Identification of cis sequences required for lytic DNA replication and packaging of murine gammaherpesvirus 68

Human gammaherpesviruses are associated with lymphomas and other malignancies. Murine gammaherpesvirus 68 (MHV-68) infection of mice has emerged as a model for understanding gammaherpesvirus pathogenesis in vivo. In contrast to human gammaherpesviruses, MHV-68 replicates in permissive cell lines in a robust manner, presenting an efficient model to study the basic mechanisms for DNA replication and recombination processes. In addition, MHV-68 also infects a broad range of cells of different tissue types and from different host species, and the viral genome persists as an episome in infected cells. These features make MHV-68 an attractive system on which to build gene delivery vectors. We have therefore undertaken a study to identify the cis elements required for MHV-68 genome replication and packaging. Here we report that an 8.4-kb MHV-68 genomic fragment between ORF66 and ORF73 conferred on the plasmid the ability to replicate; replication required the presence of either de novo viral infection or viral reactivation from latency. We further mapped the origin of lytic replication (oriLyt) to a 1.25-kb region. Moreover, we demonstrated that the terminal repeat of the viral genome is sufficient for packaging of the replicated oriLyt plasmid into mature viral particles. Functional identification of the MHV-68 oriLyt and packaging signal has laid a foundation for investigating the mechanisms controlling gammaherpesvirus DNA replication during the viral lytic phase and will also serve as a base on which to design gene delivery vectors.     

The M10 Locus of Murine Gammaherpesvirus 68 Contributes to both the Lytic and the Latent Phases of Infection

