Young-Ha Rhee

Higher intracellular levels of uridinemonophosphate under nitrogen‐limited conditions enhance metabolic flux of curdlan synthesis in Agrobacterium Species

Changes of intracellular nucleotide levels and their stimulatory effects on curdlan synthesis in Agrobacterium species were investigated under different culture conditions. Under nitrogen-limited conditions where curdlan synthesis was stimulated, intracellular levels of UMP were as high as 87 and those of AMP were 78 nmol/mg of cellular protein, while those under nitrogen-sufficient conditions were lower than 45 nmol/mg-protein. The levels of other nucleotides such as UDP, UTP, UDP-glucose, ADP, ATP, and ADP-glucose were lower than 30 nmol/mg-protein under both nitrogen-limited and sufficient conditions. The time profiles of curdlan synthesis and cellular nucleotide levels showed that curdlan synthesis had a positive relationship with intracellular levels of UMP and AMP. After the ammonium concentration in the medium fell below 0.1 g/L, intracellular levels of UMP and AMP increased, followed by curdlan synthesis. However, no significant changes in the specific activities of UMP kinase, UDP kinase, and UDP-glucose pyrophosphorylase were observed during cultivation. In vitro enzyme reactions for the synthesis of UDP-glucose, which serve as a precursor for curdlan synthesis, demonstrated that the synthesis of UDP-glucose increased with the increase of UMP concentration. In contrast, AMP had no effect on UDP-glucose synthesis at all. Addition of UMP in the medium increased the curdlan synthesis, whereas curdlan synthesis was inhibited in the presence of AMP. From these results, we concluded that only the higher intracellular UMP levels caused by nitrogen limitation in the medium enhance the metabolic flux of curdlan synthesis by promoting cellular UDP-glucose synthesis.

Accumulation of copolyesters consisting of 3-hydroxybutyrate and 3-hydroxyvalerate byAlcaligenes sp. SH-69 in batch culture

SummaryAnAlcaligenes sp. SH-69 strain capable of accumulating poly-β-hydroxy-alkanoates(PHAs) from a range of carbon sources was isolated. This organism accumulated copolyesters consisting of 3-hydroxybutyrate and 3-hydroxyvalerate after 24 h batch culture with simple carbohydrate substrates that were not generally considered as precursors of 3-hydroxyvalerate monomer units. The contents of PHA and the proportions of monomer units varied depending upon the carbon and the nitrogen sources used.

Growth Inhibitory Activities of Kalopanaxsaponins A and I against Human Pathogenic Fungi

Antifungal activities of the compounds isolated fromKalopanax pictus against representative fungi of dermatomycosis were investigated using paper disc diffusion method. It was found that kalopanaxsaponins A and I were effective in inhibiting the growth ofCandida albicans KCTC 1940 andCryptococcus neoformans KCTC 7224 with minimum inhibitory concentration (MIC) of 25 μg/ml. It showed that antifungal activity of both compounds have strong selectivity against the fungi of dermatomycosis.

Application of Mesophilic Mixed Micro-Organisms for Recovery of Valuable Metals from Spent Refinery Catalysts

Bioleaching of pre-treated spent refinery catalyst was performed using sulfur and iron oxidizing bacteria separately. Both the mixed cultures were grown at various concentration of either ferrous sulfate or elemental sulfur. Bioleaching process was conducted by varying the reaction time, effect of substrates and effect of catalyst amount. Under the most favourable transport conditions examined, (5g/L spent catalysts, 10g/L ferrous sulfate, reaction time 7 days) the mixed iron oxidizing bacteria were able to recover Ni, V, and Mo of 90, 80 and 54 % respectively. Higher catalyst and ferrous sulfate concentration showed decline result in recovery of Mo and V. Ni showed consistent results throughout the series of experiments (88 to 90% recovery). The mixed sulfur oxidizing bacteria were observed to be more effective than the iron oxidizing bacteria. The sulfur oxidizing cells were able to recover Ni, V and Mo of 88, 94 and 46 % respectively (spent catalysts 50g/L, elemental sulfur of 2% (w/v), reaction time 7 days). The oxidation of elemental sulfur to sulfate could play a vital role for extraction of the metals.