Inhibition of HIV-1 replication by a high-copy-number vector expressing antisense RNA for reverse transcriptase

We report the construction of a high-copy-number (hcn) expression vector for human cells. Amplification of this vector occurs due to the presence of an element derived from the murine DNA encoding ribosomal RNA (rDNA). HIV-1 replication in Jurkat T lymphocytes is nearly abolished when antisense RNA directed against the gene encoding reverse transcriptase is expressed from this hcn vector. The replication of the virus is only slightly reduced by the plasmid control version lacking the murine amplification-promoting element. This kind of hcn vector may represent an important improvement for the genetic engineering of eukaryotic cells and may also provide some ideas for the future gene therapy of some human diseases.

Analysis of proteins encoded by IE regions 1 and 2 of human cytomegalovirus using monoclonal antibodies generated against recombinant antigens

The genomic region of human cytomegalovirus (HCMV) encoding the major immediate-early (IE) proteins was cloned as overlapping fragments into a prokaryotic expression vector. Three recombinant polypeptides were used as antigens to generate monoclonal antibodies specifically reactive with the proteins encoded by IE region 1 and IE region 2. At least 10 different antigenic regions were identified on the IE proteins of HCMV. One monoclonal generated against an IE-2 polypeptide of 156 amino acids (termed SMX) was found to react with a viral pentapeptide, which was also a constituent of the beta-chain of human HLA-DR. This peptide was contained in the 40-kDa (p40) protein encoded by IE region 2. This protein appeared to be an abundant product of the major IE region in infected cells at late times after infection. The putative translation initiation site for p40 was mapped to position 3348 of the IE2 sequence using monoclonal antibody SMX. It is therefore proposed that p40 consists of amino acids 242-580 encoded by the IE2 gene of HCMV (strain AD169). Neither this late protein nor any other of the IE1/IE2-encoded proteins was detectable in extracellular virions.

The 86-kilodalton IE-2 protein of human cytomegalovirus is a sequence-specific DNA-binding protein that interacts directly with the negative autoregulatory response element located near the cap site of the IE-1/2 enhancer-promoter

The 86-kDa IE-2 protein of human cytomegalovirus is able to autoregulate its own expression via a short nucleotide sequence, termed the cis repression signal (CRS), that is located between the TATA box and the cap site of the IE-1/2 enhancer-promoter. Here we report that the 86-kDa IE-2 protein can interact directly with the CRS, thus demonstrating that IE-2 is a DNA-binding protein. This could be shown by both DNase I protection and gel retardation experiments using a procaryotically expressed IE-2 protein that was purified to near homogeneity. The interaction was sequence specific since a mutated form of the CRS that had previously been reported to be defective in mediating negative regulation could no longer compete for binding in DNase I protection experiments. In addition, an IE-2-reactive monoclonal antibody was able to elicit a supershift in gel retardation experiments, thus proving the presence of IE-2 within the protein-DNA complex. These results suggest that formation of a specific complex between an IE protein and a target sequence located near the cap site of its own gene promoter may be a common mechanism used by both alphaherpesviruses and betaherpesviruses to autoregulate IE gene transcription, although the sequence requirements differ between the two herpesviral subgroups.

Expression of PILOT, a putative transcription factor, requires two signals and is cyclosporin A sensitive in T cells

Few known genes (IL-2, members of the IL-8 family, interferon-gamma) are induced in T cells only through the combined effect of phorbol myristic acetate (PMA) and a Ca(2+)-ionophore, and expression of only these genes can be fully suppressed by Cyclosporin A (CyA). We have identified a putative transcription factor, designated PILOT, with an identical dual signal requirement for expression. Induction of the PILOT gene is detectable in human T cells 20 min following activation in the presence of cycloheximide and is fully suppressed by CyA. The PILOT protein has a calculated M(r) of 42.6 kDa and contains three zinc fingers of the C2H2-type at the carboxyl-terminus which are highly homologous to the zinc finger regions of the transcription factors EGR1, EGR2, and pAT 133. In contrast to T cells, in fibroblasts PILOT gene expression requires only one signal (PMA) and is not affected by CyA. This observation directly demonstrates the existence of a Ca2+ signal-dependent regulatory element obligatory for expression of some genes in T cells but not in fibroblasts. This differential expression model will be valuable in the dissection of the dual signal pathway in T cells and the effects of CyA upon it.

