View source for TetR

[[File:TetR-overview (2).png|thumb|TetR as a homodimer: Each monomer is shown in purple or salmon. The helix-turn-helix motif is shown in deep red.]]
'''Tet Repressor proteins''' (otherwise known as '''TetR''') are proteins playing an important role in conferring [[antimicrobial resistance|antibiotic resistance]] to large categories of bacterial species.
[[Tetracycline]] (Tc) is a broad family of antibiotics to which bacteria have evolved resistance. Tc normally kills bacteria by binding to the bacterial [[ribosome]] and halting protein synthesis. The expression of Tc resistance genes is regulated by the [[repressor]] TetR. TetR represses the expression of TetA, a membrane protein that pumps out substances toxic to the bacteria like Tc, by binding the ''tetA'' [[Operator (biology)|operator]].<ref name="Ramos_2005" /> In Tc-resistant bacteria, TetA will pump out Tc before it can bind to the ribosome because the repressive action of TetR on TetA is halted by binding of Tc to TetR.<ref name = "Ramos_2005">{{cite journal | vauthors = Ramos JL, Martínez-Bueno M, Molina-Henares AJ, Terán W, Watanabe K, Zhang X, Gallegos MT, Brennan R, Tobes R | display-authors = 6 | title = The TetR family of transcriptional repressors | journal = Microbiology and Molecular Biology Reviews | volume = 69 | issue = 2 | pages = 326–56 | date = June 2005 | pmid = 15944459 | pmc = 1197418 | doi = 10.1128/mmbr.69.2.326-356.2005 }}</ref> Therefore, TetR may have an important role  in helping scientists to better understand mechanisms of [[Antibiotic resistant|antibiotic resistance]] and how to treat antibiotic resistant bacteria. TetR is one of many proteins in the TetR [[protein family]], which is so named because TetR is the most well characterized member.<ref>{{Cite web|title=InterPro|url=https://www.ebi.ac.uk/interpro/entry/InterPro/IPR003012/|access-date=2020-08-06|website=www.ebi.ac.uk}}</ref>
TetR is used in artificially engineered [[gene regulatory network]]s because of its capacity for fine regulation of [[Promoter (genetics)|promoters]]. In the absence of Tc or analogs like [https://pubchem.ncbi.nlm.nih.gov/compound/Anhydrotetracycline ATc], basal expression of TetR-regulated promoters is low, but expression rises sharply in the presence of even a minute quantity of Tc. The ''tetA'' gene is also present in the widely used ''E. coli'' cloning vector [[pBR322]], where it is often referred to by the name of its tetracycline-resistance phenotype, Tet<sup>R</sup>, not to be confused with TetR.<ref name="Allard1992">{{cite journal | vauthors = Allard JD, Bertrand KP | title = Membrane topology of the pBR322 tetracycline resistance protein. TetA-PhoA gene fusions and implications for the mechanism of TetA membrane insertion | journal = The Journal of Biological Chemistry | volume = 267 | issue = 25 | pages = 17809–19 | date = September 1992 | doi = 10.1016/S0021-9258(19)37116-9 | pmid = 1517220 | doi-access = free }}</ref>
== Structure & Function ==
[[File:TetR --TcMg- complex (2).png|thumb|Tetracycline-magnesium complex (blue) bound to cavity of TetR (green). HTH motif shown in pink -note conformational change.]]
