The tryptophan biosynthetic pathway

The trp repressor proteins regulate transcription of several different operons located at separate sites on the E. coli chromosome. In addition to the trp operon, this repressor represses transcription of aroH, a single-gene operon that encodes an enzyme required for the synthesis of all aromatic amino acids, and trpR, another single-gene operon encoding the trp repressor itself. Such coordinately regulated operons constitute a regulon. Although all three operons in this regulon are repressed by the trp repressor, the extent of repression varies from about twofold for the aroH operon to seventyfold for the trp operon. This variation results from differences in the affinity of trp repressor for the specific operator in each of these operons, differences in the position of the operators relative to the −10 and −35 sequences of each promoter, and differences in the strengths of the three promoters. Thus the specific nucleotide sequences of the promoters and operators in a regulon allow the same repressor to differentially regulate transcription of the component operons.