Fine-tuned regulation of the mitotic genes in response to DNA damage
One of the biggest challenges a cell faces is stably maintaining its genome in the face of constant exposure to DNA damage from both exogenous and endogenous sources. Upon DNA damage, cells activate a conserved checkpoint that allows them to activate DNA repair machinery and arrest the cell cycle (checkpoint is reviewed in more detail in Chapter 1). The CLB2 cluster is a transcriptional cluster of thirty-three co-regulated mitotic genes that include many important regulators of mitosis. As such, this cluster is a hub for regulation of mitosis both during the cell cycle and in response to DNA damage. In response to DNA damage, budding yeast cells arrest in metaphase, but transcription of this mitotic gene cluster is low. We find that the checkpoint kinase Rad53 is responsible for inhibiting the transcriptional activator of this cluster, Ndd1, in order to maintain low levels of transcription (Chapter 2). However, since transcription of this cluster is not entirely repressed in response to DNA damage, in the absence of additional regulation, proteins that are transcribed as part of this cluster could accumulate over time. Hst3 is the histone deacetylase for histone H3 lysine 56 acetylation (H3K56-Ac), which is a cell cycle and damage-regulated chromatin mark. As H3K56-Ac must be maintained in response to DNA damage, Hst3 levels must be kept low. In addition to the transcriptional regulation of Hst3 as a member of the CLB2 cluster, Hst3 protein turnover is increased in response to DNA damage (Chapter 3) through SCFCdc4-dependent turnover. Finally, we turn to the transcriptional activator Ndd1, which is itself a hub for regulation of CLB2 cluster transcription and therefore mitosis. We find that Ndd1 is itself regulated by protein turnover during the cell cycle and in response to DNA damage (Chapter 4). Because the CLB2 cluster is composed of so many important mitotic regulators, many transcriptional and post-transcriptional mechanisms impinge on the members of this cluster, enabling fine-tuned control of mitosis during the cell cycle and in response to DNA damage.