Organization and expression of the immediate early genes of human cytomegalovirus

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Abstract
The immediate early genes of human cytomegalovirus were characterized according to map location, RNA transcripts and translation products. Three regions in the large unique component (0.709-0.751 map units) were transcribed in the prsence of an inhibitor of protein synthesis (anisomycin). A single size class of polyadenylated mRNA, 1.95 kilobases (kb), was transcribed abundantly relative to the other size classes. The predominant 1.95 kb viral RNA was transcribed from right to left on the prototype arrangement of the viral genome and spanned a region of .apprx. 2.8 kb (0.739-0.751 map units). This mRNA codes for a 75,000-dalton protein that represents the predominant immediate early protein detected in infected [human fibroblast] cells. Immunoprecipitation of viral proteins synthesized in vitro and in vivo demonstrated that the predominant immediate early protein is synthesized as a protein of 75,000 daltons, but is presumably modified in vivo, resulting in a broad banding pattern ranging from 75,000-68,000 daltons. A different immediate early viral gene (0.732-0.739 map units) is transcribed from left to right at relatively low levels. The 3'' ends of the above viral RNA terminate at .apprx. 230 base pairs apart in the region of .apprx. 0.739 map units. Five RNA size classes ranging from 2.25-1.10 kb were detected, but the 1.75 kb and 1.40 kb RNA size classes were more abundant from this region. At least 4 minor proteins are coded by these mRNA, with apparent MW ranging from 56,000-16,500. A 1.95 kb mRNA was transcribed from a 3rd region (0.709-0.728 map units). This viral mRNA was present at relatively low concentration and coded for a minor protein of 68,000 daltons. Since immediate early gene expression of human cytomegalovirus is dominated by the synthesis of an mRNA from the region of 0.739 to 0.751 map units that codes for the predominant immediate early protein found in the infected cell, this protein appears to be the major regulatory protein influencing the switch from restricted to extensive transcription.