Typical stages in a U6 snRNA (also termed class III) gene initiation (documented in vertebrates only):
*SNAPc (SNRNA Activating Protein complex; subunits: 1, 2, 3, 4, 5) (also termed PBP and PTF) binds to the PSE (Proximal Sequence Element) centered approximately 55 base pairs upstream of the start site of transcription. This assembly is greatly stimulated by the Pol II transcription factors Oct1 and STAF that bind to an enhancer-like DSE (Distal Sequence Element) at least 200 base pairs upstream of the start site of transcription. These factors and promoter elements are shared between Pol II and Pol III transcription of snRNA genes.
*SNAPc acts to assemble TFIIIB at a TATA box centered 26 base pairs upstream of the start site of transcription. It is the presence of a TATA box that specifies that the snRNA gene is transcribed by Pol III rather than Pol II.
*The TFIIIB for U6 snRNA transcription contains a smaller Brf1 paralogue, Brf2.
*TFIIIB is the transcription factor that assembles Pol III at the start site of transcription. Sequence conservation predicts that TFIIIB containing Brf2 also plays a role in promoter opening.