Introduction Text

Which parts are best described and tested

[http://partsregistry.org/wiki/index.php?title=Part:BBa_K364332 TRE-Gal4-PXR-PolyA in PSB1A3 (expression vector)]

Expression vector from standard BioBrick parts, expressing Gal4 - human Pregnane X receptor LBD composite part.

The Gal4-PXR Chimeric nuclear receptor is an artificial eukaryotic TF made of Gal4 DBD (DNA Binding Domain) and H. sapiens nuclear hormone receptor LBD (Ligand Binding Domain) which can be expressed by this expression vector.

The minimal CMV promoter ensures a continuous expression of the chimeric construct, but furthermore it can be expressed in an inducible form using the Tetracyclin-controlled transcriptional activation system. [http://partsregistry.org/wiki/index.php?title=Part:BBa_K364332 Go to the official parts registry page for this part]

PXR LBD

PXR is a xenobiotic-activated member of the nuclear receptor superfamily of transcription factors. PXR is involved in transcriptional induction of hepatic xenobiotic-catabolizing cytochrome 3A enzymes, playing a fundamental role in protecting body tissues from toxic bile acids. PXR is almost exlcusively expressed in the gastrointestinal system (stomach, duodenum, jejunum, ileum, colon and gall bladder) and liver, with lower levels in the kidney and ovary. PXR dysfunction is associated with immune disorders (sclerosing cholangitis, inflammatory bowel disease) and biliary primary cirrhosis.

Gal4 DBD

This protein is a positive regulator for the gene expression of the galactose-induced genes such as GAL1, GAL2, GAL7, GAL10, and MEL1 which encode for the enzymes used to convert galactose to glucose. This protein contains a fungal Zn(2)-Cys(6) binuclear cluster domain.

TRE-CMV

The Tetracycline Response Element (TRE) is recognized and bound by the Tetracycline repressor (TetR) protein. The TRE consists of 7 repeats separated by spacer sequences and placed upstream of CMV minimal promoter that has basal expession in the abscence of bound TetR. Tetracycline derivatives (e.g. doxycycline) bind TetR and render it incapable of binding to TRE, thereby forcing the expression of target genes.

PolyA

The PolyA tail of mRNA has multiple adenilates which is important for the nuclear export, translation and stability of mRNA in eukaryotes.

This composite artificial transcription factor will activate any reporter or any gene in general that has a UAS (Upper Activating Sequence) 3' of it's promoter. The usual binding sites of reporters, contain multiple UAS elements. In order to have a POPS output, the LBD has to recruit activators in the cell. This can be initiated by ligand binding or by recruiting a protein that has a fused strong activator like the VP activator.

With this system NHR (Nuclear Hormone Receptor) ligands or NHR interacting partners can be screened.

The NHR: cofactor-VP interaction should be also broken by a potential ligand binding, this is why this setup is also suitable for ligand identification. The benefit of the cofactor-VP interaction test is that the dynamic range of the assay is much higher than the dynamic range of the normal Gal4-NHR ligand activation assay.

Picture of gel electrophoresis: TRE-Gal4-PXR-PolyA in pSB1A3 results a new expression vector from standard Biobrick parts. The TRE-Gal4-PXR LBD-PolyA insert is 1682 bp long, without polyA which was inserted in the last step is 191 bp shorter.

Design Notes

Compatible with RFC-10 and RFC-25.

Source

Artificial and human PXR LBD.

Ligand

PXR is activated by a large number of endogenous and exogenous chemicals including steroids, antibiotics, antimycotics, bile acids, hyperforin, a constituent of the herbal antidepressant St. John's Wort, and many other herbal compounds.

PXR is activated by a range of compounds that induce CYP3A4, including dexamethasone and rifampicin.

Applications

Picture of Western blot: Gal4-PXR composite construct is highly expressed in the expression vector created from pSB1A3

References

Guhaniyogi J, Brewer G. Regulation of mRNA stability in mammalian cells. Gene. 2001 Mar 7;265(1-2):11-23.

D R Gallie. The cap and poly(A) tail function synergistically to regulate mRNA translational efficiency. Genes Dev. 1991 5: 2108-2116

1.Lehmann JM, McKee DD, Watson MA, Willson TM, Moore JT, Kliewer SA (September 1998). "The human orphan nuclear receptor PXR is activated by compounds that regulate CYP3A4 gene expression and cause drug interactions". J. Clin. Invest. 102 (5): 1016–23. doi:10.1172/JCI3703.

Ana Traven, Branka Jelicic & Mary Sopta. "Yeast Gal4: a transcriptional paradigm revisited". EMBO reports (2006) 7, 496 - 499.

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