In vitro carotenogenesis by wild type and mutants of Phycomyces blakesleeanus

Cell extracts from shake cultures of the wild type and six mutant strains of Phycomyces converted [2-¹⁴C] MVA into carotenes, squalene and prenyl phosphates. Oxygen was required for the desaturation of phytoene. When compared with the wild type, cells extracts of carB and carR mutants are much less effective in phytoene dehydrogenation and lycopene cyclization, respectively. This confirms previous conclusions about the biochemical functions of the carB and carR genes, which were based on genetic and in vivo studies. CarA strain mutants accumulate, in vivo, much less β-carotene than the wild type. This correlates with a 10-fold decrease in carotenogenesis in vitro. The addition of retinol to incubations of cell extracts of the wild type and C2 strains stimulated β-carotene formation. Both carB and carR mutants show enhanced total carotenogenic activities in vitro and the carS mutant shows a higher β-carotene-synthesizing activity than the wild type. It is suggested that the feed-back regulatory mechanism known to control this pathway operates at the level of enzyme synthesis.     

Carotenoid Mutants of Mucor circinelloides

The wild-type of the filamentous fungus Mucor circinelloides accumulates the yellow pigment β-carotene. At a continuous blue-light fluence rate of 0.1 W/m² the β-carotene content increases about eight fold over the dark controls. Among the mutants isolated after exposure of spores to either N-methyl-N'-nitro-N-nitrosoguanidine or ICR-170, a red mutant accumulating lycopene, white mutants accumulating phytoene and white mutants without carotenoids were found. The biosynthesis of carotenoids in M. circinelloides shows similarities with that of the fungus Phycomyces blakesleeanus such as the presence of mutants in the same structural genes and the induction by light of the pathway. However, negative end-product regulation by β-carotene on the biosynthetic pathway, as in Phycomyces, is absent in M. circinelloides. In contrast to Phycomyces carB and carR mutants, carotenoids in corresponding mutants of M. circinelloides are photoinduced.     

Effect of light intensity on the formation of carotenoids in normal and mutant maize leaves

Carotenoid composition in leaves of normal, lycopenic and ζ-carotenic mutants of Zea mays were investigated. In lycopenic leaves, in addition to lycopene, phytoene, phytofluene, δ- and γ-carotene, trace amounts of α- and β-carotene and antheraxanthin were identified. Low light promoted accumulation of α- and β-carotene; high light brought about an increase in antheraxanthin content. In the leaves of the ζ-carotenic mutant, phytoene, phytofluene and ζ-carotene were synthesized. Illumination of low intensity stimulated carotenoid synthesis to a slight extent. Relative amounts of carotenoid components were essentially the same as in etiolated material, except for a small increase in cis-ζ-carotene. Under high intensity illumination, carotenoids were rapidly destroyed.     

Carotenoid synthesis and phytoene synthase activity during mating of Blakeslea trispora

Carotenoid formation was investigated in wild type and carotenogenic mutants of Blakeslea trispora after mating (−) and (+) strains. The highest yields of carotenoids, especially β-carotene was observed following mating. In vitro incorporation of geranylgeranyl pyrophosphate into phytoene and β-carotene corresponded to increased carotenogenesis in the mated strains. Immuno determination of phytoene synthase protein levels revealed that the amounts of this enzyme is concurrent with the increases in carotenoid content. In fungi, phytoene synthase together with lycopene cyclase are encoded by a fusion gene crtYB or carRA with two individual domains. These domains were both heterologously expressed in an independent manner and antisera raised against both. These antisera were used, to assess protein levels in mated and non-mated B. trispora. The phytoene synthase domain was detected as an individual soluble protein with a molecular weight of 40 kDa and the lycopene cyclase an individual protein of mass about 30 kDa present in the membrane fraction following sub-cellular fractionation. This result demonstrates a post-translational cleavage of the protein transcribed from a single mRNA into independent functional phytoene synthase and lycopene cyclase.     

