Mechanism Study For SUMO Modification In Control Of RNA Pol? Transcription

Ribosome biogenesis is critical for proliferating cells and requires the coordinated activities of three eukaryotic RNA polymerases.A key regulatory point in this process is the ribosomal DNA(rDNA)transcription by Pol?,which accounts for up to about 60% of cellular RNA synthesis in proliferating cells.We recently showed that the small ubiquitin-like modifier(SUMO)system controls the global level of RNA polymerase II(Pol II)-controlled transcription in mammalian cells by regulating cyclin-dependent kinase 9(CDK9)activity.Here we present evidence that the SUMO system also plays a critical role in control of global level of transcription by RNA polymerase?.We showed that knockdown of UBC9 or multiple PIAS proteins(PIAS1,PIAS2 and PIAS4)all led to increased transcription of rDNA genes,whereas ectopic overexpression of UBC9 or PIAS1 repressed rDNA transcription.In search of the mechanisms by which SUMO represses rDNA transcription,we initially focused on sumoylation of key transcription factors required for rDNA transcription.However,no sumoylation of endogenous UBF,NCL,RRN3,TAF1 B,c-Myc and RPA194 was observed under the regular culture condition.We demonstrate that endogenous SUMO represses rDNA transcription primarily by repressing upstream-binding factor(UBF)and proto-oncogene c-Myc(C-Myc)expression and that ectopic overexpression of SUMO-associated enzymes additionally represses rDNA transcription via c-Myc SUMOylation and its subsequent degradation.The results of our study reveal a critical role of SUMOylation in the control of rDNA transcription,uncover the underlying mechanisms involved,and indicate that the SUMO system coordinates Pol I– and Pol II–mediated transcription in mammalian cells.