The effect of the human cytomegalovirus immediate early region 1 enhancer on transcription was studied in vitro with HeLa cell nuclear extract. Stimulation of in vitro transcription mediated by the enhancer element involves its recognition by specific trans-acting factors present in the nuclear extract. DNase I protection analysis was used to determine at the nucleotide level those enhancer sequences that interact with nuclear factors. At least nine sites of protein-DNA interaction were detected over -400 base pairs of enhancer sequence. The regions of nuclease protection are associated with 21-, 19-, 18-, and 17-base-pair repeat elements as well as with a unique sequence, creating a large nucleoprotein complex. The relationship between the protein binding and the activity of the immediate early region 1 enhancer is discussed.Human cytomegalovirus (HCMV), a member of the herpesvirus family, is the etiologic agent of a wide spectrum of human diseases (1, 2). Upon infection, the HCMV genes are expressed in three sequential phases MATERIALS AND METHODS Preparation of DNA. The recombinant plasmids pPSCAT, pSSCAT and pCMV(-524)CAT, pCMV(-65)CAT were obtained from J. Nelson and R. Ruger, respectively. These constructs contain various 5'-flanking sequences of the HCMV (strain AD169) IEJ gene linked to the bacterial chloramphenicol acetyltransferase gene (see Fig. 1A). Plasmids pE1, pE2, and pE3 contain BAL-31 deletion fragments (prepared by R. Ruger) overlapping the enhancer and promoter region of the IEJ gene that were cloned into the HindIII and BamHI sites of pTZl8R (see Fig. 1A). The competitor fragments (El, E2, and E3) were gel-purified from their respective plasmids after cleavage with EcoRd and HindIII restriction enzymes.Nuclear Extract Preparation and Fractionation. Nuclear extract from HeLa cells was prepared as described by Dignam et al. (20). The fractionation of the nuclear extract on columns of heparin-agarose (Bio-Rad) and DEAE-Sepharose CL-6B (Pharmacia) was performed according to the procedure of Dynan and Tjian (21). The heparin-agarose and DEAE-Sepharose columns were step-washed with 0.4 M and 0.225 M KCl, respectively, The flow-through fractions (HAO.1, DEO.1) and the step-washed fractions (HAO.4, DEO.225) from the column chromatography were used in the DNase I protection analysis.In Vitro Transcription Assays.
RESULTSThe HCMV IEJ Gene Promoter Is Transcriptionally Active in Vitro. Run-off assays were performed to determine whether the crude nuclear extract prepared from the nuclei of uninfected HeLa cells contained the necessary factors for initiating and sustaining transcription from the HCMV IEl promoter. The DNA templates pPSCAT and pSSCAT, containing the IEl promoter and 5'-flanking region, were cleaved with the restriction endonucleases Pvu II or EcoRI, which truncate the template 165 or 260 base pairs downstream of the IEl cap site, respectively, generating run-off transcripts that could be visualized by electrophoresis. For all the templates tested, the size of the run-off transc...