To prevent the development of cancer, cells must regulate the cell division cycle. Cell cycle events are coordinated by an oscillatory gene expression program, established by a conserved transcription factor (TF) network. Most TFs in the network are phosphorylated by cyclin-dependent kinases (CDKs), which regulate their activity. However, the physiological consequences of disrupting TF phosphorylation remain poorly understood. The budding yeast repressive TFs Yhp1 and Yox1 are degraded following multisite phosphorylation by CDK. Surprisingly, I discovered that blocking phosphorylation of Yhp1 and Yox1 increased fitness compared to wild type cells, despite decreased expression of several essential cell cycle genes. We found that cells expressing non-phosphorylatable Yhp1 and Yox1 accelerated the G1/S transition and delayed mitotic exit. This suggests that by lengthening mitosis mutant cells have more time to correct chromosome segregation errors, which confers a fitness advantage to cells.

Although hundreds of CDK targets have been identified, it is challenging to determine which phosphosites within a domain are required to regulate protein function. The conserved S-phase TF Hcm1 is activated by CDK-dependent phosphorylation of eight sites in its transactivation domain (TAD). Like Yox1 and Yhp1, disruption of Hcm1 TAD phosphorylation impacts cellular fitness. I leveraged these fitness phenotypes to develop a high-throughput approach, Phosphosite Scanning, that determines the importance of each phosphosite within a multisite phosphorylated domain. I identified multiple combinations of phosphosites that can activate Hcm1 and found that specific phosphorylations are required for phosphorylation throughout the TAD. These results highlight the importance of precise TF phosphoregulation and demonstrate that disruption of phosphoregulatory networks can have unexpected consequences on cellular physiology.