11.7
Co-activators and Co-repressors
Gene transcription is regulated by the synergistic action of several proteins that form a complex at a gene regulatory site. This is observed in eukar…
Transcription is regulated by activators and repressors, proteins that bind to DNA. In eukaryotes, these regulatory proteins often need additional proteins, called co-regulators, to function properly.
These co-regulators bind to activator or repressor complexes; however, they do not recognize cis-regulatory sequences, as they cannot bind directly to DNA.
A transcriptional regulator binds to a cis-regulatory sequence before the co-regulator associates; regulators and co-regulators often cannot form stable complexes unless the regulator is bound to DNA. In certain cases, RNA molecules also serve as scaffolds to hold all the proteins in a complex together.
Some of these co-regulators are enzymes that can regulate gene expression, such as histone acetyltransferases and deacetylases.
Acetyltransferases are co-activators that transfer an acetyl group to the histone leading to the loosening up of the DNA and promoting transcription. Deacetylases are co-repressors that remove acetyl groups resulting in the tight packing of DNA around the histones and preventing transcription.
For example, the co-regulator SMRT associates with the thyroid hormone receptor at the positive hormone response element and acts as a co-repressor to inhibit basal transcription. When a hormone binds to the receptor, the SMRT dissociates, and a co-activator binds to activate transcription.
However, when bound to the same receptor on a negative hormone response element, SMRT activates transcription in the absence of the hormone. Thereby acting as a co-activator.
View the full transcript and gain access to JoVE Core videos
Q1: What is the difference between co-activators and co-repressors in gene regulation?
Co-activators are proteins that enhance transcription by facilitating interactions between transcription factors and RNA polymerase, promoting gene expression. Co-repressors block transcription by preventing these interactions or recruiting chromatin-modifying enzymes that silence genes. Both work through regulation of expression during transcription and translation to fine-tune cellular responses.
Q2: How do co-activators and co-repressors interact with transcription factors?
Co-activators and co-repressors bind to transcription factors at specific DNA sites, modulating their activity. Co-activators stabilize transcription factor binding and recruit histone acetyltransferases to open chromatin. Co-repressors recruit histone deacetylases and other enzymes that compact chromatin, preventing transcription factor access to genes.
Q3: What role do co-activators play in cell-specific gene expression?
Co-activators enable cell-specific gene expression by selectively activating genes in response to cellular signals and developmental cues. Different cell types express distinct combinations of co-activators, allowing the same transcription factors to produce different outcomes. This specificity ensures appropriate gene activation patterns for each cell type's function and identity.
Q4: How do co-repressors silence genes through chromatin modification?
Co-repressors recruit histone-modifying enzymes that remove acetyl groups from histones, tightening DNA wrapping around nucleosomes. This chromatin compaction blocks transcription factor and RNA polymerase access to promoters. Co-repressors also recruit DNA methyltransferases, adding methyl marks that further silence genes and maintain repression.
Q5: What happens when co-activators and co-repressors compete for the same transcription factor?
Competition between co-activators and co-repressors determines the final transcriptional outcome of a gene. When co-activators dominate, genes are expressed; when co-repressors dominate, genes are silenced. This dynamic balance allows cells to rapidly switch gene expression states in response to environmental changes or developmental signals.
Q6: Why are co-activators and co-repressors essential for proper gene regulation?
Co-activators and co-repressors provide precise control over which genes are expressed in specific contexts. Without them, transcription factors alone cannot efficiently activate or repress genes. These regulatory proteins enable cooperative binding of transcription regulators and ensure that gene expression matches cellular needs, maintaining proper development and homeostasis.
Q7: How do co-activators and co-repressors contribute to disease when dysregulated?
Mutations or overexpression of co-activators can cause inappropriate gene activation, leading to cancer or developmental disorders. Loss of co-repressor function prevents proper gene silencing, similarly causing disease. Dysregulation disrupts the balance between activation and repression, resulting in abnormal cell proliferation, differentiation, or apoptosis.