Explain the regulation of gene expression in bacteria by induction, repression, and catabolite repression.

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Explain the regulation of gene expression in bacteria by induction, repression, and catabolite repression.



-Constitutive genes are expressed at a fixed rate
-Other genes are expressed only as needed

Repression controls the synthesis of one or several (repressible) enzymes. It is the regulatory mechanism that inhibits gene expression and decreases the synthesis of enzymes. When cells are exposed to a particular end product, the synthesis of enzymes related to that product decreases. Repression is mediated by regulatory proteins called repressors, which block the ability of RNA polymerase to initiate transcription from the repressed genes.

The process that turns on the transcription of a gene or genes is called induction. In the presence of certain chemicals (inducers), cells will synthesize more enzymes (induction).

Transcription of structural genes for catabolic enzymes (such as B-galactosidase) is induced by the absence of glucose. It is important to note that the presence of glucose inhibits metabolism of alternative carbon sources by catabolic repression (the "glucose effect"). This is due to the fact that when glucose is available the level of cAMP in the cell is low, and consequently CRP is not bound.

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