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The eukaryotic initiation complex
In contrast to the prokaryotic polymerases, eukaryotic RNA polymerases are not able to bind to the DNA on their own - they require a number of transcription factors which, together with the RNA polymerase, form the initiation complex. The assembly to the initiation complex is different for the three eukaryotic polymerases.
The initiation complex of RNA polymerase I.
This polymerase transcribes the genes for the ribosomal RNAs. The promoter consists of two important areas:
- to the core- Promoter in the area around the start site of the transcription with the TATA box, which is 20 nucleotides in front of the start site.
- and a control element 100 to 180 nucleotides in front of the start site.
The transcription factors UBFI (binds both promoter regions) and SL1 (contains the TATA box binding protein TBP) now accumulate before the RNA polymerase completes the initiation complex and the transcription of the rRNA genes begins.
The initiation complex of RNA polymerase II
This RNA polymerase transcribes the genes for the mRNAs and the snRNAs U1, U2, U4 and U5. This enzyme requires most of the transcription factors for the initiation complex. The complex of TFIIB, TFIID, TFIIH, TFIIF, TFII I, TFIIE, some other proteins and the RNA polymerase is also referred to as RNA polymerase II holoenzyme. The TATA box binding protein TBP is part of the transcription factor TFIID. In addition to the TATA box, the transcription is regulated by further sequences located in front of the TATA box, which are regulated by various further transcription factors (the transcription activators or URF for upstream regulatory factors) can be bound:
- The GC box (a GC-rich region with the consensus sequence GGGCGG) is around 40 nucleotides in front of the start point of transcription in many genes.
- The CAAT box (with the consensus sequence GGCCAATCT) is often about 70 nucleotides before the start of transcription.
- The octamer box is located further in front of the transcription start point.
These DNA elements often appear in multiple copies in the promoter area of the respective genes.
The initiation complex of RNA polymerase III
The polymerase III transcribes the genes for the 5S-RNA, the tRNAs and the U6 snRNA. The transcription factors TFIIIC, TFIIIA, and TFIIIB (contains the TATA box binding protein TBP) are required in sequence for the initiation complex before the RNA polymerase can bind.
Alberts, B .; Bray, D .; Lewis et al., J. (2002):Molecular Biology of the Cell. , ISBN: 0-8153-4072-9