Transcription Initiation: Promoters, General Factors and Polymerase Recruitment
Eukaryotic polymerase does not find a promoter alone: general factors assemble on it first and position the enzyme at the start site. The rate of transcription is set mostly at this assembly step.
What a learner can do afterwards
- Describes assembly at a promoter in order and says which step commits the enzyme.
- Explains why initiation, rather than elongation, is the usual control point.
- Contrasts the bacterial single-factor arrangement with the eukaryotic assembly.
1 · Read
Eukaryotic polymerase does not find a promoter alone: general factors assemble on it first and position the enzyme at the start site. TFIID grips the TATA box, TFIIA and TFIIB stabilise it, then polymerase arrives with TFIIF, followed by TFIIE and TFIIH. Each factor enables the next, like a checklist.
TFIIH finishes the job twice over: its helicase activity melts the promoter into an open complex, and its kinase phosphorylates the polymerase tail. That phosphorylation licenses promoter escape, the moment the enzyme breaks its initiation contacts and commits to the gene. Without it the enzyme idles at the start and no mRNA results.
Because initiation decides whether any RNA is made at all, cells regulate here rather than mid-gene. Blocking one promoter shuts off its whole gene cleanly, while stopping elongation would waste a half-built transcript and jam the template. Activators recruit or stabilise the complex and repressors block assembly or escape, setting each promoter's firing rate.
Bacteria run the same errand with striking economy: a single sigma factor guides polymerase to the minus 10 and minus 35 promoter elements. One protein provides the specificity that eukaryotes distribute across a dozen general factors. Regulation then reduces to helping or hindering sigma binding.
Factors assemble in order, escape commits the enzyme, and control lives at the start.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.