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Journal of Virology, March 2001, p. 2174-2184, Vol. 75, No. 5
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.5.2174-2184.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Type B Leukemogenic Virus Has a T-Cell-Specific Enhancer That Binds AML-1

Jennifer A. Mertz,1 Farah Mustafa,1 Shari Meyers,2 and Jaquelin P. Dudley1,*

Section of Molecular Genetics and Microbiology and Institute for Cellular and Molecular Biology, The University of Texas at Austin, Austin, Texas 78712,1 and Department of Biochemistry and Molecular Biology, Feist-Weiller Cancer Center, Louisiana State University Medical Center, Shreveport, Louisiana 711302

Received 10 October 2000/Accepted 6 December 2000

Type B leukemogenic virus (TBLV) induces rapidly appearing T-cell tumors in mice. TBLV is highly related to mouse mammary tumor virus (MMTV) except that TBLV long terminal repeats (LTRs) have a deletion of negative regulatory elements and a triplication of sequences flanking the deletion. To determine if the LTR triplication represents a viral enhancer element, we inserted the triplication upstream and downstream in either orientation relative to the thymidine kinase promoter linked to the luciferase gene. These experiments showed that upregulation of reporter gene activity by the TBLV triplication was relatively orientation independent, consistent with the activity of eukaryotic enhancer elements. TBLV enhancer activity was observed in T-cell lines but not in fibroblasts, B cells, or mammary cells, suggesting that enhancer function is cell type dependent. To analyze the transcription factor binding sites that are important for TBLV enhancer function, we prepared substitution mutations in a reconstituted C3H MMTV LTR that recapitulates the deletion observed in the TBLV LTR. Transient transfections showed that a single mutation (556M) decreased TBLV enhancer activity at least 20-fold in two different T-cell lines. This mutation greatly diminished AML-1 (recently renamed RUNX1) binding in gel shift assays with a mutant oligonucleotide, whereas AML-1 binding to a wild-type TBLV oligomer was specific, as judged by competition and supershift experiments. The 556 mutation also reduced TBLV enhancer binding of two other protein complexes, called NF-A and NF-B, that did not appear to be related to c-Myb or Ets. AML-1 overexpression in a mammary cell line enhanced expression from the TBLV LTR approximately 30-fold. These data suggest that binding of AML-1 to the TBLV enhancer, likely in combination with other factors, is necessary for optimal enhancer function.

* Corresponding author. Mailing address: Section of Molecular Genetics and Microbiology, The University of Texas at Austin, 100 W. 24th St., ESB 226, Austin, TX 78705. Phone: (512) 471-8415. Fax: (512) 471-7088. E-mail: jdudley{at}uts.cc.utexas.edu.

Journal of Virology, March 2001, p. 2174-2184, Vol. 75, No. 5
0022-538X/01/$04.00+0   DOI: 10.1128/JVI.75.5.2174-2184.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.