G4 The trp operon

The trp operon contains five structural genes encoding enzymes for tryptophan biosynthesis, a trp promoter (Ptrp) and a trp operator sequence (Otrp). The operon is transcribed only when tryptophan is scarce. 

When tryptophan is lacking, a trp repressor protein (encoded by the trpR operon) is synthesized. The trp repressor dimer is inactive, cannot bind to the trp operator and so the trp operon is transcribed to produce the enzymes that then synthesize tryptophan for the cell. When tryptophan is present, tryptophan synthesis is not needed. In this situation, acting as a corepressor, tryptophan binds to the repressor and activates it so that the repressor now binds to the trp operator and stops transcription of the trp operon. 

The trp operon is regulated by both repression (which determines whether transcription will occur or not) and attenuation (which fine tunes transcription). Other operons for amino acid biosynthetic pathways may also be regulated by both repression and attenuation or only by attenuation. 

Instant Notes in Biochemistry 2nd Edition, B.D. Hames N.M. Hooper, (c) 2000 BIOS Scientific Publishers Ltd, Oxford. 

Organization of The tryptophan (trp) operon (Fig. 1) contains five structural genes encoding 

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