Cells of the yeast Sacchammyces cerevisiae IGC 4072 grown in medium with acetic acid produced a mediated transport system for acetic acid that behaved as an electroneutral proton symport for the anionic form of the acid. The system could transport propionate and formate but not lactate and pyruvate. Uptake of labelled lactic acid was negligible, no mediated transport system activity for this acid being found. The acetate transporter was also found in cells grown in lactic acid or ethanol media, suggesting that the carrier did not require the presence of an external inducer. When cells weregrown in lactic acid medium, uptake of labelled acetic acid, at pH 5.0, wasbiphasic and consistent with the presence of two distinct transport modes for the acid. One of these components corresponded to the acetatelproton symport, and the higher affinity system corresponded to a more general monocarboxylate carrier that could also transport lactate, pyruvate and propionate. Both systems were subject to repression by glucose, fructose, sucrose, maltose or galactose. In glucose-repressed cells, the undissociated form of the acids appeared to be the only one that could cross the plasma membrane, a diffusion mechanism being involved in the acid uptake. Under these growthconditions and when the extracellular pH was lower than that of the cytosol, accumulation of the acid could also be observed, it being a function of the pH.


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