Replication of Spodoptera exigua nucleopolyhedrovirus in permissive and non-permissive lepidopteran cell lines

The Spodoptera exigua multinucleocapsid nucleopolyhedrovirus (SeMNPV) was inoculated to eight lepidopteran cell lines derived from Spodoptera exigua (Se301), Spodoptera frugiperda (SF21AEII), Spodoptera littoralis (CLS-79), Spodoptera litura (SpLi-221), Pseudaletia separata (LeSe-11), Trichoplusia ni (hi-5), Plutella xylostella (PXL/C) and Bombyx mori (BmN4). The productive infection of SeMNPV was observed only in Se301 cells. However, a dot-blot hybridization analysis revealed that SeMNPV DNA replicated in five non-permissive cell lines: SF21AEII, CLS-79, SpLi-221, hi-5 and BmN4. In addition, the virus-infected hi-5 and BmN4 cells displayed morphological changes. In contrast, CLS-79 cells inoculated with SeMNPV showed membrane blebbing at 20 hrs post inoculation (p.i.) and fragmentation of genomic DNA. All that indicated that the infected CLS-79 cells underwent apoptosis. These findings indicate that the SeMNPV replication was restricted at various points in dependence upon each cell line.

Autographa californica nucleopolyhedrovirus infection results in Sf9 cell cycle arrest at G2/M phase

Baculovirus infection results in the induction of membrane structures within the nucleoplasm of the host cells. The source of these membranes is unclear; however, using the normal dynamics of cellular membranes and the nuclear envelope as a model, it is possible that the cell cycle might play a role in the regulation of formation of these intranuclear membranes. Therefore, one goal of this study was to investigate the effect of baculovirus infection on the cell cycle of Sf9 host cells. Since few data are available on the cell cycle of insect cells, the first task was to define Sf9 cell cycle kinetics. The cell cycle phase distribution of Sf9 cells grown in suspension culture was determined to be evenly distributed (29% of the cells in G1, 33% in S, and 36% in G2/M phase), with the duration of G1 and S phases both being about 6 h and the combined duration of G2/M phase being about 8 h. When Sf9 cells were infected with AcMNPV (Autographa californica nuclear polyhedrosis virus), approximately 84% of the cells were arrested in G2/M phase by 18-24 h p.i. Concomitant with the viral-induced arrest in G2/M phase, high levels of both cdc2-associated histone H1 kinase activity and cyclin B protein were detected. By 24 h p.i. cyclin B was no longer detected; however, cdc2-associated histone H1 kinase activity remained throughout the infection. These data suggested that early in infection, cyclin B/cdc2 complex may be used to regulate the transition from G2 to M phase, but prolonged arrest may be due to a protein(s) encoded by AcMNPV. DNA hybridization analysis showed that the maximal rate of viral DNA replication occurred before G2/M arrest. We noted that viral DNA replication still occurred late in infection, when the majority of the cells were arrested in G2/M phase. Since cellular DNA replication normally does not occur during G2 or M phase, experiments were designed to determine if viral DNA replication could occur even when host cell DNA replication was arrested. Sf9 cells were arrested and "frozen" at the boundary of G1/S phase using 5-fluoro-2'deoxyuridine (FdUrd) treatment and then infected with AcMNPV In the blocked, infected cells, viral DNA replication was detected; however, cellular DNA remained at steady-state levels. These results suggested that cellular DNA replication was not necessary for viral DNA replication and show that viral DNA replication was not significantly inhibited by FdUrd treatment. It was a surprise to detect viral DNA replication when the host cells were "frozen" at G1/S phase. We wanted to determine if the viral infection was progressing to the stage of progeny virus production. Our data showed that progeny budded virus (BV) and virus-induced intranuclear microvesicles were produced in the frozen, infected cells; however, the intranuclear microvesicles had an unusual structure. They were irregular in shape and thickened compared to those observed in a normal infection. Very few enveloped nucleocapsids were visible in the nucleus of the frozen, infected cells and the occluded-derived virus envelope proteins, ODV-E66 and ODV-EC27, were not detected by Western blot analyses. Since the cells were sustained at the boundary of G1 and S phases for the duration of this experiment, the decreased amount of enveloped ODV in the nucleus could be due to several factors, including decreased levels of proteins expressed from late genes, aberrant microvesicles, or the necessity of G2/M phasing of the infected cell for efficient production and maturation of intranuclear microvesicles. These data indicate that AcMNPV infection results in cell cycle arrest in G2/M phase and this arrest may be due to a viral-encoded protein(s) that has cdc2-associated kinase activity. (ABSTRACT TRUNCATED)

