Molecular Genetics of Prokaryotes Problem Set

Problem 11: The effect of high glucose on lac operon expression

Tutorial to help answer the question

Bacteria utilize glucose first, even if other sugars are present, through a mechanism called:

A. operon repression.
B. enzyme repression.
C. catabolite repression
D. gene regulation.
E. glucose utilization.


Lactose as an energy source

E. coli prefers to use glucose as an energy source when both glucose and lactose are available. Lactose is an alternative energy source that can be used if glucose is absent. The overall rate of messenger RNA synthesis from the lac operon, and from other operons for alternate catabolic energy sources, is indirectly regulated by the concentration of glucose in the cell.  

Cyclic AMP as regulator

cAMP is the second messenger used directly in the regulation of lac operon expression in response to changing levels of glucose. cAMP is a coactivator of the CRP protein, a transcriptional enhancer for operons that encode genes for catabolism of energy sources other than glucose, including the lac operon. As glucose levels fall, cAMP levels rise and transcription of an induced lac operon is enhanced. As the levels of glucose rise, cAMP levels fall, and lac operon mRNA synthesis is not enhanced.  

Catabolite repression

This regulatory mechanism, whereby high glucose levels can repress the expression of operons required for catabolism of alternate energy sources is known as "catabolite repression."

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Updated: July 15, 1999
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