Mol Cell. 2004 Mar 26;13(6):887-93.
Dellino GI, Schwartz YB, Farkas G, McCabe D, Elgin SC, Pirrotta V.
Department of Zoology, University of Geneva, 30 quai Ernest Ansermet, CH-1211 Geneva, Switzerland.
Polycomb (PcG) complexes maintain the silent state of target genes. The mechanism of silencing is not known but has been inferred to involve chromatin packaging to block the access of transcription factors. We have studied the effect of PcG silencing on the hsp26 heat shock promoter. While silencing does decrease the accessibility of some restriction enzyme sites to some extent, it does not prevent the binding of TBP, RNA polymerase, or the heat shock factor to the hsp26 promoter, as shown by chromatin immunoprecipitation. However, we find that in the repressed state, the RNA polymerase cannot initiate transcription. We conclude that, rather than altering chromatin structure to block accessibility, PcG silencing in this construct targets directly the activity of the transcriptional machinery at the promoter.
Nucleic Acids Res. 2003 May 15;31(10):2483-94.
Lu Q, Teare JM, Granok H, Swede MJ, Xu J, Elgin SC.
Department of Biology, Washington University, St Louis, MO 63130, USA.
Previous studies of the Drosophila melanogaster hsp26 gene promoter have demonstrated the importance of a homopurine*homopyrimidine segment [primarily (CT)n*(GA)n] for chromatin structure formation and gene activation. (CT)n regions are known to bind GAGA factor, a dominant enhancer of PEV thought to play a role in generating an accessible chromatin structure. The (CT)n region can also form an H-DNA structure in vitro under acidic pH and negative supercoiling; a detailed map of that structure is reported here. To test whether the (CT)n sequence can function through H-DNA in vivo, we have analyzed a series of hsp26-lacZ transgenes with altered sequences in this region. The results indicate that a 25 bp mirror repeat within the homopurine*homopyrimidine region, while adequate for H-DNA formation, is neither necessary nor sufficient for positive regulation of hsp26 when GAGA factor-binding sites have been eliminated. The ability to form H-DNA cannot substitute for GAGA factor binding to the (CT)n sequence.
Mol Cell Biol. 2002 Sep;22(17):6148-57.
Leibovitch BA, Lu Q, Benjamin LR, Liu Y, Gilmour DS, Elgin SC.
Department of Biology, Washington University, St. Louis, Missouri 63130, USA.
The upstream regulatory region of the Drosophila melanogaster hsp26 gene includes two DNase I-hypersensitive sites (DH sites) that encompass the critical heat shock elements. This chromatin structure is required for heat shock-inducible expression and depends on two (CT)n*(GA)n elements bound by GAGA factor. To determine whether GAGA factor alone is sufficient to drive formation of the DH sites, we have created flies with an hsp26/lacZ transgene wherein the entire DNA segment known to interact with the TFIID complex has been replaced by a random sequence. The replacement results in a loss of heat shock-inducible hsp26 expression and drastically diminishes nuclease accessibility in the chromatin of the regulatory region. Chromatin immunoprecipitation experiments show that the decrease in TFIID binding does not reduce GAGA factor binding. In contrast, the loss of GAGA factor binding resulting from (CT)n mutations decreases TFIID binding. These data suggest that both GAGA factor and TFIID are necessary for formation of the appropriate chromatin structure at the hsp26 promoter and predict a regulatory mechanism in which GAGA factor binding precedes and contributes to the recruitment of TFIID.
Nucleic Acids Res. 1997 Aug 15;25(16):3345-53.
Benyajati C, Mueller L, Xu N, Pappano M, Gao J, Mosammaparast M, Conklin D, Granok H, Craig C, Elgin S.
Department of Biology, University of Rochester, Rochester, NY 14627, USA.
The GAGA transcription factor of Drosophila melanogaster is ubiquitous and plays multiple roles. Characterization of cDNA clones and detection by domain- specific antibodies has revealed that the 70-90 kDa major GAGA species are encoded by two open reading frames producing GAGA factor proteins of 519 amino acids (GAGA-519) and 581 amino acids (GAGA-581), which share a common N-terminal region that is linked to two different glutamine-rich C-termini. Purified recombinant GAGA-519 and GAGA-581 proteins can form homomeric complexes that bind specifically to a single GAGA sequence in vitro. The two GAGA isoforms also function similarly in transient transactivation assays in tissue culture cells and in chromatin remodeling experiments in vitro. Only GAGA-519 protein accumulates during the first 6 h of embryogenesis. Thereafter, both GAGA proteins are present in nearly equal amounts throughout development; in larval salivary gland nuclei they colocalize completely to specific regions along the euchromatic arms of the polytene chromosomes. Coimmunoprecipitation of GAGA-519 and GAGA-581 from crude nuclear extracts and from mixtures of purified recombinant proteins, indicates direct interactions. We suggest that homomeric complexes of GAGA-519 may function during early embryogenesis; both homomeric and heteromeric complexes of GAGA-519 and GAGA-581 may function later.
