A role for cofactors in synergistic and cell-specific activation by retinoic acid receptors and retinoid X receptor

Abstract

Transcriptional activation is thought to be mediated by DNA-bound activators through interaction with a basal transcription factor thereby stabilizing the pre-initiation complex. For such interaction cofactors such as TAFs, bridging proteins, mediators or intermediary proteins are required by binding simultaneously to the activator and the target. We have investigated the activation functions (AFs) of both RARβ and RXRα and show that both activators contain two homologous AFs. By comparing the capacity to activate transcription by these AFs on several 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 mechanisms. We found that the activity of these AFs is cell-type specific, as they are more active in certain cell lines than in others. Furthermore we observed that the AFs of RARβ and RXRα can activate transcription synergistically both as GAL-fusion protein and with full-length receptors. For AF-2 of RARβ we observed cell type-dependent differences in synergistic activation and we show that the E1A protein, which functions as a cofactor for RARβ, permits synergistic activation in cell lines in which in the absence of this cofactor transcription occurs non-synergistically. We propose a model in which several non cell type specific cofactors and cell-specific cofactors act together to form a more stable pre-initiation complex explaining the observed cell-specific synergistic activation.