Trp repressor protein is capable of intruding into other amino acid biosynthetic systems.

Escherichia coli strains with elevated intracellular levels of Trp repressor protein displayed complete growth inhibition on minimal media which contained high levels of tryptophan. The inhibition was attributable to the acquisition of a compound nutritional requirement, which could be satisfied by a combination of isoleucine, leucine, valine, threonine, serine, phenylalanine, and tyrosine. It is proposed that Trp repressor protein, at elevated levels, represses the transcription of those genes which encode enzymes for the biosynthesis of these particular amino acids. Data which support this model are presented, together with a discussion of its regulatory implications.