A systematic series of N-terminal, C-terminal, and internal deletion mutants of S. cerevisiae TFIID were expressed in vitro and tested for TATA box binding and basal level transcription activities using, respectively, DNA mobility shift and in vitro transcription assays. The domains responsible for these activities were colocalized to a surprisingly large region containing C-terminal residues 63-240. This region was noted previously to contain potentially interesting structural motifs (central basic core, direct repeats, and sigma factor homology) and, more recently, to be highly conserved among TFIID from different species. Deletion mutant cotranslation studies revealed that TFIID binds DNA as a monomer. The implications of these results for TFIID structure and function are discussed.