The heavily transcribed rDNA repeats that give rise to the ribosomal RNA are clustered in a unique chromatin structure, the nucleolus. Due to its highly repetitive nature and transcriptional activity, the nucleolus is considered a hotspot of genomic instability. Breaks in rDNA induce a transient transcriptional shut down to conserve energy and promote rDNA repair; however, how nucleolar chromatin is modified and impacts on rDNA repair is unknown. Here, we uncover that phosphorylation of serine 14 on histone H2B marks transcriptionally inactive nucleolar chromatin in response to DNA damage. We identified that the MST2 kinase localises at the nucleoli and targets phosphorylation of H2BS14p in an ATM‐dependent manner. We show that establishment of H2BS14p is necessary for damage‐induced rDNA transcriptional shut down and maintenance of genomic integrity. Ablation of MST2 kinase, or upstream activators, results in defective establishment of nucleolar H2BS14p, perturbed DNA damage repair, sensitisation to rDNA damage and increased cell lethality. We highlight the impact of chromatin regulation in the rDNA damage response and targeting of the nucleolus as an emerging cancer therapeutic approach.

Two models have been put forward for cyclin-dependent kinase (Cdk) control of the cell cycle. In the qualitative model, cell cycle events are ordered by distinct substrate specificities of successive cyclin waves. Alternatively, in the quantitative model, the gradual rise of Cdk activity from G1 phase to mitosis leads to ordered substrate phosphorylation at sequential thresholds. Here, we study the relative contributions of qualitative and quantitative Cdk control in Saccharomyces cerevisiae. All S phase and mitotic cyclins can be replaced by a single mitotic cyclin, albeit at the cost of reduced fitness. A single cyclin can also replace all G1 cyclins to support ordered cell cycle progression, fulfilling key predictions of the quantitative model. However, single-cyclin cells fail to polarize or grow buds and thus cannot survive. Our results suggest that budding yeast has become dependent on G1 cyclin specificity to couple cell cycle progression to essential morphogenetic events.

The cell cycle is an ordered series of events by which cells grow and divide to give rise to two daughter cells. In eukaryotes, cyclin–cyclin-dependent kinase (cyclin–Cdk) complexes act as master regulators of the cell division cycle by phosphorylating numerous substrates. Their activity and expression profiles are regulated in time. The budding yeast S. cerevisiae was one of the pioneering model organisms to study the cell cycle. Its genetic amenability continues to make it a favorite model to decipher the principles of how changes in cyclin-Cdk activity translate into the intricate sequence of substrate phosphorylation events that govern the cell cycle. In this chapter, we introduce robust and straightforward methods to analyze cell cycle progression in S. cerevisiae. These techniques can be utilized to describe cell cycle events and to address the effects of perturbations on accurate and timely cell cycle progression.