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Enhancers can be found in many of the same areas that silencers are found, such as upstream of the promoter by many kilobase pairs, or even downstream within the intron of the gene. [4] DNA looping is also a model function used by enhancers in order to shorten the proximity of the promoter to the enhancer.
While there are hundreds of thousands of enhancer DNA regions, [2] for a particular type of tissue only specific enhancers are brought into proximity with the promoters that they regulate. In a study of brain cortical neurons, 24,937 loops were found, bringing enhancers to their target promoters. [32] Multiple enhancers, each often at tens or ...
Since the discovery of DHSs 30 years ago, they have been used as markers of regulatory DNA regions. These regions have been shown to map many types of cis-regulatory elements including promoters, enhancers, insulators, silencers and locus control regions. A high-throughput measure of these regions is available through DNase-Seq. [2]
Enhancers are DNA sequences found in 5′ flanking regions of eukaryotic genes that affect transcription. If a transcription factor binds to an enhancer in a 5′ flanking region, the DNA strand bends in a way that the transcription factor that is bound to the enhancer can also bind the promoter of a gene.
Acetylation is usually linked to the upregulation of genes. This is the case in H3K27ac which is an active enhancer mark. It is found in distal and proximal regions of genes. It is enriched in Transcriptional start sites (TSS). H3K27ac shares a location with H3K27me3 and they interact in an antagonistic manner.
H3K4me1 is a chromatin signature of enhancers, H3K4me2 is highest toward the 5′ end of transcribing genes and H3K4me3 is highly enriched at promoters and in poised genes. H3K27me3 , H4K20me1 and H3K4me1 silence transcription in embryonic fibroblasts, macrophages, and human embryonic stem cells (ESCs).
Enhancers control cell-type-specific gene expression programs, most often by looping through long distances to come in physical proximity with the promoters of their target genes. [6] In a study of brain cortical neurons, 24,937 loops were found, bringing enhancers to promoters. [3]
These sequence regions can either be next to the transcribed region (the promoter) or separated by many kilobases (enhancers and silencers). [8] The promoter is located at the 5' end of the gene and is composed of a core promoter sequence and a proximal promoter sequence.