Transcriptional regulation of ribosomal RNA gene : actin, myosin and beyond

Sammanfattning: In the eukaryotic cell nucleus, actin and nuclear myosin 1c (NM1) are key regulators of RNA polymerase I (Pol I) transcription and function across the entire ribosomal RNA biogenesis pathway. At the gene level, actin interacts with Pol I and together with NM1 they have been suggested to facilitate assembly of Pol I at the gene promoter. At the promoter, NM1 also regulates Pol I transcription as part of the B-WICH chromatin remodeling complex containing the subunits WSFT and the ATPase SNF2h, but not actin. B-WICH is essential for the post-initiation phase of Pol I transcription. In paper I we investigated whether NM1 and actin interact to promote assembly of the BWICH complex at the gene promoter to activate transcription. We found that NM1 serves as a structural bridge between polymerase and DNA. Using cells expressing dominant negative NM1 mutants, we found that SFN2h competes with actin for NM1 binding. When NM1 interacts with SNF2h, B-WICH is stabilized at the gene promoter. NM1 can then mediate recruitment of the histone acetyl transferase PCAF to the rDNA gene. PCAF specifically acetylates histone H3 on lysine 9, which facilitates the establishment of an open chromatin state, allowing for transcription. Furthermore, these events are directly regulated by the ATPase cycle of myosin. In paper II we studied how NM1 itself is regulated. We found that the Glycogen synthase kinase (GSK) 3ß is in the same complex as NM1 and actin and that GSK3ß directly phosphorylates a single serine residue in the C-terminal tail of NM1. This is essential for rDNA binding by NM1. Furthermore, we found that GSK3ß-phosphorylation specifically occurs in G1 cells and protects NM1 for degradation by the ubiquitin proteasome system mediated by the E3-ligase UBR5. GSK3ß is a downstream effector of the Wnt signaling pathway. In paper III, we therefore investigated the potential role of Wnt signaling in ribosomal RNA synthesis. We found that upon Wnt5A expression, dishevelled (Dvl1) localizes to the nucleoli and bind to the rRNA gene, possibly by an UBF-mediated tethering mechanism. Co-immunoprecipitation assays indeed showed that UBF and Dvl1 are part of the same complex. Concomitantly with Dvl1 gene occupancy, SIRT7 is displaced from the gene. These events lead to disruption of the Pol I machinery and lead to decreased transcription.