It has been best studied in Escherichia coli, where it is a dimeric protein that regulates transcription of the 5 genes in the tryptophan operon.

When this occurs, transcription of the DNA is prevented, suppressing the products of the gene - proteins which make more tryptophan.

[5] The structure of the ligand-bound holorepressor, and the ligand-free forms have been determined by both X-ray crystallography and NMR.

[6][7][8][9][10] The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator.

If the RNA polymerase reaches the terminator (at the end of the DNA strand), the enzymes for tryptophan biosynthesis are expressed.