RNA polymerase I
RNA polymerase I (also called Pol I) transcribes DNA to synthesize rRNA (Ribosomal RNA). Pol I consists of a 12 protein subunits (polypeptide). Pol I conducts 80% of all transcription in the cell. All 12 subunits have identical or related counterparts in Pol II and Pol III.
The process of transcription by Pol I is relatively unregulated (rRNA for ribosomes is always needed in large quantities). Consequently, transcription by Pol I is a comparatively simple process with few steps requiring regulation. Pol I transcribes one large transcript over and over again. This large transcript encodes the 18S, the 5.8S and the 28S RNA molecules of the ribosome in eukaryotes. The transcripts are cleaved by snoRNA. The 5S ribosomal RNA is transcribed by Pol III. Because of the simplicity of Pol I transcription it is the fastest acting polymerase.
In the process of transcription (by any polymerase) there are three main stages:
- Initiation; the construction of the RNA polymerase complex on the gene's promoter with the help of transcription factors.
- Elongation; the actual transcription of the majority of the gene into a corresponding RNA sequence.
- Termination; the cessation of RNA transcription and the disassembly of the RNA polymerase complex.
RNAP I Initiation
Initiation: the construction of the polymerase complex on the promoter. RNAP I requires no TATA box in the promoter, instead relying on an UCS (Upstream Control Sequence).
- UBF (Upstream Binding Factor) binds the UCS.
- UCS recruits and binds a protein complex incorporating TBP (TATA Binding Protein) and three TAFs (TBP Associated Factors) called SL1 or TIF-IB. The TBP is forced to bind non-sequence specifically.
- Rrn3/TIF-IA gets phosphorylated and binds RNAP I
- RNAP I binds to the UBF/SL1 complex via Rrn3/TIF-IA, and transcription starts.
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