Nobuyuki Yoshida

Metabolic Engineering of Saccharomyces cerevisiae for Astaxanthin Production and Oxidative Stress Tolerance

ABSTRACT The red carotenoid astaxanthin possesses higher antioxidant activity than other carotenoids and has great commercial potential for use in the aquaculture, pharmaceutical, and food industries. In this study, we produced astaxanthin in the budding yeast Saccharomyces cerevisiae by introducing the genes involved in astaxanthin biosynthesis of carotenogenic microorganisms. In particular, expression of genes of the red yeast Xanthophyllomyces dendrorhous encoding phytoene desaturase (crtI product) and bifunctional phytoene synthase/lycopene cyclase (crtYB product) resulted in the accumulation of a small amount of β-carotene in S. cerevisiae. Overexpression of geranylgeranyl pyrophosphate (GGPP) synthase from S. cerevisiae (the BTS1 gene product) increased the intracellular β-carotene levels due to the accelerated conversion of farnesyl pyrophosphate to GGPP. Introduction of the X. dendrorhous crtS gene, encoding astaxanthin synthase, assumed to be the cytochrome P450 enzyme, did not lead to astaxanthin production. However, coexpression of CrtS with X. dendrorhous CrtR, a cytochrome P450 reductase, resulted in the accumulation of a small amount of astaxanthin. In addition, the β-carotene-producing yeast cells transformed by the bacterial genes crtW and crtZ, encoding β-carotene ketolase and hydroxylase, respectively, also accumulated astaxanthin and its intermediates, echinenone, canthaxanthin, and zeaxanthin. Interestingly, we found that these ketocarotenoids conferred oxidative stress tolerance on S. cerevisiae cells. This metabolic engineering has potential for overproduction of astaxanthin and breeding of novel oxidative stress-tolerant yeast strains.

Polyol production by culture of methanol-utilizing yeast.

Four methanol-utilizing yeasts, Candida boidinii, Hansenula polymorpha, Hansenula ofunaensis, and Pichia pinus, produced polyols from corresponding sugars in a methanol medium. H. polymorpha produced larger amounts of xylitol than the other yeasts. Productivity was the highest at pH 8 when 5 g (dry)/l cultured cells were incubated with 2.5 g/l urea as the nitrogen source in a medium containing 1% (v/v) methanol and 1 g/l MgSO4.7H2O. Under these conditions, 57 g/l xylitol was obtained from 110 g/l D-xylose after 3 d of cultivation. The largest amount of xylitol (58 g/l; yield, 0.62 g/g) was produced from 125 g/l, D-xylose and 5% (w/v) glycerol instead of methanol after 4 d of cultivation.

Functional analysis of fructosyl-amino acid oxidases of Aspergillus oryzae

ABSTRACT Three active fractions of fructosyl-amino acid oxidase (FAOD-Ao1, -Ao2a, and -Ao2b) were isolated from Aspergillus oryzae strain RIB40. N-terminal and internal amino acid sequences of FAOD-Ao2a corresponded to those of FAOD-Ao2b, suggesting that these two isozymes were derived from the same protein. FAOD-Ao1 and -Ao2 were different in substrate specificity and subunit assembly; FAOD-Ao2 was active toward Nε-fructosyl Nα-Z-lysine and fructosyl valine (Fru-Val), whereas FAOD-Ao1 was not active toward Fru-Val. The genes encoding the FAOD isozymes (i.e., FAOAo1 and FAOAo2) were cloned by PCR with an FAOD-specific primer set. The deduced amino acid sequences revealed that FAOD-Ao1 was 50% identical to FAOD-Ao2, and each isozyme had a peroxisome-targeting signal-1, indicating their localization in peroxisomes. The genes was expressed in Escherichia coli and rFaoAo2 showed the same characteristics as FAOD-Ao2, whereas rFaoAo1 was not active. FAOAo2 disruptant was obtained by using ptrA as a selective marker. Wild-type strain grew on the medium containing Fru-Val as the sole carbon and nitrogen sources, but strain ΔfaoAo2 did not grow. Addition of glucose or (NH4)2SO4 to the Fru-Val medium did not affect the assimilation of Fru-Val by wild-type, indicating glucose and ammonium repressions did not occur in the expression of the FAOAo2 gene. Furthermore, conidia of the wild-type strain did not germinate on the medium containing Fru-Val and NaNO2 as the sole carbon and nitrogen sources, respectively, suggesting that Fru-Val may also repress gene expression of nitrite reductase. These results indicated that FAOD is needed for utilization of fructosyl-amino acids as nitrogen sources in A. oryzae.

