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Unformatted text preview: Negative and Positive Control The trp and lac operons discussed above are examples of negative control because a repressor blocks transcription. In one case (lac operon) the repressor is active and prevents transcription. In the other case (trp) the repressor is inactive and must be activated to prevent transcription. Structural Genes Repressor or Regulator Negative Control (an active repressor inhibits transcription) Inducible Operons Inactive Active (inhibits) Repressible Operons Active Inactive (inhibits when activated) Positive Control (an active regulator promotes transcription Inactive Inactive (promotes when activated) Positive control mechanisms require the presence of an activator protein before RNA polymerase will attach. The activator protein itself must be bound to an inducer molecule before it attaches to mRNA. Genes which code for enzymes necessary for the digestion of maltose are regulated by this mechanism. Maltose acts as the inducer, binding to an activator and then to mRNA. The activator mechanism....
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