Nucleotide sequence of a portion of the Autographa californica nuclear polyhedrosis virus genome containing the EcoRI site-rich region (hr5) and an open reading frame just 5' of the p10 gene

Dual roles and evolutionary implications of P26/poxin in antagonizing intracellular cGAS-STING and extracellular melanization immunity

P26, a homolog of the viral-encoded nuclease poxin that neutralizes the cGAS-STING innate immunity, is widely distributed in various invertebrate viruses, lepidopteran insects, and parasitoid wasps. P26/poxin from certain insect viruses also retains protease activity, though its biological role remains unknown. Given that many P26s contain a signal peptide, it is surmised that P26 may possess certain extracellular functions. Here, we report that a secretory baculoviral P26 suppresses melanization, a prominent insect innate immunity against pathogen invasion. P26 targets the cofactor of a prophenoloxidase-activating protease, and its inhibitory function is independent of nuclease activity. The analysis of P26/poxin homologs from different origins suggests that the ability to inhibit the extracellular melanization pathway is limited to P26s with a signal peptide and not shared by the homologs without it. These findings highlight the independent evolution of a single viral suppressor to perform dual roles in modulating immunity during virus-host adaptation.

Bombyx mori Nucleopolyhedrovirus p26 Is Associated with Viral Late Stage Replication

Bombyx mori nucleopolyhedrovirus (BmNPV) p26 is conserved among all Lepidoptera baculoviruses that have been completely sequenced thus far, and some baculoviruses even have two copies of p26, which suggested that p26 may play an important role in the virus infection cycle. This study aimed to characterize BmNPV p26. We found that BmNPV p26 transcripts were detectable as early as 3 h post-infection (hpi), and the transcript levels rapidly increased starting from 12 hpi. Western blot analysis using an anti-p26 polyclonal antibody demonstrated that the corresponding protein was also detectable from 6 hpi in BmNPV-infected cell lysates. Immunofluorescence analysis demonstrated that p26 was mainly dispersed in the infected cell cytoplasm, whereas the over-expressed fusion protein EGFP-p26 also accumulated in the nucleus. These results indicated that p26 is an early BmNPV gene and has functions both in the cytoplasm and the nucleus. RNAi-based knockdown of p26 could produce infectious virus and normal-appearing virions but decreased budded virus (BV) production in BmNPV-infected cells at 72 hpi. Moreover, the results of further quantitative PCR (Q-PCR) analysis indicated that the gp64 and p74 transcripts levels decreased significantly. These results indicated that BmNPV p26 may be associated with BmNPV replication during the late infection stage.

Genetic modification of a baculovirus vector for increased expression in insect cells

Generating large amounts of recombinant protein in transgenic animals is often challenging and has a number of drawbacks compared to cell culture systems. The baculovirus expression vector system (BEVS) uses virus-infected insect cells to produce recombinant proteins to high levels, and these are usually processed in a similar way to the native protein. Interestingly, since the development of the BEVS, the virus most often used (Autographa californica multi-nucleopolyhedovirus; AcMNPV) has been little altered genetically from its wild-type parental virus. In this study, we modified the AcMNPV genome in an attempt to improve recombinant protein yield, by deleting genes that are non-essential in cell culture. We deleted the p26, p10 and p74 genes from the virus genome, replacing them with an antibiotic selection cassette, allowing us to isolate recombinants. We screened and identified recombinant viruses by restriction enzyme analysis, PCR and Western blot. Cell viability analysis showed that the deletions did not improve the viability of infected cells, compared to non-deletion viruses. However, expression studies showed that recombinant protein levels for the deletion viruses were significantly higher than the expression levels of non-deletion viruses. These results confirm that there is still great potential for improving the BEVS, further increasing recombinant protein expression yields and stability in insect cells.

