325genC10RegTranscp07 - Chapter10(Draft...

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Chapter 10  (Draft) Regulation of gene transcription. Cells must be able to regulate transcription and regulate it in response to  environmental conditions.   Unnecessary transcription and translation would waste energy and raw materials  and therefore, reduce reproductive output (fitness).   Alleles that contributed to this wastefulness would be selected against and they  would eventually be eliminated from the population. For efficient operation, cells must be able to do two things: Cells must be able to turn on or turn off the transcription of loci. Cells must be able to recognize environmental conditions in which they should  activate or repress transcription of the relevant loci. The expression of a gene can be regulated at various stages of expression. The  most common  is at the stage of  initiation . Gene expression is controlled by regulatory proteins Environmental signals are transmitted to genes by regulatory proteins. Two classes of regulators: Activators  (positive regulators), proteins that promote transcription Repressors  (negative regulators), proteins that inhibit transcription. Typically these regulators bind to the DNA at specific sites to perform their  function. In most cases RNA polymerase will bind only weakly to a promoter in the  absence of regulatory proteins. Recall some promoter elements are imperfect so they bind RNA pol only weakly.
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Because of this ability bind weakly, there is a low level of frequent transcription  from many genes. This low level of transcription in the absence of regulatory proteins is called the  constitutive or  basal level of transcription . A repressor often binds to the DNA in a location that overlaps the RNA pol  binding site and therefore prevents the binding of RNA pol. An activator usually helps RNA pol bind to the promoter. Typically the activator uses one domain to bind to a site on the DNA near the  promoter.  With another domain it interacts with RNA pol helping to bring RNA  pol to the promoter. This mechanism is called  recruitment  and it is an example of cooperative  binding of proteins to DNA. The interactions serve merely adhesive roles. Regulatory proteins can act at a distance from the promoter.  This is  accomplished by bending (looping) the DNA. Some regulatory proteins interact with each other after binding distant sites on  the DNA. Looping the DNA brings these proteins together.
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