C-FOS-INDUCED ACTIVATION OF A TATA-BOX-CONTAINING PROMOTER INVOLVES DIRECT-CONTACT WITH TATA-BOX-BINDING PROTEIN

Transcriptional activation in eukaryotes involves protein-protein inte ractions between regulatory transcription factors and components of th e basal transcription machinery. Here we show that c-Fos, but not a re lated protein, Fra-1, can bind the TATA-box binding protein (TBP) both in vitro and in vivo and that c-Fos can also interact with the transc ription factor IID complex. High-affinity binding to TBP requires c-Fo s activation modules which cooperate to activate transcription. One of these activation modules contains a TBP-binding motif (TBM) which was identified through its homology to TBP-binding viral activators. This motif is required for transcriptional activation, as,veil as TBP bind ing. Domain swap experiments indicate that a domain containing the TBM can confer TBP binding on Fra-1 both in vitro and in vivo. In vivo ac tivation experiments indicate that a GAL4-Fos fusion can activate a pr omoter bearing a GALA site linked to a TATA box but that this activity does not occur at high concentrations of GAL4-Fos. This inhibition (s quelching) of c-Fos activity is relieved by the presence of excess TBP , indicating that TBP is a direct functional target of c-Fos. Removing the TBM from c-Fos severely abrogates activation of a promoter contai ning a TATA box but does not affect activation of a promoter driven on ly by an initiator element. Collectively, these results suggest that c -Fos is able to activate via two distinct mechanisms, only one of whic h requires contact with TBP. Since TBP binding is not exhibited by Fra -1, TBP-mediated activation may be one characteristic that discriminat es the function of Fos-related proteins.