Characterization of a virion occlusion-defective Autographa californica multiple nucleopolyhedrovirus mutant lacking the p26, p10 and p74 genes

Nucleopolyhedroviruses (NPVs), family Baculoviridae, are insect-specific viruses with the potential to control insect pests in agriculture and forestry. NPVs are occluded in polyhedral occlusion bodies. Polyhedra protect virions from inactivation in the environment as well as assisting virions in horizontal transmission in the insect population. The process of virion occlusion in the polyhedra is undefined and the genes that regulate the virion occlusion process have not been well investigated yet. An Autographa californica multiple nucleopolyhedrovirus (AcMNPV) mutant (AcDef) that has a 2136 bp DNA deletion, including p26, p10 and p74 genes, has been isolated. No virions were detected in the polyhedra of AcDef. Restoration of all the missing sequences into AcDef led to proper virion occlusion. Individual gene deletion of either p10 or p26 could not abolish virion occlusion in the polyhedra of AcMNPV, but p10 deletion reduced virion occlusion efficiency more than threefold compared with the wild-type AcMNPV. Previous studies by other research groups on deletion of AcMNPV gene p74 suggested that p74 is a per os infectivity factor, and deletion of the p74 gene did not eliminate virion occlusion. Collectively, the three genes (p26, p10 and p74) may act in concert to regulate the virion occlusion process. Therefore, p26, p10 and p74 are all required for proper virion occlusion in the polyhedra of AcMNPV.

Camelpox virus encodes a schlafen-like protein that affects orthopoxvirus virulence

Camelpox virus (CMLV) gene 176R encodes a protein with sequence similarity to murine schlafen (m-slfn) proteins. In vivo, short and long members of the m-slfn family inhibited T-cell development, whereas in vitro, only short m-slfns caused arrest of fibroblast growth. CMLV 176 protein (v-slfn) is most closely related to short m-slfns; however, when expressed stably in mammalian cells, v-slfn did not inhibit cell growth. v-slfn is a predominantly cytoplasmic 57 kDa protein that is expressed throughout infection. Several other orthopoxviruses encode v-slfn proteins, but the v-slfn gene is fragmented in all sequenced variola virus and vaccinia virus (VACV) strains. Consistent with this, all 16 VACV strains tested do not express a v-slfn detected by polyclonal serum raised against the CMLV protein. In the absence of a small animal model to study CMLV pathogenesis, the contribution of CMLV v-slfn to orthopoxvirus virulence was studied via its expression in an attenuated strain of VACV. Recombinant viruses expressing wild-type v-slfn or v-slfn tagged at its C terminus with a haemagglutinin (HA) epitope were less virulent than control viruses. However, a virus expressing v-slfn tagged with the HA epitope at its N terminus had similar virulence to controls, implying that the N terminus has an important function. A greater recruitment of lymphocytes into infected lung tissue was observed in the presence of wild-type v-slfn but, interestingly, these cells were less activated. Thus, v-slfn is an orthopoxvirus virulence factor that affects the host immune response to infection.

Isolation of p10 gene from Bombyx mori nuclear polyhedrosis virus and study of its promoter activity in recombinant baculovirus vector system

A homologue of Autographa californica NPV (AcNPV) p10 gene was identified and cloned from Bombyx mori NPV (BmNPV). BmNPV p10 gene encodes truncated protein of 70 amino acid residues that lacks carboxyl terminus comparing with the p10 protein encoded by AcNPV. The putative TATA box sequence and the ATAAG motif which is the consensus sequence of baculovirus very late promoter were conserved. A transfer vector, pBNT1, which includes the p10 promoter region of BmNPV for foreign gene expression was constructed. By using pBNT1, a recombinant BmNPV, Bmp10-Luc, in which the p10 gene was replaced by the firefly luciferase gene, was obtained. We also obtained another recombinant virus, BmPH-Luc, in which the polyhedrin gene was replaced by the luciferase gene. The luciferase activity detected in BoMo-15AIIc insect cells infected with Bmp10-Luc was approximately 50% of that infected with BmPH-Luc, suggesting that although both the p10 and polyhedrin promoters of BrnNPV are effective in high-level expression of foreign genes, the p10 promoter is not so strong as the polyhedrin promoter.

