Cytofluorometry of lymphocytes infected with Epstein-Barr virus: effect of phosphonoacetic acid on nucleic acid.

DNA synthesis in Epstein-Barr virus (EBV)-infected lymphocytes was inhibited by phosphonoacetic acid (PAA) as measured by [3H]thymidine incorporation. PAA, at a concentration of 200 microgram/ml, inhibited [3H]thymidine incorporation by human umbilical cord lymphocytes infected with EBV strain P94 but had little effect on DNA synthesis in mitogen-stimulated cells. Transformed cell lines did not develop from infected cord cell cultures treated with 100 microgram of PAA per ml. Cytofluorometric analysis showed marked increases in cellular nucleic acid content (RNA plus DNA) as early as 9 days after infection of cord cells in the absence of PAA and before significant enhancement of [3H]thymidine incorporation became apparent. Moreover, EBV led to increases in cellular nucleic acid even when 200 microgram of PAA per ml was added to cell cultures before infection. The apparent discrepancy between results obtained by [3H]thymidine incorporation and cytofluorometry is explained either by significant inhibition of cellular DNA polymerases by PAA or by a block at the G2 + M phase of the cell cycle. The data suggest that EBV initiates alterations in cellular nucleic acid synthesis or cell division without prior replication of viral DNA by virus-induced DNA polymerases.     

Replication of Simian Virus 40 Deoxyribonucleic Acid: Analysis of the One-Step Growth Cycle

The time course of replication of simian virus 40 deoxyribonucleic acid (DNA) was investigated in growing monolayer cultures of subcloned CV1 cells. At multiplicities of infection of 30 to 60 plaque-forming units (PFU)/cell, first progeny DNA molecules (component 1) were detected by 10 hr after infection. During the following 10 to 12 hr, accumulation of virus DNA proceeded at ever increasing rates, albeit in a non-exponential fashion. The rate of synthesis then remained constant, until approximately the 40th hour postinfection, when DNA replication stopped. Under these conditions, the duration of the virus growth cycle was approximately 50 hr. The time needed for the synthesis of one DNA molecule was found to be approximately 15 min. At multiplicities of infection of 1 or less than 1 PFU/cell, the onset of the linear phase of DNA accumulation was delayed, but the final rate of DNA synthesis was the same, independent of the input multiplicity. This was taken as a proof that templates for the synthesis of viral DNA multiply in the cell during the early phase of replication. However, the probability for every replicated DNA molecule to become in turn replicative decreased constantly during that phase. This could be accounted for by assuming a limited number of replication sites in the infected cell.     

Early Inhibition of Cellular DNA Synthesis by High Multiplicities of Infectious and UV-Inactivated Reovirus

Inhibition of cellular DNA synthesis began 6 to 8 h after reovirus infection at a multiplicity of infection of 10 PFU per cell. However, as the multiplicity of infection was increased to a maximum of 10³ PFU/cell, inhibition of DNA synthesis began earlier after infection (2-4 h postinfection), and the initial rate of inhibition increased. The enhanced inhibition of DNA replication at high virus multiplicities appeared to be selective since RNA synthesis was not detectably altered as late as 9 h postinfection and inhibition of protein synthesis did not begin until 7 to 9 h after infection. Early inhibition of DNA synthesis did not appear to be related to changes in thymidine pool characteristics, thymidine kinase activity, or detectable degradation of cellular DNA. Even though the particle-to-PFU ratio was increased by ultraviolet light inactivation of virus, the ability to induce early inhibition of DNA synthesis was not diminished.     

Circular Epstein-Barr virus genomes of reduced size in a human lymphoid cell line of infectious mononucleosis origin.

Circular Epstein-Barr virus (EBV) DNA molecules have been purified and characterized from a human lymphoid cell line derived from a case of heterophile antibody-positive, blood transfusion-induced infectious mononucleosis, 883L. The circular EBV DNA in three cell lines obtained by transformation of human umbilical cord blood leukocytes with a strain of EBV originally derived from 883L was also studied. As estimated from sedimentation velocity data and electron microscopy, the circular EBV DNA molecules are 10 to 15% smaller than either the circular EBV DNA previously found intracellularly in several other types of EBV-transformed cells or the linear EBV DNA present extracellularly in virus particles. In addition, the EBV-transformed cord blood cell lines studied here differed from other EBV-transformed cells in that integrated virus DNA sequences could not be detected.     

