Abstract

Glucose is many organisms’ preferred carbon source, and its transport is complex with budding yeast using at least seven different hexose transporters. Here we address why cells might have so many. Using mathematical modelling, we argue that one role of budding yeast’s glucose-sensing network is to mitigate a rate-affinity tradeoff inherent to transport by facilitated diffusion. By combining time-lapse microscopy with dynamically changing concentrations of glucose, we demonstrate that the transporter genes are regulated by a push-pull system of repressors. As glucose rises, repression by one type of repressor is weakened, or `pulled’, and repression by the other type is strengthened, or `pushed’. In falling glucose, cells reverse the push-pull. Through this transcriptional regulation, the glucose-sensing network matches levels of transporters by their affinity to current concentrations of glucose. Together our results underpin how budding yeast excels at importing glucose, living up to its name of Saccharomyces, or sugar fungus.