Overexpression limits of fission yeast cell‐cycle regulators in vivo and in silico

Cellular systems are generally robust against fluctuations of intracellular parameters such as gene expression level. However, little is known about expression limits of genes required to halt cellular systems. In this study, using the fission yeast Schizosaccharomyces pombe, we developed a genetic ‘tug‐of‐war’ (gTOW) method to assess the overexpression limit of certain genes. Using gTOW, we determined copy number limits for 31 cell‐cycle regulators; the limits varied from 1 to >100. Comparison with orthologs of the budding yeast Saccharomyces cerevisiae suggested the presence of a conserved fragile core in the eukaryotic cell cycle. Robustness profiles of networks regulating cytokinesis in both yeasts (septation‐initiation network (SIN) and mitotic exit network (MEN)) were quite different, probably reflecting differences in their physiologic functions. Fragility in the regulation of GTPase spg1 was due to dosage imbalance against GTPase‐activating protein (GAP) byr4. Using the gTOW data, we modified a mathematical model and successfully reproduced the robustness of the S. pombe cell cycle with the model. A genetic tug‐of‐war system is used to explore the sensitivity of the fission yeast cell cycle to changes in gene dosage, revealing a deeply conserved fragile core. A mathematical model can reproduce the robustness of the cell cycle, and points to currently unknown regulatory mechanisms. A genetic tug‐of‐war system is used to explore the sensitivity of the fission yeast cell cycle to changes in gene dosage, revealing a deeply conserved fragile core. A mathematical model can reproduce the robustness of the cell cycle, and points to currently unknown regulatory mechanisms.