Solubilization and reconstitution of the membrane-bound carotenogenic enzymes from daffodil chromoplasts

Natural variation in expression of genes associated with carotenoid biosynthesis and accumulation in cassava (Manihot esculenta Crantz) storage root

BACKGROUND

Cassava (Manihot esculenta Crantz) storage root provides a staple food source for millions of people worldwide. Increasing the carotenoid content in storage root of cassava could provide improved nutritional and health benefits. Because carotenoid accumulation has been associated with storage root color, this study characterized carotenoid profiles, and abundance of key transcripts associated with carotenoid biosynthesis, from 23 landraces of cassava storage root ranging in color from white-to-yellow-to-pink. This study provides important information to plant breeding programs aimed at improving cassava storage root nutritional quality.

RESULTS

Among the 23 landraces, five carotenoid types were detected in storage root with white color, while carotenoid types ranged from 1 to 21 in storage root with pink and yellow color. The majority of storage root in these landraces ranged in color from pale-to-intense yellow. In this color group, total β-carotene, containing all-E-, 9-Z-, and 13-Z-β-carotene isomers, was the major carotenoid type detected, varying from 26.13 to 76.72 %. Although no α-carotene was observed, variable amounts of a α-ring derived xanthophyll, lutein, was detected; with greater accumulation of α-ring xanthophylls than of β-ring xanthophyll. Lycopene was detected in a landrace (Cas51) with pink color storage root, but it was not detected in storage root with yellow color. Based on microarray and qRT-PCR analyses, abundance of transcripts coding for enzymes involved in carotenoid biosynthesis were consistent with carotenoid composition determined by contrasting HPLC-Diode Array profiles from storage root of landraces IAC12, Cas64, and Cas51. Abundance of transcripts encoding for proteins regulating plastid division were also consistent with the observed differences in total β-carotene accumulation.

CONCLUSIONS

Among the 23 cassava landraces with varying storage root color and diverse carotenoid types and profiles, landrace Cas51 (pink color storage root) had low LYCb transcript abundance, whereas landrace Cas64 (intense yellow storage root) had decreased HYb transcript abundance. These results may explain the increased amounts of lycopene and total β-carotene observed in landraces Cas51 and Cas64, respectively. Overall, total carotenoid content in cassava storage root of color class representatives were associated with spatial patterns of secondary growth, color, and abundance of transcripts linked to plastid division. Finally, a partial carotenoid biosynthesis pathway is proposed.

Carotenoid biosynthesis in Arabidopsis: a colorful pathway

Plant carotenoids are a family of pigments that participate in light harvesting and are essential for photoprotection against excess light. Furthermore, they act as precursors for the production of apocarotenoid hormones such as abscisic acid and strigolactones. In this review, we summarize the current knowledge on the genes and enzymes of the carotenoid biosynthetic pathway (which is now almost completely elucidated) and on the regulation of carotenoid biosynthesis at both transcriptional and post-transcriptional levels. We also discuss the relevance of Arabidopsis as a model system for the study of carotenogenesis and how metabolic engineering approaches in this plant have taught important lessons for carotenoid biotechnology.

A transcriptional analysis of carotenoid, chlorophyll and plastidial isoprenoid biosynthesis genes during development and osmotic stress responses in Arabidopsis thaliana

Background

The carotenoids are pure isoprenoids that are essential components of the photosynthetic apparatus and are coordinately synthesized with chlorophylls in chloroplasts. However, little is known about the mechanisms that regulate carotenoid biosynthesis or the mechanisms that coordinate this synthesis with that of chlorophylls and other plastidial synthesized isoprenoid-derived compounds, including quinones, gibberellic acid and abscisic acid. Here, a comprehensive transcriptional analysis of individual carotenoid and isoprenoid-related biosynthesis pathway genes was performed in order to elucidate the role of transcriptional regulation in the coordinated synthesis of these compounds and to identify regulatory components that may mediate this process in Arabidopsis thaliana.

