Over half of the RNA synthesis in the cell takes place during transcription of ribosomal DNA (rDNA) – a nucleolar process that is tightly regulated in response to cell-cycle stage, ageing, starvation and other factors. For instance, rDNA transcription is repressed during mitosis, but we lack a complete picture of how this is controlled. Now, Valentina Sirri and colleagues (p. 489) investigate the mitotic activity of SIRT7, a nucleolar histone deacetylase that has been reported to help regulate rDNA transcription. The authors show that, in contrast to previous reports, SIRT7 associates with nucleolar organiser regions (NORs; the chromosomal sites at which rDNA genes cluster) even when rDNA transcription is repressed, and that this localisation is mediated by a direct interaction between SIRT7 and the rDNA transcription factor UBF. They next show that SIRT7 is phosphorylated by the CDK1–cyclin-B pathway during mitosis, then dephosphorylated before rDNA transcription resumes after mitosis. Moreover, the resumption of transcription requires SIRT7 activity, and the SIRT7 C-terminus becomes more reactive to cognate antibodies before transcription begins. The authors propose, therefore, that dephosphorylation of SIRT7 promotes a change in its conformation, and that this derepresses transcription. Their data underscore the complexity of rDNA transcriptional regulation.