MASSIVE ACCUMULATION OF PHYTOENE INDUCED BY NORFLURAZON IN DUNALIELLA BARDAWIL (CHLOROPHYCEAE) PREVENTS RECOVERY FROM PHOTOINHIBITION1

The effects of nanomolar to micromolar concentrations of the herbicide norflurazon were studied in Dunaliella bardawil Ben-Amotz et Avron, a β-carotene-accumulating halotolerant alga. The large amount of β-carotene which Dunaliella bardawil can contain, around 8% of the algal dry weight, is reduced to 0.2% by treatment with 100 nm norflurazon. Simultaneously, phytoene is accumulated to a similar level of about 8%. The gradual increase in phytoene content, in response to increasing norflurazon concentrations, corresponds to the decrease in β-carotene, with no evident change in other isoprenoid intermediates. Carotene-rich Dunaliella bardawil is substantially resistant to high-intensity photoinhibition. This resistance is lost in cells grown to contain low β-carotene and in the norflurazon-treated phytoene-rich cells. These observations are in agreement with the hypothesis that the accumulated β-carotene in Dunaliella bardawil protects the cells against injury by excessive irradiation.     

Macromolecular Physiology of Plastids

The composition and amount of carotenoid pigments were determined in etiolated seedling leaves of 6 barley (Hordeum vulgare L.) mutants, comprising 1 xantha and 5 tigrina mutants. All mutants had on a mole basis approximately the same content of carotenoids as the wild type. The mutants xan-u²¹, tig-n³², and tig-³³ contained significantly higher amounts of carotenes than the wild type, ranging from 32 to 68% of the total carotenoid content as compared to the 4–8% found in the wild type. In the mutants tig-b²³ and tig-o³⁴, only a slight increase in the amount of carotenes was notable. The carotene content and composition in tig-d¹² was indistinguishable from that of the wild type. The carotenes extracted from xan-u²¹, tig-b²³, tig-n³², tig-³³, and tig-o³⁴ were characterized by adsorption chromatography and spectrophotometry. Mutant xan-u²¹ is in the dark blocked in β-carotene synthesis, and accumulates the aliphatic polyenes: phytofluene, proneurosporene, poly-cis-lycopenes, neo-lycopene and lycopene. The other four mutants synthesize β-carotene, but accumulate in addition various higher saturated carotenes, the main components being ζ-carotene in tig-b²³, a lycopenic pigment in tig-n³² and tig-³³, and lycopene in tig-o³⁴. Accumulation of higher saturated carotenes appears correlated with specific aberrations of the membrane structure in plastids. The regulation of carotene and protochlorophyllide syntheses in etioplasts are closely linked as shown by the single gene mutants which affect both pathways. However, several mutants have been identified which cause defects in protochlorophyllide synthesis only.     

Isolation and Characterization of a Xanthophyll Aberrant Mutant of the Green Alga Nannochloropsis oculata

Novel mutants (xan1 and xan2) of the unicellular green alga Nannochloropsis oculata are impaired in xanthophyll biosynthesis, thereby producing aberrant levels of xanthophylls. High-performance liquid chromatography (HPLC) analysis revealed that the xan1 and xan2 mutants have double the violaxanthin (V) content, but have significantly decreased lutein content in their cells compared to the wild type. Furthermore, these mutants contain two to three times more zeaxanthin than the wild type under low light (LL) growth conditions. However, this xanthophyll aberration in N. oculata did not affect the normal growth and the major cellular chemical composition of the xan1 strain. The xanthophyll pool size of the LL-grown mutant was 1.8-fold greater than that of the wild type. Under high light (HL) growth conditions, V content was substantially decreased in both the mutant and wild types because of the epoxidation state of the xanthophylls. Under LL growth conditions, the deepoxidation states of the xanthophyll pool sizes were 0.1 and 1.2 in the wild type and the mutant, respectively. However, the deepoxidation states of the xanthophyll pool sizes were 0.78 in the wild type and 0.87 in the mutant under HL growth conditions. We observed that the level of one of the commercially important xanthophylls, zeaxanthin, was higher in the mutant than in the wild type under all culture conditions. This mutant is discussed in terms of its commercial value and potential utilization by the algal biotechnology industry for the production of zeaxanthin.     

