| Rna Polymerase I |
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| ec 2.7.7 | |
| dna replication | |
| proteins | |
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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 '''C'''ontrol '''S'''equence). # UBF (Upstream '''B'''inding '''F'''actor) binds the UCS. #UCS recruits and binds a protein complex incorporating TBP (TATA '''B'''inding '''P'''rotein) and three TAFs (TBP '''A'''ssociated '''F'''actors) 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. SEE ALSO |
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