The trp (tryptophan) repressor is a 25 kD protein which regulates transcription of the tryptophan biosynthetic pathway in bacteria. There are 5 operons which are regulated by trpR: the trpEDCBA, trpR, AroH, AroLand mtr operons.
When the amino acid tryptophan is in plentiful supply in the cell, trpR binds 2 molecules of tryptophan, which alters its structure and dynamics so that it becomes able to bind to operator DNA. When this occurs, transcription of the DNA is prevented, suppressing the products of the gene - proteins which make more tryptophan. When the cellular levels of tryptophan decline, the tryptophan molecules on the repressor fall off, allowing the repressor to return to its inactive form.
trpR also controls the regulation of its own production, through regulation of the trpR gene (Kelley & Yanovsky, 1982 PNAS USA 79 3120-3124).
The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator. The trp operon only works in the presence of tryptophan. If there isn't enough tryptophan, the repressor protein breaks off from the operator (where the repressor is normally bound) and RNA polymerase can complete its reading of the strand of DNA. If the RNA polymerase reaches the terminator (at the end of the DNA strand), the enzyme for tryptophan is made.
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