Inhibition of generation of authentic genomic termini of herpes simplex virus type 1 DNA in temperature-sensitive mutant BHK-21 cells with a mutated CCG1/TAF(II)250 gene

The differential requirement for cyclin-dependent kinase activities distinguishes two functions of herpes simplex virus type 1 ICP0

Herpes simplex virus type 1 (HSV-1) ICP0 directs the degradation of cellular proteins associated with nuclear structures called ND10, a function thought to be closely associated with its broad transactivating activity. Roscovitine (Rosco), an inhibitor of cyclin-dependent kinases (cdks), inhibits the replication of HSV-1, HSV-2, human cytomegalovirus, varicella-zoster virus, and human immunodeficiency virus type 1 by inhibiting specific steps or activities of viral regulatory proteins, indicating the broad and pleiotropic effects that cdks have on the replication of these viruses. We previously demonstrated that Rosco inhibits the transactivating activity of ICP0. In the present study, we asked whether Rosco also affects the ability of ICP0 to direct the degradation of ND10-associated proteins. For this purpose, WI-38 cells treated with cycloheximide (CHX) were mock infected or infected with wild-type HSV-1 or an ICP0(-) mutant (7134). After release from the CHX block, the infections were allowed to proceed for 2 h in the presence or absence of Rosco at a concentration known to inhibit ICP0's transactivating activity. The cells were then examined for the presence of ICP0 and selected ND10-associated antigens (promyelocytic leukemia antigen [PML], sp100, hDaxx, and NDP55) by immunofluorescence. Staining for the ND10-associated antigens was detected in </=20% of KOS-infected cells in the presence or absence of Rosco, demonstrating that Rosco-sensitive kinases are not required for ICP0's ability to direct the dispersal or degradation of these antigens. In contrast, >90% of 7134- and mock-infected cells stained positive for all ND10-associated antigens in the presence or absence of Rosco. Similar results were obtained with a non-ND10-associated antigen, DNA-PK(cs), a known target of ICP0-directed degradation. The results of the PML and DNA-PK(cs) immunofluorescence studies correlated with a decrease in the levels of these proteins as determined by Western blotting. Thus, the differential requirement for Rosco-sensitive cdk activities distinguishes ICP0's ability to direct the dispersal or degradation of cellular proteins from its transactivating activity.

Cleavage in and around the DR1 element of the A sequence of herpes simplex virus type 1 relevant to the excision of DNA fragments with length corresponding to one and two units of the A sequence

The A sequence of herpes simplex virus type 1 (HSV-1) is a region bracketed by two direct repeats named DR1. Concatemeric HSV-1 DNA, the product of DNA replication, is cleaved at a specific site on the second DR1 distal from the S component (authentic cleavage) to yield unit-length linear HSV-1 DNA prior to or during packaging of HSV-1 DNA. The presence of two DNA bands, of 0.25 kb (shorter band) and 0.5 kb (longer band), the lengths of which correspond to one and two units of the A sequence, was identified using acrylamide gel electrophoresis of HSV-1 DNA preparations extracted by the method of Hirt. Twelve DNA fragments from each band were molecularly cloned, and nucleotide sequences were determined. Both termini of eight (67%) DNA clones from the shorter band corresponded to the specific cleavage site on DR1. Five (41%) DNA clones from the longer band had a terminus corresponding to the specific cleavage site on DR1 on one side, but not on the opposite side. Thirteen (54%) of 24 termini of 12 analyzed DNA clones from the longer band were in and around DR1. Thus, cleavage events of DR1 can be classified into three categories: (i) authentic cleavage; (ii) site-specific cleavage on the third DR1 distal from the S component (secondary site-specific cleavage), which is related to the generation of the shorter DNA band in combination with authentic cleavage; and (iii) less-specific cleavage events in and around other DR1 elements which relate to the generation of the longer DNA band.

Roscovitine, a specific inhibitor of cellular cyclin-dependent kinases, inhibits herpes simplex virus DNA synthesis in the presence of viral early proteins