Murine gammaherpesvirus 68 (MHV-68) is closely related to Epstein-Barr virus and Kaposi''s sarcoma-associated herpesvirus (KSHV) and provides a small-animal model to study the pathogenesis of gammaherpesvirus (γHV) infections. According to the colinear organization of the γHV genomes, the M10 locus is situated at a position equivalent to the K12 locus of KSHV, which codes for proteins of the kaposin family. The M10 locus of MHV-68 has been predicted to code for three overlapping open reading frames (M10a, M10b, and M10c [M10a-c]) with unknown function. In addition, the M10 locus contains a lytic origin of replication (oriLyt). To elucidate the function of the M10 locus during lytic and latent infections, we investigated, both in vitro and in vivo, the following four recombinant viruses which were generated using MHV-68 cloned as a bacterial artificial chromosome: (i) a mutant virus with a deletion which affects both the coding region for M10a-c and the oriLyt; (ii) a revertant virus in which both the M10a-c coding region and the oriLyt were reverted to those of the wild type; (iii) a virus with an ectopic insertion of the oriLyt, which restores the function of the oriLyt but not the M10a-c coding region; and (iv) a mutant virus with a deletion in the oriLyt only. While the mutants were slightly attenuated with regard to lytic replication in cell culture, they showed severe growth defects in vivo. Both lytic replication and latency amplification were strongly reduced. In contrast, both the revertant virus and the virus with the ectopic oriLyt insertion grew very similarly to the parental wild-type virus both in vitro and in vivo. Thus, we provide genetic evidence that mutation of the oriLyt, and not of putative protein coding sequences within the M10a-c region, is responsible for the observed phenotype. We conclude that the oriLyt in the M10 locus plays an important role during infection of mice with MHV-68.Diseases caused by gammaherpesviruses continue to be a challenge for human health. The prototypic gamma-1 herpesvirus Epstein-Barr virus (EBV) is associated with lymphomas and nasopharyngeal carcinoma (22). Human herpesvirus 8 (also called Kaposi''s sarcoma-associated herpesvirus [KSHV]), a gamma-2 herpesvirus, is associated with lymphoproliferative disorders and Kaposi''s sarcoma (24). In vivo studies of gammaherpesvirus pathogenesis have been limited to clinical investigation of the infection because of the restricted host range of these viruses. The murine gammaherpesvirus 68 (MHV-68) is also a member of the gammaherpesvirus subfamily and is closely related to KSHV and EBV. Since there exist no good animal models for KSHV and EBV, MHV-68 serves as a small-animal model to investigate gammaherpesvirus pathogenesis (6, 9, 10, 13, 21, 25, 26, 30). MHV-68 is a natural pathogen of wild rodents (7) and is capable of infecting laboratory mice. The nucleotide sequence of MHV-68 is similar to that of EBV and even more closely related to that of KSHV (29). MHV-68 contains genes which are homologous to cellular genes or to genes of other gammaherpesviruses. In addition, it contains virus-specific genes. Many of the latency- and transformation-associated proteins of the gammaherpesviruses, for example, EBNA and LMP of EBV, appear to be encoded by virus-specific genes, yet it has been suggested that pathogenesis-associated genes of gammaherpesviruses may be contained in similarly positioned genome regions (29). The virus-specific genes of MHV-68 were originally designated M1 to M14 (29). The M10 locus has been predicted to code for three overlapping open reading frames (M10a, M10b, and M10c [M10a-c]) (29). While several MHV-68-specific genes have been shown to code for proteins with important functions, the function of M10 is still unknown. A more recent report even considered M10a-c rather unlikely to code for proteins (21). Importantly, the M10 locus also contains a lytic origin of replication (oriLyt) (3, 8). According to the colinear organization of the gammaherpesvirus genomes, the M10 locus is situated at a position equivalent to that of the K12 locus of KSHV. K12 encodes proteins of the kaposin family. Kaposin proteins are involved in cellular transformation and in stabilization of cytokine mRNAs (16-18,20). Of note, the K12 locus also contains an oriLyt (5).Here, we investigated the function of the M10 locus during lytic and latent infections by studying mutant viruses with deletions in the M10 loci, either affecting both the coding region for M10a-c and the oriLyt or the oriLyt only. While the mutants were slightly attenuated with regard to lytic replication in cell culture, they showed severe growth defects in vivo. Both lytic replication and latency amplification were strongly reduced in mice infected with the mutant viruses. In contrast, a revertant virus in which both the M10a-c coding region and the oriLyt were reverted to those of the wild type and a virus with an ectopic insertion of the oriLyt which restores the function of the oriLyt but not the M10a-c coding region grew very similarly to the parental wild-type virus both in vitro and in vivo. Thus, we provide genetic evidence that mutation of the oriLyt, and not of putative protein coding sequences within the M10a-c region, is responsible for the observed phenotype.     

The Epstein-Barr virus replication protein BBLF2/3 provides an origin-tethering function through interaction with the zinc finger DNA binding protein ZBRK1 and the KAP-1 corepressor

Herpesviruses encode a set of core proteins essential for lytic replication of their genomes. Three of these proteins form a tripartite helix-primase complex that, in the case of Epstein-Barr virus (EBV), consists of the helicase BBLF4, the primase BSLF1, and the linker protein BBLF2/3. BBLF2/3 and its homologs in the other herpesviruses remain relatively poorly characterized. To better understand the contribution to replication made by BBLF2/3, a yeast two-hybrid screen was performed with BBLF2/3 as the bait protein. This screen identified as interactors a number of cell replication-related proteins such as DNA polymerase beta and subunits of DNA polymerase delta along with the EBV-encoded DNase BGLF5. The screen also identified the DNA binding zinc finger protein ZBRK1 and the ZBRK1 corepressor KAP-1 as BBLF2/3 interactors. Interaction between BBLF2/3 and ZBRK1 and KAP-1 was confirmed in coimmunoprecipitation assays. A binding site for ZBRK1 in the EBV oriLyt enhancer was identified by electrophoretic mobility shift assay. ZBRK1, KAP-1, and the ZBRK1 binding protein BRCA1 were shown by indirect immunofluorescence to be present in replication compartments in lytically induced D98-HR1 cells, and additionally, chromatin immunoprecipitation assays determined that these proteins associated with oriLyt DNA. Replication of an oriLyt plasmid and a variant oriLyt (DeltaZBRK1) plasmid was examined in lytically induced D98-HR1 cells. Exogenous ZBRK1, KAP-1, or BRCA1 increased the efficiency of oriLyt replication, while deletion of the ZBRK1 binding site impaired replication. These experiments identify ZBRK1 as another cell protein that, through BBLF2/3, provides a tethering point on oriLyt for the EBV replication complex. The data also suggest that BBLF2/3 may serve as a contact interface for cell proteins involved in replication of EBV oriLyt.     

