Transcriptional regulation is the change in gene expression levels by altering transcription rates. [1]
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Regulation of transcription controls when transcription occurs and how much RNA is created. Transcription of a gene by RNA polymerase can be regulated by at least five mechanisms:
Regulatory protein is a term used in genetics to describe a protein involved in regulating gene expression. It is usually bound to a DNA binding site which is sometimes located near the promoter although this is not always the case. Regulatory proteins are often needed to be bound to a regulatory binding site to switch a gene on (activator) or to shut off a gene (repressor). Generally, as the organism grows more sophisticated, their cellular protein regulation becomes more complicated and indeed some human genes can be controlled by many activators and repressors working together.
In prokaryotes, regulation of transcription is needed for the cell to quickly adapt to the ever-changing outer environment. The presence or the quantity and type of nutrients determines which genes are expressed; in order to do that, genes must be regulated in some fashion. In prokaryotes, repressors bind to regions called operators that are generally located downstream from and near the promoter (normally part of the transcript). Activators bind to the upstream portion of the promoter, such as the CAP region (completely upstream from the transcript). A combination of activators, repressors and rarely enhancers (in prokaryotes) determines whether a gene is transcribed.[2]
In eukaryotes, transcriptional regulation tends to involve combinatorial interactions between several transcription factors, which allow for a sophisticated response to multiple conditions in the environment. This permits spatial and temporal differences in gene expression. Eukaryotes also make use of enhancers, distant regions of DNA that can loop back to the promoter. A major difference between eukaryotes and prokaryotes is the fact the eukaryotes have a nuclear envelope, which prevents simultaneous transcription and translation.[2] RNA interference also regulate gene expression in most eukaryotes, both by epigenetic modification of promoters and by breaking down mRNA.
Examples:
Gene Regulation can be summarized as how they respond:
In order for a gene to be expressed, several things must happen. First, there needs to be an initiating signal. This is achieved through the binding of some ligand to a receptor. Activation of g-protein-coupled receptors can have this effect; as can the binding of hormones to intra- or extracellular receptors.
This signal gives rise to the activation of a protein called a transcription factor, and recruits other members of the "transcription machine". Transcription factors generally simultaneously bind DNA as well as an RNA polymerase, as well as other agents necessary for the transcription process (HATs, scaffolding proteins, etc.). Transcription factors, and their cofactors, can be regulated through reversible structural alterations such as phosphorylation or inactivated through such mechanisms as proteolysis.
Transcription is initiated at the promoter site, as an increase in the amount of an active transcription factor binds a target DNA sequence. Other proteins, known as "scaffolding proteins" bind other cofactors and hold them in place. DNA sequences far from the point of initiation, known as enhancers, can aid in the assembly of this "transcription machinery." Histone arms are acetylated, and DNA is transcribed into RNA.
Frequently, extracellular signals induce the expression of immediate early genes (IEGs) such as c-fos, c-jun, or AP-1. These are in and of themselves transcription factors or components thereof, and can further influence gene expression.
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