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(chemistry) A substance that increases the effectiveness of a rubber vulcanization accelerator; for example, zinc oxide or litharge. A trace quantity of a substance that imparts luminescence to crystals; for example, silver or copper in zinc sulfide or cadmium sulfide pigments.
(genetics) A molecule that modifies a repressor in a way that enables it to stimulate operon transcription.
(graphic arts) accelerator
Compounds that increase the activity of enzymes.
1. an alkali, sodium carbonate, which is a component of photographic developing solution that softens and swells the gelatin of the film emulsion and provides the necessary alkaline medium for the developing agents to react with the sensitized silver halide crystals. n 2. a removable orthodontic appliance intended to function as a passive transmitter and sometimes stimulator of the forces of the perioral muscles. One in the myofunctional category of appliances also known by such names as Andresen, Bimler, Monobloc, and Frankel.
An activator is a DNA-binding protein that regulates one or more genes by increasing the rate of transcription. The activator may increase transcription by virtue of a connected domain which assists in the formation of the RNA polymerase holoenzyme, or may operate through a coactivator. A coactivator binds the DNA-binding activator and contains the domain assisting holoenzyme formation. A particular activator may bind one or more specific coactivators.
RNA polymerase (RNAP) is usually bound to the promoter region on the gene which creates a complex that sometimes undergoes a transition that allows transcription to take place. An activator essentially recruits the RNAP to its promotor region, by binding to the activator binding site itself which serves as a liaison between the RNA polymerase and the DNA. Sometimes, the activator is actually required for the RNA polymerase to transcribe DNA by changing its conformation through allostery. In this case, RNAP does not spontaneously transform into the open complex and requires the activator.
In the lac operon of the bacteria Escherichia coli, the Lac repressor is constitutively expressed and always bound to the operator region of the promotor, interfering with the ability of RNAP to bind to the promotor and transcribe the lac operon. In the presence of lactose, the repressor changes conformation and falls off the operator and RNAP is able to bind to the promotor. The activator catabolite activator protein (CAP) behaves in the exact opposite way. Cyclic adenosine monophosphate (cAMP) is present in low levels of glucose, and binds to CAP which changes to an active conformation. Only then, can CAP act as an activator and help recruit RNAP to its binding site.
In other words, for maximum expression of the lac genes, there needs to be a high level of lactose (for the repressor to fall off) and a low level of glucose (for CAP to recruit RNAP) in the cell. Otherwise, either the repressor will bind to the operator (no genes transcribed), or there is no cooperative binding by CAP thus depending on a spontaneous binding of RNAP (low levels of genes transcribed).
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 | Sci-Tech Dictionary. McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Activator (genetics)". Read more |
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