Abstract
Maf1 is the 'master' repressor of RNA polymerase III (Pol III) transcription in yeast, and is conserved in eukaryotes. Maf1 is a phospho-integrator, with unfavourable growth conditions leading to rapid Maf1 dephosphorylation, nuclear accumulation, binding to RNA Pol III at Pol III genes and transcriptional repression. Here, we establish the protein phosphatase 4 (PP4) complex as the main Maf1 phosphatase, and define the involved catalytic (Pph3), scaffold (Psy2) and regulatory subunits (Rrd1, Tip41), as well as uninvolved subunits (Psy4, Rrd2). Multiple approaches support a central role for PP4 in Maf1 dephosphorylation, Maf1 nuclear localization and the rapid repression of Pol III in the nucleus. PP4 action is likely direct, as a portion of PP4 co-precipitates with Maf1, and purified PP4 dephosphorylates Maf1 in vitro. Furthermore, Pph3 mediates (either largely or fully) rapid Maf1 dephosphorylation in response to diverse stresses, suggesting PP4 plays a key role in the integration of cell nutrition and stress conditions by Maf1 to enable Pol III regulation.