Tryptophan operon: structural gene mutation creating a "promoter" and leading to 5-methyltryptophan dependence.

A strain carrying the mutation trpA515, which maps in the "unusual" region (between the genes trpA and trpB) of the tryptophan operon of Salmonella typhimurium is capable of utilizing anthranilic acid as a growth factor only in the presence of the analog 5-methyltryptophan, normally a potent growth inhibitor. The reason for this peculiar phenotype is the creation by trpA515 of a transcription-initiating signal in the "unusual" region.