Functional assembly of the foreign carotenoid lycopene into the photosynthetic apparatus of Rhodobacter sphaeroides, achieved by replacement of the native 3-step phytoene desaturase with its 4-step counterpart from Erwinia herbicola

Genes and Pathway Reactions Related to Carotenoid Biosynthesis in Purple Bacteria

In purple bacteria, the genes of the carotenoid pathways are part of photosynthesis gene clusters which were distributed among different species by horizontal gene transfer. Their close organisation facilitated the first-time cloning of carotenogenic genes and promoted the molecular investigation of spheroidene and spirilloxanthin biosynthesis. This review highlights the cloning of the spheroidene and spirilloxanthin pathway genes and presents the current knowledge on the enzymes involved in the carotenoid biosynthesis of purple sulphur and non-sulphur bacteria. Mostly, spheroidene or spirilloxanthin biosynthesis exists in purple non-sulphur bacteria but both pathways operate simultaneously in Rubrivivax gelatinosus. In the following years, genes from other bacteria including purple sulphur bacteria with an okenone pathway were cloned. The individual steps were investigated by kinetic studies with heterologously expressed pathway genes which supported the establishment of the reaction mechanisms. In particular, the substrate and product specificities revealed the sequential order of the speroidene and spiriloxanthin pathways as well as their interactions. Information on the enzymes involved revealed that the phytoene desaturase determines the type of pathway by the formation of different products. By selection of mutants with amino acid exchanges in the putative substrate-binding site, the neurosporene-forming phytoene desaturase could be changed into a lycopene-producing enzyme and vice versa. Concerning the oxygen groups in neurosporene and lycopene, the tertiary alcohol group at C1 is formed from water and not by oxygenation, and the C2 or C4 keto groups are inserted differently by an oxygen-dependent or oxygen-independent ketolation reaction, respectively.

Co-biosynthesis of germacrene A, a precursor of β-elemene, and lycopene in engineered Escherichia coli

β-Elemene is the major component of a traditional Chinese medicine (Rhizoma Curcumae) for cancer treatment, and plant extraction is the major methods currently. Biosynthesis of β-elemene is a promising and attractive route due to its advantages, including environmentally friendly processes, renewable resources, and sustainable development. In this research, biosynthesis of germacrene A, direct precursor of β-elemene, in Escherichia coli was successfully performed and 11.99 mg/L germacrene A was obtained. Thereafter, a cobiosynthesis system for germacrene A and lycopene, another kind of isoprenoid, was constructed. Furthermore, the cultivation conditions were optimized. The germacrene A production was increased to the highest level reported to date, 364.26 mg/L, threefold increase to the strain with only germacrene A production. The cobiosynthesis system was suggested to promote the metabolic flux for germacrene A production. This research enabled germacrene A production in E. coli, and it highlights the promoting mechanism of the cobiosynthesis system for two chemicals which are both belonging to isoprenoids. KEY POINTS : • Co-production of germacrene A and lycopene in E. coli. • Promoting mechanism of cobiosynthesis of two isoprenoid compounds in E. coli. • Shake-flask production of germacrene A reached to the highest 364.26 mg/L in E. coli.

Cryo-EM Structure of the Rhodobacter sphaeroides Light-Harvesting 2 Complex at 2.1 Å

Light-harvesting 2 (LH2) antenna complexes augment the collection of solar energy in many phototrophic bacteria. Despite its frequent role as a model for such complexes, there has been no three-dimensional (3D) structure available for the LH2 from the purple phototroph Rhodobacter sphaeroides. We used cryo-electron microscopy (cryo-EM) to determine the 2.1 Å resolution structure of this LH2 antenna, which is a cylindrical assembly of nine αβ heterodimer subunits, each of which binds three bacteriochlorophyll a (BChl) molecules and one carotenoid. The high resolution of this structure reveals all of the interpigment and pigment–protein interactions that promote the assembly and energy-transfer properties of this complex. Near the cytoplasmic face of the complex there is a ring of nine BChls, which absorb maximally at 800 nm and are designated as B800; each B800 is coordinated by the N-terminal carboxymethionine of LH2-α, part of a network of interactions with nearby residues on both LH2-α and LH2-β and with the carotenoid. Nine carotenoids, which are spheroidene in the strain we analyzed, snake through the complex, traversing the membrane and interacting with a ring of 18 BChls situated toward the periplasmic side of the complex. Hydrogen bonds with C-terminal aromatic residues modify the absorption of these pigments, which are red-shifted to 850 nm. Overlaps between the macrocycles of the B850 BChls ensure rapid transfer of excitation energy around this ring of pigments, which act as the donors of energy to neighboring LH2 and reaction center light-harvesting 1 (RC–LH1) complexes.

