Fig. 1.

(A) Highly schematic representation of different classes of Pol III transcribed genes. The important function of the various promoter elements and their associated transcriptional factors is to bring TFIIIB to the start site of the transcription and to stabilize it there for the recruitment of Pol III enzymatic machinery. Type I genes are composed of a major internal element, the C box, as well as additional elements that vary among species. The single-factor TFIIIA mediates the recruitment of TFIIIC, which binds along the length of the 5S gene in the presence of TFIIIA and is required for transcription. The exact mechanism of TFIIIC recruitment to the type 1 promoter remains unclear. Type II genes consist of 2 highly conserved sequence elements, a proximal A box and a more distal B box, within the transcribed region. The distance between the A and B boxes is variable, and these elements are responsible for the recruitment of the TFIIIC complex. TFIIIC binds along the entire length of a tRNA gene, beginning just upstream of the start site of transcription and extending through the terminator (shown in red circle (dark grey in print version)), a separate control element that resides 20–25 bp downstream of the B box. For type I and type II gene promoters the proximal subunits of TFIIIC direct TFIIIB to bind upstream of the transcription start site. TFIIIB then recruits and positions Pol III over the initiation site and remains stably bound to the DNA through multiple rounds of reinitiation by Pol III. Type III genes utilize an upstream TATA box, a proximal sequence element (PSE), and a distal sequence element (DSE). The PSE functions with the TATA element to recruit the SNAPc complex (Mittal et al. 1999). The transcriptional activator Oct1 is recruited to DSE and functions in part by promoting SNAPc complex recruitment. TFIIIB, SNAPc, and Oct-1 cooperative interactions promote a stable initiation complex mediated, in part, by a positioned nucleosome (red nucleosome (dark grey in print version)) between DSE and PSE (Zhao et al. 2001). Oct1, Staf1, and SNAPc can also participate in the activation of Pol II transcription (Schaub et al. 1997). In some Pol III transcribed genes the combination of promoter element differs from those described above. Green nucleosomes represent the histone marks associated with active transcription recently observed in genome-wide studies (Barski et al. 2010; Moqtaderi et al. 2010; Oler et al. 2010). (B) Schematic representation of the homology of the TFIIIC complex subunits in Schizosaccharomyces pombe, Saccharomyces cerevisiae, and human cells. The comparison analysis for homolog identifications was reported by Huang and Maraia (2001) and completed by Dumay-Odelot et al. (2010). Intensity of color corresponds to strength of homology (Dumay-Odelot et al. 2007). The brackets enclose the 5 subunits of human TFIIIC that correspond most strongly to the yeast TFIIIC (Dumay-Odelot et al. 2007).