Biological properties and viral surface antigens of Burkitt lymphoma- and mononucleosis- derived strains of Epstein-Barr virus released from transformed marmoset cells.

Three strains of Epstein-Barr virus (EBV), two from Burkitt lymphoma (BL) and one from infectious mononucleosis (IM) were used to transform separate cultures of the same batch of primary marmoset leukocytes, and the viruses released from the transformants were compared. The three viruses shared properties of the transforming biotype of EBV, namely, stimulation of DNA synthesis and immortalization of cord blood leukocytes, and failure to induce "early antigen" in lymphoblast lines. All viruses produced more virus in transformed marmoset cells than in transformed human cells, as measured by the number of EBV genomes detected by complementary RNA/DNA hybridization, by virus capsid antigen expression, or by released virions and biologically active virus. Reference human sera and sera from primary EBV infections were used to compare the three virus strains in a virus neutralization test based on inhibition of stimulation of DNA synthesis. Specimens taken late in convalescence from patients with mononucleosis and sera from marmosets experimentally infected with virus from a patient with mononucleosis neutralized the homologous virus, as well as the two virus strains isolated from patients with BL. This finding indicates that viral antigens that elicit neutralizing antibodies are shared among the strains. However, in certain sera the neutralizing-antibody titer against one strain was consistently higher than against another strain. Furthermore, sera taken early after onset of IM contained low levels of neutralizing antibody against IM-derived virus, but failed to neutralize BL-derived virus. These latter findings suggest the existence of heterogeneity among surface antigens of EBVs. The results emphasize the biological and antigenic similarity of EBV isolates from BL and IM and do not suggest major subtype variations. It remains to be determined whether antigenic diversity such as described or virus genome variation detectable by other means is epidemiologically significant.     

Physiological State of Human Embryonic Lung Cells Affects Their Response to Human Cytomegalovirus

Cultures of human embryonic lung (HEL) cells in different physiological states were studied for their susceptibility to infection with human cytomegalovirus (CMV) with respect to production of infectious virus, synthesis of viral antigens, and virus-induced stimulation of cellular DNA synthesis. In general, subconfluent, actively growing cells yielded higher amounts of infectious virus than did confluent contact-inhibited cells. The higher yield of infectious virus was correlated with a greater percentage of cells producing viral antigens within the first 48 h after infection. In confluent cultures, 25 to 50% of the cells produced viral antigens within the first 48 h postinfection. This proportion did not change over a 10-fold range of multiplicity of infection, indicating that many of the cells in confluent cultures did not support productive infection. However, virtually all the cells in subconfluent cultures were susceptible. Also, in contrast to herpes simplex virus and pseudorabies virus, infectious CMV is not produced by cells treated with 5-fluorouracil and thymidine. Virus-induced stimulation of cellular DNA synthesis in cells infected at high multiplicities of infection could be detected only in confluent cultures, in which cellular DNA synthesis had been previously suppressed, but could not be detected in similarly treated cultures of subconfluent cells. The lack of detectable stimulation of cellular DNA synthesis in the latter was related to the fact that practically all the cells in the culture synthesized viral antigens within the first 48 h after infection, productive infection and detectable synthesis of cellular DNA being mutually exclusive.     

Inhibition of host cell protein synthesis by UV-inactivated poliovirus.

The ability of poliovirus that was irradiated with UV light at energies up to 2,160 ergs/mm2 to subsequently inhibit host cell protein synthesis was measured. The inactivation of the host cell shutoff function followed one-hit kinetics. Increasing irradiation did not affect the rate of inhibition until the multiplicity of infection after irradiation was reduced to approximately 1 PFU/cell. At higher functional multiplicities, the rate was unchanged, but an increasing lag before the onset of inhibition was observed with increasing irradiation. The energy levels required to inactivate virus-induced inhibition of host cell protein synthesis suggest that damage to virus RNA rather than to virus capsid proteins is responsible for the loss of function. When the inactivation of host cell shutoff was compared with the inactivation of other viral functions by UV irradiation, it correlated exactly with the loss of infectivity but not with other viral functions measured. Guanidine treatment, which prevents detectable viral RNA and protein synthesis, completely inhibited host cell shutoff by low multiplicities of unirradiated virus infection but not higher multiplicities. When a high multiplicity of virus was first reduced to a low titer by irradiation, host cell shutoff was still evident in the presence of guanidine. The results demonstrate that the complete inhibition of host cell protein synthesis can be accomplished by one infectious viral genome per cell.     

