chloroplast, directly or indirectly, in embryo morphogenesis and development.

Functional properties and regulatory complexity of a minimal RBCS light-responsive unit activated by phytochrome, cryptochrome, and plastid signals

Light-inducible promoters are able to respond to a wide spectrum of light through multiple photoreceptor systems. Several cis-acting elements have been identified as components of light-responsive promoter elements; however, none of these regulatory elements by itself appears to be sufficient to confer light responsiveness; rather, the combination of at least two elements seems to be required. Using phylogenetic structural analysis, we have identified conserved DNA modular arrays (CMAs) associated with light-responsive promoter regions that have been conserved throughout the evolutionary radiation of angiosperms. Here, we report the functional characterization of CMA5, a native 52-bp fragment of the Nicotiana plumbaginifolia rbcS 8B promoter, which contains an I- and a G-box cis-element. CMA5 behaves as a light-responsive minimal unit capable of activating a heterologous minimal promoter in a phytochrome-, cryptochrome-, and plastid-dependent manner. We also show that CMA5 light induction requires HY5 and that downstream negative regulators COP (constitutive photomorphogenic)/DET (de-etiolated) regulate its activity. Our results show that the simplest light-responsive promoter element from photosynthesis-associated genes described to date is the common target for different signals involved in light regulation. The possible mechanism involved in light-transcriptional regulation and tissue specificity of combinatorial elements units is discussed.

Functional analysis of the Arabidopsis thaliana GCPE protein involved in plastid isoprenoid biosynthesis

Plastid isoprenoids are synthesized via the 2-C-methyl-D-erythritol 4-phosphate pathway. A few years after its discovery, most of the Escherichia coli genes involved in the pathway have been identified, including gcpE. In this work, we have identified an Arabidopsis thaliana protein with homology to the product of this gene. The plant polypeptide, GCPE, contains two structural domains that are absent in the E. coli protein: an N-terminal extension and a central domain of 30 kDa. We demonstrate that the N-terminal region targets the Arabidopsis protein to chloroplasts in vivo, consistent with its role in plastid isoprenoid biosynthesis. Although the presence of the internal extra domain may have an effect on activity, the Arabidopsis mature GCPE was able to complement a gcpE-defective E. coli strain, indicating the plant protein is a true functional homologue of the bacterial gcpE gene product.

Biogenesis and origin of thylakoid membranes

Thylakoids are photosynthetically active membranes found in Cyanobacteria and chloroplasts. It is likely that they originated in photosynthetic bacteria, probably in close connection to the occurrence of photosystem II and oxygenic photosynthesis. In higher plants, chloroplasts develop from undifferentiated proplastids. These contain very few internal membranes and the whole thylakoid membrane system is built when chloroplast differentiation takes place. During cell and organelle division a constant synthesis of new thylakoid membrane material is required. Also, rapid adaptation to changes in light conditions and long term adaptation to a number of environmental factors are accomplished by changes in the lipid and protein content of the thylakoids. Thus regulation of synthesis and assembly of all these elements is required to ensure optimal function of these membranes.

Arabidopsis genes essential for seedling viability: isolation of insertional mutants and molecular cloning

We have undertaken a large-scale genetic screen to identify genes with a seedling-lethal mutant phenotype. From screening approximately 38,000 insertional mutant lines, we identified >500 seedling-lethal mutants, completed cosegregation analysis of the insertion and the lethal phenotype for >200 mutants, molecularly characterized 54 mutants, and provided a detailed description for 22 of them. Most of the seedling-lethal mutants seem to affect chloroplast function because they display altered pigmentation and affect genes encoding proteins predicted to have chloroplast localization. Although a high level of functional redundancy in Arabidopsis might be expected because 65% of genes are members of gene families, we found that 41% of the essential genes found in this study are members of Arabidopsis gene families. In addition, we isolated several interesting classes of mutants and genes. We found three mutants in the recently discovered nonmevalonate isoprenoid biosynthetic pathway and mutants disrupting genes similar to Tic40 and tatC, which are likely to be involved in chloroplast protein translocation. Finally, we directly compared T-DNA and Ac/Ds transposon mutagenesis methods in Arabidopsis on a genome scale. In each population, we found only about one-third of the insertion mutations cosegregated with a mutant phenotype.

