ABSTRACT
The US3 gene of human cytomegalovirus (HCMV) is expressed at immediate-early (IE) times in permissive HF cells, but not in nonpermissive rodent cells, and encodes several proteins that have been reported to have regulatory characteristics, although they are dispensable for growth in cell culture. Both spliced and unspliced forms of US3 IE transcripts are associated with the second of only two known large and complex upstream enhancer domains within the 229-kb HCMV genome, which we refer to as the IES cis-acting control region. Only the 260-bp proximal segment (from -313 to -55) of the 600-bp IES control domain, which contains multicopy NF-kappaB binding sites, proved to be necessary to transfer both high basal expression plus phorbol ester- and okadaic acid-inducible characteristics to heterologous promoters in transient assays in U-937 and K-562 cells. However, the IES control region contains a distinctive 280-bp distal domain, characterized by the presence of seven interspersed repeats of a 10-bp TGTCGCGACA palindromic consensus motif that encompasses a NruI site. This far upstream Nru repeat region (from -596 to -314) imparted up to 20-fold down-regulation effects onto strong basal heterologous promoters as well as onto the IES enhancer plus minimal promoter region in both U-937 and K-562 cells. Functional Nru repressor elements (NREs) could not be generated by multimerizing either the palindromic (P) Nru motifs alone or adjacent degenerate interrupted (I Nru motifs alone. However, multimerized forms of the combined P plus I elements reconstituted the full 20-fold cis-acting down-regulation phenotype of the intact NRE domain. The P and I forms of the Nru elements each bound independently and specifically to related cellular DNA-binding factors to form differently migrating A or B complexes, respectively, whereas the combined P plus I elements bound cooperatively to both the A and B complexes with high affinity. Interestingly, nuclear extracts from U-937, K-562, HeLa, and Vero cells all formed both the A and B NRE binding factor complexes, whereas those from HF cells produced only A complexes, and Raji, HL60, and BALB/c 3T3 cells lacked both types of binding factor complexes. The core pentameric CGACA and CGATA half sites present in both the P and I Nru motifs are related to recently described Drosophila chromosomal insulator binding sites. Therefore, in addition to its cis-repression or silencer characteristics, the NRE domain appears likely to act to shield adjacent segments of the viral genome from the chromatin-reorganizing effects of the IES-inducible enhancer. We speculate that differential expression and regulation of the IES enhancer-controlled US3 protein, either in concert with or separately from the major IE (MIE) enhancer-controlled IE1 and IE2 transactivator proteins, may play a critical role in determining HCMV permissiveness in some cell types and perhaps also in the establishment of or reactivation from latency.
