Abstract Two fission yeast mitotic activators, Cdc13 and Cdc25, have been shown to increase in concentration in correlation with cell size, and have been proposed to thereby regulate cell size at division. Here, we show that the expression of both Cdc13 and Cdc25 are, in fact, size dependent, as apposed to simply sizecorrelated due to time-dependent expression. However, we also find that their size dependence is regulated by different mechanisms. Cdc25 was known to be regulated transcriptionally. Here, we show that Cdc13 is regulated translationally. Its transcript is not expression is a size-dependent manner, rather a size-dependent concentration of protein is expressed from a size-independent concentration of mRNA. Moreover, the degradation rate of Cdc13 is not size dependent, implicating size-dependent translation in its regulation. We identify a 20-amino-acid motif, which includes the APC D-box degron, as necessary and sufficient for sizedependent expression, which allowed us to construct a size-independent allele of cdc13 . Using this allele, in combination with a size-independent allele of cdc25 , expressed from a size-independent promoter, we show that size-dependent expression of neither Cdc13 nor Cdc25 is required for size control, nor are the redundantly required for size control.

Size-Dependent Expression of the Fission Yeast Cdc13 Cyclin is Conferred by Translational Regulation