Strong trans activation of the human cytomegalovirus major immediate-early enhancer by p40tax of human T-cell leukemia virus type I via two repetitive tax-responsive sequence elements

The immediate-early 1 and 2 gene locus of human cytomegalovirus (HCMV) that encodes trans-activator proteins with effects on both homologous and heterologous promoters is expressed under control of a complex enhancer/promoter regulatory region. This enhancer contains four types of repetitive sequence elements with 17, 18, 19, and 21 bp that bind cellular transcription factors. Although the HCMV enhancer acts as a powerful stimulator of transcription in most cell types examined, human T cells do not support strong activity. The present study demonstrates that the tax gene product of human T-cell leukemia virus type I trans activates the major enhancer of HCMV more than 60-fold in the T-cell line Jurkat. When a series of chloramphenicol acetyltransferase expression plasmids containing synthetic oligonucleotides with the 17-, 18-, 19-, or 21-bp motif upstream of a minimal immediate-early 1 and 2 gene promoter was tested, two of the four repeat motifs could be identified as Tax-responsive elements. Both the 18- and the 19-bp motifs were able to act as strong Tax-responsive elements even when they were present as single copies. Thus, in addition to interacting with human immunodeficiency virus, HCMV is able to interact with a second retrovirus of clinical importance.

Analysis of proteins binding to the proximal promoter region of the human cytomegalovirus IE-1/2 enhancer/promoter reveals both consensus and aberrant recognition sequences for transcription factors Sp1 and CREB

Expression of the immediate early 1 and 2 gene (IE-1/2) of human cytomegalovirus, an important pathogen in immunosuppressed patients, is controlled by a strong enhancer/promoter. To define the promoter domain within this large cis-active region of about 550 nucleotides, DNA-protein interactions were studied. DNase I footprinting experiments using procaryotically expressed transcription factor Sp1 revealed an extensive interaction of this transcription factor with both consensus and aberrant recognition elements within the IE-1/2 promoter region. Protection of these Sp1 binding sites could also be observed when nuclear extracts prepared from HeLa cells and permissive human fibroblast cells were used. After in vitro mutagenesis of Sp1 targets and transient expression of mutagenized CAT-expression plasmids, however, no significant reduction in CAT activities was found. By analyzing a series of 5' deletion mutants of the IE-1/2 promoter region, a strong cis-acting element was localized between nucleotides -94 and -78, upstream of sites that interact with Sp1. Gel retardation experiments demonstrated binding of recombinant transcription factor CREB to this motif which reveals it as an aberrant CREB recognition sequence. Thus, this study identifies several previously unknown binding sites for transcription factors Sp1 and CREB within the proximal promoter region of the IE-1/2 gene, which differ markedly in their relevance for constitutive promoter function.

Comparative sequence analysis of human cytomegalovirus strains

Three regions of DNA from five low-passage clinical isolates of human cytomegalovirus were amplified by polymerase chain reaction. The DNA sequences as well as the predicted amino acid sequences were compared with those of the laboratory strains AD169 and Towne. The genomic regions consisted of (i) three regions from the major glycoprotein (gp58/116, unique long [UL]55), (ii) three regions from the integral membrane protein (IMP, UL100), and (iii) a region from the major immediate-early 1 and 2 (IE-1/2) enhancer/promoter. Homologies ranged from 75.8 to 100.0% on the nucleotide level and from 47 to 100% on the amino acid level. The following two patterns were observed. (i) There are regions with a high degree of conservation with few scattered point mutations (mainly in the IE-1/2 enhancer/promoter and in the IMP gene). (ii) There are clusters of highly variable regions (parts of the gp58/116 gene and of the IMP gene). Within the areas of high variability, the strains could be classified into a limited number of subtypes.

An inducible promoter mediates abundant expression from the immediate-early 2 gene region of human cytomegalovirus at late times after infection

An abundant late transcript of 1.5 kb originates from the immediate-early 2 (IE-2) gene region of human cytomegalovirus (HCMV) at late times after infection. The transcriptional start of this RNA was precisely mapped, and the putative promoter region was cloned in front of the CAT gene as reporter. This region, which comprises 78 nucleotides of IE-2 sequence upstream of the determined cap site, was strongly activated by viral superinfection at late times in the replicative cycle. As shown by RNase protection analyses, the authentic transcription start is used. No activation of this late promoter was observed after cotransfection with an expression plasmid containing the HCMV IE-1 and -2 gene region. This result suggests that, compared with early and early late promoters of HCMV, different or additional viral functions are required for the activation of true late promoters.