Characterization of the replication cycle of the Lymantria dispar nuclear polyhedrosis virus

The life cycle of the Lymantria dispar nuclear polyhedrosis virus (LdMNPV) was characterized through analysis of budded virus (BV) release, the temporal formation of polyhedra, the temporal transcription pattern of representative early, late, and hyper-expressed late genes, and the onset of DNA replication in the Ld652Y cell line. Transcripts from the LdMNPV immediate early gene G22 were detected 4 h post infection (h p.i.). The late and hyper-expressed late p39 capsid and polyhedrin genes were initially transcribed at approximately 20 and 24 h p.i., respectively. Viral DNA replication initiated at approximately 18-20 h p.i. Budded virus was released from infected cells between 24 and 36 h p.i., and polyhedra were first detected at approximately 48 h p.i.

Deletion analysis of four of eighteen late gene expression factor gene homologues of the baculovirus, BmNPV

Nucleotide sequence analysis of the genome of the baculovirus Bombyx mori nuclear polyhedrosis virus (BmNPV) identified 18 homologues of the Autographa californica NPV (AcNPV) lefs (late expression factor genes). These BmNPV lefs showed high (73-98%) amino acid sequence identities to AcNPV lefs and were localized to similar positions in the genome. One lef, p35, was previously characterized in AcNPV and BmNPV deletion experiments. Functional deletion of each of the BmNPV lef homologues was attempted here by insertion of a beta-galactosidase gene cassette into the coding region of each lef. Four of 18 BmNPV lef (39K, ie-2, lef-7, and p35) deletion mutants were successfully isolated, indicating that the other 14 BmNPV lefs were likely essential for viral replication in cell culture. Further analysis showed that deletion of lef-7, p35, and ie-2 resulted in lower levels of viral DNA replication, indicating that the BmNPV lef-7, p35, and ie-2 products have stimulating effects on DNA replication. Deletion of 39K resulted in a significantly lower level of late gene transcription and extremely low (over 10(2)-fold less at 48-80 hr p.i.) production of progeny budded virus in BmN cells. In contrast, the deletion did not affect viral DNA replication, indicating that BmNPV 39K is involved in late gene transcription. Reduced late gene expression presumably affected production and/or release of progeny budded virus particles. This was corroborated by transmission electron microscopy, which showed that virus replication was abnormal in BmN cells infected with a BmNPV mutant lacking 39K and virion production was low. Even though 39K deletion resulted in a loss of oral infectivity, the 39K deletion mutant replicated in silkworm larvae when injected into the body cavity, as did the ie-2, lef7, and p35 deletion mutants. In addition, a BmNPV homologue of the baculovirus very late expression factor gene (vif-1) found in AcNPV was essential, implying an essential function of the BmNPV vif-1 homologue at a step before the onset of very late gene expression.

Infection of a Spodoptera frugiperda cell line with Bombyx mori nucleopolyhedrovirus

The interactions of Bombyx mori nucleopolyhedrovirus (BmNPV) with Spodoptera frugiperda cells (Sf9) were investigated. S. frugiperda cells are usually considered nonpermissive for BmNPV. However, in the present study, BmNPV DNA replication was observed and an increasing infectious titre, reaching 10(4) TCID50/ml on B. mori permissive cells by 6 days post-transfection, developed in the supernatant of infected Sf9 cells. Infection of Sf9 cells by BmNPV did not induce a discernible shutoff of cellular protein synthesis and no overt cytopathic effects were observed. These data indicate that the low permissivity of Sf9 cells for BmNPV replication is associated with an inapparent infection.