Results

A global microarray expression correlation analysis revealed that the phytoene synthase gene, which encodes the first dedicated and rate-limiting enzyme of carotenogenesis, is highly co-expressed with many photosynthesis-related genes including many isoprenoid-related biosynthesis pathway genes. Chemical and mutant analysis revealed that induction of the co-expressed genes following germination was dependent on gibberellic acid and brassinosteroids (BR) but was inhibited by abscisic acid (ABA). Mutant analyses further revealed that expression of many of the genes is suppressed in dark grown plants by Phytochrome Interacting transcription Factors (PIFs) and activated by photoactivated phytochromes, which in turn degrade PIFs and mediate a coordinated induction of the genes. The promoters of PSY and the co-expressed genes were found to contain an enrichment in putative BR-auxin response elements and G-boxes, which bind PIFs, further supporting a role for BRs and PIFs in regulating expression of the genes. In osmotically stressed root tissue, transcription of Calvin cycle, methylerythritol 4-phosphate pathway and carotenoid biosynthesis genes is induced and uncoupled from that of chlorophyll biosynthesis genes in a manner that is consistent with the increased synthesis of carotenoid precursors for ABA biosynthesis. In all tissues examined, induction of β-carotene hydroxylase transcript levels are linked to an increased demand for ABA.

Conclusions

This analysis provides compelling evidence to suggest that coordinated transcriptional regulation of isoprenoid-related biosynthesis pathway genes plays a major role in coordinating the synthesis of functionally related chloroplast localized isoprenoid-derived compounds.

Isolation and characterization of the Z-ISO gene encoding a missing component of carotenoid biosynthesis in plants

Metabolic engineering of plant carotenoids in food crops has been a recent focus for improving human health. Pathway manipulation is predicated on comprehensive knowledge of this biosynthetic pathway, which has been extensively studied. However, there existed the possibility of an additional biosynthetic step thought to be dispensable because it could be compensated for by light. This step, mediated by a putative Z-ISO, was predicted to occur in the sequence of redox reactions that are coupled to an electron transport chain and convert the colorless 15-cis-phytoene to the red-colored all-trans-lycopene. The enigma of carotenogenesis in the absence of light (e.g. in endosperm, a target for improving nutritional content) argued for Z-ISO as a pathway requirement. Therefore, understanding of plant carotenoid biosynthesis was obviously incomplete. To prove the existence of Z-ISO, maize (Zea mays) and Arabidopsis (Arabidopsis thaliana) mutants were isolated and the gene identified. Functional testing of the gene product in Escherichia coli showed isomerization of the 15-cis double bond in 9,15,9'-tri-cis-zeta-carotene, proving that Z-ISO encoded the missing step. Z-ISO was found to be important for both light-exposed and "dark" tissues. Comparative genomics illuminated the origin of Z-ISO found throughout higher and lower plants, algae, diatoms, and cyanobacteria. Z-ISO evolved from an ancestor related to the NnrU (for nitrite and nitric oxide reductase U) gene required for bacterial denitrification, a pathway that produces nitrogen oxides as alternate electron acceptors for anaerobic growth. Therefore, plant carotenogenesis evolved by recruitment of genes from noncarotenogenic bacteria.

Phytoene desaturase is present in a large protein complex in the plastid membrane

Phytoene desaturase (PDS; EC 1.14.99.-) represents one of the key enzymes in the carotenoid biosynthetic pathway and is present in nearly all types of plastids in plants. To further characterize PDS, we isolated the PDS cDNA from cauliflower (BoPDS) and confirmed its function by heterologous expression in a strain of Escherichia coli containing a carotenoid-producing plasmid. The BoPDS cDNA encodes a predicted mature protein of approximately 55 kDa. In comparison with PDS from a few other plant species, BoPDS exhibited a high enzyme activity in E. coli, and its expression in plastids was independent of carotenoid levels. Plastids were purified from tissues of different plant species including cauliflower curds, tomato fruits, carrot roots and Arabidopsis leaves. By employing both Blue Native PAGE and SDS-PAGE approaches in conjunction with Western blot analysis, it was found that PDS in these plants existed in two forms. The plastid membrane form was present in a large protein complex of approximately 350 kDa, whereas the stroma version was in an approximately 660 kDa complex.