Murine gammaherpesvirus 68 contains two functional lytic origins of replication

A 1.25-kbp DNA fragment from the right side of the genome containing the lytic origin of replication (oriLyt) of murine gammaherpesvirus 68 (MHV-68) has been identified by a plasmid replication assay. Here we show that a mutant MHV-68 with a deletion of an essential part of this oriLyt, generated by using an MHV-68 bacterial artificial chromosome, was only slightly attenuated and still able to replicate but that a mutant containing an additional deletion on the left side of the genome was replication deficient. The newly identified region was sufficient to support plasmid replication, thus providing evidence for a second oriLyt.     

Contribution of C/EBP proteins to Epstein-Barr virus lytic gene expression and replication in epithelial cells

The contribution of C/EBP proteins to Epstein-Barr virus (EBV) lytic gene expression and replication in epithelial cells was examined. Nasopharyngeal carcinoma cell lines constitutively expressed C/EBPbeta and had limited C/EBPalpha expression, while the AGS gastric cancer cell line expressed significant levels of both C/EBPalpha and C/EBPbeta. Induction of the lytic cycle in EBV-positive AGS/BX1 cells with phorbol ester and sodium butyrate treatment led to a transient stimulation of C/EBPbeta expression and a prolonged increase in C/EBPalpha expression. In AGS/BX1 cells, endogenous C/EBPalpha and C/EBPbeta proteins were detected associated with the ZTA and oriLyt promoters but not the RTA promoter. Electrophoretic mobility shift assays confirmed binding of C/EBP proteins to multiple sites in the ZTA and oriLyt promoters. The response of these promoters in reporter assays to transfected C/EBPalpha and C/EBPbeta proteins was consistent with the promoter binding assays and emphasized the relative importance of C/EBPs for activation of the ZTA promoter. Mutation of the oriLyt promoter proximal C/EBP site had little effect on ZTA activation of the promoter in a reporter assay. However, this mutation impaired oriLyt DNA replication, suggesting a separate replication-specific contribution for C/EBP proteins. Finally, the overall importance of C/EBP proteins for lytic gene expression was demonstrated using CHOP10 to antagonize C/EBP DNA binding activity. Introduction of CHOP10 significantly impaired induction of the ZTA, RTA, and BMRF1 proteins in chemically treated AGS/BX1 cells. Thus, C/EBPbeta and C/EBPalpha expression are associated with lytic induction in AGS cells, and expression of C/EBP proteins in epithelial cells may contribute to the tendency of these cells to exhibit constitutive low-level ZTA promoter activity.     

In vitro synthesis and assembly of a 68 kDa outer mitochondrial membrane protein from rat liver

Outer mitochondrial membrane was purified from rat liver. Its constituent proteins were analyzed by SDS-polyacrylamide gel electrophoresis and by electrophoretic immunoblotting employing antibodies raised against total outer mitochondrial membrane. Anti-outer mitochondrial membrane antiserum reacted with only one polypeptide (15 kDa) in rough microsomes, whereas no immunological cross-reactivity was observed with other mitochondrial compartments (intermembrane space, inner membrane, or matrix) or with lysosomes or total cytosol. The antiserum was employed to characterize precursors of outer mitochondrial membrane proteins synthesized in vitro in a rabbit reticulocyte cell-free system. One product (a 68 kDa polypeptide designated OMM-68) bound efficiently to mitochondria in vitro but did not interact with either dog pancreas or rat liver microsomes, either co-translationally or post-translationally. OMM-68 was synthesized exclusively by the membrane-free class of polyribosomes. Attachment of precursor OMM-68 to mitochondria was not accompanied by processing of the polypeptide to a different size.     

