Biosynthesis of the amino acid tryptophan is an energetically expensive, complex metabolic pathway requiring multiple enzymes, which are encoded by the genes of the trp operon. Through the organization of these genes into an operon under the control of a single promoter, transcription of all of these genes can be regulated together in prokaryotes.
Binding of a repressor to the operator sequence, which lies between the promoter and the first trp gene, prevents transcription of the trp genes because the progression of RNA polymerase through the operator region is blocked. However, the trp repressor is only active when bound to tryptophan, the product of the enzymatic pathway encoded by the operon.
Which of the following BEST describes conditions allowing for transcription of the trp genes?