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Mol. Cell. Biol., Jun 1996, 2802-2813, Vol 16, No. 6
GN Filippova, S Fagerlie, EM Klenova, C Myers, Y Dehner, G Goodwin, PE Neiman, SJ Collins and VV Lobanenkov
We have isolated and analyzed human CTCF cDNA clones and show here that the
ubiquitously expressed 11-zinc-finger factor CTCF is an exceptionally
highly conserved protein displaying 93% identity between avian and human
amino acid sequences. It binds specifically to regulatory sequences in the
promoter-proximal regions of chicken, mouse, and human c-myc oncogenes.
CTCF contains two transcription repressor domains transferable to a
heterologous DNA binding domain. One CTCF binding site, conserved in mouse
and human c-myc genes, is found immediately downstream of the major P2
promoter at a sequence which maps precisely within the region of RNA
polymerase II pausing and release. Gel shift assays of nuclear extracts
from mouse and human cells show that CTCF is the predominant factor binding
to this sequence. Mutational analysis of the P2-proximal CTCF binding site
and transient-cotransfection experiments demonstrate that CTCF is a
transcriptional repressor of the human c-myc gene. Although there is 100%
sequence identity in the DNA binding domains of the avian and human CTCF
proteins, the regulatory sequences recognized by CTCF in chicken and human
c-myc promoters are clearly diverged. Mutating the contact nucleotides
confirms that CTCF binding to the human c-myc P2 promoter requires a number
of unique contact DNA bases that are absent in the chicken c-myc CTCF
binding site. Moreover, proteolytic- protection assays indicate that
several more CTCF Zn fingers are involved in contacting the human CTCF
binding site than the chicken site. Gel shift assays utilizing successively
deleted Zn finger domains indicate that CTCF Zn fingers 2 to 7 are involved
in binding to the chicken c-myc promoter, while fingers 3 to 11 mediate
CTCF binding to the human promoter. This flexibility in Zn finger usage
reveals CTCF to be a unique "multivalent" transcriptional factor and
provides the first feasible explanation of how certain homologous genes
(i.e., c-myc) of different vertebrate species are regulated by the same
factor and maintain similar expression patterns despite significant
promoter sequence divergence.
Copyright © 1996, American Society for Microbiology
An exceptionally conserved transcriptional repressor, CTCF, employs different combinations of zinc fingers to bind diverged promoter sequences of avian and mammalian c-myc oncogenes
Fred Hutchinson Cancer Research Center, Seattle, Washington 98104, USA.
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