TetR functions as a [[Protein dimer|homodimer]].<ref name="Ramos_2005" /> Each monomer consists of ten [[Alpha helix|alpha helices]] connected by loops and turns. The overall structure of TetR can be broken down into two DNA-binding domains (one per monomer) and a regulatory core, which is responsible for tetracycline recognition and dimerization. TetR dimerizes by making hydrophobic contacts within the regulatory core. There is a binding cavity for tetracycline in the outer helices of the regulatory domain. When tetracycline binds this cavity, it causes a conformational change that affects the DNA-binding domain so that TetR is no longer able to bind DNA. As a result, TetA and TetR are expressed. There is still some debate in the field whether tetracycline derivatives alone can cause this conformational change or whether tetracycline must be in complex with magnesium to bind TetR.<ref>{{cite journal | vauthors = Werten S, Dalm D, Palm GJ, Grimm CC, Hinrichs W | title = Tetracycline repressor allostery does not depend on divalent metal recognition | journal = Biochemistry | volume = 53 | issue = 50 | pages = 7990–8 | date = December 2014 | pmid = 25432019 | doi = 10.1021/bi5012805 }}</ref> (TetR typically binds tetracycline-Mg<sup>2+</sup> complexes inside bacteria, but TetR binding to tetracycline alone has been observed in vitro.){{cn|date=March 2023}}
[[File:TetR-DNAcomplex (2).png|left|thumb|TetR (purple and salmon) in complex with its target DNA sequence. HTH motifs are shown in red binding to the major grooves of the DNA. PDB: 1QPI]]
The DNA-binding domains of TetR recognize a 15 base pair [[palindromic sequence]]  of the Tet<nowiki/>A operator.<ref name="Ramos_2005" /><ref>{{cite journal | vauthors = Orth P, Schnappinger D, Hillen W, Saenger W, Hinrichs W | title = Structural basis of gene regulation by the tetracycline inducible Tet repressor-operator system | journal = Nature Structural Biology | volume = 7 | issue = 3 | pages = 215–9 | date = March 2000 | pmid = 10700280 | doi = 10.1038/73324 | s2cid = 19973826 }}</ref> These domains mainly consist of a helix-turn-helix (HTH) motif that is common in TetR protein family members (see below). However, the N-terminal residues preceding this motif have also been shown to be important for DNA binding.<ref>{{cite journal | vauthors = Berens C, Altschmied L, Hillen W | title = The role of the N terminus in Tet repressor for tet operator binding determined by a mutational analysis | journal = The Journal of Biological Chemistry | volume = 267 | issue = 3 | pages = 1945–52 | date = January 1992 | doi = 10.1016/S0021-9258(18)46038-3 | pmid = 1309804 | url = http://www.jbc.org/content/267/3/1945 | doi-access = free }}</ref> Although these residues do not directly contact the DNA, they pack against the HTH and this packing is essential for binding. The HTH motifs have mostly hydrophobic interactions with [[major groove]]s of the target DNA.<ref name="Ramos_2005" /> Binding of TetR to its target DNA sequence causes changes in both the DNA and TetR.<ref name="Huffman_2002">{{cite journal | vauthors = Huffman JL, Brennan RG | title = Prokaryotic transcription regulators: more than just the helix-turn-helix motif | journal = Current Opinion in Structural Biology | volume = 12 | issue = 1 | pages = 98–106 | date = February 2002 | pmid = 11839496 | doi = 10.1016/S0959-440X(02)00295-6 }}</ref> TetR causes widening of the major grooves as well as kinking of the DNA; one helix of the HTH motif of TetR adopts a [[310 helix|3<sub>10</sub>]] helical turn as the result of complex DNA interactions.{{cn|date=March 2023}}
== TetR Protein Family ==
[[File:HTH alignment of 3 TetR family members.png|left|thumb|HTH motif alignment of three TetR family members: MtrR (magenta), SimR (cyan), & AmtR (green)]]
As of June 2005, this family of proteins had about 2,353 members that are transcriptional regulators.<ref name = "Ramos_2005" /> (Transcriptional regulators control gene expression.) These proteins contain a helix-turn-helix (HTH) motif that is the DNA-binding domain. The second helix is considered to be most important for DNA sequence specificity and often recognizes nucleic acids within the major groove of the double helix.<ref name="Huffman_2002" /> In the majority of the family members, this motif is on the N-terminal end of the protein and is highly conserved.<ref name="Ramos_2005" /> The high conservation of the HTH motif is not observed for the other domains of the protein. The differences observed in these other regulatory domains are likely due to differences in the molecules that each family member senses.{{cn|date=March 2023}}
TetR protein family members are mostly transcriptional repressors, meaning that they prevent the expression of certain genes at the DNA level. These proteins can act on genes with various functions including antibiotic resistance, biosynthesis and metabolism, bacterial pathogenesis, and response to cell stress.{{cn|date=March 2023}}
== See also ==
* [[Tetracycline controlled transcriptional activation]]
{{-}}
== References ==
{{Reflist}}
== External links ==
*[https://archive.today/20121210214418/https://www.bio.cmu.edu/courses/03441/TermPapers/2000TermPapers/group2/resi-reg.html Regulation of Antibiotic Resistance]
[[Category:Tetracycline antibiotics]]