Figure 6.

Model for the role of SUMO in active transcription in yeast. (A) At constitutive genes, sumoylation (shown as encircled S) of transcription-related factors (gray) prior to recruitment to the promoter facilitates promoter complex assembly or recruitment of RNAP II (Pol II). (B) For inducible genes, when an activator (Ac) is present at sufficient concentration, it binds to the promoter (step 1), and recruits general transcription factors (gray) as well as Ubc9 (step 2). Note that, as for constitutive genes, some transcription-related factors may be sumoylated prior to promoter recruitment. (Step 3) However, the presence of Ubc9 and detection of SUMO at inducible genes during activation implies that sumoylation of promoter-bound factors is part of the activation process. Although targets of sumoylation have not yet been identified at induced promoters, it is possible that the activator itself is targeted, which facilitates its removal from the promoter, perhaps through SUMO-targeted ubiquitin-mediated degradation by the 26S proteasome (not shown). (Step 4) While RNAP II becomes engaged in transcription, SUMO-mediated loss of the activator (or other transcription factors), leads to deactivation of the promoter. (Step 5) The cleared promoter can now become induced again if sufficient activator is present; otherwise, transcription is shut off. Inability to deactivate the promoter by impairing sumoylation leads to prolonged activation of the induced gene, and reduced ability to shut off transcription.