EFFECT OF A MODULATOR DELETION ON TRANSCRIPTION OF THE HUMAN CYTOMEGALOVIRUS MAJOR IMMEDIATE-EARLY GENES IN INFECTED UNDIFFERENTIATED AND DIFFERENTIATED CELLS

Differentiation dependent expression of the human cytomegalovirus (HCM V) major immediate-early (MIE) genes, encoding IE1 and IE2, may partly govern virus replication in monocytic THP-1 and embryonal carcinoma ( Tera-2) cells. The modulator of the MIE promoter was shown previously in transient transfection assays to repress transcription from promote r segments in undifferentiated THP-1 and Tera-2 cells but not in diffe rentiated cells. To determine the biological importance of these findi ngs, we constructed a recombinant HCMV (r Delta MSVgpt) without a modu lator. In comparison to wild-type (WT) virus, r Delta MSVgpt exhibits a slight delay in growth in human fibroblasts, but there is no appreci able change in IE1 and IE2 transcription. Moreover, there is no apprec iable change in the early/late kinetics of transcription of RNAs colin ear with the predicted UL128 coding region, which is adjacent to the m odulator, although the size distribution and abundance of these RNAs a re altered. In infected undifferentiated THP-1 and Tera-2 cells, WT an d r Delta MSVgpt viruses produce minimal but comparable amounts of IE1 RNAs. The genomes of both viruses are detectable in similar amounts w ithin these undifferentiated cells. Induction of cellular differentiat ion before infection overcomes the block in MIE gene transcription. WT and r Delta MSVgpt infections of differentiated THP-1 cells produce s imilar levels of IE1 and IE2 RNAs. Thus, differentiation-dependent con trol of MIE gene transcription involves regulatory mechanisms other th an the modulator. Possible alternative functions of the modulator are discussed.