Maize Y9 encodes a product essential for 15-cis-zeta-carotene isomerization

Carotenoids are a diverse group of pigments found in plants, fungi, and bacteria. They serve essential functions in plants and provide health benefits for humans and animals. In plants, it was thought that conversion of the C40 carotenoid backbone, 15-cis-phytoene, to all-trans-lycopene, the geometrical isomer required by downstream enzymes, required two desaturases (phytoene desaturase and zeta-carotene desaturase [ZDS]) plus a carotene isomerase (CRTISO), in addition to light-mediated photoisomerization of the 15-cis-double bond; bacteria employ only a single enzyme, CRTI. Characterization of the maize (Zea mays) pale yellow9 (y9) locus has brought to light a new isomerase required in plant carotenoid biosynthesis. We report that maize Y9 encodes a factor required for isomerase activity upstream of CRTISO, which we term Z-ISO, an activity that catalyzes the cis- to trans-conversion of the 15-cis-bond in 9,15,9'-tri-cis-zeta-carotene, the product of phytoene desaturase, to form 9,9'-di-cis-zeta-carotene, the substrate of ZDS. We show that recessive y9 alleles condition accumulation of 9,15,9'-tri-cis-zeta-carotene in dark tissues, such as roots and etiolated leaves, in contrast to accumulation of 9,9'-di-cis-zeta-carotene in a ZDS mutant, viviparous9. We also identify a locus in Euglena gracilis, which is similarly required for Z-ISO activity. These data, taken together with the geometrical isomer substrate requirement of ZDS in evolutionarily distant plants, suggest that Z-ISO activity is not unique to maize, but will be found in all higher plants. Further analysis of this new gene-controlled step is critical to understanding regulation of this essential biosynthetic pathway.

Coordinate expression of multiple bacterial carotenoid genes in canola leading to altered carotenoid production

Carotenoids have drawn much attention recently because of their potentially positive benefits to human health as well as their utility in both food and animal feed. Previous work in canola (Brassica napus) seed over-expressing the bacterial phytoene synthase gene (crtB) demonstrated a change in carotenoid content, such that the total levels of carotenoids, including phytoene and downstream metabolites like beta-carotene, were elevated 50-fold, with the ratio of beta- to alpha-carotene being 2:1. This result raised the possibility that the composition of metabolites in this pathway could be modified further in conjunction with the increased flux obtained with crtB. Here we report on the expression of additional bacterial genes for the enzymes geranylgeranyl diphosphate synthase (crtE), phytoene desaturase (crtI) and lycopene cyclase (crtY and the plant B. napus lycopene beta-cyclase) engineered in conjunction with phytoene synthase (crtB) in transgenic canola seed. Analysis of the carotenoid levels by HPLC revealed a 90% decrease in phytoene levels for the double construct expressing crtB in conjunction with crtI. The transgenic seed from all the double constructs, including the one expressing the bacterial crtB and the plant lycopene beta-cyclase showed an increase in the levels of total carotenoid similar to that previously observed by expressing crtB alone but minimal effects were observed with respect to the ratio of beta- to alpha-carotene compared to the original construct. However, the beta- to alpha-carotene ratio was increased from 2:1 to 3:1 when a triple construct consisting of the bacterial phytoene synthase, phytoene desaturase and lycopene cyclase genes were expressed together. This result suggests that the bacterial genes may form an aggregate complex that allows in vivo activity of all three proteins through substrate channeling. This finding should allow further manipulation of the carotenoid biosynthetic pathway for downstream products with enhanced agronomic, animal feed and human nutritional values.

Cloning and characterization of a maize cDNA encoding phytoene desaturase, an enzyme of the carotenoid biosynthetic pathway

To study regulation of the plastid-localized maize carotenoid biosynthetic pathway, a cDNA encoding phytoene desaturase (PDS) was isolated and characterized. The DNA sequence of the maize Pds cDNA was determined and compared with available dicot Pds genes. The deduced PDS protein, estimated at 64.1 kDa (unprocessed), had a dinucleotide binding domain and conserved regions characteristic of other carotene desaturases. Alignment of available PDS sequences from distantly related organisms suggests that Pds has potential as a phylogenetic tool. By use of heterologous complementation in Escherichia coli, maize PDS was shown to catalyze two desaturation steps converting phytoene to zeta-carotene. RFLP (restriction fragment length polymorphism) mapping was used to place Pds on chromosome 1S near viviparous5 (vp5), and RT-PCR (reverse-transcriptase polymerase chain reaction) analysis indicated reduced Pds transcript in vp5 mutant relative to normal endosperm. Other phytoene-accumulating mutant endosperms, vp2 and white3 (w3), showed no difference in Pds transcript accumulation as compared with normal endosperm counterparts. RT-PCR analysis of Pds transcript accumulation in developing endosperm showed Pds was constitutively expressed. Therefore, endosperm carotenogensis is not regulated by increasing the level of Pds transcripts.