Essential role of residue H49 for activity of Escherichia coli 1-deoxy-D-xylulose 5-phosphate synthase, the enzyme catalyzing the first step of the 2-C-methyl-D-erythritol 4-phosphate pathway for isoprenoid Synthesis

The first step of the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for isoprenoid biosynthesis in plant plastids and most eubacteria is catalyzed by 1-deoxy-D-xylulose 5-phosphate synthase (DXS), a recently described transketolase-like enzyme. To identify key residues for DXS activity, we compared the amino acid sequence of Escherichia coli DXS with that of E. coli and yeast transketolase (TK). Alignment showed a previously undetected conserved region containing an invariant histidine residue that has been described to participate in proton transfer during TK catalysis. The possible role of the conserved residue in E. coli DXS (H49) was examined by site-directed mutagenesis. Replacement of this histidine residue with glutamine yielded a mutant DXS-H49Q enzyme that showed no detectable DXS activity. These findings are consistent with those obtained for yeast TK and demonstrate a key role of H49 for DXS activity.

Pathways of plastid-to-nucleus signaling

Several plastid signals have been identified that regulate the transcription of nuclear genes for plastid and non-plastid proteins. These signals are related to the photosynthetic metabolism of chloroplasts and include porphyrins, reactive oxygen intermediates and carotenoids. The metabolic and developmental state of the chloroplast also control cell differentiation and leaf morphogenesis, but the signaling pathways have not been characterized. Plastid-to-nucleus and light-signaling pathways are separable in some but not all cases. Retrograde signaling thus plays a central role in coordinating gene expression in the nucleus, plastid and mitochondrion, and in integrating pathways of cellular metabolism and development.

The Arabidopsis immutans mutation affects plastid differentiation and the morphogenesis of white and green sectors in variegated plants

The immutans (im) variegation mutant of Arabidopsis has green and white leaf sectors due to the action of a nuclear recessive gene, IMMUTANS (IM). This gene encodes the IM protein, which is a chloroplast homolog of the mitochondrial alternative oxidase. Because the white sectors of im accumulate the noncolored carotenoid, phytoene, IM likely serves as a redox component in phytoene desaturation. In this paper, we show that IM has a global impact on plant growth and development and is required for the differentiation of multiple plastid types, including chloroplasts, amyloplasts, and etioplasts. IM promoter activity and IM mRNAs are also expressed ubiquitously in Arabidopsis. IM transcript levels correlate with carotenoid accumulation in some, but not all, tissues. This suggests that IM function is not limited to carotenogenesis. Leaf anatomy is radically altered in the green and white sectors of im: Mesophyll cell sizes are dramatically enlarged in the green sectors and palisade cells fail to expand in the white sectors. The green im sectors also have significantly higher than normal rates of O(2) evolution and elevated chlorophyll a/b ratios, typical of those found in "sun" leaves. We conclude that the changes in structure and photosynthetic function of the green leaf sectors are part of an adaptive mechanism that attempts to compensate for a lack of photosynthesis in the white leaf sectors, while maximizing the ability of the plant to avoid photodamage.

Biolistic transformation of Arabidopsis root hairs: a novel technique to facilitate map-based cloning

The final stage of map-based gene isolation is complementation of the mutant phenotype with wild-type DNA to determine the exact location of the gene of interest. This usually involves Agrobacterium tumefaciens-mediated transformation, which is reliable and produces stable transformants. However, the process of Agrobacterium transformation may take up to three months to complete. If the mutant phenotype can be seen in a single cell, and the wild-type copy of the gene can act cell autonomously, then complementation of the whole plant is not strictly necessary. We have developed a technique for the biolistic transformation of Arabidopsis thaliana root hairs, and used this to test large insert clones for complementation of two recessive mutant phenotypes, a procedure that takes less than a day. Our results show that biolistic transformation can be used with transient assays to conduct rapid tests for complementation by large insert clones.

