Seon-Kang Choi

Biosynthesis of astaxanthin in tobacco leaves by transplastomic engineering

SUMMARY The natural pigment astaxanthin has attracted much attention because of its beneficial effects on human health, despite its expensive market price. In order to produce astaxanthin, transgenic plants have so far been generated through conventional genetic engineering of Agrobacterium-mediated gene transfer. The results of trials have revealed that the method is far from practicable because of low yields, i.e. instead of astaxanthin, large quantities of the astaxanthin intermediates, including ketocarotenoids, accumulated in the transgenic plants. In the present study, we have overcome this problem, and have succeeded in producing more than 0.5% (dry weight) astaxanthin (more than 70% of total caroteniods) in tobacco leaves, which turns their green color to reddish brown, by expressing both genes encoding CrtW (beta-carotene ketolase) and CrtZ (beta-carotene hydroxylase) from a marine bacterium Brevundimonas sp., strain SD212, in the chloroplasts. Moreover, the total carotenoid content in the transplastomic tobacco plants was 2.1-fold higher than that of wild-type tobacco. The tobacco transformants also synthesized a novel carotenoid 4-ketoantheraxanthin. There was no significant difference in the size of the aerial part of the plant between the transformants and wild-type plants at the final stage of their growth. The photosynthesis rate of the transformants was also found to be similar to that of wild-type plants under ambient CO2 concentrations of 1500 micromol photons m(-2) s(-1) light intensity.

Characterization of β-Carotene Ketolases, CrtW, from Marine Bacteria by Complementation Analysis in Escherichia coli

A complementation analysis was performed in Escherichiacoli to evaluate the efficiency of β-carotene ketolases (CrtW) from the marine bacteria Brevundimonas sp. SD212, Paracoccus sp. PC1 (Alcaligenes PC-1), and Paracoccus sp. N81106 (Agrobacteriumaurantiacum), for astaxanthin production. Each crtW gene was expressed in Escherichiacoli synthesizing zeaxanthin due to the presence of plasmid pACCAR25ΔcrtX. Carotenoids that accumulated in the resulting E.coli transformants were examined by chromatographic and spectroscopic analyses. The transformant carrying the Paracoccus sp. PC1 or N81106 crtW gene accumulated high levels of adonixanthin, which is the final astaxanthin precursor for CrtW, and astaxanthin, while the E.coli transformant with crtW from Brevundimonas sp. SD212 did not accumulate any adonixanthin and produced a high level of astaxanthin. These results show efficient conversion by CrtW of Brevundimonas sp. SD212 from adonixanthin to astaxanthin, which is a new-found characteristic of a bacterial CrtW enzyme. The phylogenetic positions between CrtW of the two genera, Brevundimonas and Paracoccus, are distant, although they fall into α-Proteobacteria.

Characterization of bacterial β-carotene 3,3′-hydroxylases, CrtZ, and P450 in astaxanthin biosynthetic pathway and adonirubin production by gene combination in Escherichia coli

Abstractβ-Carotene hydroxylase (CrtZ) is one of rate-limiting enzymes for astaxanthin production. A complementation analysis was conducted using Escherichia coli transformants to compare the catalytic efficiency of bacterial CrtZ from Brevundimonas sp. SD212, Paracoccus sp. PC1 (formerly known as Alcaligenes sp. PC-1), Paracoccus sp. N81106 (Agrobacterium aurantiacum), Pantoea ananatis (Erwinia uredovora 20D3), marine bacterium P99-3, and P450 monooxygenase (CYP175A1) from Thermus thermophilus HB27. Each crtZ or CYP175A1 gene was expressed in E. coli transformants synthesizing canthaxanthin and β-carotene due to the respective presence of plasmids pAC-Cantha and pACCAR16ΔcrtX. The carotenoids that accumulated in the resulting recombinant E. coli cells were examined by chromatographic and spectroscopic analyses. E. coli carrying Brevundimonas sp. SD212 crtZ showed the highest astaxanthin production efficiency among the transformants examined, while there was no significant difference in the catalytic efficiency for conversion from β-carotene to zeaxanthin. Recombinant E. coli expressing the CYP175A1 gene, in addition to the genes for canthaxanthin synthesis, surprisingly accumulated adonirubin (phoenicoxanthin) as the main product, although the other recombinant E. coli did not accumulate any adonirubin. The present results suggest that the Brevundimonas sp. SD212 crtZ and T. thermophilus HB27 CYP175A1 genes could, respectively, be used for the efficient production of astaxanthin and adonirubin in heterologous hosts.

