Modeling the START transition in the budding yeast cell cycle
Fig 3
Other mechanisms involved in the START transition.
(A) Role of Bck2 in the activation of SBF. Bck2 acts on Swi4 and modifies it to a less active form (indicated by the enclosing light green box). The cartoon is a concise representation of regulation by Bck2 (intermediate steps not included). Black-filled circles with ‘B’ represent the Bck2-induced modification, and active forms are enclosed in a light green box (since these complexes are not as active as Cln-activated forms). Note that only promoter-bound forms are active. Clb2 causes complex inactivation (see equations in S2 Text). (B) MBF regulation. MBF alone is inactive in a repressed state (indicated by the enclosing dark red box). It is activated either by cyclins (Cln3, Cln1,2, Clb5,6) or by Bck2 (more active, as indicated by the dark green box). MBF is primarily inactivated by its transcriptional target, Nrm1, resulting in the negative feedback highlighted by the ‘–’ sign. Clb2 is a secondary and minor inhibitor of MBF.