Gene Expression Analysis of Methylotrophic Oxidoreductases Involved in the Oligotrophic Growth of Rhodococcus erythropolis N9T-4

Rhodococcus erythropolis N9T-4 shows extremely oligotrophic growth requiring atmospheric CO2 without any additional carbon or energy source. We performed a gene expression analysis of the oxidoreductases, which are involved in methanol metabolism, under various growth and induction conditions in N9T-4. A real-time PCR analysis revealed that the genes encoding NAD-dependent formaldehyde dehydrogenase (nFADH) and N,N′-dimethyl-4-nitrosoaniline-dependent methanol dehydrogenase (MDH) were strongly expressed under the oligotrophic conditions at levels of 20–100 fold those under heterotrophic conditions, in which n-tetradecane was used as the sole carbon source, while glucose did not affect the gene expression pattern in a minimum medium. The genes encoding mycothiol-dependent formaldehyde dehydrogenase (mFADH) and formate dehydrogenase were not induced under oligotrophic conditions, although mFADH expression was observed when formaldehyde was added to the induction medium. These results suggest that N9T-4 had three distinct formaldehyde oxidation systems, and that MDH and nFADH were the key enzymes in its oligotrophic growth.

Utilization of atmospheric ammonia by an extremely oligotrophic bacterium, Rhodococcus erythropolis N9T-4.

Rhodococcus erythropolis N9T-4 shows extremely oligotrophic growth and requires CO2 for its growth. In this report, nitrogen sources for the oligotrophic growth of N9T-4 were examined. As is true for most other bacteria, N9T-4 preferred ammonium salt to nitrate as the nitrogen source on an inorganic minimum medium without carbon sources. Interestingly, N9T-4 could also grow on the minimal medium solidified by agarose or silica gel without carbon and nitrogen sources, suggesting that this bacterium is also oligotrophic for nitrogen. We can rule out the possibility of diazotrophic growth of this bacterium, because nitrogenase activity was not detected in the cells and the putative gene encoding nitrogenase was not found in N9T-4 genome. DNA microarray analysis revealed that one of the ammonium transporter genes (amtB) was strongly upregulated 40-50 fold higher under oligotrophic conditions than under heterotrophic conditions. Disruption of amtB led to a growth defect under nitrogen-limiting conditions. Furthermore, additional ammonia vapor enhanced the growth of N9T-4 on the minimum medium without nitrogen sources in a closed culture system. These results suggest that N9T-4 utilizes the trace amount of atmospheric ammonia as the nitrogen source.

Screening of carbon dioxide-requiring extreme oligotrophs from soil.

We screened soil samples for CO2-requiring extreme oligotrophs similar to Rhodococcus erythropolis N9T-4, which can grow on a basal salt agar medium without an organic carbon source. From 387 soil samples, three isolates were obtained and identified as Streptomyces spp. by 16S rDNA analysis. The isolates required gaseous CO2 for growth and grew on a basal salt medium solidified by silica gel. These results suggest that such CO2-requiring oligotrophs occur widely in nature.

The glyoxylate shunt is essential for CO2-requiring oligotrophic growth of Rhodococcus erythropolis N9T-4

Rhodococcus erythropolis N9T-4 shows extremely oligotrophic growth requiring atmospheric CO2 and forms its colonies on an inorganic basal medium (BM) without any additional carbon source. Screening of a random mutation library constructed by a unique genome deletion method that we established indicated that the aceA, aceB, and pckG genes encoding isocitrate lyase, malate synthase, and phosphoenolpyruvate carboxykinase, respectively, were requisite for survival on BM plates. The aceA- and aceB deletion mutants and the pckG deletion mutant grew well on BM plates containing L-malate and D-glucose, respectively, suggesting that the glyoxylate (GO) shunt and gluconeogenesis are essential for the oligotrophic growth of N9T-4. Interestingly, most of the enzyme activities in the TCA cycle were observed in the cell-free extract of N9T-4, with perhaps the most important exception being α-ketoglutarate dehydrogenase (KGDH) activity. Instead of the KGDH activity, we detected a remarkable level of α-ketoglutarate decarboxylase (KGD) activity, which is the activity exhibited by the E1 component of the KGDH complex in Mycobacterium tuberculosis. The recombinant KGD of N9T-4 catalyzed the decarboxylation of α-ketoglutarate to form succinic semialdehyde (SSA) in a time-dependent manner. Since N9T-4 also showed a detectable SSA dehydrogenase activity, we concluded that N9T-4 possesses a variant TCA cycle, which uses SSA rather than succinyl-CoA. These results suggest that oligotrophic N9T-4 cells utilize the GO shunt to avoid the loss of carbons as CO2 and to conserve CoA units in the TCA cycle.

