An enhancer element in the short unique region of human cytomegalovirus regulates the production of a group of abundant immediate early transcripts.

A 275-bp sequence in the short unique region of human cytomegalovirus (HCMV) strain AD169 contains a series of five imperfect 18-bp repeats with homology to the SV40 and HCMV major immediate early enhancers. Plasmids containing these putative HCMV enhancer sequences linked to the human beta-globin gene were transiently transfected into HeLa cells, and the ability of the HCMV sequences to activate beta-globin transcription was assayed. The repeat-containing region stimulated transcription in a position- and orientation-independent manner, to approximately the same degree as that of the SV40 enhancer. A promoter located immediately 3' to the enhancer in the HCMV genome is active only in the presence of the enhancer sequences. Transcription from this HCMV promoter was analysed by a combination of S1 nuclease protection mapping and Northern blotting. At immediate early times postinfection, at least four extremely abundant differentially spliced mRNAs were detected; these RNAs constitute a previously unknown class of immediate early transcript.