Characterization of Sphingomonas aldehyde dehydrogenase catalyzing the conversion of various aromatic aldehydes to their carboxylic acids

An aldehyde dehydrogenase gene, designated phnN, was isolated from a genome library of the 1,4-dimethylnaphthalene-utilizing soil bacterium, Sphingomonas sp. 14DN61. Escherichia coli expressing the phnN gene converted 1,4-dihydroxymethylnaphthalene to 1-hydroxymethyl-4-naphthoic acid. The putative amino acid sequence of the phnN gene product had 31–42% identity with those of NAD+-dependent short-chain aliphatic aldehyde dehydrogenases and a secondary alcohol dehydrogenase. The NAD(P)+-binding site and two consensus sequences involved in the active site for aldehyde dehydrogenase are conserved among these proteins. The PhnN enzyme purified from recombinant E. coli showed broad substrate specificity towards various aromatic aldehydes, i.e., 1- and 2-naphaldehydes, cinnamaldehyde, vanillin, syringaldehyde, benzaldehyde and benzaldehydes substituted with a hydroxyl, methyl, methoxy, chloro, fluoro, or nitro group were converted to their corresponding carboxylic acids. Interestingly, E. coli expressing phnN was able to biotransform a variety of not only aromatic aldehydes, but also aromatic alcohols to carboxylic acids.

Characterization of two β-carotene ketolases, CrtO and CrtW, by complementation analysis in Escherichia coli

The pathways from β-carotene to astaxanthin are crucial key steps for producing astaxanthin, one of industrially useful carotenoids, in heterologous hosts. Two β-carotene ketolases (β-carotene 4,4′-oxygenase), CrtO and CrtW, with different structure are known up to the present. In this paper, we compared the catalytic functions of a CrtO ketolase that was obtained from a marine bacterium Rhodococcus erythropolis strain PR4, CrtO derived from cyanobacterium Synechosistis sp. PCC6803, and CrtW derived from a marine bacterium Brevundimonas sp. SD212, by complementation analysis in Escherichia coli expressing the known crt genes. Results strongly suggested that a CrtO-type ketolase was unable to synthesize astaxanthin from zeaxanthin, i.e., only a CrtW-type ketolase could accept 3-hydroxy-β-ionone ring as the substrate. Their catalytic efficiency for synthesizing canthaxanthin from β-carotene was also examined. The results obtained up to the present clearly suggest that the bacterial crtW and crtZ genes are a combination of the most promising gene candidates for developing recombinant hosts that produce astaxanthin as the predominant carotenoid.

Characterization of four Rhodococcus alcohol dehydrogenase genes responsible for the oxidation of aromatic alcohols

Four genes were isolated and characterized for alcohol dehydrogenases (ADHs) catalyzing the oxidation of aromatic alcohols such as benzyl alcohol to their corresponding aldehydes, one from o-xylene-degrading Rhodococcusopacus TKN14 and the other three from n-alkane-degrading Rhodococcuserythropolis PR4. Various aromatic alcohols were bioconverted to their corresponding carboxylic acids using Escherichiacoli cells expressing each of the four ADH genes together with an aromatic aldehyde dehydrogenase gene (phnN) from Sphingomonas sp. strain 14DN61. The ADH gene (designated adhA) from strain TKN14 had the ability to biotransform a wide variety of aromatic alcohols, i.e., 2-hydroxymethyl-6-methylnaphthalene, 2-hydroxymethylnaphthalene, xylene-α,α’-diol, 3-chlorobenzyl alcohol, and vanillyl alcohol, in addition to benzyl alcohol with or without a hydroxyl, methyl, or methoxy substitution. In contrast, the three ADH genes of strain PR4 (designated adhA, adhB, and adhC) exhibited lower ability to degrade these alcohols: these genes stimulated the conversion of the alcohol substrates by only threefold or less of the control value. One exception was the conversion of 3-methoxybenzyl alcohol, which was stimulated sevenfold by adhB. A phylogenetic analysis of the amino acid sequences of these four enzymes indicated that they differed from other Zn-dependent ADHs.

3-β-Glucosyl-3′-β-quinovosyl zeaxanthin, a novel carotenoid glycoside synthesized by Escherichia coli cells expressing the Pantoea ananatis carotenoid biosynthesis gene cluster

Escherichia coli cells that express the full six carotenoid biosynthesis genes (crtE, crtB, crtI, crtY, crtZ, and crtX) of the bacterium Pantoea ananatis have been shown to biosynthesize zeaxanthin 3,3′-β-d-diglucoside. We found that this recombinant E. coli also produced a novel carotenoid glycoside that contained a rare carbohydrate moiety, quinovose (chinovose; 6-deoxy-d-glucose), which was identified as 3-β-glucosyl-3′-β-quinovosyl zeaxanthin by chromatographic and spectroscopic analyses. The chirality of the aglycone of these zeaxanthin glycosides had been shown to be 3R,3′R, in which the hydroxyl groups were formed with the CrtZ enzyme. It was here demonstrated that zeaxanthin synthesized from β-carotene with CrtR or CYP175A1, the other hydroxylase with similar catalytic function to CrtZ, possessed the same stereochemistry. It was also suggested that the singlet oxygen-quenching activity of zeaxanthin 3,3′-β-d-diglucoside, which has a chemical structure close to the new carotenoid glycoside, was superior to that of zeaxanthin.