Isolation of Epstein Barr-virus and studies of its neutralization by human IgG and complement

An isolation procedure for Epstein-Barr virus (EBV) that yields substantial quantities of purified infectious virus is described. The transforming strain of EBV was obtained from the marmoset lymphoma cell line B95-8 after stimulation with the tumor promoter, 12-O-tetradecanoylphorbol-13-acetate (TPA). Purification was achieved by dextran density gradient ultracentrifugation in the presence of bacitracin, which was included to prevent viral aggregation. When assayed in cord blood leukocytes, isolated EBV stimulated DNA synthesis and induced the formation of colonies of transformed cells. The yield of infectious virus as determined by these assays was 13 to 29%. Electron microscopic (EM) examination of negatively stained virions revealed the presence of 115-nm spherical enveloped particles containing an internal 55-nm ring-shaped nucleoid. Interactions between 3H-thymidine labeled EBV, IgG and complement (C) were examined by rate zonal ultracentrifugation. High concentrations of immune IgG aggregated the virus whereas IgG together with C induced lysis as demonstrated by release of labeled EBV nucleic acid. EM studies of the IgG and C mixtures performed in parallel revealed accumulation of protein on the viral envelope, progressive separation of the envelope from the nucleocapsid, and disintegration of the nucleoid. Approximately 25-fold less IgG was required for neutralization than for viral aggregation. Although C did not enhance the IgG dependent neutralization, physiologic concentrations of C in normal nonimmune human serum also inactivated the virus.     

Consequences of herpes simplex virus type 2 and human cell interaction at supraoptimal temperatures.

The consequences of herpes simplex virus type 2 (HSV-2) and human embryonic fibroblast cell interaction at different temperatures (37, 40, and 42 degrees C) were investigated. Incubation at 37 or 40 degrees C was permissive for HSV-2 inhibition of host DNA synthesis, induction of virus-specific DNA replication, and infectious virus production. The amount of [methyl-3H]thymidine incorporated into viral DNA and the final yield of new infectious virus were significantly reduced at 40 degrees C compared to 37 degrees C. At 42 degrees C, detectable virus-specific DNA synthesis was totally blocked. Maximum stimulation of host cell DNA synthesis at 42 degrees C was measured after a multiplicity of infection of 0.5 to 1.0 PFU/cell. By autoradiography, data indicated that HSV-2 stimulates host cell chromosomal DNA synthesis. Stimulation of thymidine kinase activity with thermostability properties in common with a virus enzyme was detected during the first 24 h of infection at 42 degrees C, after 24 h the enhanced thymidine kinase activity had properties in common with host cell isozymes. The data obtained during this investigation indicated that stimulation of host cell DNA synthesis does not require viral DNA synthesis.     

Herpes Simplex Virus Type 1 Infection of Isogenic Epstein-Barr Virus Genome-Negative and -Positive Burkitt''s Lymphoma-Derived Cell Lines

The Epstein-Barr virus (EBV) genome-negative Burkitt's lymphoma-derived cell lines BJAB and Ramos and their in vitro EBV-converted sublines BJAB-B1, BJAB-A5, BJAB-B95-8, and AW-Ramos were infected with high multiplicities of herpes simplex virus type 1 (HSV-1; 10 to 70 PFU/cell). Cultures were monitored for cell growth and HSV-1 DNA synthesis. EBV-converted BJAB cultures were more permissive for HSV-1 infection than BJAB cultures. Significant cell killing and HSV-1 DNA synthesis were observed during the first 48 h of infection in the EBV-converted BJAB cultures but not in the BJAB cultures. The EBV-converted BJAB-B1 cell line contains an appreciable fraction of EBV-negative cells. Therefore, it was cloned. EBV-positive and -negative cells were identified by using EBV-determined nuclear antigen anti-complement immunofluorescence. Two types of subclones were identified: (i) those which contained both EBV-determined nuclear antigen-positive and -negative cells and (ii) those which contained only EBV-determined nuclear antigen-negative cells. When levels of HSV-1 DNA synthesis were measured in these subclones, it was found that the former were more permissive for HSV-1 infection than the latter. Thus, the presence of the EBV genome in BJAB cells correlates with increased permissiveness of these cells for HSV-1 during the first 48 h of infection. Nonetheless, persistent HSV-1 infections were established in both BJAB and EBV-converted BJAB-B1 cultures. No differences in extent of permissiveness for HSV-1 infection were found for Ramos and EBV-converted AW-Ramos cells.     