Manipulation of the Regulatory Genes ppsR and prrA in Rhodobacter sphaeroides Enhances Lycopene Production

Rhodobacter sphaeroides is a non-sulfur purple bacterium with great metabolic versatility, capable of producing a variety of valuable compounds that include carotenoids and CoQ₁₀. In order to enhance lycopene production, we deleted the photosynthetic gene cluster repressor ppsR from a lycopene-producing Rb. sphaeroides strain (RL1) constructed in a previous study to break the control of carotenoid synthesis by the oxygen level. Also, lycopene production was further increased by overexpression of the activator prrA. The superior lycopene producer DppsR/OprrA thus obtained had a high growth rate and a lycopene production of 150.15 mg/L with a yield of 21.45 mg/g dry cell weight (DCW) under high oxygen conditions; these values were ≥6.85-fold higher than those of RL1 (19.13 mg/L; 3.32 mg/g DCW). Our findings indicate that elimination of oxygen repression led to more efficient lycopene production by DppsR/OprrA and that its increased productivity under high oxygen conditions makes it a potentially useful strain for industrial-scale lycopene production.

Atlantic Salmon (Salmo salar L., 1758) Gut Microbiota Profile Correlates with Flesh Pigmentation: Cause or Effect?

In Tasmania (Australia), during the marine phase, it has been observed that flesh pigmentation significantly drops in summer, possibly due to high water temperatures (> 20 °C). Although this deleterious effect of summer temperatures has been ascertained, there is a lack of knowledge of the actual mechanisms behind the impaired uptake and/or loss of pigments in Atlantic salmon in a challenging environment. Since the microbial community in the fish intestine significantly changes in relation to the variations of water temperature, this study was conducted to assess how the gut microbiota profile also correlates with the flesh color during temperature fluctuation. We sampled 68 fish at three time points covering the end of summer to winter at a marine farm in Tasmania, Australia. Flesh color was examined in two ways: the average color throughout and the evenness of the color between different areas of the fillet. Using 16S rRNA sequencing of the v3-v4 region, we determined that water temperature corresponded to changes in the gut microbiome both with alpha diversity (Kruskal-Wallis tests P = 0.05) and beta diversity indices (PERMANOVA P = 0.001). Also, there was a significant correlation between the microbiota and the color of the fillet (PERMANOVA P = 0.016). There was a high abundance of Pseudoalteromonadaceae, Enterobacteriaceae, Microbacteriaceae, and Vibrionaceae in the pale individuals. Conversely, carotenoid-synthesizing bacteria families (Bacillaceae, Mycoplasmataceae, Pseudomonas, Phyllobacteriaceae, and Comamonadaceae) were found in higher abundance in individuals with darker flesh color.

Metabolic Engineering of Different Microbial Hosts for Lycopene Production

As a result of the extensive use of lycopene in a variety of fields, especially the dietary supplement and health food industries, the production of lycopene has attracted considerable interest. Lycopene can be obtained through extraction from vegetables and chemical synthesis. Alternatively, the microbial production of lycopene has been extensively researched in recent years. Various types of microbial hosts have been evaluated for their potential to accumulate a high level of lycopene. Metabolic engineering of the hosts and optimization of culture conditions are performed to enhance lycopene production. After years of research, great progress has been made in lycopene production. In this review, strategies used to improve lycopene production in different microbial hosts and the advantages and disadvantages of each microbial host are summarized. In addition, future perspectives of lycopene production in different microbial hosts are discussed.

Metabolic Redesign of Rhodobacter sphaeroides for Lycopene Production

Lycopene plays an important role as an antioxidative and anticancer agent, and is an increasingly valuable commodity in the global market. Rhodobacter sphaeroides, a carotenogenic and phototrophic bacterium, is an efficient and practical host for carotenoid production. Herein, we explored the potential of metabolically engineered Rb. sphaeroides as a novel platform to produce lycopene. The basal lycopene-producing strain was generated by introducing an exogenous crtI₄ from Rhodospirillum rubrum to replace the native crtI₃ and deleting crtC in Rb. sphaeroides. Furthermore, knocking out zwf blocked the competitive pentose phosphate pathway and improved the lycopene content by 88%. Finally, the methylerythritol phosphate pathway was reinforced by integration of dxs combined with zwf deletion, which further increased the lycopene content. The final engineered strain produced lycopene to 10.32 mg/g dry cell weight. This study describes a new lycopene producer and provides insight into a photosynthetic bacterium as a host for lycopene biosynthesis.

