Genes can be switched on and off with repressor proteins. If the concentration of tryptophan inside the cell is low, RNA polymerase (blue) binds to the promoter and transcribes the five genes of the tryptophan operon (left). If the concentration of the tryptophan is high, however, the repressor protein (dark green) becomes active and binds to the operator (light green), where it blocks the binding of RNA polymerase to the promoter (right). Whenever the concentration of intracellular tryptophan drops, the repressor releases its tryptophan and is released from the DNA, allowing the polymerase to again transcribe the operon. The promoter is marked by two key blocks of DNA sequence information, the –35 and –10 regions, highlighted in yellow.  Fair Use and Copyright info