Published online 20 May 2004
Nucleic Acids Research, 2004, Vol. 32, No. 9 2829-2837
© 2004 Oxford University Press
The high-affinity Sp1 binding site in the HTLV-1 promoter contributes to Tax-independent basal expression
1 Department of Biochemistry and Molecular Biology and 2 Cell and Molecular Biology Program, Colorado State University, Fort Collins, CO 80523-1870, USA
*To whom correspondence should be addressed. Tel: +1 970 491 0420; Fax: +1 970 491 0494; Email: jnyborg{at}lamar.colostate.edu
Received February 22, 2004; Revised March 23, 2004; Accepted April 15, 2004
Transcriptional activation of human T-cell leukemia virus type 1 (HTLV-1) requires many cellular proteins and the virally encoded transcription factor Tax. Tax binds the three viral cAMP-response elements (CREs) with ATF/CREB (activating transcription factor/cAMP-response element-binding protein) and recruits the cellular coactivators CBP/p300. HTLV-1 also utilizes other cellular transcription factors that bind to the promoter to regulate transcription. One of these factors, Sp1, has been shown to bind to the viral promoter at two elements; one located within the third viral CRE, and the second located between the second and third viral CREs. The functional significance of Sp1 binding at each of these regions of the viral promoter is not completely understood. We set out to characterize Sp1 binding and to evaluate the functional significance of Sp1, both in the absence and presence of Tax. We found that Sp1 binds preferentially to the element located between the second and third viral CREs, and modestly activates transcription in vitro and in vivo. Sp1 was detected at the integrated HTLV-1 promoter in vivo. Surprisingly, point mutagenesis of the strong Sp1 binding site rendered the HTLV-1 reporter plasmid insensitive to Sp1 activation, and dramatically reduced basal transcription in vivo. These data indicate a role for Sp1 in basal level transcription of HTLV-1.