Rescue of Simian Virus 40 from Cell Lines Transformed at High and at Low Input Multiplicities by Unirradiated or Ultraviolet-irradiated Virus

The relation between simian virus 40 (SV40) input multiplicity during transformation of primary mouse kidney cultures and the subsequent rescue of SV40 from clonal lines of transformed cells has been studied. Primary mouse kidney cultures were transformed with unirradiated SV40 at input multiplicities varying from 0.06 to 200 plaque-forming units (PFU) /cell or with SV40 irradiated with ultraviolet (UV) light to a survival of 0.04 to 0.01. All of the transformed lines contained the intranuclear SV40 T antigen, but cell-free extracts prepared from the transformed cell lines failed to yield infectious virus when assayed on monkey kidney cell (CV-1) monolayers. After fusion with susceptible CV-1 cells induced by UV-inactivated Sendai, all of the lines transformed by unirradiated virus yielded infectious SV40. The frequency of induction and the incidence of successful trials did not depend on the multiplicity of infection. “Good” yielders were obtained from mouse kidney cells transformed at the low input multiplicity of 0.06 PFU /cell. In contrast, only 4 of 12 clonal lines transformed at moderately low input multiplicity, and none of the lines transformed at very low input multiplicity with UV-irradiated virus yielded infectious SV40. The four positive lines have been classified as “poor” or “rare” yielders.     

Repair and mutagenesis of herpes simplex virus in UV-irradiated monkey cells

Mutagenic repair in mammalian cells was investigated by determining the mutagenesis of UV-irradiated or unirradiated herpes simplex virus in UV-irradiated CV-1 monkey kidney cells. These results were compared with the results for UV-enhanced virus reactivation (UVER) in the same experimental situation. High and low multiplicities of infection were used to determine the effects of multiplicity reactivation (MR). UVER and MR were readily demonstrable and were approximately equal in amount in an infectious center assay. For this study, a forward-mutation assay was developed to detect virus mutants resistant to iododeoxycytidine (ICdR), probably an indication of the mutant virus being defective at its thymidine kinase locus. ICdR-resistant mutants did not have a growth advantage over wild-type virus in irradiated or unirradiated cells. Thus, higher fractions of mutant virus indicated greater mutagenesis during virus repair and/or replication. The data showed that: (1) unirradiated virus was mutated in unirradiated cells, providing a background level of mutagenesis; (2) unirradiated virus was mutated about 40% more in irradiated cells, indicating that virus replication (DNA synthesis?) became more mutagenic as a result of cell irradiation; (3) irradiated virus was mutated much more (about 6-fold) than unirradiated virus, even in unirradiated cells; (4) cell irradiation did not change the mutagenesis of irradiated virus except at high multiplicity of infection. High multiplicity of infection did not lead to higher mutagenesis in unirradiated cells. Thus the data did not demonstrate UVER or MR alone to be either error-free or error-prone. When the two processes were present simultaneously, they were mutagenic.     

Herpes Simplex Virus Type 2 Functions Expressed During Stimulation of Human Cell DNA Synthesis

Experiments were designed to identify herpes simplex virus type 2 (HSV-2)-specific functions expressed during stimulation of human embryo fibroblast DNA synthesis. Cultures were partially arrested in DNA synthesis by pretreatment with 5-fluorouracil and maintenance in low-serum (0.2%) medium during virus infection. Results showed that continuous [methyl-(3)H]thymidine uptake into cellular DNA was ninefold greater in HSV-2-infected than in mock-infected cultures measured after 24 h of incubation at 42 degrees C. Shifting mock-infected cultures from low- to high-serum (10%) medium also caused some stimulation, but [methyl-(3)H]thymidine uptake was only twofold greater than in cells maintained with low serum. Plating efficiencies of both HSV-2-infected and mock-infected cells at 42 degrees C were essentially the same and ranged from 37 to 76% between zero time and 72 h of incubation. De novo RNA and protein syntheses were continuously required for HSV-2 stimulation of cellular DNA synthesis. HSV-2 infection markedly enhanced transport, phosphorylation, and rate of incorporation of [methyl-(3)H]thymidine into cellular DNA, starting at 3 h and reaching a maximum by 12 h; after 12 h, these processes gradually declined to low levels. In mock-infected cells these processes remained at low levels throughout the observation period. Pretreatment of cells with interferon or addition of arabinofuranosylthymine at the time of virus infection inhibited stimulation caused by HSV-2. 5-Bromodeoxyuridine density-labeled experiments revealed that HSV-2 stimulates predominantly semiconservative DNA replication and some DNA repair. Stimulation of [methyl-(3)H]thymidine into cellular DNA correlated with detection of virus-specific thymidine kinase activity. In conclusion, HSV-2 stimulation of cellular DNA synthesis appeared to involve at least four virus-specific functions: induction of thymidine transport, HSV-2 thymidine kinase activity, semiconservative replication, and repair of cellular DNA.     

