The control of gene expression is critical to all living organisms. the amino acid tryptophan is important for making many proteins. when there is plenty of tryptophan in a cell, a protein binds to the gene that codes for an enzyme that will make tryptophan. when there is no tryptophan present this protein falls off the dna allowing the gene to be expressed so more tryptophan can be made. a protein that binds to a site on dna next to a gene and blocks the transcription of that gene, thus preventing the synthesis of a protein that the gene prescribes is known as a: