Tryptophan is an amino acid necessary for e. coli survival and growth. e. coli contain genes coding for enzymes that synthesize tryptophan. these genes are grouped together on a segment of dna called the tryptophan (trp) operon. cells can use these enzymes to synthesize tryptophan when it is not present in the environment. however, when tryptophan is already present in the environment, cellular resources are shifted away from manufacturing the enzymes for tryptophan synthesis. tryptophan binds with and activates the trp repressor, which then binds to the trp promoter and blocks rna polymerase. blocking rna polymerase decreases the normal transcription rate of the operon. what type of regulation does the trp operon exhibit?