TORC1 signaling modulates Cdk8-dependent GAL gene expression in Saccharomyces cerevisiae

Abstract

Cdk8 of the RNA Polymerase II mediator kinase complex regulates gene expression by phosphorylating sequence-specific transcription factors. This function is conserved amongst eukaryotes, but the signals and mechanisms regulating Cdk8 activity and phosphorylation of its substrates are unknown. Full induction of the GAL genes in yeast requires phosphorylation of the transcriptional activator Gal4 by Cdk8. We used a screen to identify regulators of the Cdk8-dependent phosphorylation on Gal4, from which we identified multiple mutants with defects in TORC1 signaling. One mutant, designated gal four throttle 1 (gft1) was identified as a recessive allele of hom3, encoding aspartokinase, and mutations in hom3 caused effects typical of inhibition of TORC1, including rapamycin sensitivity and enhanced nuclear localization of the TORC1-responsive transcription factor Gat1. Mutations in hom3 also inhibit phosphorylation of Gal4 in vivo at the Cdk8-dependent site on Gal4, as did mutations of tor1, but these mutations did not affect activity of Cdk8 assayed in vitro. Disruption of cdc55, encoding a regulatory subunit of the TORC1-regulated protein phosphatase PP2A, suppressed the effect of hom3 and tor1 mutations on GAL expression, and also restored phosphorylation of Gal4 at the Cdk8-dependent site in vivo. These observations demonstrate that TORC1 signaling regulates GAL induction through the activity of PP2A/Cdc55, and suggest that Cdk8-dependent phosphorylation of Gal4 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.

In Saccharomyces cerevisiae ρ0 Cells, UME6 Contributes to the Activation of ABC Transporter Genes and Pleiotropic Drug Resistance via RPD3 and PDR3

“…Med12 in the L-Mediator complex also contributes to the induction of PDR5 expression in ρ 0 cells but not in ρ + cells. The transcriptional mediator complex serves as the interface between gene-specific transcription factors and the RNA polymerase II machinery [58]. The L-Mediator complex in S. cerevisiae contains the core mediator complex and the Cdk8 subcomplex.…”

UME6 Is Involved in the Suppression of Basal Transcription of ABC Transporters and Drug Resistance in the ρ+ Cells of Saccharomyces cerevisiae

“…The transcriptional mediator complex serves as a link between sequence-specific transcription factors and the RNA polymerase II machinery [ 49 ]. There are two different forms in the transcriptional mediator complex in S. cerevisiae : the core Mediator and L-Mediator.…”

“…The Cdk8 subcomplex consists of Med12 (Srb8), Med13 (Srb9), the cyclin-dependent kinase Cdk8 (Srb10), and cyclin C (Srb11). The Cdk8 subcomplex has both a negative and a positive role in gene transcription [ 49 , 50 , 51 ]. Pdr1 and Pdr3 can bind to the KIX domain of a mediator subunit called Med15/Gal11 of the core Mediator and L-Mediator [ 17 ].…”