Replication of Anticarsia gemmatalis nuclear polyhedrosis virus in four lepidopteran cell lines

Anticarsia gemmatalis nuclear polyhedrosis virus (AgNPV; family Baculoviridae) is pathogenic for larvae of the velvetbean caterpillar, Anticarsia gemmatalis Hübner-an important pest of soybean. AgNPV is a viable alternative to chemical control of A. gemmatalis in Brazil, where its use as a pesticide has brought significant economic and environmental benefits. Although a significant amount of information is available on the ecological and biological control aspects of AgNPV, very little is known about the replication cycle and host specificity of this virus. We examined the susceptibility of four lepidopteran cell lines to AgNPV. Infections of the A. gemmatalis UFL-AG-286 cell line were highly productive. Ninety percent of infected cells had polyhedral inclusion bodies by 48 hr after infection, and the infectious virus titer was 10(8) IU/ml. Viral DNA replication was efficient, and the maximal rate of synthesis was between 6 and 12 hr after infection. Infections of the Spodoptera frugiperda IPLB-SF-21 cell line were productive but less efficient. Infections of Choristoneura fumiferana IPRI-CF-124T cells with this virus were poor, with only 5% of the cells forming polyhedra and an infectious virus titer of 10(6) IU/ml. The level of viral DNA replication was low, suggesting that this system was predominantly abortive. Infections of Bombyx mori BM-5 cell lines were abortive, and cells had apoptosis-like morphology. No polyhedra or increase in infectious levels were observed, and there was little or no replication of viral DNA. Our data suggest that restriction of AgNPV replication in abortive cell lines is due mainly to inability of viral DNA to replicate efficiently. The possible causes of low DNA replication are discussed. Our results suggest that the cell lines utilized in this study can provide an important model for studying mechanisms of AgNPV host specificity.

Establishment of a cell line from Spodoptera litura (Lepidoptera: Noctuidae) and replication of S. litura nuclear polyhedrosis virus in vitro

A new cell line, designated IBL-SLO1A, was established from pupal ovaries of the tobacco cutworm Spodoptera litura. Cells were grown in the TNM-FH insect cell culture medium supplemented with 10% fetal bovine serum. The subculture of cells was initiated with 1-3 x 10(5) cells/ml, a split ratio about 1:10. The growth curve of the cells fit the model of exponential growth (Y = e-0.6750 + 0.0317X R2 = 0. 98), the population doubling time during logarithmic growth at 28 degrees C was 21.9 hr. The number of chromosomes was about 140. Cytopathology characteristics of baculovirus infection were observed when the culture cells were infected with SINPV derived from alkali dissolution of occlusion bodies at 24 hr postinfection (pi). The cells lysed and released occlusion bodies into culture medium at 120 hr pi.

Spodoptera exigua multicapsid nucleopolyhedrovirus deletion mutants generated in cell culture lack virulence in vivo

The baculovirus Spodoptera exigua multicapsid nucleopolyhedrovirus (SeMNPV) has high potential for development as a bio-insecticide for control of the beet armyworm (S. exigua). It is highly infectious for S. exigua larvae and its host range is very narrow. A prerequisite for such application is the possibility of growing this virus in large quantities, e.g. in insect cell lines. It was observed, however, that polyhedra of SeMNPV plaque-purified in Se-UCR1 cells did not cause larval mortality or morbidity when fed to S. exigua larvae. As this suggested a genetic alteration in in vitro produced SeMNPV, comparative restriction analysis of in vitro and in vivo produced SeMNPV DNA was performed. The restriction patterns of viral DNA from several different plaques always differed from that of the wild-type in the same way, suggesting that a large, single deletion had occurred in the in vitro produced viral genome. In order to localize this deletion more precisely a detailed physical map of the wild-type SeMNPV genome was constructed, using the restriction endonucleases XbaI, BamHI, Bg/II, PstI, SstI, HindIII and SpeI. In addition, the entire SeMNPV genome was cloned into a library containing five overlapping cosmids and a plasmid library. About 80 restriction sites were located and the orientation of the map was set according to the location of the polyhedrin and p10 genes. The approximate size of the viral genome was 134 kbp. Based on this map it could be established that mutant SeMNPV, obtained by passage in cell culture, contained a single deletion of approximately 25 kbp between map units 12.9 and 32.3.

