In the yeast Saccharomyces cerevisiae a novel control exerted by TPS1 (=GGS1=FDP1=BYP1=CIF1 = GLC6= TSS1)-encoded trehalose-6-phosphate synthase, is essential for restriction of glucose influx into glycolysis apparently by inhibiting hexokinase activity in vivo. We show that up to 50-fold overexpression of hexokinase does not noticeably affect growth on glucose or fructose in wild-type cells. However, it causes higher levels of glucose-6-phosphate, fructose-6-phosphate and also faster accumulation of fructose-l,6-bisphosphate during the initiation of fermentation. The levels of ATP and Pi correlated inversely with the higher sugar phosphate levels. In the first minutes after glucose addition, the metabolite pattern observed was intermediate between those of the tps1 Delta mutant and the wild-type strain. Apparently, during the start-up of fermentation hexokinase is more rate-limiting in the first section of glycolysis than phosphofructokinase. We have developed a method to measure the free intracellular glucose level which is based on the simultaneous addition of D-glucose and an equal concentration of radiolabelled L-glucose. Since the latter is not transported, the free intracellular glucose level can be calculated as the difference between the total D-glucose measured (intracellular+ periplasmic/extracellular) and the total L-glucose measured (periplasmic/extracellular). The intracellular-glucose level rose in 5 min after addition of 100 mM-glucose to 0.5-2 mM in the wild-type strain, +/-10 mM in a hxk1 Delta hxk2 Delta glk1 Delta and 2-3 mM in a tps1 Delta strain. In the strains overexpressing hexokinase PII the level of free intracellular glucose was not reduced. Overexpression of hexokinase PII never produced a strong effect on the rate of ethanol production and glucose consumption. Our results show that overexpression of hexokinase does not cause the same phenotype as deletion of Tps1. However, it mimics it transiently during the initiation of fermentation. Afterwards, the Tps1-dependent control system is apparently able to restrict properly up to 50-fold higher hexokinase activity. (C) 1998 John Wiley & Sons, Ltd.