TORC1 shapes the SUMO proteome to control RNA Polymerase III activity

Loss of nutrient supply elicits alterations of the SUMO proteome and sumoylation is crucial to various cellular processes including transcription. However, the physiological significance of sumoylation of transcriptional regulators is unclear. To begin clarifying this, we mapped the SUMO proteome under nitrogen-limiting conditions in Saccharomyces cerevisiae. Interestingly, several RNA polymerase III (RNAPIII) components are major SUMO targets under normal growth conditions, including Rpc53, Rpc82, and Ret1, and nutrient starvation results in rapid desumoylation of these proteins. These findings are supported by ChIP-seq experiments that show that SUMO is highly enriched at tDNA genes. Furthermore, RNA-seq experiments revealed that preventing sumoylation results in significantly decreased tRNA transcription. TORC1 inhibition resulted in the same effect, and our data indicate that the SUMO and TORC1 pathways are both required for robust tDNA expression. Importantly, tRNA transcription was strongly reduced in cells expressing a non-sumoylatable Rpc82-4KR mutant, which correlated with a misassembled RNAPIII transcriptional complex. Our data suggest that in addition to TORC1 activity, sumoylation of RNAPIII is key to reaching full translational capacity under optimal growth conditions.