Divergence and evolution of homologous regions of Bombyx mori nuclear polyhedrosis virus

Homologous regions (hrs) (hr1,hr2-left,hr2-right,hr3,hr4-left,hr 4-right, and hr5) similar to those found in the Autographa californica nuclear polyhedrosis virus (AcNPV) genome were found in the Bombyx mori NPV (BmNPV) genome. The BmNPV hrs contained two to eight repeats of a homologous nucleotide sequence which were on average about 75 bp long. All of these homologous sequence repeats contained a 26-bp-long palindrome motif with an EcoRI or EcoRI-like site at its core. The consensus sequence of the BmNPV hrs showed 95% conservation with respect to those found in AcNPV. Nucleotide sequence analysis indicated that hr2-left and hr2-right of BmNPV evolved from an ancestor similar to hr2 of AcNPV by inversion, cleavage, and ligation. The polarities of the BmNPV and AcNPV hrs were conserved except for that of hr4-left. Within hr4-right of BmNPV, four repeats of a previously underscribed palindrome motif were found. Bmhr5D, a BmNPV mutant which lacked hr5, replicated at a rate similar to that of wild-type BmNPV in BmN cells and silkworm larvae, indicating that hr5 was not essential for viral replication. After ten passages of Bmhr5D in BmN cells, no detectable changes in its genome were observed by restriction endonuclease analysis. The evolution and divergence of the BmNPV genome are also discussed.

The hr5 transcriptional enhancer stimulates early expression from the Autographa californica nuclear polyhedrosis virus genome but is not required for virus replication

Autographa californica nuclear polyhedrosis virus (AcMNPV) contains five homologous regions (hr1 through hr5) interspersed throughout its genome. Analysis of plasmid transfections indicates that the hrs function as transcriptional enhancers and possible origins of viral DNA replication. The role of these repetitive elements in regulating expression from the AcMNPV genome was examined by constructing a series of recombinant viruses that tested the effect of hr5 on expression of the adjacent p35 gene (p35). When embedded within the viral genome, hr5 stimulated transcription from the early p35 promoter in a position- and orientation-independent manner. Moreover, hr5 and the upstream activating region of p35 were functionally interchangeable. A 28-bp imperfect palindrome, repeated six times within hr5, was the minimal sequence required for p35 promoter activation. hr5 also stimulated another early AcMNPV promoter but not a late promoter or a host-derived promoter, suggesting that enhancement is promoter specific during infection. To investigate its role during AcMNPV replication, hr5 was deleted from its normal position within the viral genome. The resulting hr5 mutants exhibited no apparent defects in replication, as judged by production of budded virus and levels of very late gene expression, even though steady-state levels of p35 RNA were reduced. These results indicated for the first time that hr5 functions as a transcriptional enhancer within the viral genome. However, the element is not required for AcMNPV replication in cultured cells. Thus, loss of one of five possible origins of DNA replication is not deleterious to viral growth. Since p26 was removed from the hr5 deletion mutants, this gene is also nonessential for viral replication.

In vitro transcription from baculovirus late gene promoters: accurate mRNA initiation by nuclear extracts prepared from infected Spodoptera frugiperda cells

Extracts prepared from nuclei of Autographa californica nuclear polyhedrosis virus-infected Spodoptera frugiperda cells were shown to support in vitro transcription from baculovirus late gene promoters. In vitro transcription was optimized for the late promoter of the 39K gene. The Mg2+ concentration was critical; concentrations higher than 1 to 2 mM did not support late transcription. Additional conditions included template (40 micrograms/ml), extract (2.5 mg/ml), and incubation time (25 min). Using a combination of runoff assays and high-resolution primer extension analyses, this system was shown to accurately initiate transcription from a variety of baculovirus late gene promoters, including those from the 39K and p39/capsid late genes and the hyperexpressed p10 and polyhedrin very late genes. In vitro transcription from the 39K late promoter was resistant to high concentrations of both alpha-amanitin (100 micrograms/ml) and tagetitoxin (4,000 U/ml), suggesting that neither RNA polymerase II nor III is responsible for the transcription of baculovirus late genes.

Competition between baculovirus polyhedrin and p10 gene expression during infection of insect cells

Polyhedrin and p10 genes are expressed concurrently during the late stage of infection. To determine whether any competition occurs between these two genes at a transcriptional and/or translational level, a series of Autographa californica nuclear polyhedrosis recombinant viruses with deletions of promoter and coding sequences of the p10 or polyhedrin gene was constructed. Two modified baculoviruses with only one of the very late promoters, single late-promoter viruses AcSLP10 and AcSLP33, were made. The polyhedrin gene was used as a reporter gene to allow direct comparison between p10 and polyhedrin-driven expression. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis and [35S]methionine labeling experiments showed that polyhedrin synthesis is enhanced in cells infected with baculoviruses carrying only one very late promoter. However, this enhancement is more significant when the polyhedrin gene is located in its natural site. Dot blot hybridization experiments carried out with total cytoplasmic RNA showed that deletion of the p10 promoter resulted in an increase of mRNAs derived from the polyhedrin promoter. In contrast, no increase of p10-promoted mRNA was detected when the polyhedrin gene promoter was deleted.

