Typical stages in a tRNA (also termed class II) gene initiation: TFIIIC (Transcription Factor for polymerase III C) binds to two intragenic (lying within the transcribed DNA sequence) control sequences, the A and B Blocks (also termed box A and box B). TFIIIC acts as an assembly factor that positions TFIIIB to bind to DNA at a site centered approximately 26 base pairs upstream of the start site of transcription. TFIIIB is the transcription factor that assembles Pol III at the start site of transcription. Once TFIIIB is bound to DNA, TFIIIC is no longer required. TFIIIB also plays an essential role in promoter opening. Class III 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. Elongation TFIIIB remains bound to DNA following initiation of transcription by Pol III (unlike bacterial σ factors and most of the basal transcription factors for Pol II transcription). This leads to a high rate of transcriptional reinitiation of Pol III-transcribed genes. Termination Polymerase III terminates transcription at small polyUs stretch (5-6). In eukaryotes, a hairpin loop is not required, but may enhance termination efficiency in humans. Transcribed RNAs The types of RNAs transcribed from RNA polymerase III include: Transfer RNAs 5S ribosomal RNA U6 spliceosomal RNA RNase P and RNase MRP RNA 7SL RNA (the RNA component of the signal recognition particle) Vault RNAs Y RNA