There are three basic types of RNA polymerase III promoters. The three types of RNA polymerase III promoters are known as type 1, type 2, and type 3 promoters. Type 1 promoters are found in the 5S genes and consist of a gene-internal element called the internal control region (ICR), that is subdivided into A block, intermediate element, and C block (Bogenhagen, 1985; Sakonju et al., 1980). Type 2 promoters are found in tRNA genes, Adenovirus 2 VAI gene, and other genes (Galli et al., 1981; Sharp et al., 1981). These promoters consists of two gene-internal elements called the A and the B boxes. Type 3 promoters consist of a distal sequence element (DSE) that serves as an enhancer, a proximal sequence element (PSE), and a TATA box (Baer et al., 1989; Lobo and Hernandez, 1989).
Some promoters combine elements from type 2 and 3 promoters. For example, the S. cerevisiae U6 promoter, also shown in the figure, contains the TATA box typical of type 3 promoters and the A and B boxes typical of type 2 promoters. Moreover, in S. pombe, nearly all tRNA and 5S genes contain a TATA box in addition to gene-internal elements, and the TATA box is required for transcription.
