DIMINISHED RESPIRATIVE GROWTH AND ENHANCED ASSIMILATIVE SUGAR UPTAKE RESULT IN HIGHER SPECIFIC FERMENTATION RATES BY THE MUTANT PICHIA-STIPITIS FPL-061

A mutant strain of Pichia stipitis, FPL-061, was obtained by selecting for growth on L-xylose in the presence of respiratory inhibitors. The specific fermentation rate of FPL-061, was higher than that of the pa rent, Pichia stipitis CBS 6054, because of its lower cell yield and gr owth rate and higher specific substrate uptake rate. With a mixture of glucose and xylose, the mutant strain FPL-061 produced 29.4 g ethanol /L with a yield of 0.42 g ethanol/g sugar consumed. By comparison, CBS 6054 produced 25.7 g ethanol/L with a yield of 0.35 g/g. The fermenta tion was most efficient at an aeration rate of 9.2 mmoles O-2 L-1 h(-1 ). At high aeration rates (22 mmoles O-2 L-1 h(-1)), the mutant cell y ield was less than that of the parent. At low aeration rates, (1.1 to 2.5 O-2 L-1 h(-1)), cell yields were similar, the ethanol formation ra tes were low, and xylitol accumulation was observed in both the strain s. Both strains respired the ethanol once sugar was exhausted. We infe r from the results that the mutant, P. stipitis FPL-061, diverts a lar ger fraction of its metabolic energy from cell growth into ethanol pro duction.