Epstein-Barr virus (EBV) encodes genes that ensure its persistence in human B lymphocytes. Some of the genes encourage B-cell proliferation; others are poised to evade or defeat immune recognition. Immune restraints on the virus, however, are typically so effective that most infections are never symptomatic. In contrast, acute infectious mononucleosis, a self-limited lymphoproliferative illness, is common in adolescents and young adults. Unbridled proliferative illnesses arise when cellular immunity is grossly defective. Treatment of EBV-associated syndromes is largely supportive. Antiviral drugs have no proven role except in patients with oral hairy leukoplakia. Vaccine development is technically feasible but is not considered a high priority for developed nations.

The positive and negative cis-acting elements that affect transcription from the human cytomegalovirus major immediate-early (MIE) promoter and the viral and cellular proteins that bind to these elements are discussed. The data obtained using in vitro transcription and transient transfection assays are reviewed and compared to recent data using recombinant viruses with exacting elements deleted. The effects of cell type and cellular differentiation on activation of transcription from the MIE promoter are compared with the effects of mitogens and virion-associated tegument proteins that directly or indirectly activate protein kinase pathways. The repressor and enhancer regions upstream of the MIE promoter in the large unique component of the viral genome are compared to the elements upstream of the more simplified IEUS3 promoter in the short unique component of the viral genome.

Inactivity of the human cytomegalovirus (HCMV) major immediate-early regulatory region (MIERR),Human cytomegalovirus (HCMV) replicates in wide variety of cell types, including central nervous system (CNS) neurons (reviewed in reference 82). The virus replicates poorly or negligibly in lymphocytes, neutrophils, and immature neurons (61,65,73,80,82), and it resides latently in monocytes and their precursors (34,41,42,59,60,80,83,87,88,91). The mechanisms that preclude viral replication in nonpermissive cell types are poorly understood. One possible mechanism involves silencing of the viral major immediate-early (MIE) genes, whose products (e.g., IE1 p72 and IE2 p86) are required for initiating viral replication (32,37,62,70,84). These genes are vigorously transcribed in productive (lytic) infection but are relatively inactive in latently or other nonpermissively infected cell types (41,42,45,59,66,80,88). Hence, MIE gene expression and viral replication may be silenced by the same mechanism(s).The NTera2/D1 (NT2) cell line, derived from a human teratocarinoma (5), is a useful model in which to examine the coordinate regulation of HCMV replication and MIE gene expression (30,31,45,66). These cells resemble embryonal cells and are able to differentiate predominantly into CNS neurons when induced with retinoic acid (RA) (4, 40, 72). Embryonal NT2 cells do not permit HCMV replication, whereas differentiated cells do (30,31). These findings are consonant with the differentiation-dependent nature of HCMV replication in cultured primary CNS neurons (73). The lack of HCMV replication in embryonal NT2 cells corresponds to a block in viral MIE gene expression (45,66). This block is abrogated by RA pretreatment, which induces cellular differentiation prior to infection (45,66). However, the block persists if RA treatment is delayed to the postinfection (p.i.) period (31). Thus, establishing the RA-induced cellular condition prior to infection is key to promoting viral replication and MIE gene expression. HCMV's fate within embryonal NT2 cells is not fully known, although viral genomes are associated with cell nuclei at 5 h p.i. (66). DNase I mapping of the MIE regulatory region (MIERR) of these viral genomes reveals a hypersensitivity profile that differs considerably from that produced in the . This finding implies that superstructure or chromatin organization of viral genomes differs among the two isogenic cell types.Previous studies analyzing MIERR segments in in vitro, transfection, and transgenic animal studies indicate that this

The human cytomegalovirus (HCMV) major immediate-early (MIE) genes, encoding IE1 p72 and IE2 p86, are activated by a complex enhancer region (base positions -65 to -550) that operates in a cell type-and differentiation-dependent manner. The expression of MIE genes is required for HCMV replication. Previous studies analyzing functions of MIE promoter-enhancer segments suggest that the distal enhancer region variably modifies MIE promoter activity, depending on cell type, stimuli, or state of differentiation. To further understand the mechanism by which the MIE promoter is regulated, we constructed and analyzed several different recombinant HCMVs that lack the distal enhancer region (-300 to -582, -640, or -1108). In human fibroblasts, the HCMVs without the distal enhancer replicate normally at high multiplicity of infection (MOI) but replicate poorly at low MOI in comparison to wild-type virus (WT) or HCMVs that lack the neighboring upstream unique region and modulator (-582 or -640 to -1108). The growth aberrancy was normalized after restoring the distal enhancer in a virus lacking this region. For HCMVs without a distal enhancer, the impairment in replication at low MOI corresponds to a deficiency in production of MIE RNAs compared to WT or virus lacking the unique region and modulator. An underproduction of viral US3 RNA was also evident at low MOI. Whether lower production of IE1 p72 and IE2 p86 causes a reduction in expression of the immediateearly (IE) class US3 gene remains to be determined. We conclude that the MIE distal enhancer region possesses a mechanism for augmenting viral IE gene expression and genome replication at low MOI, but this regulatory function is unnecessary at high MOI.Human cytomegalovirus (HCMV) replicates in many cell types, including endothelial, smooth muscle, fibroblast, hepatocyte, neuronal, glial, and macrophage cells (reviewed in reference 45). It replicates poorly or negligibly in lymphocytes, neutrophils, and certain embryonal cells (11,12,25,44,45), and it resides latently in monocytes and their precursors (16,21,22,33,34,42,46,52,53,57). The mechanisms that govern HCMV replication or latency are poorly understood. Products of the HCMV major immediate-early (MIE) genes (e.g., IE1 p72 and IE2 p86) are required for viral replication (15,18,36,38,47). They are not expressed in latently or certain nonpermissively infected cell types (21,22,25,33,37,42,53). Hence, the regulation of their expression may be a pivotal step in controlling viral replication.The MIE regulatory region controls transcription of its genes through interplay of both positive and negative cis-acting elements. The enhancer component of the MIE regulatory region contains many of these cis-acting elements. The enhancer's boundaries are inexact but are often considered to span base positions -65 to -550 with respect to the ϩ1 start site of MIE RNAs (32). A variety of cellular and viral proteins interact with the enhancer's cis-acting elements to regulate activity of MIE promoter segments when assayed in transfec...

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediateearly protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.