METHIONINE AND SULFATE AS COMPETING AND COMPLEMENTARY SOURCES OF SULFUR FOR YEAST DURING FERMENTATION

Study of the uptake of sulphate and methionine by an ale yeast from a range of media showed that utilisation of sulphate was fairly strictly controlled but assimilation of methionine was not. Cells never took u p more than about 0.3 mMol sulphate per litre whilst methionine, up to an initial concentration of 10 mMol per litre, was completely absorbe d. Sulphate-grown cells had low intracellular pools of amino acids and methinonine was never detected. Methionine-grown cells contained meth ionine in both cytosol and vacuole and the concentration of several ot her amino acids also increased in such a way to suggest that methionin e catabolism was occurring. With mixed sulphur sources methionine prev ented uptake of sulphate when the concentration of sulphate was high b ut not when it was low suggesting the presence of two sulphate transpo rters with different control properties. Sulphate did not influence up take of methionine. Addition of other amino acids to the medium did re duce the rate and extent of methionine uptake but not the intracellula r pool sizes. Pilot plant studies suggested that SO2 production in a b rewery is more likely to be a reflection of the overall nutritive stat us of the wort rather than be connected to the initial methionine conc entration.