Production of polyhedra of the Autographa californica nuclear polyhedrosis virus using the Sf21 and Tn5B1-4 cell lines and comparison with host-derived polyhedra by bioassay

Both wild-type and recombinant baculoviruses are becoming more attractive for the control of insect pests. Thus, there is an increased incentive to address and resolve logistical problems associated with large-scale production of these viruses. In this study, we have compared the potential of two insect cell lines, Tn5B1-4 and Sf21, for the production of polyhedra and compared the efficacy of both cell culture-derived and host-derived viruses by bioassay. The efficacy of both wild-type AcMNPV and AcAaIT, a recombinant baculovirus expressing an insect-specific scorpion toxin, were compared. Yields of polyhedra from Tn5B1-4 were sixfold higher than those from the cell line Sf21. Morphological analysis of polyhedra derived from cell culture showed greater variability in size relative to host-derived polyhedra. The maximum size of cell culture-derived polyhedra was over 1.5 times larger than that of insect-derived polyhedra. The efficacy of AcMNPV and AcAaIT derived from cell culture, or from amplification in larvae of Trichoplusia ni or Heliothis virescens, was compared by bioassay in H. virescens. There was a significant difference between the slopes for lethal time data for host-derived and cell culture-derived wild-type virus. Mortality occurred at a faster rate following infection with host-derived virus. No significant difference was seen for the recombinant virus AcAaIT. Lethal doses of cell- and host-derived polyhedra were not significantly different. The reasons for and implications of this for pest control are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo and in vitro characterization of several isolates of spodoptera exigua nuclear polyhedrosis virus

Spodoptera exigua nuclear polyhedrosis viruses (SeNPVs), isolated form five geographically distinct regions of Japan and Thailand, were characterized by their DNA restriction endonuclease pattern, level of virus production in a continuous cell line of S. exigua and biological activity to S. exigua larvae. The EcoRI and PstI fragments exhibited similar overall patterns with minor differences. Digestion of virus DNA from a plaque-purified isolate, SeNPV-I1, with PstI yielded 14 fragments and the estimated genome size was approximately 123 kbp. The SeNPV wild isolate from Kagoshima, SeNPV-KW, showed the highest yield of extracellular virus (ECV) in the Se301 cell line of S. exigua among five wild isolates, but there was no significant difference in the level of polyhedral inclusion body (PIB) formation. In comparative studies of biological activity using 2nd-instar S. exigua larvae, SeNPV-KW had the highest virulence with an LD50 value of 3.0 PIBs per larva. When 16 clones, plaque-purified from the Isahaya isolate, SeNPV-IW, were examined for genetic relatedness, seven distinct EcoRI patterns were observed, indicating that SeNPV-IW wild isolate consisted of a mixture of different genotypes.

Expression of the IE1 transactivator of Autographa californica nuclear polyhedrosis virus during viral infection

The immediate-early IE1 protein of Autographa californica nuclear polyhedrosis virus (AcMNPV) is an important regulator of viral gene transcription. To provide a tool for further analysis of the expression and function of IE1, a polyclonal antiserum was raised against IE1 expressed in bacteria. Immunoblot analysis of infected cell lysates was used to monitor the accumulation of IE1 throughout the viral life cycle. When extracts were prepared in the presence of phosphatase inhibitors, only one protein band was detected on SDS-polyacrylamide gels. However, in the absence of phosphatase inhibitors, at least four distinct electrophoretic species were detected. Mobility shift assays were conducted using an enhancer DNA probe and whole cell extracts prepared at different times postinfection. Results indicated that the enhancer-binding activity of IE1 increased from 4 to 72 hr postinfection. DNA-protein complexes formed with infected cell extracts migrated more slowly than those formed with transfected cell extracts. This effect was more pronounced with extracts prepared in the presence of phosphatase inhibitors. Supershift experiments with IE1 antiserum confirmed that IE1 was a component of DNA-protein complexes in both transfected and infected cell extracts. A titration experiment was done to determine the minimal amounts of IE1 required for activation of the 39k promoter in the presence and absence of a cis-linked enhancer element. These analyses indicated that the intracellular levels of IE1 are not sufficient for enhancer-independent activation of the 39k promoter during the early phase of viral infection. Quantitative immunoblots revealed that the amount of IE1 in budded virus was less than 0.68 mole per mole of viral DNA, suggesting that IE1 is not a structural protein of AcNPV.