Proteome rebalancing in transgenic <Emphasis Type="Italic">Camelina</Emphasis> occurs within the enlarged proteome induced by β-carotene accumulation and storage protein suppression

Oilseed crops are global commodities for their oil and protein seed content. We have engineered the oilseed Camelina sativa to exhibit increased protein content with a slight decrease in oil content. The introduction of a phytoene synthase gene with an RNAi cassette directed to suppress the storage protein 2S albumin resulted in seeds with an 11–24 % elevation in overall protein. The phytoene synthase cassette alone produced enhanced β-carotene content of an average 275 ± 6.10 μg/g dry seed and an overall altered seed composition of 11 % less protein and comparable nontransgenic amounts of both oil and carbohydrates. Stacking an RNAi to suppress the major 2S storage protein resulted in seeds that contain elevated protein and slight decrease in oil and carbohydrate amounts showing that Camelina rebalances its proteome within an enlarged protein content genotype. In both β-carotene enhanced seeds with/without RNAi2S suppression, the seed size was noticeably enlarged compared to nontransgenic counterpart seeds. Metabolic analysis of maturing seeds indicate that the enhanced β-carotene trait had the larger effect than the RNAi2S suppression on the seed metabolome. The use of a GRAS (generally regarded as safe) β-carotene as a visual marker in a floral dip transformation system, such as Camelina, might eliminate the need for costly regulatory and controversial antibiotic resistance markers. β-carotene enhanced RNAi2S suppressed Camelina seeds could be further developed as a rapid heterologous protein production platform in a nonfood crop leveraging its enlarged protein content and visual marker.     

Mutants and Intersexual Heterokaryons of Blakeslea trispora for Production of β-Carotene and Lycopene

The industrial production of β-carotene with the zygomycete Blakeslea trispora involves the joint cultivation of mycelia of opposite sex in the presence of β-ionone and other chemical activators. We have obtained improved strains by mutation and heterokaryosis. We chose wild strains on the basis of their growth and carotene content in single and mated cultures. Following exposure of their spores to N-methyl-N′-nitro-N-nitrosoguanidine, we obtained high-carotene mutants, which were more productive than their parents but similar to them in having β-carotene as the main product. Further increases in carotene content were obtained after a new round of mutagenesis in one of the mutants. The production was shifted to lycopene in cultures incubated in the presence of nicotine and in lycopene-rich mutants derived from the wild strains. The highest production levels were achieved in intersexual heterokaryons, which contained mutant nuclei of opposite sex. These contained up to 39 mg of β-carotene or 15 mg of lycopene per g (dry mass) under standard laboratory conditions in which the original wild strains contained about 0.3 mg of β-carotene per g (dry mass). β-Ionone did not increase the carotene content of these strains. Not all wild strains lent themselves to these improvements, either because they produced few mutants or because they did not increase their carotene production in mated cultures.     

ACCUMULATION OF β-CAROTENE IN HALOTOLERANT ALGE: PURIFICATION AND CHARACTERIZATION OF β-CAROTENE-RICH GLOBULES FROM DUNALIELLA BARDAWIL (CHLOROPHYCEAE)1