1-Deoxy-D-xylulose 5-phosphate reductoisomerase and plastid isoprenoid biosynthesis during tomato fruit ripening

The recently discovered 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway for the biosynthesis of plastid isoprenoids (including carotenoids) is not fully elucidated yet despite its central importance for plant life. It is known, however, that the first reaction completely specific to the pathway is the conversion of 1-deoxy-D-xylulose 5-phosphate (DXP) into MEP by the enzyme DXP reductoisomerase (DXR). We have identified a tomato cDNA encoding a protein with homology to DXR and in vivo activity, and show that the levels of the corresponding DXR mRNA and encoded protein in fruit tissues are similar before and during the massive accumulation of carotenoids characteristic of fruit ripening. The results are consistent with a non-limiting role of DXR, and support previous work proposing DXP synthase (DXS) as the first regulatory enzyme for plastid isoprenoid biosynthesis in tomato fruit. Inhibition of DXR activity by fosmidomycin showed that plastid isoprenoid biosynthesis is required for tomato fruit carotenogenesis but not for other ripening processes. In addition, dormancy was reduced in seeds from fosmidomycin-treated fruit but not in seeds from the tomato yellow ripe mutant (defective in phytoene synthase-1, PSY1), suggesting that the isoform PSY2 might channel the production of carotenoids for abscisic acid biosynthesis. Furthermore, the complete arrest of tomato seedling development using fosmidomycin confirms a key role of the MEP pathway in plant development.

Metabolic engineering of essential oil yield and composition in mint by altering expression of deoxyxylulose phosphate reductoisomerase and menthofuran synthase

Peppermint (Mentha x piperita L.) was independently transformed with a homologous sense version of the 1-deoxy-d-xylulose-5-phosphate reductoisomerase cDNA and with a homologous antisense version of the menthofuran synthase cDNA, both driven by the CaMV 35S promoter. Two groups of transgenic plants were regenerated in the reductoisomerase experiments, one of which remained normal in appearance and development; another was deficient in chlorophyll production and grew slowly. Transgenic plants of normal appearance and growth habit expressed the reductoisomerase transgene strongly and constitutively, as determined by RNA blot analysis and direct enzyme assay, and these plants accumulated substantially more essential oil (about 50% yield increase) without change in monoterpene composition compared with wild-type. Chlorophyll-deficient plants did not afford detectable reductoisomerase mRNA or enzyme activity and yielded less essential oil than did wild-type plants, indicating cosuppression of the reductoisomerase gene. Plants transformed with the antisense version of the menthofuran synthase cDNA were normal in appearance but produced less than half of this undesirable monoterpene oil component than did wild-type mint grown under unstressed or stressed conditions. These experiments demonstrate that essential oil quantity and quality can be regulated by metabolic engineering. Thus, alteration of the committed step of the mevalonate-independent pathway for supply of terpenoid precursors improves flux through the pathway that leads to increased monoterpene production, and antisense manipulation of a selected downstream monoterpene biosynthetic step leads to improved oil composition.

1-Deoxy-D-xylulose-5-phosphate synthase, a limiting enzyme for plastidic isoprenoid biosynthesis in plants

The initial step of the plastidic 2C-methyl-D-erythritol 4-phosphate (MEP) pathway that produces isopentenyl diphosphate is catalyzed by 1-deoxy-d-xylulose-5-phosphate synthase. To investigate whether or not 1-deoxy-d-xylulose-5-phosphate synthase catalyzes a limiting step in the MEP pathway in plants, we produced transgenic Arabidopsis plants that over- or underexpress this enzyme. Compared with non-transgenic wild-type plants, the transgenic plants accumulate different levels of various isoprenoids such as chlorophylls, tocopherols, carotenoids, abscisic acid, and gibberellins. Phenotypically, the transgenic plants had slight alterations in growth and germination rates. Because the levels of several plastidic isoprenoids correlate with changes in 1-deoxy-D-xylulose-5-phosphate synthase levels, we conclude that this enzyme catalyzes one of the rate-limiting steps of the MEP biosynthetic pathway. Furthermore, since the product of the MEP pathway is isopentenyl diphosphate, our results suggest that in plastids the pool of isopentenyl diphosphate is limiting to isprenoid production.