Quality control of plasma membrane proteins by Saccharomyces cerevisiae Nedd4-like ubiquitin ligase Rsp5p under environmental stress conditions.

ABSTRACT In Saccharomyces cerevisiae, when a rich nitrogen source such as ammonium is added to the culture medium, the general amino acid permease Gap1p is ubiquitinated by the yeast Nedd4-like ubiquitin ligase Rsp5p, followed by its endocytosis to the vacuole. The arrestin-like Bul1/2p adaptors for Rsp5p specifically mediate this process. In this study, to investigate the downregulation of Gap1p in response to environmental stresses, we determined the intracellular trafficking of Gap1p under various stress conditions. An increase in the extracellular ethanol concentration induced ubiquitination and trafficking of Gap1p from the plasma membrane to the vacuole in wild-type cells, whereas Gap1p remained stable on the plasma membrane under the same conditions in rsp5A401E and Δend3 cells. A 14C-labeled citrulline uptake assay using a nonubiquitinated form of Gap1p (Gap1pK9R/K16R) revealed that ethanol stress caused a dramatic decrease of Gap1p activity. These results suggest that Gap1p is inactivated and ubiquitinated by Rsp5p for endocytosis when S. cerevisiae cells are exposed to a high concentration of ethanol. It is noteworthy that this endocytosis occurs in a Bul1/2p-independent manner, whereas ammonium-triggered downregulation of Gap1p was almost completely inhibited in Δbul1/2 cells. We also found that other environmental stresses, such as high temperature, H2O2, and LiCl, also promoted endocytosis of Gap1p. Similar intracellular trafficking caused by ethanol occurred in other plasma membrane proteins (Agp1p, Tat2p, and Gnp1p). Our findings suggest that stress-induced quality control is a common process requiring Rsp5p for plasma membrane proteins in yeast.

A Novel NAD-Dependent Dehydrogenase, Highly Specific for 1,5-Anhydro-d-Glucitol, from Trichoderma longibrachiatum Strain 11-3

ABSTRACT A novel NAD-dependent dehydrogenase highly specific for 1,5-anhydro-d-glucitol (1,5-AG) was found in the cell extract of an imperfect fungus, Trichoderma longibrachiatum strain 11-3. This fungus used 1,5-AG as a sole carbon source for growth and transformed 1,5-AG into glucose. 1,5-AG dehydrogenase (AGH) was purified to homogeneity, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The molecular mass of the purified enzyme was estimated to be 36 and 141 kDa by SDS-PAGE and by gel filtration, respectively, suggesting that the enzyme was homotetrameric. The enzyme was highly specific for 1,5-AG and did not exhibit activity with any sugar or sugar alcohol tested in this study other than 1,5-AG. A linear relationship between the initial rate of the enzyme reaction and the concentration of 1,5-AG at the physiological level was observed. The presence of glucose in abundance did not interfere with the relationship. The optimum temperature for the enzyme reaction was 50°C, and the enzyme was stable at temperatures up to 70°C. These results suggested that AGH is a novel enzyme and is useful for specifically diagnosing diabetes mellitus.

Carbon monoxide utilization of an extremely oligotrophic bacterium, Rhodococcus erythropolis N9T-4.

CO utilization by a CO(2)-requiring extremely oligotrophic bacterium, Rhodococcus erythropolis N9T-4 was found when CO(2) was removed from the culture environment before cultivation, suggesting that this bacterium can convert CO into CO(2). However, the gene encoding putative CO dehydrogenase large subunit in N9T-4 was not induced by CO.