![]() This looping is the result of interactions between the proteins bound to the enhancer and those bound to the promoter. Enhancers can be thousands of nucleotides away from the promoters with which they interact, but they are brought into proximity by the looping of DNA. For example, some tumor-promoting viruses transform healthy cells by inserting strong promoters in the vicinity of growth-stimulating genes, while translocations in some cancer cells place genes that should be "turned off" in the proximity of strong promoters or enhancers.Įnhancer sequences do what their name suggests: They act to enhance the rate at which genes are transcribed, and their effects can be quite powerful. Alteration of promoter strength can have deleterious effects upon a cell, often resulting in disease. The terms "strong" and "weak" are often used to describe promoters and enhancers, according to their effects on transcription rates and thereby on gene expression. Because eukaryotic DNA is tightly packaged as chromatin, transcription also requires a number of specialized proteins that help make the template strand accessible. Enhancer sequences control gene activation by binding with activator proteins and altering the 3-D structure of the DNA to help "attract" RNA pol II, thus regulating transcription. Many eukaryotic genes also possess enhancer sequences, which can be found at considerable distances from the genes they affect. There are a number of different sigma subunits that bind to different promoters and therefore assist in turning genes on and off as conditions change.Įukaryotic promoters are more complex than their prokaryotic counterparts, in part because eukaryotes have the aforementioned three classes of RNA polymerase that transcribe different sets of genes. The sigma subunit conveys promoter specificity to RNA polymerase that is, it is responsible for telling RNA polymerase where to bind. In any case, upon binding, the RNA pol " core enzyme" binds to another subunit called the sigma subunit to form a holoezyme capable of unwinding the DNA double helix in order to facilitate access to the gene. Many genes also have the consensus sequence TTGCCA at a position 35 bases upstream of the start site, and some have what is called an upstream element, which is an A-T rich region 40 to 60 nucleotides upstream that enhances the rate of transcription (Figure 3). Although substitutions do occur, each box nonetheless resembles this consensus fairly closely. Not all Pribnow boxes have this exact nucleotide sequence these nucleotides are simply the most common ones found at each site. In prokaryotes, most genes have a sequence called the Pribnow box, with the consensus sequence TATAAT positioned about ten base pairs away from the site that serves as the location of transcription initiation. In bacteria, promoters are usually composed of three sequence elements, whereas in eukaryotes, there are as many as seven elements. The first step in transcription is initiation, when the RNA pol binds to the DNA upstream (5′) of the gene at a specialized sequence called a promoter (Figure 2a).
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