Analysis of human cytomegalovirus oriLyt sequence requirements in the context of the viral genome

During the lytic phase of infection, replication of herpesvirus genomes initiates at the lytic origin of replication, oriLyt. Many herpesviruses harbor more than one lytic origin, but so far, only one oriLyt has been identified for human cytomegalovirus (HCMV). Evidence for the existence of additional lytic origins of HCMV has remained elusive. On the basis of transient replication assays with cloned viral fragments, HCMV oriLyt was described as a core region of 1.5 kbp (minimal oriLyt) flanked by auxiliary sequences required for maximal replication activity (complete oriLyt). It remained unclear whether minimal oriLyt alone can drive the replication of HCMV in the absence of its accessory regions. To investigate the sequence requirements of oriLyt in the context of the viral genome, mutant genomes were constructed lacking either minimal or complete oriLyt. These genomes were not infectious, suggesting that HCMV contains only one lytic origin of replication. Either minimal or complete oriLyt was then ectopically reinserted into the oriLyt-depleted genomes. Only the mutant genomes carrying complete oriLyt led to infectious progeny. Remarkably, inversion of the 1.5-kbp core origin relative to its flanking regions resulted in a replication-defective genome. Mutant genomes carrying minimal oriLyt plus the left flanking region gave rise to minifoci, but genomes harboring minimal oriLyt together with the right flanking region were noninfectious. We conclude that the previously defined minimal lytic origin is not sufficient to drive replication of the HCMV genome. Rather, our results underline the importance of the accessory regions and their correct arrangement for the function of HCMV oriLyt.     

Interaction of Kaposi's sarcoma-associated herpesvirus ORF59 with oriLyt is dependent on binding with K-Rta

Kaposi's sarcoma-associated herpesvirus (KSHV)/human herpesvirus 8 (HHV-8) displays two distinct life stages, latency and lytic reactivation. Progression through the lytic cycle and replication of the viral genome constitute an essential step toward the production of infectious virus and human disease. KSHV K-RTA has been shown to be the major transactivator required for the initiation of lytic reactivation. In the transient-cotransfection replication assay, K-Rta is the only noncore protein required for DNA synthesis. K-Rta was shown to interact with both C/EBPα binding motifs and the R response elements (RRE) within oriLyt. It is postulated that K-Rta acts in part to facilitate the recruitment of replication factors to oriLyt. In order to define the role of K-Rta in the initiation of lytic DNA synthesis, we show an interaction with ORF59, the DNA polymerase processivity factor (PF), one of the eight virally encoded proteins necessary for origin-dependent DNA replication. Using the chromatin immunoprecipitation (ChIP) assay, both K-Rta and ORF59 interact with the RRE and C/EBPα binding motifs within oriLyt in cells harboring the KSHV bacterial artificial chromosome (BAC). A transient-transfection ChIP assay demonstrated that the interaction of ORF59 with oriLyt is dependent on binding with K-Rta and that ORF59 fails to bind to oriLyt in the absence of K-Rta. Also, using the cotransfection replication assay, overexpression of the interaction domain of K-Rta with ORF59 has a dominant negative effect on oriLyt amplification, suggesting that the interaction of K-Rta with ORF59 is essential for DNA synthesis and supporting the hypothesis that K-Rta facilitates the formation of a replication complex at oriLyt.     

Intragenomic linear amplification of human herpesvirus 6B oriLyt suggests acquisition of oriLyt by transposition.