Catharanthus roseus genes regulated differentially by mollicute infections

A differential display of mRNAs was used to isolate periwinkle cDNAs differentially expressed following infection with one of three mollicutes: Spiroplasma citri, Candidatus Phytoplasma aurantifolia, and stolbur phytoplasma. Twenty-four differentially expressed cDNAs were characterized by Northern blots and sequence analysis. Eight of them had homologies with genes in databanks coding for proteins involved in photosynthesis, sugar transport, response to stress, or pathways of phytosterol synthesis. The regulation of these genes in periwinkle plants infected by additional phloem-restricted bacteria showed that they were not specific to a given mollicute, but correlations with particular symptoms could be established. Expression of transketolase was down regulated following infection with a pathogenic strain of S. citri. No down regulation was observed for the nonphytopathogenic mutant GMT553, which is deficient for fructose utilization.

Analysis of the expression of CLA1, a gene that encodes the 1-deoxyxylulose 5-phosphate synthase of the 2-C-methyl-D-erythritol-4-phosphate pathway in Arabidopsis

The discovery of the 2-C-methyl-D-erythritol-4-phosphate pathway for the biosynthesis of isoprenoids raises the important question of the nature and regulation of the enzymes involved in this pathway. CLA1, a gene previously isolated from Arabidopsis, encodes the first enzyme of the 2-C-methyl-D-erythritol-4-phosphate pathway, 1-deoxy-D-xylulose-5-phosphate synthase. We demonstrate this enzyme activity by complementation of the cla1-1 mutant phenotype and by direct enzymatic assays. Based on mRNA and protein expression patterns this enzyme is expressed mainly in developing photosynthetic and non-photosynthetic tissues. The beta-glucuronidase expression pattern driven from the CLA1 gene regulatory region supports the northern and protein data while also showing that this gene has some level of expression in most tissues of the plant. A mutation in the CLA1 gene interferes with the normal development of chloroplasts and etioplasts, but does not seem to affect amyloplast structure. Microscopic analysis also shows a pleiotropic effect of the CLA1 gene mutation in mesophyll tissue formation.

Carotenoid biosynthesis during tomato fruit development: regulatory role of 1-deoxy-D-xylulose 5-phosphate synthase

Plant isoprenoids represent a heterogeneous group of compounds which play essential roles not only in growth and development, but also in the interaction of plants with their environment. Higher plants contain two pathways for the biosynthesis of isoprenoids: the mevalonate pathway, located in the cytosol/endoplasmic reticulum, and the recently discovered mevalonate-independent pathway (Rohmer pathway), located in the plastids. In order to evaluate the function of the Rohmer pathway in the regulation of the synthesis of plastidial isoprenoids, we have isolated a tomato cDNA encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS), the first enzyme of the pathway. We demonstrate in vivo activity and plastid targeting of plant DXS. Expression analysis of the tomato DXS gene indicates developmental and organ-specific regulation of mRNA accumulation and a strong correlation with carotenoid synthesis during fruit development. 1-Deoxy-D-xylulose feeding experiments, together with expression analysis of DXS and PSY1 (encoding the fruit-specific isoform of phytoene synthase) in wild-type and yellow flesh mutant fruits, indicate that DXS catalyses the first potentially regulatory step in carotenoid biosynthesis during early fruit ripening. Our results change the current view that PSY1 is the only regulatory enzyme in tomato fruit carotenogenesis, and point towards a coordinated role of both DXS and PSY1 in the control of fruit carotenoid synthesis.

