The F Box Protein Dsg1/Mdm30 Is a Transcriptional Coactivator that Stimulates Gal4 Turnover and Cotranscriptional mRNA Processing

We report here that the prototypical yeast transcription factor Gal4 undergoes two distinct modes of ubiquitin-mediated proteolysis: one that occurs independent of transcription and restricts Gal4 function, and another that is transcription coupled and essential for productive activation of Gal4 target genes. Destruction of transcriptionally active Gal4 depends on an F box protein called Dsg1/Mdm30. In the absence of Dsg1, Gal4 is stable, nonubiquitylated, and unable to productively stimulate transcription. Analysis of the phenotype of dsg1-null yeast reveals a striking disconnect between GAL gene RNA and protein levels; in the absence of Dsg1, Gal4 target genes are transcribed, but the resulting RNAs are not translated. The translational defects of these RNAs are related to defects in phosphorylation of the RNA polymerase II carboxy-terminal domain, which in turn affects recruitment of RNA processing machinery. We propose that Gal4 ubiquitylation and destruction are required for initiation-competent transcription complexes to transition to fully mature elongating complexes capable of appropriate mRNA processing.