Figure 1. The ubiquitin timer model for Gal4. (A) In cells grown under non-inducing conditions, Gal80 binds Gal4 and blocks its activation domain. The GAL genes are off. (B) Upon galactose induction, SCF^Mdm30 monoubiquitinates Gal4, which stabilizes its binding to the GAL gene promoter, as ubiquitinated Gal4 is protected from the promoter-stripping activity of the 19S subunit of the proteasome. SCF^Mdm30 further polyubiquitinates Gal80, leading to its degradation by the 26S proteasome. This frees Gal4’s activation domain and allows it to recruit the RNA Polymerase II holoenzyme. The GAL genes are switched on. (C) SCF^Mdm30 adds ubiquitin to the ubiquitin moiety of monoubiquitinated Gal4, which continues to activate transcription of the GAL genes. (D) SCF^Mdm30 extends the ubiquitin chain to three, while Gal4 continues to activate transcription of the GAL genes. (E) Once SCF^Mdm30 has added a fourth ubiquitin moiety, polyubiquitinated Gal4 is degraded by the 26S proteasome and the GAL genes are switched off.