In vitro expression and activity of lycopene cyclase and beta-carotene hydroxylase from Erwinia herbicola

The cyclisation of lycopene to beta-carotene and the hydroxylation of beta-carotene to zeaxanthin are common enzymatic steps in the biosynthesis of carotenoids in a wide range of bacteria, fungi, and plants. We have individually expressed in E. coli the two genes coding for these enzymatic steps in Erwinia herbicola. The cyclase and hydroxylase enzymes have apparent molecular weights of 43 kDa and 22 kDa, respectively, as determined by SDS-PAGE. Hydroxylase in vitro activity was obtained only in the cytoplasmic fraction. Cyclase also demonstrated enzyme activity in a crude cell-free lysate, although to a lesser extent.

Production of beta-carotene in Zymomonas mobilis and Agrobacterium tumefaciens by introduction of the biosynthesis genes from Erwinia uredovora

The Erwinia uredovora crtB, crtE, crtI, and crtY genes required for beta-carotene biosynthesis were introduced by conjugal transfer into an ethanol-producing bacterium, Zymomonas mobilis, and a phytopathogenic bacterium, Agrobacterium tumefaciens, in which no carotenoid is synthesized. The transconjugants of Z. mobilis and A. tumefaciens carrying these genes appeared as yellow colonies and produced 220 and 350 micrograms of beta-carotene per g of dry weight, respectively, in the stationary phase in liquid culture.

Elucidation of the Erwinia uredovora carotenoid biosynthetic pathway by functional analysis of gene products expressed in Escherichia coli

The most important function of carotenoid pigments, especially beta-carotene in higher plants, is to protect organisms against photooxidative damage (G. Britton, in T. W. Goodwin, ed., Plant Pigments--1988, 1988; N. I. Krinsky, in O. Isler, H. Gutmann, and U. Solms, ed., Carotenoids--1971, 1971). beta-Carotene also functions as a precursor of vitamin A in mammals (G. A. J. Pitt, in I. Osler, H. Gutmann, and U. Solms, ed., Carotenoids--1971, 1971). The enzymes and genes which mediate the biosynthesis of cyclic carotenoids such as beta-carotene are virtually unknown. We have elucidated for the first time the pathway for biosynthesis of these carotenoids at the level of enzyme-catalyzed reactions, using bacterial carotenoid biosynthesis genes. These genes were cloned from a phytopathogenic bacterium, Erwinia uredovora 20D3 (ATCC 19321), in Escherichia coli and located on a 6,918-bp fragment whose nucleotide sequence was determined. Six open reading frames were found and designated the crtE, crtX, crtY, crtI, crtB, and crtZ genes in reference to the carotenoid biosynthesis genes of a photosynthetic bacterium, Rhodobacter capsulatus; only crtZ had the opposite orientation from the others. The carotenoid biosynthetic pathway in Erwinia uredovora was clarified by analyzing carotenoids accumulated in E. coli transformants in which some of these six genes were expressed, as follows: geranylgeranyl PPiCrtB----prephytoene PPiCrtE----phytoeneCrtI---- lycopeneCrtY----beta-caroteneCrtZ----zeaxanthinCrtX--- -zeaxanthin-beta- diglucoside. The carotenoids in this pathway appear to be close to those in higher plants rather than to those in bacteria. Also significant is that only one gene product (CrtI) for the conversion of phytoene to lycopene is required, a conversion in which four sequential desaturations should occur via the intermediates phytofluene, zeta-carotene, and neurosporene.

Quinone compounds are able to replace molecular oxygen as terminal electron acceptor in phytoene desaturation in chromoplasts of Narcissus pseudonarcissus L

The desaturation of phytoene to zeta-carotene and of zeta-carotene to lycopene employs molecular oxygen as the terminal electron acceptor. 2,3,5,6-Tetramethyl-1,4-benzoquinone (duroquinone) and other artificial quinones are able to replace oxygen, which demonstrates that oxygen does not act in a mixed-function oxygenase-like mechanism at the desaturase itself, but at a spatially separate site. Evidence for additional redox elements mediating between desaturase and oxygen is presented.

