The tryptophan operon in E. coli consists of 5 genes (trpE, trpD, trpC, trpB and trpA) that code for a set of proteins that function as a biochemical pathway to synthesize the amino acid tryptophan starting with the precursor chorismic acid. Another protein, TrpR is coded elsewhere and is the Trp Repressor.

Unlike the Lac operon, which is induced in the presence of lactose so that it can be utilized, the Trp operon is a repressible system. This makes sense because the cell only needs to synthesize tryptophan when it is needed, so when there is plenty of tryptophan available the operon need not be expressed. The amino acid (trp) itself is the effector molecule and it works like a classic Negative Repressible system as illustrated here.

A video about negative control of the trp operon by the TrpR repressor:

Prokaryotic Regulation

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