Different types of RNA polymerase III promoters. The type 1 promoter of the Xenopus laevis 5S RNA gene consists of an internal control region (ICR), which can be subdivided into A box (+50 to +60), intermediate element (IE, +67 to +72), and C box (+80 to +90). The type 2 promoter of the X. laevis tRNALeugene consists of an A box (+8 to +19) and a B box (+52 to +62). The type 3 promoter of the Homo sapiens U6 snRNA gene consists of a distal sequence element (DSE, −215 to −240) that enhances transcription and a core promoter composed of a proximal sequence element (PSE, −65 to −48) and a TATA box (−32 to −25). TheSaccharomyces cerevisiae promoter is a hybrid promoter consisting of a TATA box (−30 to −23), an A box (+21 to +31), and a B box located downstream of the U6 coding region (from +234 to +244 relative to the start site of transcription).
