Abstract: The transcription factor TFIID is a large multiprotein complex, composed of the TATA box-binding protein (TBP) and 14 TBP-associated factors (TAFs), which plays a key role in the regulation of gene expression by RNA polymerase II. The three-dimensional structure of yeast (y) TFIID, determined at approximately 3 nm resolution by electron microscopy and image analysis, resembles a molecular clamp formed by three major lobes connected by thin linking domains. The yTFIID is structurally similar to the human factor although the clamp appears more closed in the yeast complex, probably reflecting the conformational flexibility of the structure. Immunolabelling experiments showed that nine TAFs that contain the histone fold structural motif were located in three distinct substructures of TFIID. The distribution of these TAFs showed that the previously reported pair-wise interactions between histone fold domain (HFD)-containing TAFs are likely to occur in the native yTFIID complex. Most of the HFD-containing TAFs have been found in two distinct lobes, thus revealing an unexpected and novel molecular organization of TFIID.