Transcriptional regulation of cell line-dependent, baculovirus-mediated expression of foreign genes

Cell line-dependent expression of foreign genes in the baculovirus system was investigated using a recombinant vAc beta hCG-luc virus carrying two reporter genes--beta subunit of human chorionic gonadotropin (beta hCG) and luciferase (luc)--placed under the transcriptional control of the Autographa californica nuclear polyhedrosis virus (AcNPV) polyhedrin gene promoters. Five different lepidopteran cell lines derived from Spodoptera frugiperda (Sf21 and Sf9), Bombyx mori (BmN and Bm5), and Trichoplusia ni (TN368) were used as host cells. TN368 expressed both beta hCG and LUC to maximum levels, followed by BmN, Sf21, and Sf9 in descending order. Bm5 did not show any evidence of synthesis of the two proteins. Dot blot analysis of DNA from the vAc beta hCG-luc-infected cells revealed that the level of entry of viral DNA was the same for all the five cell lines. After the completion of viral DNA replication (18 hr post infection), the level of viral DNA was the same for all the cell lines except for Bm5 where viral DNA replication did not take place and the residual virus was cleared from the cells. Analysis of RNA from the four expressing cell lines revealed a direct correlation between protein levels and levels of mRNA, suggesting transcriptional control. Differences in mRNA stability between cell lines was also evident. Gel retardation analysis of a host factor binding to transcriptionally important sequence motifs within the AcNPV polyhedrin gene promoter revealed an inverse correlation between the levels of this polyhedrin promoter-binding protein (PPBP) and reporter gene expression.(ABSTRACT TRUNCATED AT 250 WORDS)

Plaque assay for Spodoptera exigua and Autographa californica nuclear polyhedrosis viruses in a newly established cell line of the beet armyworm, Spodoptera exigua

The nuclear polyhedrosis viruses of Spodoptera exigua (SeNPV) and Autographa californica (AcNPV) produced plaques in a newly established cell line of the beet armyworm, Spodoptera exigua. Plaques were composed of infected cells containing many polyhedra and were visible without any staining procedure. Dose-response assays showed a direct correlation between the number of plaques and the inoculum size. Growth kinetic studies of the two viruses, using the developed plaque assay system, revealed that the release of extracellular viruses began 6 hrs post infection (p.i.) and the titer reached a plateau 48 hrs p.i. The sensitivity of this plaque assay system was 100 times greater for the heterologous AcNPV than for the homologous SeNPV.

Interaction of Trichoplusia ni granulosis virus-encoded enhancin with the midgut epithelium and peritrophic membrane of four lepidopteran insects

Enhancin, an infectivity-enhancing protein from Trichoplusia ni granulosis virus (TnGV) was tested for its ability to increase Autographa californica multiple nucleocapsid nuclear polyhedrosis virus (AcMNPV) infection in the larvae of four lepidopteran insects. Enhancin increased the mortality of AcMNPV infection in all the four insect species tested. Peritrophic membrane (PM) assays showed altered protein profiles in PMs treated with enhancin in all the four species. This supports the hypothesis that enhancin affects virus infection by altering the structural integrity of the PMs. The binding of enhancin to the midgut brush border membranes (BBMs) was determined and specific binding sites were found on the BBM of Pseudaletia unipuncta. No specific binding sites were found on the BBMs of T. ni, Helicoverpa zea or Spodoptera exigua. Therefore, specific binding of enhancin to the midgut cell membrane may not be necessary for the enhancement of baculovirus infection in insects.

