Proteolytic and scanning electron microscopy studies indicate that S. cerevisiae TFIIIC consists of two globular domains separated by a flexible linker, one of which, designated tau B, binds strongly to the B box, and the other, designated tau A, binds weakly to the A box, of type 2 promoters (Schultz et al., 1989). DNA footprinting and protein-protein interaction studies (Hsieh et al., 1999a; Hsieh et al., 1999b; Kovelman and Roeder, 1992; Shen et al., 1996; Yoshinaga et al., 1989) support the models shown in the figure. The components of Brf1-TFIIIB (see TFIIIB entries) are shown in grey, and TFIIIA is shown in blue. Sites of strong protein-DNA cross-linking are indicated by small ovals. Black and grey rectangles show protein-protein contacts observed in human and S. cerevisiae TFIIIC subunits, respectively. The general arrangement of the TFIIIC subunits on type 1 and 2 promoters is strikingly similar (Bartholomew et al., 1990; Braun et al., 1992a).
On type 1 promoters, S. cerevisiae TFIIIA cross-links strongly to the A box and more weakly over most of the gene, suggesting that it extends over most of the gene (Braun et al., 1992a). Tfc3 is shifted downstream as compared to its position in the tRNA gene, with a main cross-link at the 3 end of the C box and another one further downstream. The Tfc6 subunit cross-links at the end of the gene, like in type 2 genes. There is no indication that the Tfc7 subunit contacts DNA in type 1 genes, but the Tfc1 subunit cross-links strongly upstream of the A box. The Tfc4 subunit crosslinks to sites around and upstream of the start site of transcription (Braun et al., 1992a).
Numerous protein-protein contacts between various TFIIIC subunits have been described, which are symbolized by small rectangles in the figure. The black rectangles indicate contacts identified with human TFIIIC subunits, the grey rectangles with S. cerevisiae TFIIIC subunits. Thus, Tfc7 interacts directly with Tfc1 (Manaud et al., 1998). TTFIIIC90 interacts with TFIIIC220, TFIIIC110, and TFIIIC63 (Hsieh et al., 1999). TFIIIC102 interacts with TFIIIC63 (Hsieh et al., 1999). Various TFIIIC subunits also interact directly with Brf1-TFIIIB subunits, as shown in the figure. These protein-protein contacts are discussed below.