NF-κB activation of the cytomegalovirus enhancer is mediated by a viral transactivator and by T cell stimulation

Abstract

The expression of cytomegalovirus α (immediate early) genes is under control of an enhancer that carries signals for strong constitutive expression as well as response element for transactivation by viral proteins. We have used synthetic oligonucleotides representing the 16, 18 and 19 bp repeat elements within the enhancer to investigate the role of virus-induced cellular transcription factors in enhancer activation. We show that the transcription factor NF-κB, which binds to the 18 bp repeat, plays a central role in enhancer activation in infected human fibroblasts and that activation is mediated by the product of the viral gene iel. The simian immunodeficiency virus κB site can functionally substitute for the 18 bp element in transient transactivation assays and can also compete efficiently for specific binding to the 18 bp repeat element in vitro. Point mutations in the NG-κB site within the 18 bp element disrupt iel-mediated transactivation and binding. We have found that the characteristics of the 18 bp binding factor from human fibroblasts are indistinguishable from NG-κB induced by phorbol ester plus mitogen treatment of T lymphocytes, as determined by gel mobility shift assay as well as protection of the binding site from chemical cleavage. Furthermore, T cell stimulation mediates activation of the viral enhancer via κB sites, an observation that may be important in the interaction of cytomegalovirus with the naturally infected human host. Thus, NF-κB plays a central role as a target for enhancer activation via viral and cellular factors.