Clonal variation in the Spodoptera frugiperda IPLB-SF21-AE insect cell population

Clones have been isolated from the heterogeneous Spodoptera frugiperda IPLB-SF21-AE insect cell population. Five of these clones, in addition to the parent cell line and the SF9 cell line (another clonal isolate of the parent cell line), have been compared in regards to morphology, growth, budded virus synthesis, and recombinant protein synthesis. No significant differences in cell morphology were found among these cell lines. There was, however, a significant difference in the average cell size, with diameters ranging from 9.30 +/- 0.184 to 11.11 +/- 0.22 microns and from 9.17 +/- 0.05 to 11.25 +/- 0.24 microns for cells growing in Excell 401 serum-free medium in spinner flask cultures and in TNM-FH medium supplemented with 10% FBS in tissue flask cultures, respectively. While no significant differences in the growth rates were found in TNM-FH medium containing 10% calf serum, significant differences were found in Excell 401 serum-free medium, with population doubling times ranging from 38.5 +/- 6.6 to 64.5 +/- 6.4 h in spinner flask studies. Significant differences in expression levels of Escherichia coli beta-galactosidase (beta-gal) were also found in both 12-well plates and spinner flasks. In the 12-well plate studies, the peak levels of beta-galactosidase obtained by these cell lines ranged from 0.332 +/- 0.091 to 0.805 +/- 0.117 mg/10(6) cells and from 0.580 +/- 0.130 to 1.458 +/- 0.132 mg/10(6) cells in Excell 401 and Hyclone Hy-Q serum-free media, respectively. In the spinner flask studies, peak expression levels ranged from 0.128 +/- 0.053 to 0.573 +/- 0.215 mg/10(6) cells in Excell 401 serum-free medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Orgyia pseudotsugata baculovirus p10 and polyhedron envelope protein genes: analysis of their relative expression levels and role in polyhedron structure

To investigate the regulation of p10 and polyhedron envelope protein (PEP) gene expression and their role in polyhedron development, Orgyia pseudotsugata multinucleocapsid nuclear polyhedrosis viruses lacking these genes were constructed. Recombinant viruses were produced, in which the p10 gene, the PEP gene or both genes were disrupted with the beta-glucuronidase (GUS) or beta-galactosidase (lacZ) genes. GUS activity under the control of the PEP protein promoter was observed later in infection and its maximal expression was less than 10% the level for p10 promoter-GUS constructs. Tissues from O. pseudotsugata larvae infected with these recombinants were examined by electron microscopy. Cells from insects infected with the p10- viruses lacked p10-associated fibrillar structures, but fragments of polyhedron envelope-like structures were observed on the surface of some polyhedra. Immunogold labelling of cells infected with the p10-GUS+ virus with an antibody directed against PEP showed that the PEP was concentrated at the surface of polyhedra. Although polyhedra produced by p10 and PEP gene deletion mutants demonstrated what appeared to be a polyhedron envelope by transmission electron microscopy, scanning electron microscopy showed that they had irregular, pitted surfaces that were different from wild-type polyhedra. These data suggested that both p10 and PEP are important for the proper formation of the periphery of polyhedra.