Arbuscular mycorrhizal fungi induce the non-mevalonate methylerythritol phosphate pathway of isoprenoid biosynthesis correlated with accumulation of the 'yellow pigment' and other apocarotenoids

Plants and certain bacteria use a non-mevalonate alternative route for the biosynthesis of many isoprenoids, including carotenoids. This route has been discovered only recently and has been designated the deoxyxylulose phosphate pathway or methylerythritol phosphate (MEP) pathway. We report here that colonisation of roots from wheat, maize, rice and barley by the arbuscular mycorrhizal fungal symbiont Glomus intraradices involves strong induction of transcript levels of two of the pivotal enzymes of the MEP pathway, 1-deoxy-D-xylulose 5-phosphate synthase (DXS) and 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR). This induction is temporarily and spatially correlated with specific and concomitant accumulation of two classes of apocarotenoids, namely glycosylated C13 cyclohexenone derivatives and mycorradicin (C14) conjugates, the latter being a major component of the long-known 'yellow pigment'. A total of six cyclohexenone derivatives were characterised from mycorrhizal wheat and maize roots. Furthermore, the acyclic structure of mycorradicin described previously only from maize has been identified from mycorrhizal wheat roots after alkaline treatment of an 'apocarotenoid complex' of yellow root constituents. We propose a hypothetical scheme for biogenesis of both types of apocarotenoids from a common oxocarotenoid (xanthophyll) precursor. This is the first report demonstrating (i) that the plastidic MEP pathway is active in plant roots and (ii) that it can be induced by a fungus.

A Synechococcus leopoliensis SAUG 1402-1 operon harboring the 1-deoxyxylulose 5-phosphate synthase gene and two additional open reading frames is functionally involved in the dimethylallyl diphosphate synthesis

Experiments have been performed to prove the existence and the functionality of the novel mevalonate independent 1-deoxyxylulose 5-phosphate isoprenoid biosynthesis pathway in cyanobacteria. For this purpose, a segment of the 1-deoxyxylulose 5-phosphate synthase gene (dxs) was amplified from Synechococcus leopoliensis SAUG 1402-1 DNA via PCR using oligonucleotides for conserved regions of dxs. Subsequent hybridization screening of a genomic cosmid library of S. leopoliensis with this segment has led to the identification of an 18.7 kbp segment of the S. leopoliensis genome on which a dxs homologous gene and two adjacent open reading frames organized in one operon could be localized by DNA sequencing. The three genes of the operon were separately expressed in Escherichia coli, proving that the identified cyanobacterial dxs is functionally involved in the formation of dimethylallyl diphosphate, one basic intermediate of isoprenoid biosynthesis.

Inhibitors of the nonmevalonate pathway of isoprenoid biosynthesis as antimalarial drugs

A mevalonate-independent pathway of isoprenoid biosynthesis present in Plasmodium falciparum was shown to represent an effective target for chemotherapy of malaria. This pathway includes 1-deoxy-D-xylulose 5-phosphate (DOXP) as a key metabolite. The presence of two genes encoding the enzymes DOXP synthase and DOXP reductoisomerase suggests that isoprenoid biosynthesis in P. falciparum depends on the DOXP pathway. This pathway is probably located in the apicoplast. The recombinant P. falciparum DOXP reductoisomerase was inhibited by fosmidomycin and its derivative, FR-900098. Both drugs suppressed the in vitro growth of multidrug-resistant P. falciparum strains. After therapy with these drugs, mice infected with the rodent malaria parasite P. vinckei were cured.

Cloning and heterologous expression of a cDNA encoding 1-deoxy-D-xylulose-5-phosphate reductoisomerase of Arabidopsis thaliana

Various plant isoprenoids are synthesized via the non-mevalonate pathway of isopentenyl diphosphate formation. In this pathway, 1-deoxy-D-xylulose 5-phosphate (DOXP), the first intermediate, is transformed to 2-C-methyl-D-erythritol 4-phosphate (MEP) by an enzyme which was recently cloned from Escherichia coli. In order to find a plant homologue of this 1-deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) we cloned a cDNA fragment from Arabidopsis thaliana which has high homology to the E. coli DXR. By expression of this fragment in E. coli we could demonstrate that it encodes a protein which transforms DOXP to MEP. The antibiotic fosmidomycin specifically inhibits this DXR enzyme activity.