We identified some passage lineages of human herpesvirus 6 variant B (HHV-6B) strain Z29 that contain as many as 12 tandem copies of a genomic segment that corresponds almost precisely to a previously identified minimal efficient origin of lytic replication (oriLyt). Analysis of nucleotide sequences in the vicinity of the amplified segment suggests that the amplification occurred as a two-step process, with the first step being a rare sequence duplication mediated through directly repeated sequences located near the termini of the amplified segment and the second step occurring via homologous recombination through the duplicated sequence. These results demonstrate that oriLyt has been amplified in some virus stocks and indicate that (i) origin amplification confers a growth advantage on the virus in cell culture and (ii) laboratory-passaged HHV-6B genomes can accommodate additional nucleotide sequences and thus may be useful gene transfer vectors. The structures of the amplified segment and its adjacent sequences together suggest that HHV-6B or a progenitor virus acquired oriLyt by transposition from an unknown source.     

In vitro replication of adeno-associated virus DNA.

The study of eukaryotic viral DNA replication in vitro has led to the identification of cellular enzymes involved in DNA replication. Adeno-associated virus (AAV) is distinct from previously reported systems in that it is believed to replicate entirely by leading-strand DNA synthesis and requires coinfection with adenovirus to establish completely permissive replication. In previous work, we demonstrated that two of the AAV nonstructural proteins, Rep78 and -68, are site-specific endonucleases and DNA helicases that are capable of resolving covalently closed AAV termini, a key step in AAV DNA replication. We have now cloned the AAV nonstructural proteins Rep78, Rep68, and Rep52 in the baculovirus expression system. Using the baculovirus-expressed proteins, we have developed an efficient in vitro AAV DNA replication system which mimics the in vivo behavior of AAV in every respect. With no-end AAV DNA as the starting substrate, the reaction required an adenovirus-infected cell extract and the presence of either Rep78 or Rep68. Rep52, as expected, did not support DNA replication. A mutant in the AAV terminal resolution site (trs) was defective for DNA replication in the in vitro assay. Little, if any, product was formed in the absence of the adenovirus-infected HeLa cell extract. In general, uninfected HeLa extracts were less efficient in supporting AAV DNA replication than adenovirus-infected extracts. Thus, the requirement for adenovirus infection in vivo was partially duplicated in vitro. The reduced ability of uninfected HeLa extracts to support complete DNA replication was not due to a defect in terminal resolution but rather to a defect in the reinitiation reaction or in elongation. Rep78 produced a characteristic monomer-dimer pattern of replicative intermediates, but surprisingly, Rep68 produced little, if any, dimer replicative form. The reaction had a significant lag (30 min) before incorporation of 32P-deoxynucleoside triphosphate could be detected in DpnI-resistant monomer replicative form and was linear for at least 4 h after the lag. The rate of incorporation in the reaction was comparable to that in the simian virus 40 in vitro system. Replication of the complete AAV DNA molecule was demonstrated by the following criteria. (i) Most of the monomer and dimer product DNAs were completely resistant to digestion with DpnI. (ii) Virtually all of the starting substrate was converted to heavy-light or heavy-heavy product DNA in the presence of bromo-dUTP when examined on CsCl density gradients.(ABSTRACT TRUNCATED AT 400 WORDS)     

The Epstein-Barr Virus Lytic Transactivator Zta Interacts with the Helicase-Primase Replication Proteins