A mutant baculovirus with a temperature-sensitive IE-1 transregulatory protein

We have mapped the mutation responsible for the temperature-sensitive (ts) phenotype of tsB821, a mutant of the baculovirus Autographa californica nuclear polyhedrosis virus (H. H. Lee and L. K. Miller, J. Virol. 31:240-252, 1979), to a single nucleotide which changes alanine 432 of the multifunctional regulatory protein IE-1 to a valine. Mapping was done with a combination of marker rescue and transient expression assays, hybrid gene construction by overlap PCR gene splicing, and nucleotide sequence analysis. Cells infected with tsB821 at high multiplicities of infection showed a spectrum of responses from severe cytopathic effects, including apoptosis, to a lack of obvious signs of infection. Protein synthesis in tsB821-infected cells at the restrictive temperature appeared similar to uninfected cell protein synthesis, but viral DNA replication and budded virus production were observed, albeit in a delayed manner. The dependence of early and late promoter activity on the wild-type IE-1 gene, ie-1, was observed in transient expression assays. However, the dependence of early promoter activity on ie-1 was strongest in the absence of other viral genes. Thus, other viral genes appear to be able to compensate, at least in part, for the lack, or low levels, of ie-1 in transient expression assays using early promoters. The mutant should prove useful in further defining the function(s) of IE-1.

Functional mapping of regions of the Autographa californica nuclear polyhedrosis viral genome required for DNA replication

Previous results showed that plasmids containing one of the eight putative origins (ori's) of Autographa californica nuclear polyhedrosis virus (AcMNPV) are replicated after transfection into Spodoptera frugiperda cells if essential trans-acting factors are supplied by AcMNPV infection (Kool et al., Virology, 192, 94-101, 1993a; Kool et al., J. Gen. Virol., in press, 1993b; Leisy and Rohrmann, Virology, 196, 722-730, 1993). In this report a transient complementation assay is described in which four cotransfected cosmid clones, instead of AcMNPV infection, provided essential trans-acting factors for plasmid DNA replication. In this assay plasmid replication was found to be independent of the presence, in cis, of a viral ori. No replication of plasmids occurred when one of the cosmids was omitted from the transfection mixture. This result indicated that this assay is a valid approach for identification of AcMNPV replication genes. We further used the assay to define essential regions in the four required cosmids. Six regions of the AcMNPV genome, EcoRI-I (map unit 0.3-5.8), EcoRI-O (map unit 6.9-8.7), SstI-F (map unit 38.9-45.0), EcoRI-D (map unit 59.9-68.3), a BamHI-SstII fragment of BamHI-B (map unit 84.3-89.7), and EcoRI-B (map unit 90.0-100), with at least seven genes, were found to be essential for plasmid DNA replication. These regions contain the putative DNA polymerase gene (SstI-F), the helicase-like gene (EcoRI-D), and the region where most of the trans-activating immediate--early genes of AcMNPV are located (EcoRI-B). For SstI-F it was shown that this region contains besides the DNA polymerase gene at least one other replication gene. These results show that it will now be possible to define the set of AcMNPV genes necessary and sufficient for DNA replication.

Extension of Autographa californica nuclear polyhedrosis virus host range by interspecific replacement of a short DNA sequence in the p143 helicase gene

Recombinant baculoviruses obtained by coinfection of insect cells with Autographa californica and Bombyx mori nuclear polyhedrosis viruses (AcNPV and BmNPV, respectively) possess a wider in vitro host range than either parent virus. To localize the DNA sequences responsible for this species specificity, we used a two-step method of production and selection of recombinant viruses with altered specificity. Sf9 cells, which are permissive for AcNPV, were first cotransfected with genomic AcNPV DNA and a complete or incomplete set of BmNPV restriction fragments. AcNPV-BmNPV recombinants from the Sf9 supernatant were then selected on the basis of ability to replicate in B. mori Bm5 cells, which are not permissive for AcNPV. Cotransfection of AcNPV DNA with the 7.6-kbp BmNPV Sma I-C fragment was sufficient to produce recombinants able to infect both Sf9 and Bm5 cells. A series of cotransfections with subclones of this fragment defined a 79-nt sequence within the p143 helicase gene capable of extending AcNPV host range in vitro. In this 79-nt region, BmNPV and AcNPV differ at six positions, corresponding to four amino acid substitutions. The involvement of the 79-nt region in species specificity control was confirmed by cotransfecting AcNPV DNA and gel-purified polymerase chain reaction products derived from the BmNPV p143 gene. Replacement in the AcNPV genome of three AcNPV-specific amino acids by the three corresponding BmNPV-specific amino acids at positions 556, 564, and 577 of the p143 protein extends AcNPV host range to B. mori larvae.