FIGURE 19-13 Regulation of S phase and mitotic cyclin levels in budding yeast. In late anaphase, the anaphase-promoting complex (APC/C) ubiquitinylates S phase and mitotic cyclins. The activity of this ubiquitin-protein ligase is directed toward mitotic cyclins by a specificity factor called Cdh1. Cdh1 activity is regulated by phosphorylation. During exit from mitosis and G₁, Cdh1 is dephosphorylated and active; during S phase and mitosis, Cdh1 is phosphorylated and dissociates from APC/C, and APC/C becomes inactive. The G₁/S phase CDKs, which themselves are not APC/C^Cdh1 substrates, phosphorylate Cdh1 at the G₁–S phase transition. A specific phosphatase called Cdc14 removes the regulatory phosphate from the specificity factor late in anaphase.