The Epstein-Barr virus transactivator Zta triggers lytic gene expression and is essential for replication of the lytic origin, oriLyt. Previous analysis indicated that the Zta activation domain contributed a replication-specific function. We now show that the Zta activation domain interacts with components of the EBV helicase-primase complex. The three helicase-primase proteins BBLF4 (helicase), BSLF1 (primase), and BBLF2/3 (primase-associated factor) were expressed fused to the Myc epitope. When expression plasmids for BBLF4 or BBLF2/3 plus BSLF1 (primase subcomplex) were separately transfected, the proteins localized to the cytoplasm. Interaction between Zta and the components of the helicase-primase complex was tested by examining the ability of Zta to alter the intracellular localization of these proteins. Cotransfection of Zta with Myc-BBLF4 resulted in nuclear translocation of Myc-BBLF4; similarly, cotransfection of Zta with the primase subcomplex led to nuclear translocation of the Myc-BSLF1 and Myc-BBLF2/3 proteins. This relocalization provides evidence for an interaction between Zta and the helicase and Zta and the primase subcomplex. An affinity assay using glutathione S-transferase–Zta fusion proteins demonstrated that Myc-BBLF4 and Myc-BBLF2/3 plus BSLF1 bound to the Zta activation domain (amino acids 1 to 133). In the nuclear relocalization assay, the amino-terminal 25 amino acids of Zta were required for efficient interaction with the primase subcomplex but not for interaction with BBLF4. Evidence for interaction between oriLyt bound Zta and the helicase-primase complex was obtained in a superactivation assay using an oriLyt-chloramphenicol acetyltransferase (CAT) reporter. Zta activated expression from a CAT reporter containing the complete oriLyt region and regulated by the oriLyt BHLF1 promoter. Cotransfection of the helicase-primase proteins, one of which was fused to a heterologous activation domain, led to Zta-dependent superactivation of CAT expression. This assay also provided evidence for an interaction between the single-stranded DNA binding protein, BALF2, and the Zta-tethered helicase-primase complex. The helicase-primase interaction is consistent with a role for Zta in stabilizing the formation of an origin-bound replication complex.     

Analysis of the Effects of Charge Cluster Mutations in Adeno-Associated Virus Rep68 Protein In Vitro

The Rep78 and Rep68 proteins of adeno-associated virus type 2 (AAV) are multifunctional proteins which are required for viral replication, regulation of AAV promoters, and preferential integration of the AAV genome into a region of human chromosome 19. These proteins bind the hairpin structures formed by the AAV inverted terminal repeat (ITR) origins of replication, make site- and strand-specific endonuclease cuts within the AAV ITRs, and display nucleoside triphosphate-dependent helicase activities. Additionally, several mutant Rep proteins display negative dominance in helicase and/or endonuclease assays when they are mixed with wild-type Rep78 or Rep68, suggesting that multimerization may be required for the helicase and endonuclease functions. Using overlap extension PCR mutagenesis, we introduced mutations within clusters of charged residues throughout the Rep68 moiety of a maltose binding protein-Rep68 fusion protein (MBP-Rep68Δ) expressed in Escherichia coli cells. Several mutations disrupted the endonuclease and helicase activities; however, only one amino-terminal-charge cluster mutant protein (D40A-D42A-D44A) completely lost AAV hairpin DNA binding activity. Charge cluster mutations within two other regions abolished both endonuclease and helicase activities. One region contains a predicted alpha-helical structure (amino acids 371 to 393), and the other contains a putative 3,4 heptad repeat (coiled-coil) structure (amino acids 441 to 483). The defects displayed by these mutant proteins correlated with a weaker association with wild-type Rep68 protein, as measured in coimmunoprecipitation assays. These experiments suggest that these regions of the Rep molecule are involved in Rep oligomerization events critical for both helicase and endonuclease activities.     

Induction of 68,000-dalton heat shock proteins by cyclopentenone prostaglandins. Its association with prostaglandin-induced G1 block in cell cycle progression