We have previously shown that two inhibitors specific for cellular cyclin-dependent kinases (cdks), Roscovitine (Rosco) and Olomoucine (Olo), block the replication of herpes simplex virus (HSV). Based on these results, we demonstrated that HSV replication requires cellular cdks that are sensitive to these drugs (L. M. Schang, J. Phillips, and P. A. Schaffer. J. Virol. 72:5626-5637, 1998). We further established that at least two distinct steps in the viral replication cycle require cdks: transcription of immediate-early (IE) genes and transcription of early (E) genes (L. M. Schang, A. Rosenberg, and P. A. Schaffer, J. Virol. 73:2161-2172, 1999). Since Rosco inhibits HSV replication efficiently even when added to infected cells at 6 h postinfection, we postulated that cdks may also be required for viral functions that occur after E gene expression. In the study presented herein, we tested this hypothesis directly by measuring the efficiency of viral replication, viral DNA synthesis, and expression of several viral genes during infections in which Rosco was added after E proteins had already been synthesized. Rosco inhibited HSV replication, and specifically viral DNA synthesis, when the drug was added at the time of release from a 12-h phosphonoacetic acid (PAA)-induced block in viral DNA synthesis. Inhibition of DNA synthesis was not a consequence of inhibition of expression of IE or E genes in that Rosco had no effect on steady-state levels of two E transcripts under the same conditions in which it inhibited viral DNA synthesis. Moreover, viral DNA synthesis was inhibited by Rosco even in the absence of protein synthesis. In a second series of experiments, the replication of four HSV mutants harboring temperature-sensitive mutations in genes essential for viral DNA replication was inhibited when Rosco was added at the time of shift-down from the nonpermissive to the permissive temperature. Viral DNA synthesis was inhibited by Rosco under these conditions, whereas expression of viral E genes was not affected. We conclude that cellular Rosco-sensitive cdks are required for replication of viral DNA in the presence of viral E proteins. This requirement may indicate that HSV DNA synthesis is functionally linked to transcription, which requires cdks, or that both viral transcription and DNA replication, independently, require viral or cellular factors activated by Rosco-sensitive cdks.

Proteolytic cleavage of the amino terminus of the U(L)15 gene product of herpes simplex virus type 1 is coupled with maturation of viral DNA into unit-length genomes

The U(L)15 gene of herpes simplex virus type 1 (HSV-1), like U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33, is required for cleavage of concatameric DNA into genomic lengths and for packaging of cleaved genomes into preformed capsids. A previous study indicated that the U(L)15 gene encodes minor capsid proteins. In the present study, we have shown that the amino-terminal 509 amino acids of the U(L)15-encoded protein are sufficient to confer capsid association inasmuch as a carboxyl-terminally truncated form of the U(L)15-encoded protein with an M(r) of approximately 55,000 readily associated with capsids. This and previous studies have shown that, whereas three U(L)15-encoded proteins with apparent M(r)s of 83,000, 80,000, and 79,000 associated with wild-type B capsids, only the full-length 83,000-M(r) protein associated with B capsids purified from cells infected with viruses lacking functional U(L)6, U(L)17, U(L)28, U(L)32, and U(L)33 genes (B. Salmon and J. D. Baines, J. Virol. 72:3045-3050, 1998). Thus, all viral mutants that fail to cleave viral DNA into genomic-length molecules also fail to produce capsid-associated U(L)15 80,000- and 79,000-M(r) proteins. In contrast, the 80,000- and 79,000-M(r) proteins were readily detected in capsids purified from cells infected with a U(L)25 null virus that cleaves, but does not package, DNA. The conclusion that the amino terminus of the 83,000-M(r) protein is truncated to produce the 80,000- and/or 79,000-M(r) protein was supported by the following observations. (i) Whereas the C termini of the 83,000-, 80, 000-, and 79,000-M(r) proteins are identical, immunoreactivity dependent on the first 35 amino acids of the U(L)15 83,000-M(r) protein was absent from the 80,000- and 79,000-M(r) proteins. (ii) The 79,000- and 80,000-M(r) proteins were detected in capsids from cells infected with HSV-1(U(L)15M36V), an engineered virus encoding valine rather than methionine at codon 36. Thus, initiation at codon 36 is unlikely to account for production of the 80,000- and/or 79, 000-M(r) protein. Taken together, these data strongly suggest that capsid-associated U(L)15-encoded protein is proteolytically cleaved near the N terminus and indicate that this modification is tightly linked to maturation of genomic DNA.

Mechanism and application of genetic recombination in herpesviruses

Herpes simplex virus type 1 (HSV-1) is a ubiquitous human pathogen that latently infects sensory ganglia and encodes over 80 genes in a 152 kbp DNA genome. This well characterised virus provides a model for analysing genetic recombination in herpesviruses, a fundamental biological process by which new combinations of genetic materials are generated. The frequency of homologous recombination was estimated to be 0.0048-0.007 (0.48%-0.7%)/kb of the HSV-1 genome, determined using physical markers. The double-strand break repair model, the current model of homologous recombination, adequately explains L-S inversion of herpesvirus genomes and the recombinogenicity of the a sequence. Several herpesvirus genomes, including HSV-1 consist of a unique sequence bracketed by a pair of inverted repeat sequences. This arrangement is attributed to illegitimate recombination between molecules arranged in an inverse orientation. Junctions of unique and repeated sequences that correspond to the crossover site of illegitimate recombination are recombinogenic. Recombination is important for virus evolution, construction of mutated virus, gene therapy and vaccination in which the potential for recombination between engineered input virus and wild type virus has to be considered.

