Figure 1.

Molecular mechanism for cell-cycle control in budding yeast. A solid arrow represents a chemical reaction and a dotted arrow represents an enzymatic activity on a reaction. Dashed arrows represent multisite phosphorylation chains. A T-shaped arrow with balls on the cross bar indicates a reversible binding reaction. Parameter values for multisite phosphorylation/dephosphorylation of regulatory proteins: for Whi5, N=6 and q=2; for SBF, N=4 and q=0; for Cdh1, N=10 and q=0; for Net1, N=8 and q=5. Hi5, Hbf and Ht1 are three unregulated phosphatases, which oppose cyclin-dependent kinases on the Whi5, SBF and Net1 phosphorylation chains, respectively. The synthesis and degradation reactions of Whi5, SBF, Cdh1, Net1, Hbf, Hi5 and Ht1 are not shown here. The diagram includes synthesis and degradation of MbS, the mRNA for ClbS, which is the only regulated mRNA species in the model. The mRNAs encoding all other proteins in the model are assumed to be synthesized and degraded by simple birth-death processes.