Activation of human immunodeficiency virus gene expression by ultraviolet light in stably transfected human cells does not require the enhancer element.

Ultraviolet light (UV) exposure of cells infected with human immunodeficiency virus type 1 (HIV) or transfected with HIV reporter genes increases virus-directed gene expression. Here we report the mapping of the UV response on the long terminal repeat (LTR) by using human cells stably transfected with HIV promoter plasmids harboring different mutations and controlling the expression of the chloramphenicol acetyltransferase (cat) reporter gene. Promoter mutation analysis revealed that no specific upstream region of the LTR was associated with UV activation, although a significant decrease was observed with mutations in the basal promoter elements Sp1 and TATA, Most importantly, UV activation was not diminished by removal of the -119 to -69 region encompassing the LTR enhancer region or, more specifically, by point mutations in the NF-kappa B binding elements. Consistent with this result, we found that the phorbol ester (PMA) response, which is known to act through the enhancer, occurred independently and was synergistic with the UV response, Removal of the -119 to -69 region did not affect UV activation; however, it resulted in total abrogation of the PMA response. These results suggest that UV activation is distinct from NF-kappa B activation and does not act through the enhancer in stably transfected cells. This is in dramatic contrast to what is found with transient expression analysis of these responses, Lastly, RNA protection experiments revealed that UV may act on preassembled basal transcription complexes by allowing elongation of nascent short mRNAs generated from the LTR. We conclude that neither the enhancer nor any other single upstream LTR element is necessary for UV activation of HIVcat expression in stably transfected cells. Instead, UV activation appears to require only an intact basal promoter.