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Home >>Regulation of Gene Expression - in Prokaryotes >> Control of the Operon Attenuation
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Control of the Operon Attenuation - In addition to the standard, negative control scheme (lac operon) the trp operon employs another mechanism of control called "attenuation". Repression of the trp operon is considerably weak. Thus considerable transcription of the trp operon can occur even in the presence of repressor.

In fact, in attenuator mutants where only repression can operate, the fully repressed level of transcription is 70-fold lower than the fully depressed level. The attenuation system permits another 10 fold control over the operon's activity. This is valuable because synthesis of tryptophan requires considerable energy

In the trp operon, an attenuator region lies between the operator and the first structural gene. The messenger RNA transcribed from the attenuator region termed the leader transcribe has four (1, 2, 3 and 4) sub regions of the messenger .

RNA and has base sequences that are complementary to each other so that three different stem loop structures can form in the messenger RNA. Depending on circumstances, regions 1-2 and 3-4 can form two stem-loop structures or region 2-3 can form a single stem loop. When one stem loop structure is formed, the others are preempted.

When excess of tryptophan is present in the medium, the transcription of the gene continues past the leader peptide. Three different outcomes can take place. The moving ribosome overlaps regions 1 and 2 of the transcript and allows the stem-loop for 3-4 to form.

This stem-loop structure causes transcription to terminate. Hence this stem-loop (3-4) structure is called as the terminator or attenuator stem. When there is no tryptophan, the trp operon must be activated. Accordingly, there must be a means of overriding attenuation.

In this case, the stem loop form will also form, but instead of forming two stem loops (1-2 and 3-4) only a single stem loop (2-3) is formed. In this configuration, transcription is not terminated so that eventually, the whole operon is transcribed and translated raising the level of tryptophan in the cell. The stem-loop 2-3 structure is referred to as the preemptor stem.

 
 

 
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