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{{short description|Transcription factor}}
{{Infobox protein family
| Symbol = Trp_repressor
| Name = Trp repressor protein
| image = TrpR.jpg
| width =
| caption = [[Ribbon diagram]] of the trpR protein
| Pfam = PF01371
| Pfam_clan = CL0123
| InterPro = IPR000831
| SMART =
| PROSITE =
| MEROPS =
| SCOP = 2wrp
| TCDB =
| OPM family =
| OPM protein =
| CAZy =
| CDD =
}}
{{Infobox nonhuman protein
| Name = Trp operon repressor
| image =
| width =
| caption =
| Organism = ''Escherichia coli''
| TaxID = 511145
| Symbol = trpR
| AltSymbols =
| EntrezGene = 948917
| PDB =
| RefSeqmRNA =
| RefSeqProtein = NP_418810
| UniProt = P0A881
| ECnumber =
| Chromosome = genome
| EntrezChromosome = NC_000913
| GenLoc_start = 4630733
| GenLoc_end = 4631157
}}

'''Tryptophan repressor''' (or '''trp repressor''') is a [[transcription factor]] involved in controlling [[amino acid]] metabolism. It has been best studied in ''[[Escherichia coli]]'', where it is a dimeric protein that regulates transcription of the 5 genes in the tryptophan [[operon]].<ref>{{cite journal |vauthors=Santillan M, Mackey MC |title=Dynamic regulation of the tryptophan operon: A modeling study and comparison with experimental data |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=98 |issue=4 |pages=1364–9 |year=2001 |pmid=11171956 |doi=10.1073/pnas.98.4.1364 |pmc=29262|bibcode=2001PNAS...98.1364S |doi-access=free }}</ref> When the amino acid [[tryptophan]] is plentiful in the cell, it binds to the protein, which causes a conformational change in the protein.<ref>{{cite journal |author=Zhang RG, Joachimiak A, Lawson CL, Schevitz RW, Otwinowski Z, [[Paul Sigler|Sigler PB]] |title=The crystal structure of trp aporepressor at 1.8 A shows how binding tryptophan enhances DNA affinity |journal=Nature |volume=327 |issue=6123 |pages=591–7 |year=1987 |pmid=3600756 |doi=10.1038/327591a0|bibcode=1987Natur.327..591Z |s2cid=4335349 }}</ref> The repressor complex then binds to its operator sequence in the genes it regulates, shutting off the genes.<ref>{{cite journal |vauthors=Jeeves M, Evans PD, Parslow RA, Jaseja M, Hyde EI |title=Studies of the Escherichia coli Trp repressor binding to its five operators and to variant operator sequences |journal=Eur. J. Biochem. |volume=265 |issue=3 |pages=919–28 |year=1999 |pmid=10518785 |doi=10.1046/j.1432-1327.1999.00792.x|doi-access=free }}</ref><ref>{{cite journal |vauthors=Arvidson DN, Arvidson CG, Lawson CL, Miner J, Adams C, Youderian P |title=The tryptophan repressor sequence is highly conserved among the Enterobacteriaceae |journal=Nucleic Acids Res. |volume=22 |issue=10 |pages=1821–9 |year=1994 |pmid=8208606 |doi=10.1093/nar/22.10.1821 |pmc=308080}}</ref>

One of the genes regulated by trp repressor, [[trpR]], codes for the tryptophan repressor protein itself. This is a form of [[negative feedback|feedback regulation]]. However, these genes are located on different operons.

The (tryptophan) repressor is a 25 kD [[protein]] homodimer which regulates [[transcription (genetics)|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.<ref name="pmid7048301">{{cite journal |vauthors=Kelley RL, Yanofsky C | title = Trp aporepressor production is controlled by autogenous regulation and inefficient translation | journal = Proc. Natl. Acad. Sci. U.S.A. | volume = 79 | issue = 10 | pages = 3120–4 |date=May 1982 | pmid = 7048301 | pmc = 346365 | doi = 10.1073/pnas.79.10.3120| bibcode = 1982PNAS...79.3120K | doi-access = free }}</ref>

The structure of the [[ligand]]-bound holorepressor, and the ligand-free forms have been determined by both [[X-ray crystallography]] and [[Protein NMR|NMR]].<ref>{{cite journal |author=Schevitz RW, Otwinowski Z, Joachimiak A, Lawson CL, [[Paul Sigler|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|bibcode=1985Natur.317..782S |s2cid=4340128 }}</ref><ref>{{cite journal |author=Otwinowski Z |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 |name-list-style=vanc|author2=Schevitz RW |author3=Zhang RG |display-authors=3 |last4=Lawson |first4=C. L. |last5=Joachimiak |first5=A. |last6=Marmorstein |first6=R. Q. |last7=Luisi |first7=B. F. |author8-link=Paul Sigler |last8=Sigler |first8=P. B.|bibcode=1988Natur.335..321O |s2cid=9358980 }}</ref><ref>{{cite journal |vauthors=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|bibcode=1993Natur.366..178L |s2cid=4309487 }}</ref><ref>{{cite journal |vauthors=Zhao D, Arrowsmith CH, Jia X, Jardetzky O|authorlink2=Cheryl Arrowsmith|authorlink4=Oleg Jardetzky |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|doi-access=free }}</ref><ref>{{cite journal |author=Zhang H |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 |name-list-style=vanc|author2=Zhao D |author3=Revington M |display-authors=3 |last4=Lee |first4=W |last5=Jia |first5=X |last6=Arrowsmith |first6=C |last7=Jardetzky |first7=O}}</ref>

The trp operon consists of a regulatory gene, a promoter, an operator, and a terminator. The trp operon is active only when cellular tryptophan is scarce. 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 enzymes for tryptophan biosynthesis are expressed.

==See also==
* [[trp operon]]
* [[Paul Sigler]]

==References==
{{reflist|2}}

{{Transcription}}

[[Category:Proteins]]

Tryptophan repressor - Revision history

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