Abstract
Transcriptional repression of ribosomal components and tRNAs is coordinately regulated in response to a wide variety of environmental stresses. Part of this response involves the convergence of different nutritional and stress signaling pathways on Maf1, a protein that is essential for repressing transcription by RNA polymerase (pol) III in Saccharomyces cerevisiae. Here we identify the functions buffering yeast cells that are unable to down-regulate transcription by RNA pol III. MAF1 genetic interactions identified in screens of non-essential gene-deletions and conditionally expressed essential genes reveal a highly interconnected network of 64 genes involved in ribosome biogenesis, RNA pol II transcription, tRNA modification, ubiquitin-dependent proteolysis and other processes. A survey of non-essential MAF1 synthetic sick/lethal (SSL) genes identified six gene-deletions that are defective in transcriptional repression of ribosomal protein (RP) genes following rapamycin treatment. This subset of MAF1 SSL genes included MED20 which encodes a head module subunit of the RNA pol II Mediator complex. Genetic interactions between MAF1 and subunits in each structural module of Mediator were investigated to examine the functional relationship between these transcriptional regulators. Gene expression profiling identified a prominent and highly selective role for Med20 in the repression of RP gene transcription under multiple conditions. In addition, attenuated repression of RP genes by rapamycin was observed in a strain deleted for the Mediator tail module subunit Med16. The data suggest that Mediator and Maf1 function in parallel pathways to negatively regulate RP mRNA and tRNA synthesis.
Highlights
Nuclear gene transcription in proliferating cells is dedicated primarily to the synthesis of ribosomes and tRNAs
We found that deletions in a subset of MAF1 synthetic sick/lethal (SSL) genes, including subunits of the RNA polymerase II Mediator complex, lead to defects in transcriptional repression of ribosomal protein (RP) genes
Synthetic Genetic Array Analysis of MAF1 A maf1D strain was screened in triplicate against an ordered array of,4700 viable gene-deletion strains and the relative growth of the double mutants was scored by computer-based image analysis [15]
Summary
Nuclear gene transcription in proliferating cells is dedicated primarily to the synthesis of ribosomes and tRNAs. As illustrated by studies in Saccharomyces cerevisiae, the doubling of cell mass with each cell cycle involves the production of ,200,000 ribosomes along with 3–6 million molecules of tRNA and consumes .80% of the nucleotides needed for transcription during this ,100 minute interval [1,2,3]. As illustrated by studies in Saccharomyces cerevisiae, the doubling of cell mass with each cell cycle involves the production of ,200,000 ribosomes along with 3–6 million molecules of tRNA and consumes .80% of the nucleotides needed for transcription during this ,100 minute interval [1,2,3] This expenditure of metabolic energy is tightly regulated by diverse signaling pathways that sense the quality and quantity of nutrients or environmental stresses [1,2]. Substantial gaps in understanding remain concerning the components and structure of these pathways, their targets and mechanisms of action
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