TOR signaling modulates Cdk8-dependent GAL gene expression in Saccharomyces cerevisiae

Cdk8 of the RNA Polymerase II mediator complex regulates genes by phosphorylating sequence specific transcription factors. Despite conserved importance for eukaryotic transcriptional regulation, the signals regulating Cdk8 are unknown. Full induction of the yeast genes requires phosphorylation of Gal4 by Cdk8, and we exploited this requirement for growth of yeast on galactose to identify mutants affecting Cdk8 activity. Several mutants from the screen produced defects in TOR signaling. A mutant designated al our throttle () 1, , was identified as an allele of , encoding aspartokinase. Defects in / caused hypersensitivity to rapamycin, and constitutive nuclear localization of Gat1. Furthermore, mutations of or , encoding TORC1 components, also prevented expression in yeast, and was determined to be allelic to . Disruption of , encoding a subunit of PP2A regulated by TOR signaling, suppressed the effect of / / , and mutations on expression in yeast, but not of / disruptions or Gal4 S699A mutation. Mutations of / and did not affect kinase activity of Cdk8 , but caused loss of Gal4 phosphorylation . These observations demonstrate that TOR signaling regulates induction through the activity of PP2A/ Cdc55, and are consistent with the contention that Cdk8-dependent phosphorylation of Gal4 S699 is opposed by PP2A/ Cdc55 dephosphorylation. These results provide insight into how induction of transcription by a specific inducer can be modulated by global nutritional signals through regulation of Cdk8-dependent phosphorylation.