Tuesday, November 4, 2008

Q: Explain how protein synthesis can be controlled at a transcriptional level in prokaryotes

In prokaryotes, genes that encode enzymes of a metabolic pathway are usually clustered together on the chromosome in a region called an operon. A typical operon consists of regulatory sequences, like promoter, operator, terminator, and structural genes. Lac operon is an inducible operon that can be under negative control by lac repressor or positive control by CAP. In the absence of lactose, repressor binds to operator and blocks RNA polymerase from binding to promoter, thus no / low level of transcription of structural genes. In the presence of lactose, lactose will bind to repressor and change its conformation and prevent repressor from binding to operator, thus RNA polymerase can bind to promoter and transcription occurs. When both glucose and lactose are present, bacteria can selectively catabolise glucose instead of lactose. In the absence of glucose, the concentration of cAMP is high and cAMP binds to CAP and activates CAP, thus CAP binds to CAP-binding site and enhances rate of transcription. In the presence of glucose, concentration of cAMP is low and CAP not activated, thus CAP does not bind to CAP-binding site and transcription of structural genes occur as a low / basal rate.

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