Discordant expression of the immediate-early 1 and 2 gene regions of human cytomegalovirus at early times after infection involves posttranscriptional processing events

Expression of the immediate-early 1 and 2 (IE-1 and IE-2) gene region of human cytomegalovirus (HCMV) was studied during initial phases of the replicative cycle. Accumulation of RNA from IE-1 and -2 was found to be differential. Transcripts from IE-2 reached peak levels very early in infection between 3 and 5 h, whereas IE-1 RNA peak levels were detected later, between 6 and 8 h. A strong decrease in steady-state levels of a 2.2-kb IE-2 RNA was observed at a time when IE-1 transcripts showed a further increase in abundance. Northern (RNA) blot experiments revealed that expression of both the IE-1 RNA and the 2.2-kb IE-2 transcript is controlled by the IE-1 enhancer-promoter. Nuclear run-on experiments demonstrated equal rates of primary transcription for IE-1 and -2 at a time when different steady-state levels of RNA were observed. Concomitant with down-regulation of IE-2 RNAs, a decrease in the size of the IE-1 transcript was detected. At 2 to 5 h after infection the IE-1 transcript migrated at 1.95 kb, whereas later in the replicative cycle the RNA was found at 1.8 kb. RNase H blot analysis revealed that this size discrepancy is due to a shorter poly(A) tail of the IE-1 RNA at early times after infection. These experiments suggest that in addition to transcriptional regulation, specific posttranscriptional mechanisms are involved in controlling expression from the IE-1 and -2 gene region of HCMV.

Transcriptional analysis of the eight-kilobase mRNA encoding the major capsid protein of human cytomegalovirus

The 8-kilobase mRNA coding for the major capsid protein (MCP) of human cytomegalovirus was precisely mapped. Two 5' ends of the transcript were located within HindIII fragment a, 29 and 34 base pairs, respectively, downstream of the sequence TATTAGA. The 3' end was localized within HindIII fragment b of the viral genome. The MCP transcript was synthesized at late times after infection and was not detected before viral DNA replication. In addition, the MCP promoter region could be identified, which strongly responded to viral trans activation at early and late times after infection in a transient expression assay.

Cell type-specific induction of the major immediate early enhancer of human cytomegalovirus by cyclic AMP

The major immediate early enhancer of human cytomegalovirus (HCMV) is known to exert a strong constitutive transcription stimulation in a broad spectrum of cells. This basal activity can be augmented considerably by elevated levels of intracellular cAMP in a cell type-specific manner. Cyclic AMP induction was observed in several lymphoid cell lines and in HeLa cells. One of the functionally important enhancer sequence modules, the 19 bp repeat element, mediates this effect as a cAMP-responsive element (CRE). It acts more efficiently than the corresponding sequence from the human chorionic gonadotropin gene. It is suggested that protein kinase C is involved in the pathway which leads to the activation of CRE-containing genes in lymphoid cells. Gel retardation assays indicated that similar, but not identical complexes are formed between nuclear protein extracts and the CREs of HCMV and the gonadotropin gene.

The role of a repetitive palindromic sequence element in the human cytomegalovirus major immediate early enhancer