THE 1-DEOXY-D-XYLULOSE-5-PHOSPHATE PATHWAY OF ISOPRENOID BIOSYNTHESIS IN PLANTS

In plants the biosynthesis of prenyllipids and isoprenoids proceeds via two independent pathways: (a) the cytosolic classical acetate/mevalonate pathway for the biosynthesis of sterols, sesquiterpenes, triterpenoids; and (b) the alternative, non-mevalonate 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway for the biosynthesis of plastidic isoprenoids, such as carotenoids, phytol (a side-chain of chlorophylls), plastoquinone-9, isoprene, mono-, and diterpenes. Both pathways form the active C5-unit isopentenyl diphosphate (IPP) as the precursor from which all other isoprenoids are formed via head-to-tail addition. This review summarizes current knowledge of the novel 1-deoxy-d-xylulose-5-phosphate (DOXP) pathway for isopentenyl diphosphate biosynthesis, apparently located in plastids. The DOXP pathway of IPP formation starts from D-glyceraldehyde-3-phosphate (GA-3-P) and pyruvate, with DOXP-synthase as the starting enzyme. This pathway provides new insight into the regulation of chloroplast metabolism.

Monoterpenes in essential oils. Biosynthesis and properties

Monoterpenes are compounds found in the essential oils extracted from many plants, including fruits, vegetables, spices and herbs. These compounds contribute to the flavor and aroma of plant from which they are extracted. Monoterpenes are acyclic, monocyclic, or bicyclic C30 compounds synthesized by monoterpene synthases using geranyl pyrophosphate (GPP) as substrate. GPP is also the precursor in the synthesis of farnesyl pyrophosphate (FPP) and geranyl-geranyl pyrophosphate (GGPP), two important compounds in cell metabolism of animals, plants and yeast. Monoterpene cyclases produce cyclic monoterpenes through a multistep mechanism involving a universal intermediate, a terpinyl cation which can be transformed to several compounds. Experimental studies, using animal cancer models, have demonstrated that some monoterpenes possess anticarcinogenic properties, acting at different cellular and molecular levels. From these discoveries it seems clear that monoterpenes could be considered as effective, nontoxic dietary antitumorigenic agents that hold promise as a novel class of anticancer drugs.

Expression of prokaryotic 1-deoxy-D-xylulose-5-phosphatases in Escherichia coli increases carotenoid and ubiquinone biosynthesis

Isopentenyl diphosphate (IPP) acts as the common, five-carbon building block in the biosynthesis of all isoprenoids. The first reaction of IPP biosynthesis in Escherichia coli is the formation of 1-deoxy-D-xylulose-5-phosphate, catalysed by 1-deoxy-D-xylulose-5-phosphate synthase (DXPS). E. coli engineered to produce lycopene, was transformed with dxps genes cloned from Bacillus subtilis and Synechocystis sp. 6803. Increases in lycopene levels were observed in strains expressing exogenous DXPS compared to controls. The recombinant strains also exhibited elevated levels of ubiquinone-8. These increases corresponded with enhanced DXP synthase activity in the recombinant E. coli strains.

New Arabidopsis cue mutants suggest a close connection between plastid- and phytochrome regulation of nuclear gene expression

We searched for new components that are involved in the positive regulation of nuclear gene expression by light by extending a screen for Arabidopsis cue (chlorophyll a/b-binding [CAB] protein-underexpressed) mutants (H.-M. Li, K. Culligan, R.A. Dixon, J. Chory [1995] Plant Cell 7: 1599-1610). cue mutants display reduced expression of the CAB3 gene, which encodes light-harvesting chlorophyll protein, the main chloroplast antenna. The new mutants can be divided into (a) phytochrome-deficient mutants (hy1 and phyB), (b) virescent or delayed-greening mutants (cue3, cue6, and cue8), and (c) uniformly pale mutants (cue4 and cue9). For each of the mutants, the reduction in CAB expression correlates with the visible phenotype, defective chloroplast development, and reduced abundance of the light-harvesting chlorophyll protein. Levels of protochlorophyllide oxidoreductase (POR) were reduced to varying degrees in etiolated mutant seedlings. In the dark, whereas the virescent mutants displayed reduced CAB expression and the lowest levels of POR protein, the other mutants expressed CAB and accumulated POR at near wild-type levels. All of the mutants, with the exception of cue6, were compromised in their ability to derepress CAB expression in response to phytochrome activation. Based on these results, we propose that the previously postulated plastid-derived signal is closely involved in the pathway through which phytochrome regulates the expression of nuclear genes encoding plastid proteins.

