Abstract
Ribosome biogenesis is critical for proliferating cells and requires the coordinated activities of three eukaryotic RNA polymerases. 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 activity. Here, we present evidence that the SUMO system also plays a critical role in the control of Pol I transcription. Using an siRNA-based knockdown approach, we found that multiple SUMO E3 ligases of the PIAS (protein inhibitor of activated STAT) family are involved in SUMO-mediated repression of ribosomal DNA (rDNA) gene transcription. We demonstrate that endogenous SUMO represses rDNA transcription primarily by repressing upstream-binding factor and proto-oncogene 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.
Highlights
Ribosome biogenesis is critical for proliferating cells and requires the coordinated activities of three eukaryotic RNA polymerases
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 activity
Knockdown of UBC9 elevated the level of ribosomal DNA (rDNA) transcription, suggesting that the Pol I–mediated rDNA transcription is repressed by the endogenous SUMO system
Summary
Vents the formation of P-TEFb complex [23] that is required for efficient transcriptional elongation of Pol II–transcribed genes. By EU incorporation assay, we confirmed that knockdown of UBC9 led to an elevated level of rDNA transcription in CDK9WT but not in CDK9K/R cells (Fig. 4H) Together, these data indicate that the rDNA transcription in CDK9K/R cells is not repressed by endogenous SUMO, indicating that endogenous SUMO represses rDNA transcription primarily through its ability to repress transcription of UBF and c-Myc via CDK9 SUMOylation. We confirmed by immunostaining assay that ectopic overexpression of PIAS1 and SUMO led to a reduced level of c-Myc proteins in the CDK9WT cells and in CDK9K/R cells (Fig. 5C), whereas down-regulation of UBF was only observed in the CDK9WT cells (Fig. S6C). Ectopic overexpression of the SUMO system components can repress rDNA transcription through a SUMOylation-mediated degradation of c-Myc proteins
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