Abstract
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.
