The BamHI J fragment of human cytomegalovirus (HCMV) AD169 located at 0.815 to 0.855 map units in the unique short component of the genome was demonstrated to be responsive to the HCMV IE proteins by using a transient chloramphenicol acetyltransferase (cat) gene expression system. The BamHI J fragment was cloned into a cat gene expression plasmid and then cotransfected with a plasmid that expresses the immediate early (IE) genes of HCMV AD169 into the HCMV permissive cell line MRC-5. The results indicated that the BamHl 1 fragment enhanced cat gene expression 10-fold when the HCMV IE proteins were present. The BamHI J fragment was demonstrated to have properties of an inducible enhancer. In the presence of the HCMV IE proteins, it enhances cat gene expression when positioned in either orientation both upstream and downstream from the cat gene; it enhances transcription from the herpes simplex virus type 1 (HSV-1) thymidine kinase gene and the simian virus 40 (SV40) early gene promoters; and it requires a cis-positioned promoter for enhancer activity.
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