Trans-regulation and localization of orthologous maltose transporters in the interspecies lager yeast hybrid.

In the interspecies lager yeast hybrid there are MAL loci involved in maltose and maltotriose utilization derived from each parent (Saccharomyces cerevisiae and Saccharomyces eubayanus). We show that trans-regulation across hybrid subgenomes occurs for MAL genes. However, gene expression is less efficient with non-native activators (trans-activation) compared to native activators (cis-activation). MAL genes were induced by maltose and repressed by glucose irrespective of host. Despite the strong expression of S. cerevisiae-type genes in the S. eubayanus host, a very low amount of transporter protein was actually observed in cells. This suggests that proper formation and configuration of the S. cerevisiae transporters is not efficient in S. eubayanus. The S. eubayanus-type Malx1 transporter was present in the plasma membrane in high amounts in all hosts (S. cerevisiae, S. eubayanus and Saccharomyces pastorianus) at all times. However, the S. cerevisiae-type transporters appeared sequentially in the plasma membrane; scMalx1 was localized in the plasma membrane during early to late linear growth and subsequently withdrawn to intracellular compartments. In contrast, the scAgt1 transporter was found in the plasma membrane mainly in the stationary phase of growth. Different localization patterns may explain why certain transporter orthologues in natural S. pastorianus strains were lost to mutation.