TOR signaling modulates Cdk8-dependentGALgene expression inSaccharomyces cerevisiae

AbstractCdk8 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 yeastGALgenes requires phosphorylation of Gal4 by Cdk8, and we exploited this requirement for growth ofgal3yeast on galactose to identify mutants affecting Cdk8 activity. Several mutants from the screen produced defects in TOR signaling. A mutant designatedgalfour throttle (gft) 1,gft1, was identified as an allele ofhom3, encoding aspartokinase. Defects ingft1/hom3caused hypersensitivity to rapamycin, and constitutive nuclear localization of Gat1. Furthermore, mutations oftor1ortco89, encoding TORC1 components, also preventedGALexpression ingal3yeast, andtco89was determined to be allelic togft7. Disruption ofcdc55, encoding a subunit of PP2A regulated by TOR signaling, suppressed the effect ofgft1/hom3, gft7/tco89, andtor1mutations onGALexpression ingal3yeast, but not ofcdk8/srb10disruptions or Gal4 S699A mutation. Mutations ofgft1/hom3andtor1did not affect kinase activity of Cdk8in vitro, but caused loss of Gal4 phosphorylationin vivo. These observations demonstrate that TOR signaling regulatesGALinduction 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.