Tryptophan repressor bound to DNA. Molecular model of the tryptophan (trp) repressor (grey and green, and orange and yellow, across bottom) bound to DNA (deoxyribonucleic) molecules (blue and orange, and green and purple, across top). The trp repressor inhibits the expression of genes that code for the amino acid tryptophan, the trp operon, in Escherichia coli bacteria. When there is a plentiful supply of tryptophan in the environment the protein binds to the trp operon inhibiting gene expression. When levels of tryptophan in the cell fall, the repressor drops off and the bacterium begins to produce its own tryptophan.