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A more recent version of this article appeared on September 1, 2005
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Submitted on May 4, 2005
Accepted on June 16, 2005
Manitoba Institute of Cell Biology, University of Manitoba, Winnipeg, Manitoba, R3E 0V9 Canada
Monitoring Editor: William Tansey
Sp1 and Sp3 are ubiquitously expressed mammalian transcription factors which activate or repress the expression of a wide variety of genes and are believed to compete for the same DNA binding site. We used indirect immunofluorescence microscopy and image deconvolution to show that Sp1 and Sp3 are organized into distinct nonoverlapping domains in human breast and ovarian cells. Domains of Sp1 and Sp3 infrequently associate with sites of transcription. Sp3 partitions with the tightly bound nuclear protein fraction of hormone responsive MCF-7 breast cancer cells while only a subpopulation of Sp1 is found in that fraction. Both Sp1 and Sp3 are bound to the nuclear matrix, and the nuclear matrix-associated sites of Sp1 and Sp3 are different. Indirect immunofluorescence studies demonstrate that Sp1 and Sp3 associate with histone deacetylases 1 and 2 and with the estrogen receptor 
, albeit at low frequencies in MCF-7 cells. Chromatin immunoprecipitation (ChIP) and reChIP assays revealed that although both Sp1 and Sp3 bind to the estrogen-responsive trefoil factor 1 promoter in MCF-7 cells, they do not occupy the same promoter. Our results demonstrate the different features of Sp1 and Sp3, providing further evidence that Sp3 is not a functional equivalent of Sp1.
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