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GAGA factor isoforms. Two classes of cDNAs have been recovered for the GAGA factor to date (Soeller et al, 1993; Benyajati et al, paper above). The predicted proteins both contain a single C2-H2 type Zn-finger. The amino-terminus - the BTB or POZ domain - shares homology to a large number of eukaryotic and viral proteins and may serve as a protein-protein interaction domain (Zollman et al, 1994; Bardwell and Treisman, 1994). These GAGA factor isoforms diverge at amino acid 378; both proteins contain a glutamine-rich carboxy terminus. |
Mol Cell Biol. 1993 May;13(5):2802-14.
Lu Q, Wallrath LL, Granok H, Elgin SC.
Department of Biology, Washington University, St. Louis, Missouri 63130.
Previous analysis of the hsp26 gene of Drosophila melanogaster has shown that in addition to the TATA box and the proximal and distal heat shock elements (HSEs) (centered at -59 and -340, relative to the start site of transcription), a segment of (CT)n repeats at -135 to -85 is required for full heat shock inducibility (R.L. Glaser, G.H. Thomas, E.S. Siegfried, S.C.R. Elgin, and J.T. Lis, J. Mol. Biol. 211:751-761, 1990). This (CT)n element appears to contribute to formation of the wild-type chromatin structure of hsp26, an organized nucleosome array that leaves the HSEs in nucleosome-free, DNase I-hypersensitive (DH) sites (Q. Lu, L.L. Wallrath, B.D. Allan, R.L. Glaser, J.T. Lis, and S.C.R. Elgin, J. Mol. Biol. 225:985-998, 1992). Inspection of the sequences upstream of hsp26 has revealed an additional (CT)n element at -347 to -341, adjacent to the distal HSE. We have analyzed the contribution of this distal (CT)n element (-347 to -341), the proximal (CT)n element (-135 to -85), and the two HSEs both to the formation of the chromatin structure and to heat shock inducibility. hsp26 constructs containing site-directed mutations, deletions, substitutions, or rearrangements of these sequence elements have been fused in frame to the Escherichia coli lacZ gene and reintroduced into the D. melanogaster genome by P-element-mediated germ line transformation. Chromatin structure of the transgenes was analyzed (prior to gene activation) by DNase I or restriction enzyme treatment of isolated nuclei, and heat-inducible expression was monitored by measuring beta-galactosidase activity. The results indicate that mutations, deletions, or substitutions of either the distal or the proximal (CT)n element affect the chromatin structure and heat-inducible expression of the transgenes. These (CT)n repeats are associated with a nonhistone protein(s) in vivo and are bound by a purified Drosophila protein, the GAGA factor, in vitro. In contrast, the HSEs are required for heat-inducible expression but play only a minor role in establishing the chromatin structure of the transgenes. Previous analysis indicates that prior to heat shock, these HSEs appear to be free of protein. Our results suggest that GAGA factor, an abundant protein factor required for normal expression of many Drosophila genes, and heat shock factor, a specific transcription factor activated upon heat shock, play distinct roles in gene regulation: the GAGA factor establishes and/or maintains the DH sites prior to heat shock induction, while the activated heat shock factor recognizes and binds HSEs located within the DH sites to trigger transcription.
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Pre-heat shock chromatin structure of hsp26 transgenes. The D.melanogaster hsp26 gene is assembled into a precise chromatin structure prior to heat shock. A specifically positioned nucleosome (yellow disk) lies between two nucleosome-free DNase I hypersensitive sites (DH sites; red bars). These DH sites encompass the heat shock elements (HSEs) and (CT)n elements which bind GAGA factor. GAGA bound to high affinity sites is colored mauve. The TATA binding protein (TBP; blue) is present at the promoter prior to heat shock, and RNA polymerase II (green) has paused after synthesizing a short transcript (see Lis and Wu, 1993). After heat shock, heat shock transcription factor binds to the HSEs and triggers transcription; there is a perturbation of the downstream nucleosome array during transcription. |
Science. 1989 Sep 29;245(4925):1487-90.
Gilmour DS, Thomas GH, Elgin SC.
Department of Biology, Washington University, St. Louis, MO 63130.
Proteins from Drosophila nuclei that bind to regions of alternating C and T residues present in the promoters of the heat shock genes hsp70 and hsp26 and the histone genes his3 and his4 have been purified. These proteins bind to isolated linear DNA, and genomic footprinting analyses indicate that they are bound to DNA in nuclei. In supercoiled plasmids at low pH, some of these DNA sequences adopt triple-helical structures which, if they form in vivo, could significantly affect chromatin structure. The nuclear proteins described here, and not necessarily the deformed conformation of the DNA, may be responsible for maintaining a potentially inducible promoter structure before transcriptional activation.
EMBO J. 1988 Jul;7(7):2191-201.
Thomas GH, Elgin SC.
Department of Biology, Washington University, St. Louis, MO 63130.
Genomic footprinting on the Drosophila hsp26 promoter in isolated nuclei has shown that a TATA box binding factor is present before and after induction by heat shock, while three of the seven heat shock consensus sequences 5' of the gene are occupied (presumably by heat shock factor, HSF) specifically on heat shock. The sites of HSF interaction are separated by greater than 200 bp of which approximately 150 bp are bound to the surface of a nucleosome. The juxtaposition of these various macromolecules on the DNA suggests a basis for the major DNase I hypersensitive site 5' of hsp26 and a novel tertiary structure for the promoter complex.