Cracking the Code of the Crabtree effect: Enhancing S. cerevisiae's Potential as a Cell Factory

One of the most intriguing traits of the common baker’s yeast Saccharomyces cerevisiae is the Crabtree effect. When glucose levels exceed a specific threshold, S. cerevisiae actively represses respiration so that most carbon is routed towards ethanol production. While this dramatically reduces metabolic efficiency, it does allow faster ATP generation and hampers the growth of ethanol-sensitive competitors. In an industrial setting, however, the Crabtree effect limits the cell biomass yield due to reduced ATP production per molecule of consumed glucose. Despite its crucial ecological and industrial importance, the molecular determinants of the Crabtree effect remain largely elusive. We used a combination of high-throughput screens and QTL analyses to investigate differences in Crabtree repression between genetically divergent S. cerevisiae strains, and uncover which genes and alleles underlie the different behaviors. We performed two high-throughput screens in our lab which revealed that different S. cerevisiae strains, originating from diverse ecological niches, varied widely in Crabtree repression. Moreover, measuring fitness in respiratory and fermentative conditions suggests a fitness tradeoff between both conditions. Next, we performed multiple QTL mapping assays to identify the molecular determinants underlying Crabtree repression. Through this analysis we identified several alleles associated in modulating the Crabtree effect. Together, our findings demonstrate that Crabtree repression varies greatly among natural S. cerevisiae variants and can be assessed using high-throughput reporter assays. The integration of fitness assays and genetic mapping offers a powerful framework to identify candidate loci which play a significant role in the Crabtree regulation pathway, yielding targets to generate superior industrial strains.