The Arabinose Operon - the attachment of RNA polymerase to...

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The Arabinose Operon The arabinose operon uses both positive and negative control. The operon contains three different genes (ara A, ara B, and ara D) that code for three enzymes needed to convert arabinose to a usable form. A fourth gene codes for a protein (ara C) that acts to regulate the structural genes. The regulator protein (ara C) is needed for transcription of the three structural genes (ara A, ara B, and ara D). It binds to its own structural gene preventing its own transcription, thus autoregulating its own level. When the level of ara C is low, transcription occurs and more ara C is synthesized. Ara C also binds to other sites within the operon, inhibiting transcription of the three structural genes. The genes therefore are normally not active.
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When arabinose is present, it binds to ara C causing it to change shape. The new shape promotes
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Unformatted text preview: the attachment of RNA polymerase to the DNA, thus allowing transcription to occur. The Lac Operon also has Positive Control In addition to the negative control mechanism described earlier in this chapter, the lac operon is also regulated by a positive control mechanism. E. coli prefer glucose when both glucose and lactose are present. When allolactose binds to and inhibits the repressor, the genes that code for enzymes needed to digest lactose are transcribed but only at a low level. Much of the cell's energy comes from glucose instead of lactose. When the level of glucose is low, the level of cAMP increases. Cyclic AMP activates a molecule (CAP), which then binds to the lac promoter and enhances the rate of transcription....
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