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J. Cell Biol.,
Volume 141, Number 2, April 20, 1998 335-348
Ernest Orlando Lawrence Berkeley National Laboratory, Life Science Division, University of California, Berkeley, California
94720
Special AT-rich sequence-binding protein 1 (SATB1), a DNA-binding protein expressed predominantly in thymocytes, recognizes an ATC sequence
context that consists of a cluster of sequence stretches
with well-mixed A's, T's, and C's without G's on one strand. Such regions confer a high propensity for stable
base unpairing. Using an in vivo cross-linking strategy,
specialized genomic sequences (0.1-1.1 kbp) that bind
to SATB1 in human lymphoblastic cell line Jurkat cells
were individually isolated and characterized. All in vivo
SATB1-binding sequences examined contained typical ATC sequence contexts, with some exhibiting homology to autonomously replicating sequences from the
yeast Saccharomyces cerevisiae that function as replication origins in yeast cells. In addition, LINE 1 elements,
satellite 2 sequences, and CpG island-containing DNA
were identified. To examine the higher-order packaging of these in vivo SATB1-binding sequences, high-resolution in situ fluorescence hybridization was performed with both nuclear "halos" with distended loops
and the nuclear matrix after the majority of DNA had
been removed by nuclease digestion. In vivo SATB1-binding sequences hybridized to genomic DNA as single spots within the residual nucleus circumscribed by
the halo of DNA and remained as single spots in the
nuclear matrix, indicating that these sequences are localized at the base of chromatin loops. In human breast cancer SK-BR-3 cells that do not express SATB1, at
least one such sequence was found not anchored onto
the nuclear matrix. These findings provide the first evidence that a cell type-specific factor such as SATB1
binds to the base of chromatin loops in vivo and suggests that a specific chromatin loop domain structure is involved in T cell-specific gene regulation.
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