Prenyl lipid and fatty-acid synthesis in isolatedAcetabularia chloroplasts

A gentle procedure allowed the isolation of intact and highly active chloroplasts from the unicellular green algaAcetabularia mediterranea. These chloroplasts incorporated carbon from NaH(14)CO3 into fatty acids and prenyl lipids at a rate of about 20-50 nmol carbon· (mg chlorophyll)(-1)·h(-1). Most of the fatty acids formed in vitro were esterified in galactolipids. The main prenyl lipids synthesized were the chlorophyll side chain, intermediates of the carotenogenic path, α-and β-carotene, as well as lutein. Large amounts of [1-(14)C]acetate were incorporated, but exclusively into fatty acids.Isopentenyl diphosphate was a good substrate for prenyl-lipid formation in hypotonically treated chloroplasts. The envelope of intact chloroplasts, however, was impermeable to this compound. Intermediates of the mevalonate pathway were not accepted as precursors under conditions whereisopentenyl diphosphate was well incorporated. The results show that the lipid biosynthetic pathways in the plastids ofAcetabularia, a member of the ancient family of Dasycladaceae, are very similar to those in higher-plant plastids.

Chlorophyll-free chromoplasts from daffodil contain most of the enzymes for chlorophyll synthesis in a highly active form

Chromoplasts isolated from chlorophyll-free daffodil flowers utilize in vitro delta-aminolevulinic acid (ALA) as precursor for the synthesis of large amounts of at least nine different products. Their identification as intermediates of the chlorophyll biosynthetic pathway demonstrates the presence of the majority of the respective enzymes in this nongreen plastid preparation. Porphobilinogen synthase was investigated more closely and found to be similar in its properties to the corresponding enzyme from other plastid sources. Protoporphyrin IX was also accepted as a substrate by chromoplast homogenate; here, as in the case of ALA as a substrate, Mg-protoporphyrin IX monomethyl ester was the last product formed. Formation of the isocyclic chlorophyll ring was not observed.

Molecular oxygen and the state of geometric isomerism of intermediates are essential in the carotene desaturation and cyclization reactions in daffodil chromoplasts

The membrane-bound carotenogenic reaction sequence in daffodil chromoplasts can be subdivided in vitro into three reaction segments by varying the incubation parameters O2 and light. In the first segment, 15-cis-phytoene is desaturated to 15-cis-zeta-carotene (trans elimination of hydrogen) in the dark and in the presence of O2 as an essential cofactor. A photoisomerization of the 15-cis double bond of the accumulated zeta-carotene to trans is the prerequisite for the function of the second segment, the desaturation to 7,9,9',7'-tetra-cis-lycopene (prolycopene, cis elimination of hydrogen). The role of O2 as an electron acceptor is discussed and evidence for an oxidoreductase acting as a redox mediator between the desaturase (forming the polyene chromophore) and O2 is presented. A certain analogy to the desaturation of stearoyl-carrier protein, which also occurs in plastids, is proposed. The third segment, the cyclization of prolycopene, is active only in the absence of O2 and involves additional cis-trans isomerization reactions.

Nucleotide sequence, organization, and nature of the protein products of the carotenoid biosynthesis gene cluster of Rhodobacter capsulatus

Carotenoid pigments are essential for the protection of both photosynthetic and non-photosynthetic tissues from photooxidative damage. Although carotenoid biosynthesis has been studied in many organisms from bacteria to higher plants, little is known about carotenoid biosynthetic enzymes, or the nature and regulation of the genes encoding them. We report here the first DNA sequence of carotenoid genes from any organism. We have determined the complete nucleotide sequence (11,039 bp) of a gene cluster encoding seven of the eight previously known carotenoid genes (crtA, B, C, D, E, F, I) and a new gene, designated crtK, from Rhodobacter capsulatus, a purple non-sulfur photosynthetic bacterium. The 5' flanking regions of crtA, I, D and E contain a highly conserved palindromic sequence homologous to the consensus binding site for a variety of prokaryotic DNA-binding regulatory proteins. This putative regulatory palindrome is also found 5' to the puc operon, encoding the light-harvesting II antenna polypeptides. Escherichia coli-like sigma 70 promoter sequences are located 5' to crtI and crtD, suggesting for the first time that such promoters may exist in purple photosynthetic bacteria. The crt genes form a minimum of four distinct operons, crtA, crtIBK, crtDC and crtEF, based on inversions of transcriptional orientation within the gene cluster. Possible rho-independent transcription terminators are located 3' to crtI, B, K, C and F. The 3' end of crtA may overlap transcription initiation signals for a downstream gene required for bacteriochlorophyll biosynthesis. We have also observed two regions of exceptional amino acid homology between CrtI and CrtD, both of which are dehydrogenases.

