Trp repressor

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", "AroL"and "mtr" operons.

Mechanism

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 structure of the ligand-bound holorepressor, and the ligand-free forms have been determined by both X-ray crystallography and NMR. [cite journal |author=Schevitz RW, Otwinowski Z, Joachimiak A, Lawson CL, Sigler PB |title=The three-dimensional structure of trp repressor |journal=Nature |volume=317 |issue=6040 |pages=782–6 |year=1985 |pmid=3903514 |doi=10.1038/317782a0] [cite journal |author=Otwinowski Z, Schevitz RW, Zhang RG, "et al" |title=Crystal structure of trp repressor/operator complex at atomic resolution |journal=Nature |volume=335 |issue=6188 |pages=321–9 |year=1988 |pmid=3419502 |doi=10.1038/335321a0] [cite journal |author=Lawson CL, Carey J |title=Tandem binding in crystals of a trp repressor/operator half-site complex |journal=Nature |volume=366 |issue=6451 |pages=178–82 |year=1993 |pmid=8232559 |doi=10.1038/366178a0] [cite journal |author=Zhao D, Arrowsmith CH, Jia X, Jardetzky O |title=Refined solution structures of the Escherichia coli trp holo- and aporepressor |journal=J. Mol. Biol. |volume=229 |issue=3 |pages=735–46 |year=1993 |pmid=8433368 |doi=10.1006/jmbi.1993.1076] [cite journal |author=Zhang H, Zhao D, Revington M, "et al" |title=The solution structures of the trp repressor-operator DNA complex |journal=J. Mol. Biol. |volume=238 |issue=4 |pages=592–614 |year=1994 |pmid=8176748 |doi=10.1006/jmbi.1994.1317]

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.

ee also

*lac operon
*Protein biosynthesis
*Transcription factor
*Trp operon

References

External links

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