Dunaliella bardawil Ben-Amotz & Avron, but not most other Dunaliella species, has a unique property of being able to accumulate, in addition to glycerol, large amounts of β-carotene when cultivated under appropriate conditions. These include high light intensity, a high sodium chloride concentration, nitrate deficiency and extreme temperatures. Under conditions of maximal carotene accumulation D. bardawil contains at least 8% of its dry weight as β-carotene while D. salina grown under similar conditions contains only about 0.3%. Electron micrographs of D. bardawil grown under conditions of high β-carotene accumulation show many β-carotene containing globules located in the interthylakoid spaces of the chloroplast. The same algae grown under conditions where β-carotene does not accumulate, contain few to no β-carotene globules. The β-carotene-rich globules were released from the algae into an aqueous medium by a two-stage osmotic shock technique and further purified by centrifugal ion on 10% sucrose. The isolated purified globules were shown by electron microscopy to be free of significant contamination and composed of membrane-free osmiophilic droplets with an average diameter of 150 nm. Reversed phase high performance liquid chromatography of a total pigment extract of the cells revealed the presence of β-carotene as the major pigment, together with chlorophylls a and b, α-carotene and the xanthophylls lutein, neoxauthin and zeaxanthin. β-Carotene accounted for essentially all the pigment in the purified globules. Analysis of the algal and globule β-carotene fractions by HPLC showed that the β-carotene was composed of approximately equal amounts of all-trans β-carotene and of its 9-cis isomer. Intact D. bardawil cells contained on a dry weight basis about 30% glycerol, 30% protein, 18% lipid, 11% carbohydrate, 9%β-carotene and 1% chlorophyll. The β-carotene globules were composed of practically only neutral lipids, more than half of which was β-carotene. It is suggested that the β-carotene globules may serve to protect D. bardawil against injury by the high intensity irradiation to which this alga is usually exposed in nature.     

Biochemical analysis of isoallelic series at the al- i locus of Neurospora crassa

The neutral carotenoids of 3 phenotypically distinct albino-1 (al- i) strains, a wild type, 2 heterokaryons containing 2 al- i, alleles and 1 heterokaryon containing al- i+al-2 markers were analyzed. All al- i strains and the al- i heterokaryons contained large amounts of phytoene and only traces of higher carotenoids such as -carotene and lycopene which are responsible for the phenotypic variation at this locus (from pure white to lemon yellow). The biochemical lesion for al- i mutants affects phytoene dehydrogenase and enzyme leakiness accounts for the gene polymorphism. There is no evidence for interallelic complementation at the al- i locus.     

β-Carotene producing mutants of Phaffia rhodozyma

Like other carotenoid-producing organisms, Phaffia rhodozyma, a red astaxanthin-producing yeast, is supposed to synthesize carotenoids by the following steps: formation of phytoene from geranylgeranyl pyrophosphate, dehydrogenation of phytoene to lycopene, cyclization of lycopene to -carotene and oxidation of the latter to astaxanthin. Mutagenic treatments generated in P. rhodozyma a wide diversity of colour variants ranging from white to dark red. The identification of the corresponding carotenoid compounds revealed the occurrence of -carotene-accumulating strains, phytoene-accumulating strains, and strains lacking any carotenoid compound. These classes of strains are likely to result from alterations in, respectively, the oxidation of -carotene, phytoene dehydrogenation and the phytoene synthetase step. Except for the cyclization of lycopene to -carotene, all the steps of carotenogenesis in P. rhodozyma are represented by the above mutants. Furthermore, astaxanthin-overproducing mutants were also selected; they are likely to be affected in some upstream step, and certainly before -carotene, as after an additional mutagenesis they generated oxidaseless strains that, in this case, overproduce -carotene. The latter strains appear very promising for biotechnological production of natural -carotene.     

Metabolic Engineering of the Carotenoid Biosynthetic Pathway in the Yeast Xanthophyllomyces dendrorhous (Phaffia rhodozyma)

