Cell size and Cln-Cdc28 complexes mediate entry into meiosis by modulating cell growth

In the yeast Saccharomyces cerevisiae, mitotic cell cycle progression depends upon the G(1)-phase cyclin-dependent kinase Cln-Cdc28 and cell growth to a minimum cell size. In contrast, Cln-Cdc28 inhibits entry into meiosis, and a cell growth requirement for sporulation has not been established. Here, we report that entry into meiosis also depends upon cell growth. Moreover, sporulation and cell growth rates were proportional to cell size; large cells grew rapidly and sporulated sooner while smaller cells grew slowly and sporulated later. In addition, Cln2 protein levels were higher in smaller cells suggesting that Cln-Cdc28 activity represses meiosis in smaller cells by preventing cell growth. In support of this hypothesis, loss of Clns, or the presence of a cdc28 mutation increased cell growth specifically in smaller cells and accelerated meiosis in these cells. Finally, overexpression of CLNs repressed meiosis in smaller cells, but not in large cells. Taken together, these results demonstrate that Cln-Cdc28 represses entry into meiosis in part by inhibiting cell growth.