Engineered photoproteins that give rise to photosynthetically-incompetent bacteria are effective as photovoltaic materials for biohybrid photoelectrochemical cells

Reaction centre/light harvesting proteins such as the RCLH1X complex from Rhodobacter sphaeroides carry out highly quantum-efficient conversion of solar energy through ultrafast energy transfer and charge separation, and these pigment-proteins have been incorporated into biohybrid photoelectrochemical cells for a variety of applications. In this work we demonstrate that, despite not being able to support normal photosynthetic growth of Rhodobacter sphaeroides, an engineered variant of this RCLH1X complex lacking the PufX protein and with an enlarged light harvesting antenna is unimpaired in its capacity for photocurrent generation in two types of bio-photoelectrochemical cells. Removal of PufX also did not impair the ability of the RCLH1 complex to act as an acceptor of energy from synthetic light harvesting quantum dots. Unexpectedly, the removal of PufX led to a marked improvement in the overall stability of the RCLH1 complex under heat stress. We conclude that PufX-deficient RCLH1 complexes are fully functional in solar energy conversion in a device setting and that their enhanced structural stability could make them a preferred choice over their native PufX-containing counterpart. Our findings on the competence of RCLH1 complexes for light energy conversion in vitro are discussed with reference to the reason why these PufX-deficient proteins are not capable of light energy conversion in vivo.

Engineering of a calcium-ion binding site into the RC-LH1-PufX complex of Rhodobacter sphaeroides to enable ion-dependent spectral red-shifting

The reaction centre-light harvesting 1 (RC-LH1) complex of Thermochromatium (Tch.) tepidum has a unique calcium-ion binding site that enhances thermal stability and red-shifts the absorption of LH1 from 880nm to 915nm in the presence of calcium-ions. The LH1 antenna of mesophilic species of phototrophic bacteria such as Rhodobacter (Rba.) sphaeroides does not possess such properties. We have engineered calcium-ion binding into the LH1 antenna of Rba. sphaeroides by progressively modifying the native LH1 polypeptides with sequences from Tch. tepidum. We show that acquisition of the C-terminal domains from LH1 α and β of Tch. tepidum is sufficient to activate calcium-ion binding and the extent of red-shifting increases with the proportion of Tch. tepidum sequence incorporated. However, full exchange of the LH1 polypeptides with those of Tch. tepidum results in misassembled core complexes. Isolated α and β polypeptides from our most successful mutant were reconstituted in vitro with BChl a to form an LH1-type complex, which was stabilised 3-fold by calcium-ions. Additionally, carotenoid specificity was changed from spheroidene found in Rba. sphaeroides to spirilloxanthin found in Tch. tepidum, with the latter enhancing in vitro formation of LH1. These data show that the C-terminal LH1 α/β domains of Tch. tepidum behave autonomously, and are able to transmit calcium-ion induced conformational changes to BChls bound to the rest of a foreign antenna complex. Thus, elements of foreign antenna complexes, such as calcium-ion binding and blue/red switching of absorption, can be ported into Rhodobacter sphaeroides using careful design processes.

New insights into the photochemistry of carotenoid spheroidenone in light-harvesting complex 2 from the purple bacterium Rhodobacter sphaeroides

Light-harvesting complex 2 (LH2) from the semi-aerobically grown purple phototrophic bacterium Rhodobacter sphaeroides was studied using optical (static and time-resolved) and resonance Raman spectroscopies. This antenna complex comprises bacteriochlorophyll (BChl) a and the carotenoid spheroidenone, a ketolated derivative of spheroidene. The results indicate that the spheroidenone-LH2 complex contains two spectral forms of the carotenoid: (1) a minor, "blue" form with an S₂ (1¹B _u⁺ ) spectral origin band at 522 nm, shifted from the position in organic media simply by the high polarizability of the binding site, and (2) the major, "red" form with the origin band at 562 nm that is associated with a pool of pigments that more strongly interact with protein residues, most likely via hydrogen bonding. Application of targeted modeling of excited-state decay pathways after carotenoid excitation suggests that the high (92%) carotenoid-to-BChl energy transfer efficiency in this LH2 system, relative to LH2 complexes binding carotenoids with comparable double-bond conjugation lengths, derives mainly from resonance energy transfer from spheroidenone S₂ (1¹B _u⁺ ) state to BChl a via the Q_x state of the latter, accounting for 60% of the total transfer. The elevated S₂ (1¹B _u⁺ ) → Q_x transfer efficiency is apparently associated with substantially decreased energy gap (increased spectral overlap) between the virtual S₂ (1¹B _u⁺ ) → S₀ (1¹A _g^- ) carotenoid emission and Q_x absorption of BChl a. This reduced energetic gap is the ultimate consequence of strong carotenoid-protein interactions, including the inferred hydrogen bonding.

Extract from a mutant Rhodobacter sphaeroides as an enriched carotenoid source

Background

The extract Lycogen™ from the phototrophic bacterium Rhodobacter sphaeroides (WL-APD911) has attracted significant attention because of its promising potential as a bioactive mixture, attributed in part to its anti-inflammatory properties and anti-oxidative activity.

Objective

This study aims to investigate the components of Lycogen™ and its anti-inflammatory properties and anti-oxidative activity.

