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Journal of Virology, July 2000, p. 6348-6357, Vol. 74, No. 14
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
CDP Is a Repressor of Mouse Mammary Tumor Virus
Expression in the Mammary Gland
Quan
Zhu,
Keqin
Gregg,
Mary
Lozano,
Jinqi
Liu, and
Jaquelin P.
Dudley*
Section of Molecular Genetics and
Microbiology and Institute for Cellular and Molecular Biology, The
University of Texas at Austin, Austin, Texas 78705
Received 7 January 2000/Accepted 19 April 2000
Mouse mammary tumor virus (MMTV) transcription is highest in the
lactating mammary gland but is detectable in a variety of other
tissues. Previous results have shown that MMTV expression is suppressed
in lymphoid and other tissues through the binding of the
homeodomain-containing repressor special AT-rich binding protein 1 to a
negative regulatory element (NRE) in the MMTV long terminal repeat
(LTR). Another homeoprotein repressor, CCAAT displacement protein
(CDP), also binds to the MMTV NRE, but a role for CDP in MMTV
transcriptional suppression has not yet been demonstrated. In this
paper, we show that the level of CDP decreases during development of
the mammary gland and that this decline in CDP level correlates with
the known increase in MMTV expression observed during mammary gland
differentiation. Moreover, CDP overexpression was able to suppress MMTV
LTR-reporter gene activity up to 20-fold in transient-transfection
assays of mouse mammary cells. To determine if this effect was due to
direct binding of CDP to the promoter-proximal NRE, we performed DNase
I protection assays to map two CDP-binding sites from +835 to +845 and
+920 to +931 relative to the first base of the LTR. Mutations
engineered into each of these sites decreased CDP binding to the
proximal NRE, whereas a combination of these mutations further reduced
binding. Subsequently, each of these mutations was introduced into the
full-length MMTV LTR upstream of the luciferase reporter gene. Analysis
of stable transfectants of LTR constructs showed that CDP binding site
mutations in the proximal NRE elevated reporter gene expression two- to
sixfold compared to wild-type LTR constructs. Thus, MMTV expression
increases during mammary gland development, in part due to decreased
CDP levels and CDP binding to the LTR. Together, these experiments provide the first evidence that CDP acts as a repressor of MMTV transcription in the mammary gland.
*
Corresponding author. Mailing address: Section of
Molecular Genetics and Microbiology, 100 W. 24th St., The University of Texas at Austin, Austin, TX 78705. Phone: (512) 471-8415. Fax: (512)
471-7088. E-mail: jdudley{at}uts.cc.utexas.edu.
Journal of Virology, July 2000, p. 6348-6357, Vol. 74, No. 14
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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