Cyclopentenone prostaglandins (PGs) such as PGA2 and delta 12-PGJ2 act specifically on cells in the G1 phase and induce block of cell cycle progression (Ohno, K., Sakai, T., Fukushima, M., Narumiya, S., and Fujiwara, M. (1988) J. Pharmacol. Exp. Ther. 245, 294-298). In this study, we characterized proteins induced by these PGs in HeLa S3 cells of synchronized growth and examined its association with the cell cycle block. HeLa S3 cells transiently expressed two 68-kDa proteins of isoelectric points of 5.5 and 5.6 in the G1 phase of cell cycle. When G1-enriched cells were incubated with either PGA2 or delta 12-PGJ2, synthesis of these proteins was markedly enhanced. Enhancement by delta 12-PGJ2 was persistent and irreversible, whereas that by PGA2 was reversible. delta 12-PGJ2 also enhanced the synthesis of two additional 68-kDa proteins with isoelectric points of 5.8 and 5.9. On two-dimensional gel electrophoresis, these proteins overlapped exactly with the 68-kDa heat shock proteins induced in cells treated at 43 degrees C for 90 min. They were also indistinguishable from the heat shock proteins in limited proteolysis. When delta 12-PGJ2 was incubated with G2/M phase cells, it induced only a small and transient increase in the 68-kDa proteins. These results suggest that cyclopentenone PGs extensively induce 68-kDa heat shock proteins in the G1 phase HeLa S3 cells and this induction is closely associated with the G1 block of cell cycle progression caused by these PGs.     

Open reading frames UL44, IRS1/TRS1, and UL36-38 are required for transient complementation of human cytomegalovirus oriLyt-dependent DNA synthesis.

Previous results showed that plasmids containing human cytomegalovirus (HCMV) oriLyt are replicated after transfection into permissive cells if essential trans-acting factors are supplied by HCMV infection (D. G. Anders, M. A. Kacica, G. S. Pari, and S. M. Punturieri, J. Virol. 66:3373-3384, 1992). We have now used oriLyt as a reporter of HCMV DNA replication in a transient complementation assay in which cotransfected cosmid clones, instead of HCMV infection, provided essential trans-acting factors. Complemented replication was oriLyt dependent and phosphonoformic acid sensitive and produced tandem arrays typical of HCMV lytic-phase DNA synthesis. Thus, this assay provides a valid genetic test to find previously unidentified genes that are essential for DNA synthesis and to corroborate functional predictions made by nucleotide sequence comparisons and biochemical analyses. Five cosmids were necessary and sufficient to produce origin-dependent DNA synthesis; all but one of these required cosmids contain at least one candidate homolog of herpes simplex virus type 1 replication genes. We further used the assay to define essential regions in two of the required cosmids, pCM1017 and pCM1052. Results presented show that UL44, proposed on the basis of biochemical evidence to be the HCMV DNA polymerase accessory protein, was required for complementation. In addition, three genomic regions encoding regulatory proteins also were needed to produce origin-dependent DNA synthesis in this assay: (i) IRS1/TRS1, which cooperates with the major immediate-early proteins to activate UL44 expression; (ii) UL36-38; and (iii) the major immediate-early region comprising IE1 and IE2. Combined, these results unequivocally establish the utility of this approach for mapping HCMV replication genes. Thus, it will now be possible to define the set of HCMV genes necessary and sufficient for initiating and performing lytic-phase DNA synthesis as well as to identify those virus genes needed for their expression in human fibroblasts.     

Effect of stimulating maize germination on cell cycle proteins

The germination process can be accelerated if seeds are stimulated either by adding cytokinins or by osmopriming. Under these conditions, cells in maize ( Zea mays ) embryo axes shorten the time at which the first round of DNA replication and mitosis takes place, thus advancing the cell cycle. Using heterologous antibodies against different cell cycle proteins, we have followed the behaviour of several markers for G1 phase (cyclin D, E2F and p53) and a marker of G2 phase (cyclin B) under either control or "accelerated" germination conditions. The results showed two classes of behaviour: either there was no variation in the amount of the protein present under control or accelerated germination conditions, represented by cyclin Band E2F‐type proteins, or the amount of the proteins was drastically reduced, more rapidly under accelerated germination, as was the case for cyclin D‐ and p53‐type proteins. Although the cyclin D‐type protein was synthesized de novo during germination, the balance was towards degradation so that there was no cyclin D detected 15 h after germination in benzyladenine‐treated and osmoprimed seeds. A Cdk4‐type protein seemed to be present in cyclin D immunoprecipitates and its kinase activity paralleled the fluctuations of the cyclin amount during germination. These data are discussed in the context of early seed germination.     