Design and results

The mutant strain R. sphaeroides (WL-APD911) whose carotenoid 1,2-hydratase gene has been altered by chemical mutagenesis was used for the production of a new carotenoid. The strain was grown at 30°C on Luria–Bertani (LB) agar plates. After a 4-day culture period, the mutant strain displayed a 3.5-fold increase in carotenoid content, relative to the wild type. In the DPPH test, Lycogen™ showed more potent anti-oxidative activity than lycopene from the wild-type strain. Primary skin irritation test with hamsters showed no irritation response in hamster skins after 30 days of treatment with 0.2% Lycogen™. Chemical investigations of Lycogen™ using nuclear magnetic resonance (NMR) ¹H, ¹³C, and COSY/DQCOSY spectra have identified spheroidenone and methoxyneurosporene. Quantitative analysis of these identified compounds based on spectral intensities indicates that spheroidenone and methoxyneurosporene are major components (approximately 1:1); very small quantities of other derivatives are also present in the sample.

Conclusions

In this study, we identified the major carotenoid compounds contained in Lycogen™, including spheroidenone and methoxyneurosporene by high-resolution NMR spectroscopy analysis. The carotenoid content of this mutant strain of R. sphaeroides was 3.5-fold higher than that in normal strain. Furthermore, Lycogen™ from the mutant strain is more potent than lycopene from the wild-type strain and does not cause irritation in hamster skins. These findings suggest that this mutant strain has the potential to be used as an enriched carotenoid source.

Functional characteristics of spirilloxanthin and keto-bearing Analogues in light-harvesting LH2 complexes from Rhodobacter sphaeroides with a genetically modified carotenoid synthesis pathway

Light-harvesting 2 (LH2) complexes from a genetically modified strain of the purple photosynthetic bacterium Rhodobacter (Rba.) sphaeroides were studied using static and ultrafast optical methods and resonance Raman spectroscopy. Carotenoid synthesis in the Rba. sphaeroides strain was engineered to redirect carotenoid production away from spheroidene into the spirilloxanthin synthesis pathway. The strain assembles LH2 antennas with substantial amounts of spirilloxanthin (total double-bond conjugation length N=13) if grown anaerobically and of keto-bearing long-chain analogs [2-ketoanhydrorhodovibrin (N=13), 2-ketospirilloxanthin (N=14) and 2,2'-diketospirilloxanthin (N=15)] if grown semi-aerobically (with ratios that depend on growth conditions). We present the photophysical, electronic, and vibrational properties of these carotenoids, both isolated in organic media and assembled within LH2 complexes. Measurements of excited-state energy transfer to the array of excitonically coupled bacteriochlorophyll a molecules (B850) show that the mean lifetime of the first singlet excited state (S1) of the long-chain (N≥13) carotenoids does not change appreciably between organic media and the protein environment. In each case, the S1 state appears to lie lower in energy than that of B850. The energy-transfer yield is ~0.4 in LH2 (from the strain grown aerobically or semi-aerobically), which is less than half that achieved for LH2 that contains short-chain (N≤11) analogues. Collectively, the results suggest that the S1 excited state of the long-chain (N≥13) carotenoids participates little if at all in carotenoid-to-BChl a energy transfer, which occurs predominantly via the carotenoid S2 excited state in these antennas.

Assembly of functional photosystem complexes in Rhodobacter sphaeroides incorporating carotenoids from the spirilloxanthin pathway

Carotenoids protect the photosynthetic apparatus against harmful radicals arising from the presence of both light and oxygen. They also act as accessory pigments for harvesting solar energy, and are required for stable assembly of many light-harvesting complexes. In the phototrophic bacterium Rhodobacter (Rba.) sphaeroides phytoene desaturase (CrtI) catalyses three sequential desaturations of the colourless carotenoid phytoene, extending the number of conjugated carbon–carbon double bonds, N, from three to nine and producing the yellow carotenoid neurosporene; subsequent modifications produce the yellow/red carotenoids spheroidene/spheroidenone (N = 10/11). Genomic crtI replacements were used to swap the native three-step Rba. sphaeroides CrtI for the four-step Pantoea agglomerans enzyme, which re-routed carotenoid biosynthesis and culminated in the production of 2,2′-diketo-spirilloxanthin under semi-aerobic conditions. The new carotenoid pathway was elucidated using a combination of HPLC and mass spectrometry. Premature termination of this new pathway by inactivating crtC or crtD produced strains with lycopene or rhodopin as major carotenoids. All of the spirilloxanthin series carotenoids are accepted by the assembly pathways for LH2 and RC–LH1–PufX complexes. The efficiency of carotenoid-to-bacteriochlorophyll energy transfer for 2,2′-diketo-spirilloxanthin (15 conjugated C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]>C bonds; N = 15) in LH2 complexes is low, at 35%. High energy transfer efficiencies were obtained for neurosporene (N = 9; 94%), spheroidene (N = 10; 96%) and spheroidenone (N = 11; 95%), whereas intermediate values were measured for lycopene (N = 11; 64%), rhodopin (N = 11; 62%) and spirilloxanthin (N = 13; 39%). The variety and stability of these novel Rba. sphaeroides antenna complexes make them useful experimental models for investigating the energy transfer dynamics of carotenoids in bacterial photosynthesis.

