A

tryptophan corepressor

(b) tryptophan present; operon repressed

Figure 6.12. Transcription of the trp operon is controlled by the concentration of the amino acid tryptophan. (a) No tryptophan, operon transcribed. (b) Tryptophan present, operon repressed.

way the cell carefully controls the concentration of free amino acids. The tryptophan (trp) operon is made up of five structural genes encoding enzymes that synthesize the amino acid tryptophan (Fig. 6.12). This is a repressible operon. The cell regulates the amount of tryptophan produced by preventing transcription of the trp operon mRNA when there is sufficient tryptophan about. As with the lac operon, the transcription of the trp operon is controlled by a regulatory protein. The gene trp R encodes an inactive repressor protein that is called an aporepressor. Tryptophan binds to this to produce an active repressor complex and is therefore called the corepressor. The active repressor complex binds to the operator sequence of the trp operon and prevents the attachment of RNA polymerase to the trp promoter sequence. Therefore, when the concentration of tryptophan in the cell is high, the active repressor complex will form, and transcription of the trp operon is prevented. However, when the amount of tryptophan in the cell decreases, the active repressor complex cannot be formed. RNA polymerase binds to the promoter, transcription of the trp operon proceeds, and the enzymes needed to synthesize tryptophan are produced. This is an example of negative feedback (page 303).

Many other operons are regulated by similar mechanisms in which specific regulatory proteins interact with specific small molecules.

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