A ROLE FOR COFACTORS IN SYNERGISTIC AND CELL-SPECIFIC ACTIVATION BY RETINOIC ACID RECEPTORS AND RETINOID-X RECEPTOR

Transcriptional activation is thought to be mediated by DNA-bound acti vators through interaction with a basal transcription factor thereby s tabilizing the pre-initiation complex. For such interaction cofactors such as TAFs, bridging proteins, mediators or intermediary proteins ar e required by binding simultaneously to the activator and the target. We have investigated the activation functions (AFs) of both RAR beta a nd RXR alpha and show that both activators contain two homologous AFs. By comparing the capacity to activate transcription by these AFs on s everal promoters, both as full-length receptors and as fusion-proteins of AFs with the DNA-binding domain of the yeast transcription factor GAL-4, we were able to show that these AFs function by different mecha nisms. We found that the activity of these AFs is cell-type specific, as they are more active in certain cell lines than in others. Furtherm ore we observed that the AFs of RAR beta and RXR alpha can activate tr anscription synergistically both as GAL-fusion protein and with full-l ength receptors. For AF-2 of RAR beta we observed cell type-dependent differences in synergistic activation and we show that the E1A protein , which functions as a cofactor for RAR beta, permits synergistic acti vation in cell lines in which in the absence of this cofactor transcri ption occurs non-synergistically. We propose a model in which several non cell type specific cofactors and cell-specific cofactors act toget her to form a more stable pre-initiation complex explaining the observ ed cell-specific synergistic activation.