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The spirilloxanthin biosynthetic pathway has been constructed in Rba. sphaeroides.

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The new carotenoids are accepted by the photosystem assembly pathways.

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These pigments are efficiently integrated into LH2 and RC–LH1–PufX complexes.

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Carotenoid–BChl energy transfer drops with the number of conjugated CC bonds (N).

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The lowest efficiency, 35%, is for the N = 15 carotenoid 2,2′ diketospirilloxanthin.

The LH2 complexes are assembled in the cells of purple sulfur bacterium Ectothiorhodospira haloalkaliphila with inhibition of carotenoid biosynthesis

The effect of the inhibitor of carotenoid (Car) biosynthesis, diphenylamine (DPA), on the cells of the purple sulfur bacterium Ectothiorhodospira (Ect.) haloalkaliphila has been studied. There occurs an inhibition of the biosynthesis of colored Cars (≥99 %) at 71 μM DPA. Considering "empty" Car pockets (Moskalenko and Makhneva 2012) the content of Cars in the DPA-treated samples is first calculated more correctly. The total content of the colored Cars in the sample at 71 μM DPA does not exceed 1 % of the wild type. In the DPA-treated cells (membranes) a complete set of pigment-protein complexes is retained. The LH2 complex at 71 μM DPA is isolated, which is identical to the LH2 complex of the wild type in near IR absorption spectra. This suggests that the principles for assembling this LH2 complex in vivo in the absence of colored Cars remain the same. These results are in full agreement with the data obtained earlier for Allochromatium (Alc.) minutissimum (Moskalenko and Makhneva 2012). They are as follows: (1) DPA almost entirely inhibits the biosynthesis of the colored Cars in Ect. haloalkaliphila cells. (2) In the DPA-treated samples non-colored Cars are detected at 53.25 μM DPA (as traces) and at 71 μM DPA. (3) DPA may affect both phytoene synthase (at ≤71 μM DPA) and phytoene desaturase (at ≥53.25 μM DPA). (4) The assembly of LH2 complex does occur without any colored Cars.

High-level production of the industrial product lycopene by the photosynthetic bacterium Rhodospirillum rubrum

The biosynthesis of the major carotenoid spirilloxanthin by the purple nonsulfur bacterium Rhodospirillum rubrum is thought to occur via a linear pathway proceeding through phytoene and, later, lycopene as intermediates. This assumption is based solely on early chemical evidence (B. H. Davies, Biochem. J. 116:93-99, 1970). In most purple bacteria, the desaturation of phytoene, catalyzed by the enzyme phytoene desaturase (CrtI), leads to neurosporene, involving only three dehydrogenation steps and not four as in the case of lycopene. We show here that the chromosomal insertion of a kanamycin resistance cassette into the crtC-crtD region of the partial carotenoid gene cluster, whose gene products are responsible for the downstream processing of lycopene, leads to the accumulation of the latter as the major carotenoid. We provide spectroscopic and biochemical evidence that in vivo, lycopene is incorporated into the light-harvesting complex 1 as efficiently as the methoxylated carotenoids spirilloxanthin (in the wild type) and 3,4,3',4'-tetrahydrospirilloxanthin (in a crtD mutant), both under semiaerobic, chemoheterotrophic, and photosynthetic, anaerobic conditions. Quantitative growth experiments conducted in dark, semiaerobic conditions, using a growth medium for high cell density and high intracellular membrane levels, which are suitable for the conventional industrial production in the absence of light, yielded lycopene at up to 2 mg/g (dry weight) of cells or up to 15 mg/liter of culture. These values are comparable to those of many previously described Escherichia coli strains engineered for lycopene production. This study provides the first genetic proof that the R. rubrum CrtI produces lycopene exclusively as an end product.

Anti-Inflammatory Property of Biomaterial Carotenoids Production by Rhodobacter Sphaeroides WL-APD911

A Rhodobacter sphaeroides WL-APD911 with the capability of producing the end products of neurosporene and ξ-carotene, rather than the normal end products of spheroidene, was isolated from hundreds of strains by using chemical mutagenesis. The strain WL-APD911 grew well in LB medium, and the colored carotenoids were produced within 3 days of incubation at 30°C. The Rs-M of colored carotenoid extracts obtained from strain WL-APD911 by using methanol extraction inhibited the NO production and iNOS expression by LPS-induced RAW 264.7 cells. This probability of reason for the Rs-M as the best source was to prompt the anti-inflammation and anti-oxidation in nutraceutical products. Finally, the carotenoid extracts (Rs-M) with neurosporene and ξ-carotene was called lycogene to be a newly biomaterial.