Development of baculovirus triple and quadruple expression vectors: co-expression of three or four bluetongue virus proteins and the synthesis of bluetongue virus-like particles in insect cells

Baculovirus multiple gene transfer vectors pAcAB3 and pAcAB4 have been developed to facilitate the insertion of three or four foreign genes respectively into the Autographa californica nuclear polyhedrosis virus (AcNPV) genome by a single co-transfection experiment. The pAcAB3 vector contains a polyhedrin promoter and two p10 promoters on either side of the polyhedrin promoter but in opposite orientations. The pAcAB4 vector has an additional polyhedrin promoter in opposite orientation to the first copy that is in juxtaposition to the first p10 promoter. Each of these derived vectors (pAcAB3, pAcAB4) have been used for the simultaneous expression of three or four bluetongue virus (BTV) genes respectively. When Spodoptera frugiperda cells were infected with the recombinant virus (AcBT-3/2/7/5) expressing the four major structural genes of BTV, double-capsid, virus-like particles consisting of VP2, VP3, VP5 and VP7 of BTV were assembled.

The structural and functional organization of the Autographa californica nuclear polyhedrosis virus genome

Baculoviruses are used as biological control agents of insect pests in agriculture and forestry. The multiple-nucleocapsid nuclear polyhedrosis virus of Autographa californica (AcMNPV) is the prototype baculovirus. Recently, this virus has become widely used as vector for the high-level expression of foreign genes in insect cells. An updated physical map of restriction sites as well as the location of open reading frames (ORFs) and transcripts are presented. Most characteristic is the dispersal of "early", "late", and "very late" genes over the genome and the presence of nested sets of 3' and 5' coterminal transcripts.

Identification and characterization of the p35 gene of Bombyx mori nuclear polyhedrosis virus that prevents virus-induced apoptosis

Nucleotide sequence analysis of the Bombyx mori nuclear polyhedrosis virus (BmNPV) genome revealed the existence of a gene homologous to the p35 gene of Autographa californica NPV (AcNPV), which has been shown to prevent virus-induced apoptosis. The BmNPV p35 gene showed 96.1% nucleotide and 89.6% predicted amino acid sequence identity to the AcNPV p35 gene. A mutant BmNPV (BmP35Z) lacking a functional p35 gene induced apoptosis-like cell degradation in infected BmN cells. However, unlike the p35-deleted AcNPV mutant (vAcAnh), BmP35Z replicated normally and produced polyhedral inclusion bodies. The patterns of protein synthesis and the percentages of viable BmN cells remaining following infection with either wild-type BmNPV or BmP35Z were nearly identical. BmP35Z also replicated in silkworm larvae without showing any apparent apoptotic response in infected hemocytes, fat body, or other tissues. Time to death of larvae infected with BmP35Z was similar to that for wild-type-infected larvae, and significant numbers of polyhedral inclusion bodies were produced. These results indicate that viral factors (or genes) other than p35 or host cell factors play a role in inducing, accelerating, or interfering with apoptotic processes. The evolution of baculovirus genomes is also discussed with reference to comparative analysis of the p35 and p94 gene sequences. The p94 gene is found immediately upstream of p35 in AcNPV; in BmNPV, however, the p94 gene was nearly completely missing, presumably because of large deletions in a BmNPV ancestor virus having a gene similar to the AcNPV p94 gene.

The Autographa californica baculovirus genome: evidence for multiple replication origins

The Autographa californica multinucleocapsid nuclear polyhedrosis virus (AcMNPV), which is used for the overexpression of eukaryotic genes and is being engineered for possible use as a viral insecticide, has a circular, supercoiled genome of approximately 128 kilobases. Despite its widespread use, little is known about the mechanism by which AcMNPV replicates. Evidence is presented in this report that AcMNPV origins of DNA replication are repeated sequences each containing several closely related imperfect palindromes that are present in six regions distributed around the genome. Although AcMNPV infection-dependent plasmid replication was initiated by a single complete palindrome, the amount of replication was substantially increased in plasmids containing six or eight palindromes.

