Differential gene expression drives cell-cycle-dependent transition from monopolar to bipolar growth in fission yeast

Cell polarity is important for maintaining cell structure and function. In S. pombe, after division, cells are monopolar and grow from the old end that existed from the previous generation. Cells transition to bipolar growth once the cell reaches a certain size, when the new end generated after division initiates growth. Bipolarity occurs as a consequence of cell growth, where the two cell ends have sufficient growth-promoting factors. However, the transition to bipolarity does not occur in G1-arrested cells even with enhanced cell growth, suggesting a role for the cell-cycle phase in this process. To identify how the cell cycle impacts monopolar to bipolar transition, we performed high-throughput mRNA sequencing to detect differentially expressed genes in G1/S-arrested and G2-phase cells of the cell cycle mutant cdc10-129. Gene expression analysis using the DESeq2 R package identified 65 unique genes upregulated in G1/S phase and 35 in G2 phase cells. Enrichment analysis shows that G1/S phase cells upregulated the MAPK pheromone-response pathway, protein folding, rRNA processing, and heat-shock protein binding. G2 phase cells showed upregulation of plasma membrane maintenance and cell wall organization. Protein-protein interaction networks identified the cdc15-hob3-rho1-bgs1 hub in G2 phase cells. These genes are known to promote bipolar growth and regulate cell wall biogenesis. In G1/S phase cells spk1-byr2-ste11 hub was identified, which is required for pheromone-response and nutritional stress-dependent G1-arrest. We find that Spk1 also prevents precocious bipolar growth. Thus, we hypothesize that the pheromone response pathway, in addition to promoting G1-G0 transition, also prevents bipolar growth. In nutrient-rich conditions, this pathway is downregulated, G1-arrest is alleviated, the cell cycle progresses, and bipolarity occurs. Our findings suggest that in nutrition-rich conditions, stress response pathways are downregulated enabling transition from monopolar to bipolar growth, along with cell cycle progression.