The augmentation of human natural killer cell activity by interferon-gamma is not associated with the induction of the interferon-alpha-inducible proteins

Treatment of partly purified large granular lymphocytes (LGL) with either IFN-alpha or IFN-gamma for 2 hr augmented their NK cell activity. This augmentation was completely inhibited by the addition of 10 micrograms/ml of cycloheximide. In contrast, when the effects of IFN-gamma on the synthesis of specific proteins in these cells was directly studied by use of two-dimensional gel electrophoresis, we found that IFN-gamma was unable to induce any of the earlier detected, IFN-alpha/IFN-beta-inducible proteins within 18 hr of incubation. No additional, IFN-gamma-induced proteins were detected in either the partly purified LGL or purified T cells. In contrast, the effects of the two factors were comparable in the glioma cell line 251 MG. This shows i) that the effects of IFN-alpha and IFN-gamma are dependent on the responder cell type, ii) that there exists at least one mechanism that can augment NK cell activity that is not dependent on the increased synthesis of the IFN-alpha-inducible proteins, and iii) that either the nine IFN-alpha-inducible proteins are not involved in any leukocyte function that is augmentable by both IFN-alpha and IFN-gamma, or that the two factors exert their actions in leukocyte through different mechanisms.     

Identification and analysis of a lytic-phase origin of DNA replication in human herpesvirus 7.

Human herpesvirus 7 (HHV-7) DNA sequences colinear with the HHV-6 lytic-phase origin of DNA replication (oriLyt) were amplified by PCR. Plasmid constructs containing these sequences were replicated in HHV-7-infected cord blood mononuclear cells but not in HHV-6-infected cells. In contrast, plasmids bearing HHV-6 oriLyt were replicated in both HHV-6- and HHV-7-infected cells. Finally, the minimal HHV-7 DNA element necessary for replicator activity was mapped to a 600-bp region which contains two sites with high homology to the consensus binding site for the HHV-6 origin binding protein. At least one of these binding sites was shown to be essential for replicator function of HHV-7 oriLyt.     

Human cytomegalovirus UL84 interacts with an RNA stem-loop sequence found within the RNA/DNA hybrid region of oriLyt

Human cytomegalovirus (HCMV) lytic DNA replication is initiated at the complex cis-acting oriLyt region, which spans nearly 3 kb. DNA synthesis requires six core proteins together with UL84 and IE2. Previously, two essential regions were identified within oriLyt. Essential region I (nucleotides [nt] 92209 to 92573) can be replaced with the constitutively active simian virus 40 promoter, which in turn eliminates the requirement for IE2 in the origin-dependent transient-replication assay. Essential region II (nt 92979 to 93513) contains two elements of interest: an RNA/DNA hybrid domain and an inverted repeat sequence capable of forming a stem-loop structure. Our studies now reveal for the first time that UL84 interacts with a stem-loop RNA oligonucleotide in vitro, and although UL84 interacted with other nucleic acid substrates, a specific interaction occurred only with the RNA stem-loop. Increasing concentrations of purified UL84 produced a remarkable downward-staircase pattern, which is not due to a nuclease activity but is dependent upon the presence of secondary structures, suggesting that UL84 modifies the conformation of the RNA substrate. Cross-linking experiments show that UL84 possibly changes the conformation of the RNA substrate. The addition of purified IE2 to the in vitro binding reaction did not affect binding to the stem-loop structure. Chromatin immunoprecipitation assays performed using infected cells and purified virus show that UL84 is bound to oriLyt in a region adjacent to the RNA/DNA hybrid and the stem-loop structure. These results solidify UL84 as the potential initiator of HCMV DNA replication through a unique interaction with a conserved RNA stem-loop structure within oriLyt.