The biochemical basis for structural diversity in the carotenoids of chlorophototrophic bacteria

Ongoing work has led to the identification of most of the biochemical steps in carotenoid biosynthesis in chlorophototrophic bacteria. In carotenogenesis, a relatively small number of modifications leads to a great diversity of carotenoid structures. This review examines the individual steps in the pathway, discusses how each contributes to structural diversity among carotenoids, and summarizes recent progress in elucidating the biosynthetic pathways for carotenoids in chlorophototrophs.

Kinetic variations determine the product pattern of phytoene desaturase from Rubrivivax gelatinosus

In bacteria and fungi, the degree of carotenoid desaturation is determined by a single enzyme, the CrtI-type phytoene desaturase. In different organisms, this enzyme can carry out either three, four or even five desaturation steps. The purple bacterium Rubrivivax gelatinosus is the only known species in which reaction products of a 3-step and a 4-step desaturation (i.e. neurosporene and lycopene derivatives) accumulate simultaneously. The properties of this phytoene desaturation to catalyze neurosporene or lycopene were analyzed by heterologous complementations in Escherichia coli and by in vitro studies. They demonstrated that high enzyme concentrations or low phytoene supply favor the formation of lycopene. Under these conditions, CrtI from Rhodobacter spheroides can be forced in vitro to lycopene formation although this carotene is not synthesized in this species. All results can be explained by a model based on the competition between phytoene and neurosporene for the substrate binding site of phytoene desaturase. Mutations in CrtI from Rvi. gelatinosus have been generated resulting in increased lycopene formation in Escherichia coli. This modification in catalysis is due to increased amounts of CrtI protein.

Progress on molecular breeding and metabolic engineering of biosynthesis pathways of C(30), C(35), C(40), C(45), C(50) carotenoids

At least 700 natural carotenoids have been characterized; they can be classified into C(30), C(40) and C(50) subfamilies. The first step of C(40) pathway is the combination of two molecules of geranylgeranyl pyrophosphate to synthesize phytoene by phytoene synthase (CrtB or PSY). Most natural carotenoids originate from different types and levels of desaturation by phytoene desaturase (CrtI or PDS+ZDS), cyclization by lycopene cyclase (CrtY or LYC) and other modifications by different modifying enzyme (CrtA, CrtU, CrtZ or BCH, CrtX, CrtO, etc.) of this C(40) backbone. The first step of C(30) pathway is the combination of two molecules of FDP to synthesize diapophytoene by diapophytoene synthase (CrtM). But natural C(30) pathway only goes through a few steps of desaturation to form diaponeurosporene by diapophytoene desaturase (CrtN). Natural C(50) carotenoid decaprenoxanthin is synthesized starting from the C(40) carotenoid lycopene by the addition of 2 C(5) units. Concerned the importance of carotenoids, more and more attention has been concentrated on achieving novel carotenoids. The method being used successfully is to construct carotenoids biosynthesis pathways by metabolic engineering. The strategy of metabolic engineering is to engineer a small number of stringent upstream enzymes (CrtB, CrtI, CrtY, CrtM, or CrtN), then use a lot of promiscuous downstream enzymes to obtain large number of novel carotenoids. Two key enzymes phytoene desaturase (CrtI(m)) and lycopene cyclase (CrtY(m)) have been modified and used with a series of downstream modifying enzymes with broad substrate specificity, such as monooxygenase (CrtA), carotene desaturase (CrtU), carotene hydroxylase (CrtZ), zeaxanthin glycosylase (CrtX) and carotene ketolase (CrtO) to extend successfully natural C(30) and C(40) pathways in E. coli. Existing C(30) synthase CrtM to synthesize carotenoids with different chain length have been engineered and a series of novel carotenoids have been achieved using downstream modifying enzymes. C(35) carotenoid biosynthesis pathway has been constructed in E. coli as described. C(45) and C(50) carotenoid biosynthesis pathways have also been constructed in E. coli, but it is still necessary to extend these two pathways. Those novel acyclic or cyclic carotenoids have a potential ability to protect against photooxidation and radical-mediated peroxidation reactions which makes them interesting pharmaceutical candidates.

Astaxanthin formation in the marine photosynthetic bacterium Rhodovulum sulfidophilum expressing crtI, crtY, crtW and crtZ

This study reports the production of astaxanthin in the photosynthetic bacterium Rhodovulum sulfidophilum, which has adequate precursor pools and storage capabilities for heterologous carotenoid production. Chemical mutagenesis was carried out using ethylmethane sulfonate to produce mutants with a modified carotenoid biosynthesis pathway downstream of phytoene. Stable green- and gray-colored mutants were selected. Green mutants contained neurosporene or chloroxanthin as their major carotenoid (>90%), while the gray mutants accumulated phytoene. We previously demonstrated the production of beta-carotene in Rhodovulum sulfidophilum by cloning the Erythrobacter longus crtI (phytoene dehydrogenase) and crtY (lycopene cyclase) genes. In the present study, an expression vector for astaxanthin production was constructed that contained the Paracoccus crtW (beta-carotene oxygenase) and crtZ (beta-carotene hydroxylase) genes in addition to the E. longus crtI and crtY genes. A transconjugant, which can synthesize astaxanthin, was successfully generated (2.0 microg g(-1) DCW).

