Transcription Factor IID Recruitment and Sp1 Activation

Cyclin D1 is an oncogene that regulates progression through the G1 phase of the cell cycle. A temperaturesensitive missense mutation in the transcription factor TAF1/TAFII250 induces the mutant ts13 cells to arrest in late G1 by decreasing transcription of cell cycle regulators, including cyclin D1. Here we provide evidence that TAF1 serves two independent functions, one at the core promoter and one at the upstream activating Sp1 sites of the cyclin D1 gene. Using in vivo genomic footprinting, we have identified protein-DNA interactions within the cyclin D1 core promoter that are disrupted upon inactivation of TAF1 in ts13 cells. This 33-bp segment, which we termed the TAF1-dependent element 1 (TDE1), contains an initiation site that displays homology to the consensus motif and is sufficient to confer a requirement for TAF1 function. Electrophoretic mobility shift assays reveal that binding of ts13-TAF1-containing TFIID complexes to the cyclin D1 TDE1 occurs at 25 °C but not at 37 °C in vitro and involves the initiator element. Temperature-dependent DNA binding activity is also observed for TAF1-TAF2 heterodimers assembled with the ts13 mutant but not the wild-type TAF1 protein. These data suggest that a function of TAF is required for the interaction of TFIID with the cyclin D1 initiator. Our finding that recruitment of TFIID, by insertion of a TBP binding site upstream of the TDE1, restores basal but not activated transcription supports the model that TAF1 carries out two independent functions at the cyclin D1 promoter.