The type of carbon source not the growth rate it supports can determine diauxie
How cells choose between potential carbon sources is a classic example of cellular decision-making, and we know that many organisms prioritise glucose. Yet there has been little investigation of whether other sugars are also preferred, blinkering our view of carbon sensing. Here we study eukaryotic budding yeast and its growth on mixtures of palatinose, an isomer of sucrose, with other sugars. We find that yeast prioritise galactose over palatinose, but not sucrose or fructose, despite all three of these sugars being able to support faster growth than palatinose. Our results therefore disfavour carbon flux-sensing as the sole mechanism. By using genetic perturbations and transcriptomics, we show that repression is active and through Gal4, the master regulator of the GAL regulon. Cells enforce their preference for galactose over palatinose by preventing runaway positive feedback in the MAL regulon, whose genes enable palatinose catabolism. They do so both by repressing MAL11, the gene encoding the palatinose transporter, and by first expressing the isomaltases, IMA1 and IMA5, which cleave palatinose and so prevent its intracellular concentration becoming enough to induce further MAL expression. Our results demonstrate that budding yeast actively maintain a preference for carbon sources other than glucose and that such preferences have been selected by more than differences in growth rates. They imply that carbon-sensing strategies even in unicellular organisms are more complex than previously thought.