The major enhancer, extending from nucleotides -530 to -120 upstream of the transcription initiation site of immediate early (IE) genes 1 and 2 in human cytomegalovirus (HCMV), contains four groups of repeated sequence motifs that consist of 17, 18, 19 or 21 bp, respectively. One of these elements, the 19 bp repeat, is a symmetrical palindrome that is also part of IE regulatory sequences of other cytomegalovirus-type herpesviruses, but not of unrelated members of the herpesvirus group. Synthetic oligonucleotides representing the 19 bp repeat unit strongly reduced the activity of the IE1/2 enhancer/promoter in cotransfection assays after transient expression. The HCMV enhancer can substitute for the 72 bp repeats of simian virus 40 (SV40). Replication-competent deletion mutants of SV40/HCMV enhancer recombinants were constructed that contained a single palindromic 19 bp repeat with a central cleavage site for AhaII. If deletions were introduced into the single remaining 19 bp repeat most of the mutant viruses were still replication-competent in CV-1 monkey kidney cells. Insertion of two nucleotides into the single AhaII site did not significantly alter transient SV40 T antigen expression. Deletion of four nucleotides or more from the single 19 bp palindrome reduced the stimulation of T antigen synthesis by the HCMV enhancer/SV40 promoter unit down to about 50%. More extended deletions (28 to 80 bp) did not further reduce T antigen expression. All mutants without an intact 19 bp repeat contained the 18 bp and/or the 21 bp sequence motif. DNase I footprinting and gel retardation assays indicated sequence-specific protein binding by the 19 bp palindrome. Altered palindromes, correlating with reduced enhancer activity, lost most of their protein-binding properties. Thus, the 19 bp repeat element is one of several protein-binding sites that contribute to enhancer strength. However, the 19 bp sequence motif can be deleted entirely to leave reduced activity. The HCMV IE1/2 upstream sequence appears to be the perfect model of an enhancer as a complex of multiple binding sites for trans-activating proteins in a modular fashion.

The 160,000-Mr virion protein encoded at the right end of the herpesvirus saimiri genome is homologous to the 140,000-Mr membrane antigen encoded at the left end of the Epstein-Barr virus genome

The sequence of 4.4 kilobase pairs (kbp) from the conventional right terminus of the A + T-rich light-DNA (L-DNA) sequences of the herpesvirus saimiri (HVS) genome contains a leftward-directed open reading frame (ORF) for a 1,299-residue protein. The molecular weight predicted for the protein (143,000) is in good agreement with the estimates of 150,000 to 160,000 for the major nonglycosylated polypeptide of the virion tegument (the 160K polypeptide), previously shown to be encoded by this region of the genome. The first initiation codon of the ORF is only 250 nucleotides from the junction of the L-DNA component with the G + C-rich terminal reiterations (i.e., heavy or H-DNA) of the genome. An unusually A + T-rich sequence (43 of 45 nucleotides are A or T, relative to a mean composition of 40% G + C for the ORF) occurs some 75 bp 5' to this initiation codon, and the first adenylation signal (AATAAA) on this DNA strand occurs 18 bp 3' to the termination codon. The amino acid sequence predicted for the 160K protein of HVS is homologous over most of its length to the 1,318-residue protein encoded by the leftmost major ORF of the G + C-rich genome of Epstein-Barr virus (BNRF1, the 140K nonglycosylated membrane antigen). No homology to either of these proteins is evident among the products predicted from the complete sequence of the alpha herpesvirus varicella-zoster virus. Thus gamma herpesviruses with coding sequences which differ in mean nucleotide composition by some 20% G + C have homologous proteins encoded at similar positions with respect to genome termini, with the right end of HVS being homologous to the left end of Epstein-Barr virus.

Organization of terminal reiterations in the virion DNA of herpesvirus saimiri

The population of herpesvirus saimiri (HVS) genomes extracted from extracellular virions are double-stranded, linear DNA molecules of about 160 kilobase pairs (kbp) each composed of a central segment of 110 to 112 kbp and 36% (G + C) (i.e. 'light' or L-DNA) linked to direct reiterations of a 1.44 kbp repeat unit of 71% (G + C) (i.e. 'heavy' or H-DNA) at each terminus. In this paper, we show that the population of HVS DNA molecules contains approximately equal concentrations of genomes with all possible integral numbers of complete repeat units (i.e. from greater than 30 to 1) at either 'left' or 'right' ends but that all molecular ends are derived by a unique cleavage at a site close to the single ApaI restriction endonuclease site of the H-DNA repeat unit. Junctions of proximal H-DNA repeat units with L-DNA occur at, or very close to, the sequence present at the molecular ends. The transition from L- to H-DNA occurs abruptly at this site at the 'right' end of the L-DNA component but some rearranged restriction enzyme cleavage sites typical of H-DNA are found within the first 0.8 kbp of the L-DNA sequences at the 'left' H-L DNA junction. HVS appears to provide an extreme example of the general process whereby herpesvirus DNAs are matured from concatemeric intermediates by a site-specific cleavage/recombination process involving random choice between equivalent sites for the initiation of the process and with choices between alternative termination sites being limited by a headful packaging mechanism.