Role of the simian virus 40 gene A product in regulation of DNA synthesis in transformed cells.

Cells transformed by tsA mutants of simian virus 40 (SV40) are temperature sensitive for the maintenance of the transformed phenotype. The kinetics of induction of DNA synthesis were determined for hamster cell transformants shifted to the permissive temperature after a 48-h serum arrest at the nonpermissive temperature. DNAsynthesis was initiated in the tsA transformants by 8 h after shiftdown was maximal by 12 h. The presence or absence of fetal bovine serum at the time of temperature shift had no effect on the kinetics of initiation of DNA synthesis. Analysis of TTP in tsA transformants revealed similar levels of incorporation of [3H]thymidine into TTP at both permissive and nonpermissive temperatures. Autoradiography revealed that by 12 h after a shift to the permissive temperature, approximately 50% of the cells exhibited labeled nuclei after a 60-min pulse with [3H]thymidine, indicating that a majority of the cells were actively synthesizing DNA. By 8 to 12 h after a shiftup of confluent tsA transformants to the nonpermissive temperature, the number of labeled nuclei was reduced to approximately 16%, regardless of serum concentration. These data indicate that the SV40 gene A product, either directly or indirectly, regulates cellular DNA synthesis in transformed cells.     

Effects of adenine arabinoside on lymphocytes infected with Epstein-Barr virus.

Low concentrations of adenine arabinoside inhibited growth of two Epstein-Barr virus producer cell lines in culture, while not significantly affecting a nonproducer cell line and a B-cell-negative line. These observations were extended to include freshly infected cells. Mitogen-stimulated human umbilical cord blood lymphocytes were unaffected by the drug at concentration levels that inhibited [3H]thymidine incorporation into the DNA of Epstein-Barr virus-stimulated cells. DNA synthesis in Epstein-Barr virus-superinfected Raji cells was also adversely affected by adenine arabinoside. However, these same low concentrations of adenine arabinoside in the triphosphate form produced less effect on DNA synthesis in nuclear systems and DNA polymerase assays than on growth or DNA synthesis in whole cells. Therefore the effects reported here of low concentrations of the drug on whole cells may be only in part related to DNA polymerase inhibition. The work reported here suggests that adenine arabinoside has multiple sites of action in infected cells.     

Effect of acyclovir [9-(2-hydroxyethoxymethyl)guanine] on Epstein-Barr virus DNA replication.

The effect of acyclovir [9-(2-hydroxyethoxymethyl)guanine] on Epstein-Barr virus (EBV) DNA replication in the lymphoblastoid cell lines P3HR-1 and Raji is reported. Acyclovir at a concentration of 100 microM completely inhibited EBV DNA synthesis in superinfected Raji cells, but did not inhibit DNA synthesis in mock-infected cells. The number of EBV genome equivalents per cell in the virus-producing cell line P3HR-1 was significantly reduced by acyclovir, whereas the number of latent EBV genomes in Raji cells was not affected by the drug. In situ cytohybridization performed on untreated P3HR-1 cultures revealed the presence of relatively large amounts of EBV DNA in 15 to 20% of the cells. After a 100 microM drug treatment, no P3HR-1 cells contained levels of EBV DNA detectable by in situ cytohybridization. Indirect immunofluorescence studies demonstrated that during treatment with 100 microM acyclovir for 7 days, the percentage of P3HR-1 cells expressing viral capsid antigen was reduced. The EBV DNA remaining in P3HR-1 cells after treatment with 100 microM acyclovir (approximately 14 genomes per cell) had the properties of covalently closed circular DNA with an average molecular weight of 108 X 10(6), as determined by contour length measurements.     

