Mechanisms of appearance of the Pasteur effect in Saccharomyces cerevisiae: inactivation of sugar transport systems

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Abstract
S. cerevisiae does not show a noticeable Pasteur effect (activation of sugar catabolism by anaerobiosis) when growing with an excess of sugar and N source, but it does do so after exhaustion of the N source in the medium (resting state). This different behavior of growth and resting S. cerevisiae seems to be due to differences in the contribution of respiration to catabolism under both states. Growing S. cerevisiae respired only 3-20% of the catabolized sugar, depending on the sugar present; the remainder was fermented. Resting S. cerevisiae respired as much as 25-100% of the catabolized sugar. Evidently, a shift to anaerobiosis would have much greater energetic consequences in resting than in growing S. cerevisiae. In resting S. cerevisiae, anaerobiosis would strongly decrease the formation of ATP; as a consequence, various regulatory mechanisms would switch on, producing the observed increase of the rate of glycolysis. The greater significance that respiration reached in resting cells was not due to an increase of the respiratory capacity itself, but to a loss of fermentation which turned respiration into the main catabolic pathway. The main mechanism involved in the loss of fermentation occurring during N starvation was a progressive inactivation of the sugar transport systems that reduced the rate of fermentation to < 10% of the value in growing cells. Inactivation of the sugar transports seems a consequence of the turnover of the sugar carriers whose apparent half-lives were 2-7 h.