The crtYB locus was used as an integrative platform for the construction of specific carotenoid biosynthetic mutants in the astaxanthin-producing yeast Xanthophyllomyces dendrorhous. The crtYB gene of X. dendrorhous, encoding a chimeric carotenoid biosynthetic enzyme, could be inactivated by both single and double crossover events, resulting in non-carotenoid-producing transformants. In addition, the crtYB gene, linked to either its homologous or a glyceraldehyde-3-phosphate dehydrogenase promoter, was overexpressed in the wild type and a β-carotene-accumulating mutant of X. dendrorhous. In several transformants containing multiple copies of the crtYB gene, the total carotenoid content was higher than in the control strain. This increase was mainly due to an increase of the β-carotene and echinone content, whereas the total content of astaxanthin was unaffected or even lower. Overexpression of the phytoene synthase-encoding gene (crtI) had a large impact on the ratio between mono- and bicyclic carotenoids. Furthermore, we showed that in metabolic engineered X. dendrorhous strains, the competition between the enzymes phytoene desaturase and lycopene cyclase for lycopene governs the metabolic flux either via β-carotene to astaxanthin or via 3,4-didehydrolycopene to 3-hydroxy-3′-4′-didehydro-β-ψ-caroten-4-one (HDCO). The monocylic carotenoid torulene and HDCO, normally produced as minority carotenoids, were the main carotenoids produced in these strains.     

Analysis of proteomic changes in colored mutants of Xanthophyllomyces dendrorhous (Phaffia rhodozyma)

The yeast Xanthophyllomyces dendrorhous synthesizes astaxanthin as its most prevalent xanthophyll derivative. Comparisons between the protein profiles of mutant lines of this yeast can provide insight into the carotenogenic pathway. Differently colored mutants (red, orange, pink, yellow, and white) were obtained from this yeast species, and their protein profiles were determined using two-dimensional polyacrylamide gel electrophoresis (2DE). Individual proteins differentially expressed were identified using mass spectrometry. The red mutants hyperproduced total carotenoids (mainly astaxanthin), while in white and orange mutants, mutagenesis affected the phytoene dehydrogenase activity as indicated by the accumulation of phytoene. Inactivation of astaxanthin synthase after the mutagenic treatment was evident in β-carotene accumulating mutants. Differences in the proteomic profiles of wild-type X. dendrorhous and its colored mutants were demonstrated using 2DE. Of the total number of spots detected in each gel (297–417), 128 proteins were present in all strains. The red mutant showed the greatest number of matches with respect to the wild type (305 spots), while the white and yellow mutants, which had reduced concentrations of total carotenoids, presented the highest correlation coefficient (0.6) between each other. A number of differentially expressed proteins were sequenced, indicating that tricarboxylic acid cycle and stress response proteins are closely related to the carotenogenic process.     

Carotenogenesis in Phycomyces

Time course studies of carotenoid production and of mycelial growth in liquid cultures of Phycomyces blakesleeanus wild type [NRRL 1555 (?)], red mutants C9, C10 and C13 and the heterokaryon C2 * C9 are reported. The ratios of the concentrations of lycopene, γ-carotene and β-carotene in the red mutant C13 and in the heterokaryon C2 * C9 during the growth periods were measured. In these strains the concentration of lycopene is close to its final value after 2 days of growth, at a time at which β-carotene is just beginning to be produced. It is suggested that the β-carotene produced late is possibly synthesized via β-zeacarotene.     

Evaluation of colors in green mutants isolated from purple bacteria as a host for colorimetric whole-cell biosensors

The change in carotenoid-based bacterial color from yellow to red can be applied to whole-cell biosensors. We generated several green mutants to emphasize the color change in such biosensors. The blue-green crtI-deleted mutant, Rhodopseudomonas palustris no.711, accumulated the colorless carotenoid precursor, phytoene. Green Rhodovulum sulfidophilum M31 accumulated neurosporene, a downstream product of phytoene. Another green mutant, Rhodobacter sphaeroides Ga, accumulated neurosporene and chloroxanthin, which are both downstream products of phytoene. All green mutants accumulated bacteriochlorophyll a. Photosynthetic membrane obtained from the green mutants all exhibited decreased absorption of wavelength range at 510–570 nm. Therefore, these indicate that the greenish bacterial colors were mainly caused by the existence of bacteriochlorophyll a and the changes in carotenoid composition in photosynthetic membrane. The colors of the green mutants and their wild-type strains were plotted in the CIE-L*a*b* color space, and the color difference (ΔE*ab) values between a green mutant and its wild type were calculated. ΔE*ab values were higher in the green mutants than in Rdv. sulfidophilum CDM2, the yellowish host strain of reported biosensors. These data indicate that change in bacterial color from green to red is more distinguishable than that from yellow to red as a reporter signal of carotenoid-based whole-cell biosensors.     

