Enhancers, accessibility and combinatorial control
Enhancer-bound proteins can increase transcription from distant promoters through three-dimensional contacts. Factor activity, chromatin access and combinations of regulators help determine expression.
What a learner can do afterwards
- Explains how a sequence thousands of bases away affects a promoter.
- Says why the same enhancer is active in one cell type and silent in another.
- Predicts the expression pattern from a stated combination of available factors.
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Enhancers are regulatory DNA sequences where activators can bind. They may lie near a promoter or thousands of bases away. DNA folding can bring enhancer-bound proteins near promoter machinery. Coactivators such as Mediator can help communication. Many enhancers tolerate changes in spacing or orientation, but their genomic setting still matters.
Cells with the same enhancer sequence can differ in factor availability, factor activity and chromatin access. Closed chromatin often limits binding, although some pioneer factors can bind nucleosomal DNA and help open a region. If required activators cannot access an enhancer, its contribution can fall. Other enhancers or basal initiation may still support transcription.
Combinatorial control means several regulatory inputs jointly affect expression. Real effects can be graded. For an on/off question, apply its stated rule. Suppose activation above basal output requires an accessible enhancer, both A and B, and no repressor C. Missing B prevents this activation. C also prevents it even when A and B are present.
Enhancer contacts, access and regulator combinations influence transcription. Apply the stated rule without confusing lost activation with total silence.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Regulation of transcription and Regulatory genes are Khan Academy videos linked through YouTube.
More to read
Distant sites in folded DNA
An enhancer contains sites where regulatory proteins can bind. A promoter may be distant along the DNA yet close to those proteins after folding. Coactivators can help enhancer-bound regulators influence transcription machinery.
Many enhancers function after changes in spacing or orientation. This does not mean every placement works equally well. Chromatin organisation and available promoter contacts still affect regulatory activity.
Checking a regulatory rule
A prediction requires a rule and the cell's conditions. If activation needs two activators and absence of a repressor, check all three conditions. Having one activator does not satisfy the complete rule.
The predicted output also needs a definition. Failure to activate above basal output is not proof of zero transcription. Other enhancers or basal initiation may remain active. Real expression can vary gradually rather than follow an on/off rule.
Access and factor activity
Nucleosomes can limit access to regulatory DNA. Two cells can share an enhancer sequence but differ in factor activity and access, producing different expression patterns.
Access is not fixed independently of proteins. Some pioneer factors can bind nucleosomal DNA and help make regions accessible to other factors. A factor list alone is therefore incomplete, while unchanged DNA alone does not establish unchanged enhancer activity.
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