If CAP binds - able to bind to trp operon and inhibit...

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If CAP binds, the lac operon is weakly activated but can now fully be activated if lactose is present . EXAMPLE FOR CONTROL AT THE LEVEL OF ELONGATION Regulation of elongation: additional mechanism of control for gene expression. therefore called transcriptional attenuation . Used only for limited number of genes, best described for E. coli trp operon . Trp stands for tryptophan: operon contains 5 structural genes important in the synthesis of the amino acid tryptophan. Tryptophan is only synthesized in E. coli , if it is not available in the food source. Regulation of the trp operon at the level of initiation of transcription Initiation of transcription blocked when abundant tryptophan is available: High level of tryptophan ---- tryptophan can bind to repressor ---- repressor is now
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Unformatted text preview: able to bind to trp operon and inhibit initiation of transcription (Fig. 6.11). But transcriptional initiation never totally blocked. Transcriptional attenuation increases stringency of inhibition . Mechanism of transcriptional attenuation : (Fig. 6.12) animation later Requires a specific regulatory sequence called attenuator . It lies somewhere downstream of the transcription start site in the transcribed and translated region of the operon . The attenuator contains a premature termination signal (a GC-rich inverted repeat followed by adenines, sequences 3 and 4 in Figure). If transcription and translation proceed normally (under high tryptophan levels) the mRNA is terminated prematurely and no complete protein can be synthesized....
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