Binding of the cAMPCAP complex to DNA produces a sharp bend in DNA that activates transcription

gene, trpE, contains a long 5' untranslated region (5' UTR) that is transcribed but does not encode any of these enzymes. Instead, this 5' UTR plays an important role in another regulatory mechanism, discussed in the next section. Upstream of the structural genes is the trp promoter. When tryptophan levels are low, RNA polymerase binds to the promoter and transcribes the five structural genes into a single mRNA, which is then translated into enzymes that convert chorismate into tryptophan.

Some distance from the trp operon is a regulator gene, trpR, which encodes a repressor that alone cannot bind DNA (see Figure 16.14). Like the lac repressor, the tryptophan repressor has two binding sites, one that binds to DNA at the operator site and another that binds to tryptophan (the activator). Binding with tryptophan causes a conformational change in the repressor that makes it capable of binding to DNA at the operator site, which overlaps the promoter (see Figure 16.14). When the operator is occupied by the tryptophan repressor, RNA polymerase cannot bind to the promoter and the structural genes cannot be transcribed. Thus, when cellular levels of tryptophan are low, transcription of the trp operon takes place and more tryptophan is synthesized; when cellular levels of tryptophan are high, transcription of the trp operon is inhibited and the synthesis of more tryptophan does not take place.

When tryptophan is low

Regulator gene (trpR)

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