Sukhyeong Cho

Enhanced 2,3-Butanediol Production by Optimizing Fermentation Conditions and Engineering Klebsiella oxytoca M1 through Overexpression of Acetoin Reductase

Microbial production of 2,3-butanediol (2,3-BDO) has been attracting increasing interest because of its high value and various industrial applications. In this study, high production of 2,3-BDO using a previously isolated bacterium Klebsiella oxytoca M1 was carried out by optimizing fermentation conditions and overexpressing acetoin reductase (AR). Supplying complex nitrogen sources and using NaOH as a neutralizing agent were found to enhance specific production and yield of 2,3-BDO. In fed-batch fermentations, 2,3-BDO production increased with the agitation speed (109.6 g/L at 300 rpm vs. 118.5 g/L at 400 rpm) along with significantly reduced formation of by-product, but the yield at 400 rpm was lower than that at 300 rpm (0.40 g/g vs. 0.34 g/g) due to acetoin accumulation at 400 rpm. Because AR catalyzing both acetoin reduction and 2,3-BDO oxidation in K. oxytoca M1 revealed more than 8-fold higher reduction activity than oxidation activity, the engineered K. oxytoca M1 overexpressing the budC encoding AR was used in fed-batch fermentation. Finally, acetoin accumulation was significantly reduced by 43% and enhancement of 2,3-BDO concentration (142.5 g/L), yield (0.42 g/g) and productivity (1.47 g/L/h) was achieved compared to performance with the parent strain. This is by far the highest titer of 2,3-BDO achieved by K. oxytoca strains. This notable result could be obtained by finding favorable fermentation conditions for 2,3-BDO production as well as by utilizing the distinct characteristic of AR in K. oxytoca M1 revealing the nature of reductase.

Synthesis and characterization of single-crystal Cu(In,Ga)Se2 nanowires: high Ga contents and growth behaviour

Precise control over the defect density, a high Ga content, and uniform stoichiometry are critical for controlling the physical and optical properties of Cu(In,Ga)Se2 (CIGS) nanowires (NWs). In this study, we investigated the synthesis of epitaxially grown, single-crystal CIGS NWs by a vapour-phase transport method using multiple sources of Ga2Se3, In2Se3, and Cu2Se as the precursors. No catalysts were employed, and r-cut Al2O3 substrates were used for the fabrication of the NWs. The synthesized CIGS NWs had a uniform composition along their length, and the NWs with the highest Ga/(In + Ga) content ratio (0.8) had a chalcopyrite structure. The bandgap energy of the CIGS NWs was higher than that of typical CIGS thin films grown by co-evaporation methods because of the high Ga content ratio. These single-crystal CIGS NWs offer an attractive platform for exploring various concepts related to hierarchical nanostructures and devices based on fully epitaxial semiconductor structures.

Increased incorporation of gaseous CO2 into succinate by Escherichia coli overexpressing carbonic anhydrase and phosphoenolpyruvate carboxylase genes.

Carbon dioxide (CO2) is an abundant and cheap carbon source that is partly responsible for global warming in the atmosphere. The objective of this study was to construct a recombinant E. coli strain that can show enhanced production of succinate derived from CO2. In this study, we confirmed the enhancement of utilization by analyzing succinate containing one carbon-13 (13C) derived from 13CO2. Firstly, the carbonic anhydrase gene (SP(-)HCCA) derived from Hahella chejuensis KCTC 2396 was over-expressed to enhance carbon flux toward bicarbonate ion (HCO3-) synthesis in E. coli. The phosphoenolpyruvate carboxylase gene (ppc) was over-expressed to enhance the production of oxaloacetate by enhancing the carbon flux. Compared with the control strain, the percentage of the succinate containing one 13C (succinate119) to total succinate was enhanced by approximately 2.80-fold and the amount of succinate119 also increased by approximately 4.09-fold in SGJS120. Secondly, the lactate dehydrogenase gene (ldhA) was deleted to re-direct the utilization of the carbon source from glucose to enhance succinate production in SGJS120. However, ldhA deletion did not increase CO2 utilization in SJGS120. Finally, the phosphotransferase system gene (ptsG) and pyruvate kinase F gene (pykF) were deleted to increase the amount of phosphoenolpyruvate (PEP). SGJS126 (pykF deletion strain) showed the highest increase, which was 6.05-fold higher than the control strain. From the results, SP(-)HCCA overexpression and pykF deletion may be useful for enhancing CO2 utilization in E. coli. Additionally, engineered strains showed the potential to reduce the cost of succinate production by using an industrially cheaper carbon source such as CO2 and converting CO2 to a valuable chemical.

High Production of 2,3-Butanediol (2,3-BD) by Raoultella ornithinolytica B6 via Optimizing Fermentation Conditions and Overexpressing 2,3-BD Synthesis Genes

Biological production of 2,3-butandiol (2,3-BD) has received great attention as an alternative to the petroleum-based 2,3-BD production. In this study, a high production of 2,3-BD in fed-batch fermentation was investigated with a newly isolated bacterium designated as Raoultella ornithinolytica B6. The isolate produced 2,3-BD as the main product using hexoses (glucose, galactose, and fructose), pentose (xylose) and disaccharide (sucrose). The effects of temperature, pH-control schemes, and agitation speeds on 2,3-BD production were explored to optimize the fermentation conditions. Notably, cell growth and 2,3-BD production by R. ornithinolytica B6 were higher at 25°C than at 30°C. When three pH control schemes (no pH control, pH control at 7, and pH control at 5.5 after the pH was decreased to 5.5 during fermentation) were tested, the best 2,3-BD titer and productivity along with reduced by-product formation were achieved with pH control at 5.5. Among different agitation speeds (300, 400, and 500 rpm), the optimum agitation speed was 400 rpm with 2,3-BD titer of 68.27 g/L, but acetic acid was accumulated up to 23.32 g/L. Further enhancement of the 2,3-BD titer (112.19 g/L), yield (0.38 g/g), and productivity (1.35 g/L/h) as well as a significant reduction of acetic acid accumulation (9.71 g/L) was achieved by the overexpression of homologous budABC genes, the 2,3-BD-synthesis genes involved in the conversion of pyruvate to 2,3-BD. This is the first report presenting a high 2,3-BD production by R.ornithinolytica which has attracted little attention with respect to 2,3-BD production, extending the microbial spectrum of 2,3-BD producers.

Complete genome sequence of Klebsiella oxytoca M1, isolated from Manripo area of South Korea

Here we report the full genome sequence of Klesiella oxytoca M1, isolated from Manripo area of South Korea. The strain K. oxytoca M1 is able to produce either 2,3-butanediol or acetoin selectively by controlling the pH and temperature.

Production of uracil from methane by a newly isolated Methylomonas sp. SW1

Methane is an abundant, inexpensive one-carbon feedstock and one of the most powerful greenhouse gases. Because it does not compete with food demand, it is considered a promising carbon feedstock for the production of valuable products using methanotrophic bacteria. Here, we isolated a novel methanotrophic bacterium, Methylomonas sp. SW1, from a sewage sample obtained from Wonju City Water Supply Drainage Center, Republic of Korea. The conditions for uracil production by Methylomonas sp. SW1, such as Cu2+ concentration and temperature were investigated and optimized. As a result, Methylomonas sp. SW1 produced uracil from methane as a sole carbon source with a titer of 2.1mg/L in 84h without genetic engineering under the optimized condition. The results in this study demonstrate the feasibility of using Methylomonas sp. SW1 for the production of uracil from methane. This is the first report of uracil production from gas feedstock by methanotrophic bacteria.