In vitro transactivation of baculovirus early genes by nuclear extracts from Autographa californica nuclear polyhedrosis virus-infected Spodoptera frugiperda cells

Nuclear extracts, prepared from Autographa californica nuclear polyhedrosis virus-infected Spodoptera frugiperda cells during a time course of infection, were analyzed for activation of early gene transcription and for late gene transcription. The templates used in the in vitro transcription assays contained promoters for baculovirus genes that have been classified as immediate early, delayed early, and late. The promoters were derived from the baculovirus 39K, p26, gp64, and DNA polymerase genes. In addition, the adenovirus major late promoter was included in these studies. We found that transcription from promoters classified as immediate early or delayed early was accurately initiated by using extracts from uninfected cells. Furthermore, transcription from all early promoters tested was found to be transactivated by nuclear extracts prepared at 4 and 8 h postinfection. However, baculovirus enhancer-dependent transcriptional activation was not observed in tests with templates containing the hr5 enhancer sequence. Transcription from baculovirus late promoters was also not observed. A decline in transcription by nuclear extracts prepared from cells late in infection was associated with the presence of DNase activity.

Transcription analysis of the EcoRI D region of the baculovirus Autographa californica nuclear polyhedrosis virus identifies an early 4-kilobase RNA encoding the essential p143 gene

We have investigated the transcriptional activity of the 60.1- to 68.3-map-unit region of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV). Twelve transcripts mapping to this region were expressed at various times during infection. An early 4.0-kb transcript, potentially coding for a 143-kDa peptide essential for viral DNA replication, was maximally abundant at 6 h postinfection (p.i.). Transcripts of 0.5, 1.1, 1.4, 2.1, and 3.1 kb were most abundant at 12 h p.i., while two large transcripts of 5.2 and 6.8 kb were expressed maximally at 24 h p.i. In the presence of cycloheximide, and in ts8-infected cells at the nonpermissive temperature, only the 4.0-kb RNA was expressed. Northern (RNA) blot analysis using DNA subfragments from the EcoRI D fragment as probes suggested that many of the transcripts overlapped. Strand-specific cRNA probes revealed that the majority of the RNAs were transcribed in the counterclockwise direction. S1 nuclease and primer extension analysis were used to map the 5' ends of transcripts coded within the 60.1- to 64.8-map-unit region. Mapping of the 3' ends of the 1.1-, 4.0-, 5.2-, and 6.8-kb transcripts suggested that these RNAs were all coterminal at their 3' ends. A minicistron was found between the early 4.0-kb transcription start site and the predicted ATG start codon of the p143 gene. Several similar sequence motifs were identified in the promoter regions of the p143 gene and the AcMNPV DNA polymerase gene.

Prevention of apoptosis by a baculovirus gene during infection of insect cells

Programmed cell death is an active process of self destruction that is important in both the development and maintenance of multicellular animals. The molecular mechanisms controlling activation or suppression of programmed cell death are largely unknown. Apoptosis, a morphologically and biochemically defined type of programmed cell death commonly seen in vertebrates, was found to be initiated during baculovirus replication in insect cells. A specific viral gene product, p35, was identified as being responsible for blocking the apoptotic response. Identification of the function of this gene will allow further definition of the molecular pathways involved in the regulation of programmed cell death and may identify the role of apoptosis in invertebrate viral defense systems.

Identification of p74, a gene essential for virulence of baculovirus occlusion bodies

DNA sequencing of the HindIII-P fragment of the baculovirus Autographa californica nuclear polyhedrosis virus downstream of a major late protein, p10, revealed the presence of an open reading frame (ORF) 1935 nucleotides in length and in opposite polarity to p10. The gene product is considered essential for virus virulence in Trichoplusia ni larvae since infection with occlusion bodies from a mutant, Ac228z, in which portions of adjacent carboxy-termini from peptides p74 and p10 were deleted, failed to kill larvae, whereas virus with deletions in p10 alone were as infectious to larvae as wild-type virus. The ORF has the potential to code for a polypeptide of 645 amino acid residues (Mr 73,819) and was designated p74. Time course analysis of RNA from infected cells using primer extension assays suggested that the gene's promoter was weak and was most active at 16-20 hr postinfection. The transcription initiation site of the RNA was located at -90/-91 bases upstream of the start codon. The p74 gene was cloned into a baculovirus expression vector and a recombinant virus was produced which overexpressed the p74 protein.