Effects of carotenoid inhibition on the photosynthetic RC-LH1 complex in purple sulphur bacterium Thiorhodospira sibirica

Core complexes (LH1-RC) were isolated using preparative gel electrophoresis from photosynthetic membranes of the purple bacterium, Thiorhodospira sibirica, grown in the absence or presence of the carotenoid biosynthesis inhibitor, diphenylamine. The biosynthesis of carotenoids is affected by diphenylamine both quantitavely and qualitatively: after inhibition, the level of carotenoids in core complexes reaches only 10% of the normal content, as analyzed by HPLC and absorption spectroscopy. The normally grown bacterium biosynthesizes spirilloxanthin, rhodopin, anhydrorhodovibrin and lycopene, whereas after inhibition only neurosporene, zeta-carotene and their derivatives are found in the complexes. There is no concomitant accumulation of appreciable amounts of colorless carotenoid precursors. Interestingly, the main absorption band of the core light harvesting complex isolated from carotenoid-inhibited cells, shows a red shift to 889 nm, instead of a blue shift observed in many carotenoid-deficient species of purple photosynthetic bacteria. The stability of isolated core complexes against n-octyl-beta-D: -glucopyranoside clearly depends on the presence of carotenoids. Subcomplexes resulting from the detergent treatment, were characterized by non-denaturating gel electrophoresis combined with in situ absorption spectroscopy. Core complexes with the native carotenoid complement dissociate into three subcomplexes: (a) LH1 complexes partially depleted of carotenoids, with an unusual spectrum in the NIR region (lambdamax = 791, 818, 847 and 875 nm), (b) reaction centers associated with fragments of LH1, (c) small amounts of a carotenoidless B820 subcomplex. The core complex from the carotenoid-deficient bacterium is much less stable and yields only the two sub-complexes (b) and (c). We conclude that carotenoids contribute critically to stability and interactions of the core complexes with detergents.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

Diversifying carotenoid biosynthetic pathways by directed evolution

Microorganisms and plants synthesize a diverse array of natural products, many of which have proven indispensable to human health and well-being. Although many thousands of these have been characterized, the space of possible natural products--those that could be made biosynthetically--remains largely unexplored. For decades, this space has largely been the domain of chemists, who have synthesized scores of natural product analogs and have found many with improved or novel functions. New natural products have also been made in recombinant organisms, via engineered biosynthetic pathways. Recently, methods inspired by natural evolution have begun to be applied to the search for new natural products. These methods force pathways to evolve in convenient laboratory organisms, where the products of new pathways can be identified and characterized in high-throughput screening programs. Carotenoid biosynthetic pathways have served as a convenient experimental system with which to demonstrate these ideas. Researchers have mixed, matched, and mutated carotenoid biosynthetic enzymes and screened libraries of these "evolved" pathways for the emergence of new carotenoid products. This has led to dozens of new pathway products not previously known to be made by the assembled enzymes. These new products include whole families of carotenoids built from backbones not found in nature. This review details the strategies and specific methods that have been employed to generate new carotenoid biosynthetic pathways in the laboratory. The potential application of laboratory evolution to other biosynthetic pathways is also discussed.

Two Distinct crt Gene Clusters for Two Different Functional Classes of Carotenoid in Bradyrhizobium

Aerobic photosynthetic bacteria possess the unusual characteristic of producing different classes of carotenoids. In this study, we demonstrate the presence of two distinct crt gene clusters involved in the synthesis of spirilloxanthin and canthaxanthin in a Bradyrhizobium strain. Each cluster contains the genes crtE, crtB, and crtI leading to the common precursor lycopene. We show that spirilloxanthin is associated with the photosynthetic complexes, while canthaxanthin protects the bacteria from oxidative stress. Only the spirilloxanthin crt genes are regulated by light via the control of a bacteriophytochrome. Despite this difference in regulation, the biosyntheses of both carotenoids are strongly interconnected at the level of the common precursors. Phylogenetic analysis suggests that the canthaxanthin crt gene cluster has been acquired by a lateral gene transfer. This acquisition may constitute a major selective advantage for this class of bacteria, which photosynthesize only under conditions where harmful reactive oxygen species are generated.