Dedicated roles of plastid transketolases during the early onset of isoprenoid biogenesis in pepper fruits1

Isopentenyl diphosphate (IPP), which is produced from mevalonic acid or other nonmevalonic substrates, is the universal precursor of isoprenoids in nature. Despite the presence of several isoprenoid compounds in plastids, enzymes of the mevalonate pathway leading to IPP formation have never been isolated or identified to our knowledge. We now describe the characterization of two pepper (Capsicum annuum L.) cDNAs, CapTKT1 and CapTKT2, that encode transketolases having distinct and dedicated specificities. CapTKT1 is primarily involved in plastidial pentose phosphate and glycolytic cycle integration, whereas CapTKT2 initiates the synthesis of isoprenoids in plastids via the nonmevalonic acid pathway. From pyruvate and glyceraldehyde-3-phosphate, CapTKT2 catalyzes the formation of 1-deoxy-xylulose-5-phosphate, the IPP precursor. CapTKT1 is almost constitutively expressed during the chloroplast-to-chromoplast transition, whereas CapTKT2 is overexpressed during this period, probably to furnish the IPP necessary for increased carotenoid biosynthesis. Because deoxy-xylulose phosphate is shared by the plastid pathways of isoprenoid, thiamine (vitamin B1), and pyridoxine (vitamin B6) biosynthesis, our results may explain why albino phenotypes usually occur in thiamine-deficient plants.

Distribution of the mevalonate and glyceraldehyde phosphate/pyruvate pathways for isoprenoid biosynthesis in unicellular algae and the cyanobacterium Synechocystis PCC 6714

Isopentenyl diphosphate, the universal isoprenoid precursor, can be produced by two different biosynthetic routes: either via the acetate/mevalonate (MVA) pathway, or via the more recently identified MVA-independent glyceraldehyde phosphate/pyruvate pathway. These two pathways are easily differentiated by incorporation of [1-13C]glucose and analysis of the resulting labelling patterns found in the isoprenoids. This method was successfully applied to several unicellular algae raised under heterotrophic growth conditions and allowed for the identification of the pathways that were utilized for isoprenoid biosynthesis. All isoprenoids examined (sterols, phytol, carotenoids) of the green algae Chlorella fusca and Chlamydomonas reinhardtii were synthesized via the GAP/pyruvate pathway, as in another previously investigated green alga, Scenedesmus obliquus, which was also shown in this study to synthesize ubiquinone by the same MVA-independent route. In the red alga Cyanidium caldarium and in the Chrysophyte Ochromonas danica a clear dichotomy was observed: as in higher plants, sterols were formed via the MVA route, whereas chloroplast isoprenoids (phytol in Cy. caldarium and O. danica and beta-carotene in O. danica) were synthesized via the GAP/pyruvate route. In contrast, the Euglenophyte Euglena gracilis synthesized ergosterol, as well as phytol, via the acetate/MVA route. Similar feeding experiments were performed with the cyanobacterium Synechocystis PCC 6714 using [1-13C]- and [6-13C]-glucose. The two isoprenoids examined, phytol and beta-carotene, were shown to have the typical labelling pattern derived from the GAP/pyruvate route.

NUCLEAR CONTROL OF PLASTID AND MITOCHONDRIAL DEVELOPMENT IN HIGHER PLANTS

The nucleus must coordinate organelle biogenesis and function on a cell and tissue-specific basis throughout plant development. The vast majority of plastid and mitochondrial proteins and components involved in organelle biogenesis are encoded by nuclear genes. Molecular characterization of nuclear mutants has illuminated chloroplast development and function. Fewer mutants exist that affect mitochondria, but molecular and biochemical approaches have contributed to a greater understanding of this organelle. Similarities between organelles and prokaryotic regulatory molecules have been found, supporting the prokaryotic origin of chloroplasts and mitochondria. A striking characteristic for both mitochondria and chloroplast is that most regulation is posttranscriptional.

