FIGURE 1:

Wiring diagram of the START-2013 model. The model is based on START-2004 (Chen et al., 2004), with significant changes as outlined in the Results section. The Start module (top left) now has a mechanism for cell size control, Whi5 inhibition of SBF, positive feedback from G1–S cyclins to SBF, and MBF via inhibition of Whi5 and direct phosphorylation, and negative feedback from Nrm1 and Clb2. It also includes control of nucleocytoplasmic shuttling of Whi5 and Swi6. The remainder of the wiring diagram is largely the same as that of START-2004, except that now Pds1 expression is constitutive and PPX (a hypothetical protein phosphatase that is now known to be PP2A-Cdc55) is inhibited by Esp1 rather than directly by Pds1. IE, a hypothetical intermediary enzyme in START-2004, is likely a Clb2-CDK–dependent phosphorylated form of anaphase-promoting complex (APC). We propose that a hypothetical protein phosphatase (hyp PP) dephosphorylates APC to prevent its premature association.