Casein kinase 2 inhibits HomolD-directed transcription by Rrn7 in Schizosaccharomyces pombe

In Schizosaccharomycespombe, ribosomal protein gene (RPG) promoters contain a TATA analogue element called the HomolD box. The HomolD-binding protein Rrn7 forms a complex with the RNA polymeraseII machinery. Despite the importance of ribosome biogenesis to cell survival, the mechanisms involved in the regulation of transcription of eukaryotic RPGs are unknown. In this study, we identified Rrn7 as a new substrate of the pleiotropic casein kinase2 (CK2), which is a regulator of basal transcription. Recombinant Rrn7 from S.pombe, which is often used as a model organism for studying eukaryotic transcription, interacted with CK2 invitro and invivo. Furthermore, CK2-mediated phosphorylation of Rrn7 inhibited its HomolD-directed transcriptional activity and ability to bind to an oligonucleotide containing a HomolD box invitro. Mutation of Rrn7 at Thr67 abolished these effects, indicating that this residue is a critical CK2 phosphorylation site. Finally, Rrn7 interacted with the regulatory subunit of CK2 invivo, inhibition of CK2 invivo potentiated ribosomal protein gene transcription, and chromatin immunoprecipitation analyses identified that the catalytic subunit of CK2 was associated with the rpk5 gene promoter in S.pombe. Taken together, these data suggest that CK2 inhibits ribosomal protein gene transcription in S.pombe via phosphorylation of Rrn7 at Thr67.