The trp Operon of E coli

The lac operon just discussed is an inducible operon, one in which transcription does not normally take place and must be turned on. Other operons are repressible; transcription in these operons is normally turned on and must be repressed. The tryptophan (trp) operon in E. coli, which controls the biosynthesis of the amino acid trypto-phan, is an example of a repressible operon.

The trp operon contains five structural genes (trpE, trpD, trpC, trpB, and trpA), which produce components of three enzymes (two of the enzymes consist of two polypeptide chains). These enzymes convert chorismate into tryptophan (<Figure 16.14). The first structural

cAMP-CAP complex

^-y-J Transcription

Promoter start site

^-y-J Transcription

Promoter start site

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