In the lack of tryptophan shown in the figure, a trp repressor protein encoded by a split operon, trpR that is synthesized and forms a dimer.  Furthermore, this is inactive and so is unable to bind to the trp operator and the structural genes of the trp operon are transcribed.  When tryptophan is there in the figure, the enzymes for tryptophan  biosynthesis  are  not  needed  and  so  expression  of  these  genes  is turned off. This is achieved by tryptophan binding to the repressor to activate it so which it now stops transcription and binds to the operator of the structural genes. In the role tryptophan is said to be a co-repressor.

Figure:  Regulation of the trp operon (a) transcription in the absence of tryptophan (b) no transcription in the presence of tryptophan.