ABSTRACT
Saccharomyces cerevisiae and Schizosaccharomyces pombe cells were grown on D‐glucose, D‐galactose, D‐fructose, D‐mannose, maltose, trehalose and ethanol. All these substrates were separately added to cells thus grown and the onset and rate of acidification mediated by the plasma membrane H^+^‐ATPase were determined. Irrespective of the growth substrate, the best triggers of acidification in both species were fructose, mannose and glucose (with average rates of 5.2, 5.0 and 4.8 nmol H^+^ per min per mg dry weight, respectively, for S. cerevisiae, and 4.5, 6.8 and 5.8 for S. pombe). These were followed in S. cerevisiae by galactose in Gal‐, Man‐ and Tre‐grown cells (about 0.40 nmol H+) and by maltose in Mal‐ and Tre‐grown cells (about 0.15 nmol H+). Trehalose elicited some response in only ethanol‐grown cells while ethanol itself was completely ineffective in activating the H^+^‐ATPase. In S. pombe, however, maltose caused an acidification rate of 3.6 nmol H^+^ per min per mg dry wt., followed by EtOH (().38), Gal (0.13) and Tre (0.05). 6‐Deoxy‐D‐glucose and 2‐deoxy‐D‐glucose, not metabolized or improperly metabolized analogues of glucose, had no effect whatsoever. ‐ It appears that the sensor triggering the ATPase‐activating pathway is a complex responding both to a glucose‐type sugar (Glc, Man, Fru) and possibly identical with one of the glucose carriers, and to one of its metabolites, most probably fructose‐6‐phosphate.