<P> The trp operon is an operon--a group of genes that is used, or transcribed, together--that codes for the components for production of tryptophan . The trp operon is present in many bacteria, but was first characterized in Escherichia coli . The operon is regulated so that when tryptophan is present in the environment, the genes for tryptophan synthesis are not expressed . It was an important experimental system for learning about gene regulation, and is commonly used to teach gene regulation . </P> <P> Discovered in 1953 by Jacques Monod and colleagues, the trp operon in E. coli was the first repressible operon to be discovered . While the lac operon can be activated by a chemical (allolactose), the tryptophan (Trp) operon is inhibited by a chemical (tryptophan). This operon contains five structural genes: trp E, trp D, trp C, trp B, and trp A, which encode tryptophan synthetase . It also contains a repressive regulator gene called trp R. trp R has a promoter where RNA polymerase binds and synthesizes mRNA for a regulatory protein . The protein that is synthesized by trp R then binds to the operator which then causes the transcription to be blocked . In the lac operon, allolactose binds to the repressor protein, allowing gene transcription, while in the trp operon, tryptophan binds to the repressor protein effectively blocking gene transcription . In both situations, repression is that of RNA polymerase transcribing the genes in the operon . Also unlike the lac operon, the trp operon contains a leader peptide and an attenuator sequence which allows for graded regulation . </P>

The structural genes of the trp operon are not expressed when
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