The strong enhancer element in the immediate early region of the human cytomegalovirus genome

The human cytomegalovirus (HCMV), a member of the herpesvirus group, was found to possess a strong transcription enhancer in the immediate early gene region. Co-transfection of enhancerless SV40 DNA with randomly fragmented HCMV DNA yielded two SV40-like recombinant viruses , each containing HCMV DNA fragments that were substituting for the missing SV40 enhancer. The two inserts , 341 and 262 bp in length , are overlapping segments of genuine viral DNA representing part of the 5'flanking region of the major immedistte early gene i n HCMV. Studies with deletion mutants showed that different nonoverlapping subsets of the HCMV enhancer region can substitute for the 72 bp repeats of SV40. Transient expression assays indicated that the HCMV enhancer is significantly stronger than the SV40 element, activating cis-linked heterologous promoters in a wide spectrum of cultured cells. It appears that the HCMV enhancer is positively regulated by viral immediate early genes.