DROSOPHILA TFIIA DIRECTS COOPERATIVE DNA-BINDING WITH TBP AND MEDIATES TRANSCRIPTIONAL ACTIVATION

Drosophila transcription factor IIA (TFIIA) is composed of three subun its (30, 20, and 14 kD) that function during initiation of transcripti on. We reported previously the characterization of cDNAs that encode a precursor (dTFIIA-L) of the Drosophila TFIIA 30- and 20-kD subunits. In the absence of the smallest subunit, dTFIIA-S (14 kD), the unproces sed large subunit failed to exhibit any detectable promoter binding or transcriptional activity. Here, we report the molecular cloning and e xpression of dTFIIA-S, which has allowed the assembly of holo-dTFIIA ( dTFIIA-L/S). Subunit interaction studies indicate that dTFIIA-S binds to an amino-terminal domain of dTFIIA-L, which likely corresponds to t he endogenous 30-kD processed species. In addition, both dTFIIA-S and the carboxy-terminal domain of dTFIIA-L, which corresponds to the 20-k D species, independently interact weakly with the TATA-binding protein (TBP). In contrast, the holo-dTFIIA (L/S) binds TBP with high affinit y. The dTFIIA-L/S complex also binds cooperatively with TBP to TATA bo x DNA sequences, generating an extended DNase footprint pattern. The r econstituted holo-dTFIIA is able to stimulate basal transcription of s everal core promoter templates. Interestingly, dTFIIA-L/S is also able to significantly enhance transcriptional activation by upstream trans cription factors including Sp1, VP16, and NTF-1. These results suggest that dTFIIA is a multifunctional transcription factor capable of infl uencing DNA binding as well as interactions with the basal machinery, thereby enhancing activator-dependent transcription.