GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

20. METABOLIC REGULATION

20.3. MECHANISMS OF ENZYME SYNTHESIS (INDUCTION AND REPRESSION)

20.3.2. End-product repression of the tryptophan operon

The Mechanism of end-product repression is illustrated in Fig. 20.7.

The Tryptophan Operon in E. coli contains structural genes for the synthesis of five Enzymes involved in The conversion of chorismate to tryptophan. An operator and a promoter are located at the beginning of the operon. The repressible operon model was proposed in 1961 by the French scientists F. Jacob and J. Monod. A regulator Gene, located at a distance from the operon, encodes an effector protein known as the aporepressor. In the presence of the end-product tryptophan (the corepressor), which has a high affinity for the aporepressor, an active repressor is formed. The repressor binds to the operator, thereby blocking METABOLISM/31.html">Transcription. A decrease in tryptophan concentration leads to the release of the operator and the resumption of mRNA synthesis. In the absence of the end-product, enzyme synthesis proceeds. An effective repressor that blocks the operator (the "activated" repressor) is formed only when the end-product, or corepressor, binds to the aporepressor.

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Fig. 20.7. Model of biosynthetic enzyme regulation via end-product repression