Heterologous expression, purification, and enzymatic characterization of the acyclic carotenoid 1,2-hydratase from Rubrivivax gelatinosus

The carotenoid 1,2-hydratase CrtC from Rubrivivax gelatinosus has been expressed in Escherichia coli in an active form and purified by affinity chromatography. The enzyme catalyzes the conversion of various acyclic carotenes including 1-hydroxy derivatives. This broad substrate specificity reflects the participation of CrtC in 1'-HO-spheroidene and in spirilloxanthin biosynthesis. Enzyme kinetic studies including the determination of substrate specificity constants indicate that among the alternative biosynthetic routes to 1'-HO-spheroidene the one via spheroidene is the dominating pathway. In contrast to CrtC from Rvi. gelatinosus, the equivalent enzyme from Rhodobacter capsulatus, a closely related bacterium which lacks the biosynthetic branch to spirilloxanthin and accumulates spheroidene instead of substantial amounts of 1'-HO-spheroidene, is extremely poor in converting 1-HO-carotenoids. The individual catalytic properties of both carotenoid 1,2-hydratases reflect the in situ carotenogenic pathways in both purple photosynthetic bacteria.

A C35 carotenoid biosynthetic pathway

Upon coexpression with Erwinia geranylgeranyldiphosphate (GGDP) synthase in Escherichia coli, C(30) carotenoid synthase CrtM from Staphylococcus aureus produces novel carotenoids with the asymmetrical C(35) backbone. The products of condensation of farnesyldiphosphate and GDP, C(35) structures comprise 40 to 60% of total carotenoid accumulated. Carotene desaturases and carotene cyclases from C(40) or C(30) pathways accepted and converted the C(35) substrate, thus creating a C(35) carotenoid biosynthetic pathway in E. coli. Directed evolution to modulate desaturase step number, together with combinatorial expression of the desaturase variants with lycopene cyclases, allowed us to produce at least 10 compounds not previously described. This result highlights the plastic and expansible nature of carotenoid pathways and illustrates how combinatorial biosynthesis coupled with directed evolution can rapidly access diverse chemical structures.

Genes involved in the biosynthesis of photosynthetic pigments in the purple sulfur photosynthetic bacterium Thiocapsa roseopersicina

A pigment mutant strain of the purple sulfur photosynthetic bacterium Thiocapsa roseopersicina BBS was isolated by plasposon mutagenesis. Nineteen open reading frame, most of which are thought to be genes involved in the biosynthesis of carotenoids, bacteriochlorophyll, and the photosynthetic reaction center, were identified surrounding the plasposon in a 22-kb-long chromosomal locus. The general arrangement of the photosynthetic genes was similar to that in other purple photosynthetic bacteria; however, the locations of a few genes occurring in this region were unusual. Most of the gene products showed the highest similarity to the corresponding proteins in Rubrivivax gelatinosus. The plasposon was inserted into the crtD gene, likely inactivating crtC as well, and the carotenoid composition of the mutant strain corresponded to the aborted spirilloxanthin pathway. Homologous and heterologous complementation experiments indicated a conserved function of CrtC and CrtD in the purple photosynthetic bacteria. The crtDC and crtE genes were shown to be regulated by oxygen, and a role of CrtJ in aerobic repression was suggested.

Dynamics of energy transfer from lycopene to bacteriochlorophyll in genetically-modified LH2 complexes of Rhodobacter sphaeroides

LH2 complexes from Rb. sphaeroides were modified genetically so that lycopene, with 11 saturated double bonds, replaced the native carotenoids which contain 10 saturated double bonds. Tuning the S1 level of the carotenoid in LH2 in this way affected the dynamics of energy transfer within LH2, which were investigated using both steady-state and time-resolved techniques. The S1 energy of lycopene in n-hexane was determined to be approximately 12 500 +/- 150 cm(-1), by direct measurement of the S1-S2 transient absorption spectrum using a femtosecond IR-probing technique, thus placing an upper limit on the S1 energy of lycopene in the LH2 complex. Fluorescence emission and excitation spectra demonstrated that energy can be transferred from lycopene to the bacteriochlorophyll molecules within this LH2 complex. The energy-transfer dynamics within the mutant complex were compared to wild-type LH2 from Rb. sphaeroides containing the carotenoid spheroidene and from Rs. molischianum, in which lycopene is the native carotenoid. The results show that the overall efficiency for Crt --> B850 energy transfer is approximately 80% in lyco-LH2 and approximately 95% in WT-LH2 of Rb. sphaeroides. The difference in overall Crt --> BChl transfer efficiency of lyco-LH2 and WT-LH2 mainly relates to the low efficiency of the Crt S(1) --> BChl pathway for complexes containing lycopene, which was 20% in lyco-LH2. These results show that in an LH2 complex where the Crt S1 energy is sufficiently high to provide efficient spectral overlap with both B800 and B850 Q(y) states, energy transfer via the Crt S1 state occurs to both pigments. However, the introduction of lycopene into the Rb. sphaeroides LH2 complex lowers the S1 level of the carotenoid sufficiently to prevent efficient transfer of energy to the B800 Q(y) state, leaving only the Crt S1 --> B850 channel, strongly suggesting that Crt S1 --> BChl energy transfer is controlled by the relative Crt S1 and BChl Q(y) energies.