Derepression of RNA Polymerase III Transcription by Phosphorylation and Nuclear Export of Its Negative Regulator, Maf1

Maf1 is the global repressor of RNA polymerase III (Pol III) in yeast Saccharomyces cerevisiae. Transcription regulation by Maf1 is important under stress conditions and during the switch between fermentation and respiration. Under repressive conditions on nonfermentable carbon sources, Maf1 is dephosphorylated and located predominantly in the nucleus. When cells were shifted to glucose medium, Maf1 became phosphorylated and concomitantly relocated to the cytoplasm. This relocation was dependent on Msn5, a carrier responsible for export of several other phosphoproteins out of the nucleus. Using coimmunoprecipitation, Maf1 was found to interact with Msn5. When msn5-Delta cells were transferred to glucose, Maf1 remained in the nucleus. Remarkably, despite constitutive presence in the nucleus, Maf1 was dephosphorylated and phosphorylated normally in the msn5-Delta mutant, and Pol III was under proper regulation. That phosphorylation of Maf1 and Pol III derepression are tightly linked was shown by studying tRNA transcription in Maf1 mutants with an altered pattern of phosphorylation. In summary, we conclude that phosphorylation of Maf1 inside the nucleus acts both directly by decreasing of Maf1-mediated repression of Pol III and indirectly by stimulation of Msn5 binding and export of nuclear Maf1 to the cytoplasm.