<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> <P> It is an example of repressible negative regulation of gene expression . Within the operon's regulatory sequence, the operator is blocked by the repressor protein in the presence of tryptophan (thereby preventing transcription) and is liberated in tryptophan's absence (thereby allowing transcription). The process of attenuation (explained below) complements this regulatory action . </P> <P> The operon operates by a negative repressible feedback mechanism . The repressor for the trp operon is produced upstream by the trpR gene, which is constitutively expressed at a low level . Synthesized trpR monomers associate into dimers . When tryptophan is present, these tryptophan repressor dimers bind to tryptophan, causing a change in the repressor conformation, allowing the repressor to bind to the operator . This prevents RNA polymerase from binding to and transcribing the operon, so tryptophan is not produced from its precursor . When tryptophan is not present, the repressor is in its inactive conformation and cannot bind the operator region, so transcription is not inhibited by the repressor . </P>

What is the function of the hairpin loop in the mrna transcript produced by the trp operon