When tryptophan is absent, the tryptophan repressor protein is in an inactive state, according to Dr. Michael Gregory of Clinton Community College. When the amino acid tryptophan becomes plentiful in the cell, it binds to and activates the tryptophan repressor, which blocks the synthesis of additional tryptophan.
All proteins are produced based on an mRNA blueprint that is provided by cellular DNA. The gene code in the DNA for tryptophan synthesis is always being transcribed for continuous production of tryptophan. When the tryptophan repressor is activated due to high tryptophan levels, the repressor binds to and blocks the segment of DNA containing the tryptophan gene code. This ceases the production of tryptophan until levels subside, at which time the tryptophan repressor returns to an inactive state and tryptophan synthesis continues.