HIGH SPECIFIC RATES OF GLUCOSE-UTILIZATION UNDER CONDITIONS OF RESTRICTED GROWTH ARE REQUIRED FOR CITRIC-ACID ACCUMULATION BY YARROWIA-LIPOLYTICA IMK-2

Yarrowia lipolytica strain IMK 2, a yeast capable of producing and exc reting citric acid, only accumulated citric acid when cell growth was restricted by specific nutrient limitations. Effective citrate accumul ation was linked with the ability of cells to maintain a high rate of glucose utilisation when growth was limited. Cells limited by nitrogen (N), sulphur (S), magnesium (Mg) or phosphorus (P) accumulated betwee n 50 and 220 mM citric acid after 168 h, with N and S limitation resul ting in the highest specific rates of production. In contrast, potassi um (K)-limited cells accumulated 6 mM citric acid in the same time per iod. Cells limited by K or lower levels of Mg or P had rates of glucos e utilisation that were less than 50% of those measured in cells limit ed by N or S. Although limitation of strain IMK 2 by Mg or P led to ci trate production, significant accumulation occurred only when the thre shold concentration of the limiting nutrient was exceeded. There was n o large accumulation of other tricarboxylic acid (TCA) cycle acids, ac etate, pyruvate, lactate or ethanol, although K-limited cells produced quantities of intracellular glycogen. Mannitol was accumulated under N, 8 and P limitation (up to 40 mM), as were small quantities of 2-oxo glutarate, which preceded the accumulation of citrate and all of the o ther TCA cycle acids measured. A clear difference was evident between the rates of glucose utilisation under N limitation between two citrat e-accumulating strains (IMK 2 and Candida guilliermondii IMK 1) and a citrate non-accumulating strain, Y. lipolytica YB 423. Although these strains had similar rates of glucose utilisation during exponential gr owth, both strains IMK 1 and IMK 2 had specific rates of glucose utili sation under N limitation more than double that of strain YB 423.