Transcription of herpes simplex virus immediate-early and early genes is inhibited by roscovitine, an inhibitor specific for cellular cyclin-dependent kinases

Although herpes simplex virus (HSV) replicates in noncycling as well as cycling cells, including terminally differentiated neurons, it has recently been shown that viral replication requires the activities of cellular cyclin-dependent kinases (cdks) (L. M. Schang, J. Phillips, and P. A. Schaffer, J. Virol. 72:5626-5637, 1998). Since we were unable to isolate HSV mutants resistant to two cdk inhibitors, Olomoucine and Roscovitine (Rosco), we hypothesized that cdks may be required for more than one viral function during HSV replication. In the experiments presented here, we tested this hypothesis by measuring the efficiency of (i) viral replication; (ii) expression of selected immediate-early (IE) (ICP0 and ICP4), early (E) (ICP8 and TK), and late (L) (gC) genes; and (iii) viral DNA synthesis in infected cultures to which Rosco was added after IE or IE and E proteins had already been synthesized. Rosco inhibited HSV replication, transcription of IE and E genes, and viral DNA synthesis when added at 1, 2, or 6 h postinfection or after release from a 6-h cycloheximide block. Transcription of a representative L gene, gC, was also inhibited by Rosco under all conditions examined. We conclude from these studies that cellular cdks are required for transcription of E as well as IE genes. In contrast, steady-state levels of at least one cellular housekeeping gene were not affected by Rosco. The requirement of viral IE and E transcription for cellular cdks may reflect either a requirement for specific cdk-activated cellular and/or viral transcription factors or a more global requirement for cdks in the transcriptional activation of the viral genome.

Requirement for cellular cyclin-dependent kinases in herpes simplex virus replication and transcription

Several observations indicate that late-G1/S-phase-specific cellular functions may be required for herpes simplex virus (HSV) replication: (i) certain mutant HSV strains are replication impaired during infection of cells in the G0/G1 but not in the G1/S phase of the cell cycle, (ii) several late-G1/S-phase-specific cellular proteins and functions are induced during infection, and (iii) the activity of a cellular protein essential for expression of viral immediate-early (IE) genes, HCF, is normally required during the late G1/S phase of the cell cycle. To test the hypothesis that late-G1/S-phase-specific cellular functions are necessary for HSV replication, HEL or Vero cells were infected in the presence of the cell cycle inhibitors roscovitine (Rosco) and olomoucine (Olo). Both drugs inhibit cyclin-dependent kinase 1 (cdk-1) and cdk-2 (required for cell cycle progression into the late G1/S phase) and cdk-5 (inactive in cycling cells) but not cdk-4 or cdk-6 (active at early G1). We found that HSV replication was inhibited by Rosco and Olo but not by lovastatin (a cell cycle inhibitor that does not inhibit cdk activity), staurosporine (a broad-spectrum protein serine-threonine kinase inhibitor), PD98059 (an inhibitor specific for erk-1 and -2) or iso-Olo (a structural isomer of Olo that does not inhibit cdk activity). The concentrations of Rosco and Olo required to inhibit cell cycle progression and viral replication in both HEL and Vero cells were similar. Inhibition of viral replication was found not to be mediated by drug-induced cytotoxicity. Efforts to isolate Rosco- or Olo-resistant HSV mutants were unsuccessful, indicating that these drugs do not act by inhibiting a single viral target. Viral DNA replication and accumulation of IE and early viral RNAs were inhibited in the presence of cell cycle-inhibitory concentrations of Rosco or Olo. We therefore conclude that one or more cdks active from late G1 onward or inactive in nonneuronal cells are required for accumulation of HSV transcripts, viral DNA replication, and production of infectious virus.

Sp1 binds to the precise locus of end processing within the terminal repeats of Epstein-Barr virus DNA

Interconversion between the linear genome of Epstein-Barr virus (EBV) present in virions and intracellular circular EBV DNA is a novel DNA recombination process. A previously characterized DNA binding activity called terminal repeat or tandem repeat binding protein (TRBP) was found to recognize several G-rich recombinogenic sequences in the EBV genome and in cellular DNA. TRBP was also found to be an autoantigen recognized by sera from certain patients with undifferentiated connective-tissue disorders. Here the transcription factor Sp1 has been identified as a component of TRBP and has been shown to be an autoantigen. Sp1 bound to recombination junctions of EBV DNA, such as those in the terminal repeats and in the large internal repeats, as well as to recombinogenic regions of cellular DNA, such as variable-number tandem repeats and switch regions of the immunoglobulin genes. We defined the ends of the linear EBV genome present in virions and showed that Sp1 binds to the sequence (GGGGTGGGGCATGGG) within EBV terminal repeats at the precise locus of interconversion of linear and circular viral DNA. Sp1 may be involved in DNA recombination.