Kinetics of Nucleic Acid Synthesis in Human Embryonic Kidney Cultures Infected with Adenovirus 2 or 12: Inhibition of Cellular Deoxyribonucleic Acid Synthesis

Infection of human embryonic kidney (HEK) cell cultures with adenovirus types 2 or 12 resulted in an initial drop in the rate of incorporation of (3)H-thymidine into deoxyribonucleic acid (DNA) during the early latent period of virus growth, followed by a marked rise in label uptake. It was shown by cesium chloride isopycnic centrifugation that, after adenovirus 2 infection, there was a decrease in the rate of incorporation of thymidine into cellular DNA. Moreover, DNA-DNA hybridization experiments revealed that, by 28 to 32 hr after infection with either adenovirus 2 or 12, the amount of isolated pulse-labeled DNA capable of hybridizing with HEK cell DNA was reduced by approximately 60 to 70%. Autoradiographic measurements showed that the inhibition of cellular DNA synthesis was due to a decrease in the ability of an infected cell to synthesize DNA. The adenovirus-induced inhibition of host cell DNA synthesis was not due to degradation of cellular DNA. (3)H-thymidine incorporated into cellular DNA at the time of infection remained acid-precipitable, and labeled material was not incorporated into viral DNA. Furthermore, when zone sedimentation through neutral or alkaline sucrose density gradients was employed, no detectable change was observed in the sedimentation rate of this cellular DNA at various times after infection with adenovirus 2 or 12. In addition, there was no increase in deoxyribonuclease activity in cells infected with either virus. Cultures infected for 38 hr with adenovirus 2 or 12 incorporated three to four times as much (3)H-uridine into ribonucleic acid (RNA) as did non-infected cultures. Furthermore, the net RNA synthesized by infected cultures substantially exceeded that of control cultures. The activity of thymidine kinase was induced, but there was no stimulation of uridine kinase.     

Herpes simplex type 1 activation by Epstein-Barr virus nuclear antigen 1

We have investigated the effect of Epstein-Barr virus nuclear antigen 1 (EBNA-1), a nuclear protein encoded by EBV, on herpes simplex virus type 1 (HSV-1) infection either in cells constitutively expressing EBNA-1 or in transient expression assays. Rat-1 cells and rat embryo fibroblasts (REF) immortalized by c-myc or E1A were transfected with a specific EBV DNA fragment coding for EBNA-1. Cloned cell lines which constitutively expressed this antigen were infected with HSV-1. Our results indicate that in EBNA-1-expressing cells, virus growth was higher than in control cells for different virus strains or rodent cell lines. This increase was maximal when cells were infected at low multiplicity, as determined by virus growth, and correlated with the stimulation of viral DNA synthesis. REF + c-myc and Vero cells were contransfected by an EBNA-1 expression vector driven by Moloney murine leukaemia virus LTR and HSV-1 immediate-early (α0) or early thymidine kinase upstream promoter regulatory regions linked to chloramphenicol acetyltransferase (CAT) coding sequences as effectors. In both cell lines, stimulation of CAT expression by EBNA-1 was observed only with the immediate-early promoter. These results suggest that EBNA-1 can transactivate immediate-early HSV-1 expression.     

Virus DNA replication and protein synthesis in Amsacta moorei entomopoxvirus-infected Estigmene acrea cells

Amsacta moorei entomopoxvirus DNA synthesis was detected in Estigmene acrea cells by [³H]thymidine incorporation 12 hr after virus inoculation. Hybridization of ³²P-labeled Amsacta entomopoxvirus DNA to the DNA from virus-infected cells indicated that viral-specific DNA synthesis was initiated between 6 and 12 hr after virus inoculation. A rapid increase in the rate of virus DNA synthesis was detected from 12 to 24 hr after virus inoculation. Amsacta entomopoxvirus protein biosynthesis in E. acrea cells was studied by [su35S]methionine incorporation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Extracellular virus and virus-containing occlusion bodies were first detected in virus-infected cell cultures 18 hr after virus inoculation. Thirty-seven virus structural proteins, ranging in molecular weight from 13,000 to 208,000 were detected in both occluded and nonoccluded forms of the virus. The biosynthesis of virus structural proteins increased rapidly from 18 to 34 hr after infection. A major viral-induced protein corresponding in molecular weight to viral occlusion body protein (110,000) was detected approximately 24 hr after virus inoculation.