Two types of transcription factors regulate eukaryotic transcription: General transcription factors that bind to the core promoter region to assist with the binding of RNA polymerase and Specific transcription factors that bind to various regions outside of the core promoter region and interact with the proteins at the core promoter to enhance or repress the activity of the polymerase.
In some eukaryotic genes, some regions help increase or enhance transcription. These regions, called enhancers, are not necessarily close to the genes they enhance. They can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or maybe thousands of nucleotides away. Enhancer regions are binding sequences, or sites, for transcription factors. When a DNA-bending protein binds to an enhancer, the shape of the DNA changes. This shape change allows the interaction between the activators bound to the enhancers and the transcription factors bound to the promoter region and the RNA polymerase to occur. Whereas DNA is generally depicted as a straight line in two dimensions, it is a three-dimensional object. Therefore, a nucleotide sequence of thousands of nucleotides away can fold over and interact with a specific promoter.
Transcriptional repressors can bind to promoter or enhancer regions and block transcription. Like the transcriptional activators, repressors respond to external stimuli to prevent the binding of activating transcription factors. A corepressor is a protein that decreases gene expression by binding to a transcription factor that contains a DNA-binding domain. The corepressor is unable to bind DNA by itself. The corepressor can repress transcriptional initiation by recruiting histone deacetylase, which catalyzes the removal of acetyl groups from lysine residues. This increases the positive charge on histones, which strengthens the interaction between the histones and DNA, making the DNA less accessible to the process of transcription.
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Key Points
• Enhancers can be located upstream of a gene, within the coding region of the gene, downstream of a gene, or thousands of nucleotides away.
• When a DNA-bending protein binds to the enhancer, the shape of the DNA changes, which allows interactions between the activators and transcription factors to occur.
• Repressors respond to external stimuli to prevent the binding of activating transcription factors.
• Corepressors can repress transcriptional initiation by recruiting histone deacetylase.
• Histone deacetylation increases the positive charge on histones, which strengthens the interaction between the histones and DNA, making the DNA less accessible to transcription.
Key Terms
enhancer: a short region of DNA that can increase transcription of genes
repressor: any protein that binds to DNA and thus regulates the expression of genes by decreasing the rate of transcription
activator: any chemical or agent which regulates one or more genes by increasing the rate of transcription
transcription factor: a protein that binds to specific DNA sequences, thereby controlling the flow (or transcription) of genetic information from DNA to mRNA
eukaryotic: organisms whose cells have a nucleus enclosed within membranes
upstream: refers to relative positions of genetic code in DNA or RNA
nucleotides: form the basic structural unit of nucleic acids such as DNA
DNA-binding proteins: proteins that have DNA-binding domains
RNA polymerase: an enzyme that turns DNA into RNA
corepressor: a protein that decreases gene expression by binding to a transcription factor that contains a DNA-binding domain
histone deacetylase: which strengthens the interaction between the histones and DNA
