TFIID is a multiprotein complex composed of TBP and several TAFIIs. Small amino-terminal segments (TAF N-terminal domain (TAND)) of Drosophila TAFII230 (dTAFII230) and yeast TAFII145 (yTAFII145) bind strongly to TBP and inhibit TBP-DNA interactions. yTAFII145 TAND (yTAND) was divided into two subdomains, yTANDI10-37 and yTANDII46-71, that function cooperatively. Here, we identify dTANDII within the amino terminus of dTAFII230 at 118-143 amino acids in addition to dTANDI18-77, reported previously. dTANDII exhibits pronounced sequence similarity to yTANDII, and the two were shown to be functionally equivalent in binding to TBP and inhibiting TBP-DNA interactions in vitro. Alanine scanning mutation analysis demonstrated that Phe-57 (yTANDII) and Tyr-129 (dTANDII) are critically required for the interaction with TBP. Yeast strains containing mutant yTAFII145 lacking yTANDI or yTANDII showed a temperature-sensitive growth phenotype. The conserved core of dTANDII could substitute for the yTANDII core, and Phe-57 or Tyr-129 described above was critically required for the function of this segment in promoting normal cell growth at 37 degreesC. In these respects, the impact of yTANDII mutations on cell growth paralleled their effects on TBP binding in vitro, strongly suggesting that the yTAFII145-TBP interaction and its negative effects on TFIID binding to core promoters are physiologically important.