Transcription initiation: promoters, general factors and polymerase recruitment
RNA polymerase II uses general transcription factors to initiate at a promoter. Recruitment, opening and promoter escape can regulate RNA production, alongside control during elongation.
What a learner can do afterwards
- Describes a model assembly pathway at a TATA-containing polymerase II promoter and explains promoter escape.
- Explains how initiation control limits new RNA synthesis while recognising regulation during elongation.
- Contrasts bacterial sigma-assisted initiation with polymerase II general-factor assembly.
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In homologous repair, a donor DNA sequence guides DNA synthesis. In transcription, a gene's template DNA guides RNA synthesis. RNA polymerase II makes pre-mRNA with general transcription factors. In a model TATA-containing promoter, TBP within TFIID binds TATA. TFIIA and TFIIB help organise the complex. Polymerase II joins with TFIIF, then TFIIE and TFIIH join. Many promoters lack TATA, and assembly need not follow this exact order.
TFIIH helps open promoter DNA through ATP-driven DNA translocation. Its kinase phosphorylates the polymerase's C-terminal domain, or CTD, supporting the transition out of initiation. During promoter escape, polymerase releases initiation contacts and moves away from the promoter. Later pausing or termination can still occur. Loss of TFIIH kinase activity can impair productive transcription without proving that all RNA synthesis stops.
Activators can recruit or stabilise initiation machinery; repressors can hinder it. Reducing initiation avoids starting unnecessary RNA chains, but does not remove existing mRNA. Elongation is also regulated. Polymerase can pause near a promoter and resume after pause release. Pausing differs from termination: a pause need not discard the RNA or jam the template.
In bacteria, sigma associates with core RNA polymerase and helps recognise promoters. Many promoters used by the main sigma factor have minus 10 and minus 35 elements. Bacteria use different sigma factors and other regulators. Polymerase II instead uses its general-factor complex. Neither system's regulation is limited to initial promoter binding.
General factors help polymerase II initiate. Promoter opening, escape and later elongation control affect productive RNA synthesis.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Regulation of transcription is a Khan Academy video linked through YouTube.
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Starting RNA synthesis
A promoter helps position the machinery that starts transcription. RNA polymerase II uses general factors to assemble there and expose template DNA. Initial RNA synthesis can precede movement away from the promoter.
Promoter escape releases initiation contacts. It does not promise a completed transcript: polymerase may later pause or terminate. Recruitment, opening, escape and continued elongation are separate events to consider when explaining RNA output.
A TATA-promoter model
One assembly model begins when TBP within TFIID binds a TATA box. TFIIA and TFIIB help organise the complex. Polymerase II joins with TFIIF, followed by TFIIE and TFIIH. Promoters without TATA and alternative assembly routes also exist.
TFIIH contributes DNA-opening activity and CTD phosphorylation. These support initiation transitions, but losing its kinase does not by itself establish that the cell makes no RNA.
Bacterial promoter recognition
Bacterial core RNA polymerase works with a sigma factor to recognise promoters. Many promoters used by the main sigma factor contain minus 10 and minus 35 elements. Alternative sigma factors help direct transcription towards different promoter sets.
This differs from polymerase II general-factor assembly. It does not mean bacterial regulation has only one input. Other regulators and events after promoter binding also influence transcription.
Initiation and pause release
Reducing initiation limits how many new RNA chains begin. It does not directly remove messages already present. Activators and repressors can influence initiation machinery by supporting or hindering assembly.
Elongation provides additional control. A polymerase paused near a promoter may resume when the pause is released. The RNA chain need not be discarded. Measure the relevant stage rather than treating every reduction in RNA output as failed recruitment.
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