Differential gene expression drives cell-cycle-dependent transition from monopolar to bipolar growth in <i>Schizosaccharomyces pombe</i>

Cell polarity is important for maintaining cell structure and function. In S. pombe, after division, cells grow monopolarly from the old end, then transition to bipolar growth at a certain size when the new end activates. However, G1-arrested cells do not become bipolar despite continued growth, suggesting a role for the cell cycle 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-arrested and G2-phase cells of the cell-cycle mutant cdc10-129. DESeq2 analysis identified 65 unique genes upregulated in G1 phase and 35 in G2 phase. Enrichment analysis shows that G1 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. In G2 phase cells, protein-protein interaction networks identified the cdc15-hob3-rho1-bgs1 hub, known to promote bipolar growth and regulate cell wall biogenesis. In G1 phase cells, the spk1-byr2-ste11 hub involved in pheromone-response and nutritional stress-dependent G1-arrest was identified. In agreement with these findings, we find that spk1Δ cells are precociously bipolar, indicating a role for this kinase in preventing bipolar growth. We hypothesize that bipolar growth in G2 cells requires a combination of factors that favor cell growth. In G2, stress response pathways are downregulated while anabolic pathways are upregulated enabling transition from monopolar to bipolar growth.