Gal11p dosage-compensates transcriptional activator deletions via Taf14p

Transcriptional activators work by recruiting transcription factors that are required for the process of transcription to their target genes. We have used the Split-Ubiquitin system to identify eight transcription factors that interacted with both the transcriptional activators Gal4p and Gcn4p in living cells. The over-expression of one of the activator-interacting proteins, Gal11p, partially suppressed GAL4 and GCN4 deletions. We have isolated two point mutants in Gal11p, F848L and F869S that were defective for the dosage compensation. We have identified 35 transcription factors that interacted with Gal11p in living cells, and the only protein-protein interaction affected by the Gal11p mutations was the one between Gal11p and Taf14p. We have further shown that the suppression of a GAL4 deletion by high levels of Gal11p required Taf14p, and that over-expression of Gal11p recruited Taf14p to the GAL1 promoter together with Tbp1p, Swi2p and Srb7p. Gal11p interacted with Mig1p, indicating that Mig1/2p could have recruited Gal11p to the GAL1 promoter in the absence of Gal4p. Our results suggest that transcriptional activators work by raising the local concentration of the limiting factor Gal11p, and that Gal11p works by recruiting Mediator and Taf14p-containing transcription factors like TFIID and SWI/SNF and by competing general repressors like Ssn6p-Tup1p off the target promoters.