Activities of cellulase and other extracellular enzymes during lignin solubilization by Streptomyces viridosporus

Summary Two mutant strains of the lignin degrading bacterium Streptomyces viridosporus strain T7A with enhanced abilities to produce a soluble lignin degradation intermediate, acid-precipitable polymeric lignin (APPL) and several mutants derepressed for cellulase production were compared with the wild type to examine the roles of cellulase and selected other extracellular enzymes in lignin solubilization by S. viridosporus. The two APPL-overproducing mutants, T-81 and T-138, had higher cellulase activities than the wild type. Mutants specifically derepressed for cellulase were also isolated and were found to produce more APPL than the wild type. The results are indicative of some involvement of cellulase in the lignin solubilization process. The lignin solubilized from corn (Zea mays) lignocellulose by the mutants was slightly different chemically as compared to wild type solubilized lignin in that it had a higher coumaric acid ester content. The production of extracellular coumarate ester esterase, aromatic aldehyde oxidase, and xylanase was also examined in the mutants. Xylanase and aromatic aldehyde oxidase production did not differ significantly between the mutants and the wild type. Mutant T-81 was found to have a slightly lower activity for esterase as compared with the wild type. It was concluded that xylanase, oxidase and esterase are not the enzymes directly responsible for enhanced lignin solubilization. The results, however, do implicate cellulase in the process.Paper number 86 511 of the Idaho Agricultural Experiment Station     

Carotene-superproducing mutants ofPhycomyces blakesleeanus

The wild-type mycelia of the fungusPhycomyces blakesleeanus are yellow because they contain small amounts of β-carotene. Some mutations lead to large increases in β-carotene content. These “deep yellow” mutants carry recessive mutations in either of two genes,carD andcarS, not linked to each other or to other genes related to carotenogenesis. ThecarS mutants contain up to 100 times more β-carotene than the wild type; thecarD mutants, up to 20 times.carS mutants are unable to form zygospores and their carotenogenesis is not activated by retinol; on the other hand,carD mutants complete the sexual cycle and respond to retinol.carS mutations are epistatic overcarD mutations. The product of genecarS mediates the end-product regulation of the pathway; it is suggested that thecarD gene product increases the amount or the activity of thecarS gene product.     

Reconstruction of the astaxanthin biosynthesis pathway in rice endosperm reveals a metabolic bottleneck at the level of endogenous β-carotene hydroxylase activity

Astaxanthin is a high-value ketocarotenoid rarely found in plants. It is derived from β-carotene by the 3-hydroxylation and 4-ketolation of both ionone end groups, in reactions catalyzed by β-carotene hydroxylase and β-carotene ketolase, respectively. We investigated the feasibility of introducing an extended carotenoid biosynthesis pathway into rice endosperm to achieve the production of astaxanthin. This allowed us to identify potential metabolic bottlenecks that have thus far prevented the accumulation of this valuable compound in storage tissues such as cereal grains. Rice endosperm does not usually accumulate carotenoids because phytoene synthase, the enzyme responsible for the first committed step in the pathway, is not present in this tissue. We therefore expressed maize phytoene synthase 1 (ZmPSY1), Pantoea ananatis phytoene desaturase (PaCRTI) and a synthetic Chlamydomonas reinhardtii β-carotene ketolase (sCrBKT) in transgenic rice plants under the control of endosperm-specific promoters. The resulting grains predominantly accumulated the diketocarotenoids canthaxanthin, adonirubin and astaxanthin as well as low levels of monoketocarotenoids. The predominance of canthaxanthin and adonirubin indicated the presence of a hydroxylation bottleneck in the ketocarotenoid pathway. This final rate-limiting step must therefore be overcome to maximize the accumulation of astaxanthin, the end product of the pathway.