The metazoan-specific type 3 promoters, which are exemplified by the human U6 promoter, recruit a complex variously called the snRNA activating protein complex (SNAPc) (Sadowski et al., 1993), the PSE binding protein (PBP) (Waldschmidt et al., 1991), or the PSE transcription factor (PTF) (Murphy et al., 1992). The complex contains five types of subunits and binds to the PSE. Type 3 promoters also recruit Brf2-TFIIIB through a combination of protein-protein contacts with SNAPc and a direct association of the TBP component of Brf2-TFIIIB with the TATA box. This then allows RNA polymerase III to join the complex.
The down stream element (DSE) of type 3 promoters, which enhances transcription from the core promoter, almost invariably contains an octamer sequence and an SPH element (also called NONOCT element)(Cheung et al., 1993; Danzeiser et al., 1993; Kunkel et al., 1996; Myslinski et al., 1992). The octamer sequence recruits the POU domain protein Oct-1 (Herr et al., 1988; Sturm et al., 1988), and the SPH element recruits a zinc finger protein known as Staf or SPH binding factor (SBF), which has been cloned from humans (Myslinski et al., 1998; Rincon et al., 1998).