A family of transketolases that directs isoprenoid biosynthesis via a mevalonate-independent pathway

Isopentenyl diphosphate, the common precursor of all isoprenoids, has been widely assumed to be synthesized by the acetate/mevalonate pathway in all organisms. However, based on in vivo feeding experiments, isopentenyl diphosphate formation in several eubacteria, a green alga, and plant chloroplasts has been demonstrated very recently to originate via a mevalonate-independent route from pyruvate and glyceraldehyde 3-phosphate as precursors. Here we describe the cloning from peppermint (Mentha x piperita) and heterologous expression in Escherichia coli of 1-deoxy-D-xylulose-5-phosphate synthase, the enzyme that catalyzes the first reaction of this pyruvate/glyceraldehyde 3-phosphate pathway. This synthase gene contains an ORF of 2,172 base pairs. When the proposed plastid targeting sequence is excluded, the deduced amino acid sequence indicates the peppermint synthase to be about 650 residues in length, corresponding to a native size of roughly 71 kDa. The enzyme appears to represent a novel class of highly conserved transketolases and likely plays a key role in the biosynthesis of plastid-derived isoprenoids essential for growth, development, and defense in plants.

Cloning and characterization of a gene from Escherichia coli encoding a transketolase-like enzyme that catalyzes the synthesis of D-1-deoxyxylulose 5-phosphate, a common precursor for isoprenoid, thiamin, and pyridoxol biosynthesis

For many years it was accepted that isopentenyl diphosphate, the common precursor of all isoprenoids, was synthesized through the well known acetate/mevalonate pathway. However, recent studies have shown that some bacteria, including Escherichia coli, use a mevalonate-independent pathway for the synthesis of isopentenyl diphosphate. The occurrence of this alternative pathway has also been reported in green algae and higher plants. The first reaction of this pathway consists of the condensation of (hydroxyethyl)thiamin derived from pyruvate with the C1 aldehyde group of D-glyceraldehyde 3-phosphate to yield D-1-deoxyxylulose 5-phosphate. In E. coli, D-1-deoxyxylulose 5-phosphate is also a precursor for the biosynthesis of thiamin and pyridoxol. Here we report the molecular cloning and characterization of a gene from E. coli, designated dxs, that encodes D-1-deoxyxylulose-5-phosphate synthase. The dxs gene was identified as part of an operon that also contains ispA, the gene that encodes farnesyl-diphosphate synthase. D-1-Deoxyxylulose-5-phosphate synthase belongs to a family of transketolase-like proteins that are highly conserved in evolution.

Identification of a thiamin-dependent synthase in Escherichia coli required for the formation of the 1-deoxy-D-xylulose 5-phosphate precursor to isoprenoids, thiamin, and pyridoxol

In Escherichia coli, 1-deoxy-D-xylulose (or its 5-phosphate, DXP) is the biosynthetic precursor to isopentenyl diphosphate [Broers, S. T. J. (1994) Dissertation (Eidgenössische Technische Hochschule, Zürich)], thiamin, and pyridoxol [Himmeldirk, K., Kennedy, I. A., Hill, R. E., Sayer, B. G. & Spenser, I. D. (1996) Chem. Commun. 1187-1188]. Here we show that an open reading frame at 9 min on the chromosomal map of E. coli encodes an enzyme (deoxyxylulose-5-phosphate synthase, DXP synthase) that catalyzes a thiamin diphosphate-dependent acyloin condensation reaction between C atoms 2 and 3 of pyruvate and glyceraldehyde 3-phosphate to yield DXP. We have cloned and overexpressed the gene (dxs), and the enzyme was purified 17-fold to a specific activity of 0.85 unit/mg of protein. The reaction catalyzed by DXP synthase yielded exclusively DXP, which was characterized by 1H and 31P NMR spectroscopy. Although DXP synthase of E. coli shows sequence similarity to both transketolases and the E1 subunit of pyruvate dehydrogenase, it is a member of a distinct protein family, and putative DXP synthase sequences appear to be widespread in bacteria and plant chloroplasts.