Biosynthesis of monoterpene hydrocarbons by isolated chromoplasts from daffodil flowers

Incubation studies using [1-14C]isopentenyl diphosphate and [1-2H2]isopentenyl diphosphate as substrates revealed that isolated chromoplasts from flowers of Narcissus pseudonarcissus L. are able to synthesize monoterpene hydrocarbons and linalool in high yields. The enzymes involved are soluble in the chromoplast stroma. It is hypothesized that in the plant cell plastids are the site of monoterpene biosynthesis, whereas the formation of sesquiterpenes may be restricted to the cytoplasm/endoplasmic reticulum.

Formation of isopentenyl diphosphate via mevalonate does not occur within etioplasts and etiochloroplasts of mustard (Sinapis alba L.) seedlings

Seedlings from the white mustard, Sinapis alba, grown under continuous far-red light exhibit enhanced plastid enzyme activities when compared with dark-grown seedlings (for review, see Mohr 1981). These activities are even more pronounced upon illumination with white light during the etioplast/chloroplast transformation. Etioplasts and etiochloroplasts from the cotyledons of such seedlings show high prenyl-lipid-synthesizing activities when [1-(14)C]isopentenyl diphosphate is used as the precursor. They lack, however, any enzymatic activities for the formation of isopentenyl diphosphate via the mevalonate pathway, i.e. hydroxymethylglutaryl-CoA reductase, mevalonate kinase, phosphomevalonate kinase and diphosphomevalonate decarboxylase, which are present and easily detectable within the endoplasmic reticulum and cytoplasm. These results corroborate the view that the cytoplasm of the plant cell is the only site of isopentenyl-diphosphate formation via the mevalonate pathway.

Carotenoid and chlorophyll biosynthesis in isolated plastids from mustard seedling cotyledons (Sinapis alba L.) during etioplast-chloroplast conversion

Etioplasts and etiochloroplasts, isolated from seedlings of white mustard (Sinapis alba L.) grown in continuous far-red light, and chloroplasts isolated from cotyledons and primary leaves of white-light-grown seedlings exhibit high prenyl-lipid-forming activities. Only the etioplasts and etiochloroplasts, and to a much lesser extent chloroplasts from cotyledons are capable of forming carotenes from isopentenyl diphosphate as substrate, whereas in chloroplasts from primary leaves no such activities could be detected. By subfractionation experiments, it could be demonstrated that the phytoene-synthase complex in etioplasts and etiochloroplasts is present in a soluble form in the stroma, whereas the subsequent enzymes, i.e. the dehydrogenase, cis-trans isomerase and cyclase are bound to both membrane fractions, the prolamellar bodies/prothylakoids and the envelopes. In good agreement with previous results using isolated chromoplasts and chloroplasts, it is concluded that the phytoene-synthase complex may change its topology from a peripheral membrane protein in non-green plastids to a tightly membrane-associated protein in chloroplasts. This change is apparently paralleled by altered functional properties which render the complex undetectable in isolated chloroplasts. Further experiments concerning the reduction of chlorophyll a containing a geranylgeranyl side chain to chlorophyll a indicate that the light-induced etioplast-chloroplast conversion is accompanied by a certain reorganization of the polyprenoid-forming enzymatic equipment.

Carotenoid synthesis and pleiotropic effects in carotenoid-deficient seedlings of maize

Plastid-envelope membranes from seedlings ofZea mays L. made carotenoid-deficient by either norflurazon treatment or mutation lack an activity permitting conversion of phytoene to β-carotene. This activity in membrane fractions was measured by coincubation in vitro with a soluble system from spinach chloroplasts capable of converting [(14)C]isopentenyl pyrophosphate into phytoene. When grown in light, the carotenoid-deficient seedlings lack many soluble chloroplast proteins, including NADP-dependent malic enzyme (EC 1.1.1.40), pyruvate phosphate dikinase (EC 2.7.9.1), and ribulose-1,5-bisphosphate carboxylase (EC 4.1.1.39), but apparently still contain the soluble activities permitting synthesis of phytoene.