TRANSCRIPTION FACTOR SP1 MEDIATES CELL-SPECIFIC TRANSACTIVATION OF THE HUMAN CYTOMEGALOVIRUS DNA-POLYMERASE GENE PROMOTER BY IMMEDIATE-EARLY PROTEIN IE86 IN GLIOBLASTOMA U373MG CELLS

Human cytomegalovirus (HCMV) gene expression is highly cell and tissue specific. Cell factor-mediated regulatory interactions are involved i n regulating the restricted expression of the HCMV major immediate-ear ly (IE) gene (J. F. Baskar, P. P. Smith, G. Nilaver, R. A. Jupp, S. Ho ffmann, N. J. Peffer, D. J. Tenney, A. M. Colberg-Poley, P. Ghazal, an d J. A. Nelson, 70:3207-3213, 1996). To gain an understanding of HCMV early gene activation, we studied the effect of each of the three majo r IE proteins, IE72, IE86, and IE55, on the HCMV DNA polymerase gene ( pol; UL54) promoter. In transient-expression assays, the IE86 protein alone was able to transactivate the pol promoter, but IE72 and IE55 we re not, in permissive U373MG cells. However, we were unable to detect IE86-mediated transactivation in nonpermissive HeLa or C33-A cells. Us ing electrophoretic mobility shift assays (EMSAs), we found that expre ssion of the IE86 protein in U373MG cells resulted in specific binding of a DNA complex to an inverted-repeat element, IR1, of the pol promo ter. Antibody supershifting and EMSA-Western blotting experiments furt her showed that IE86 and the cellular transcription factor Sp1 were co mponents of the IR1 DNA-binding complex. Furthermore, we found that bi nding of DNA by Sp1 was dramatically increased in the presence of IE86 . Interestingly, this IE86-induced DNA-binding activity of Sp1 was inh ibited by a repressor activity presented in HeLa cells. In summary, ou r study suggests that a viral regulatory protein can modulate the DNA binding activity of a cellular transcription factor, resulting in cell -specific transactivation of viral genes.