Molecular Genetics of Prokaryotes Problem Set

Problem 13: Effect of high lactose and low glucose on lac operon

Tutorial to help answer the question

Which of the following growth media would you expect to result in synthesis of high levels of mRNA for the enzymes of the E. coli lac operon?

A. high glucose, high lactose.

B. no glucose, no lactose.

C. high glucose, low lactose.

D. no glucose, high lactose.

E. none of these.

Tutorial

Regulation of mRNA synthesis

mRNA synthesis from the E. coli lac operon is inducible. mRNA synthesis does not occur unless lactose is available for catabolism. To review the mechanism of induction of lac mRNA synthesis by lactose, see problem 2.

The level of mRNA synthesis from the lac operon is also regulated by the intracellular concentration of glucose via a mechanism called catabolite repression. To review catabolite repression, see problems 11 and 12.

Briefly, maximum rates of lac mRNA synthesis occur only at low levels of glucose. As suggested by the diagram, cAMP present in cells in response to low glucose levels binds to the CRP protein and the cAMP-CRP complex enhances the level of mRNA synthesis from the lac operon as much as 50-fold. CRP alone cannot activate transcription.

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Updated: July 15, 1999
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