Location, sequence, transcriptional mapping, and temporal expression of the gp64 envelope glycoprotein gene of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus

The gene encoding gp64, the envelope glycoprotein of the budded virus (BV) of Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus (OpMNPV), was mapped to the HindIII-E fragment of of the viral genome and expression of the gp64 gene was examined at various times postinfection. To locate the gp64 gene, a cross-reacting monoclonal antibody (AcV5) (A. W. Hohmann and P. Faulkner, 1983, Virology, 125, 432-444) directed against the gp64 protein of the Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) was used to screen a lambda gt11 expression library of OpMNPV and insert DNAs from immunopositive recombinants were used for Southern hybridization mapping. The gp64 gene was sequenced and transcription of the gp64 gene was examined by Northern blot, S1 nuclease, and primer extension analysis. Two sets of gp64 transcripts were detected during infection: a single early transcript which initiated at -43 nt and four late transcripts which initiated at -152, -167, -174, and -175 nt relative to the start of the gp64 open reading frame. Comparison of the gp64 early transcription initiation site with several other early baculovirus genes revealed a four-nucleotide consensus sequence (CAGT) which is conserved at the early transcription initiation sites of the IE-1 and 39K genes. The four late gp64 transcripts initiated at two of the four upstream ATAAG motifs. All gp64 mRNAs appear to be coterminal at the 3' end. Analysis of the nucleotide sequence of the gp64 gene revealed that the late gp64 mRNAs are bicistronic, consisting of a three amino acid minicistron located 70 nt upstream of the 509 amino acid gp64 open reading frame. Early transcripts do not contain the minicistron. The 1527-nt gp64 open reading frame encodes a predicted protein of 509 amino acids with a molecular weight of 58 kDa. The predicted gp64 protein contains seven potential N-linked glycosylation sites and hydrophobic N- and C-termini characteristic of signal and membrane anchor sequences found on envelope glycoproteins. By western blot analyses and indirect immunofluorescence microscopy, we show that the gp64 protein is present at early times (6 hr) postinfection and accumulates in the infected cell, moving to the periphery at later times postinfection. Western blot comparisons of the temporal expression of the gp64 protein with the p39 capsid protein revealed that these two virion structural protein genes differ significantly in the timing of their initial expression. The upstream regulatory regions, open reading frames, and predicted proteins from the OpMNPV and AcMNPV gp64 genes were compared.(ABSTRACT TRUNCATED AT 400 WORDS)

Functional analysis of the p10 gene 5' leader sequence of the Autographa californica nuclear polyhedrosis virus

Deletions were made in the 5' non-coding (leader) sequence of the Autographa californica nuclear polyhedrosis virus (AcNPV) p10 gene which progressively removed nucleotides upstream from the ATG translation initiation codon. The effect of these deletions on p10 gene expression was studied using a transient expression assay. Fragments containing the putative promoter and the entire or partly deleted 5' leader sequence of the p10 gene were inserted in front of the chloramphenicol acetyltransferase (CAT) gene in the pSVO-CAT construct. Transfection of AcNPV-infected Spodoptera frugiperda cells with these plasmids resulted in higher CAT expression with increasing representation of the 5' leader sequence. The lowest level of CAT expression was found with a construct containing only 10% of the 5' leader sequence, but this was enhanced on average by 50-fold if the entire 5' leader sequence was retained. The results indicate that the entire 5' leader sequence of the p10 gene is necessary for the high level of expression. The normal transcription initiation site was utilized in the transient expression of CAT. The data are discussed in relation to the strong promoter of the baculovirus polyhedrin gene.

Comparison of the p26 gene region of two baculoviruses

A 1.1-kb region of DNA containing the p26 gene of the multicapsid nuclear polyhedrosis virus of Orgyia pseudotsugata (OpMNPV) was sequenced, transcriptionally mapped, and compared to the same region in the MNPV of Autographa californica (AcMNPV). The mRNA start site of the p26 gene occurs about 22 nucleotides downstream from an A/T-rich putative promoter sequence that is highly conserved between AcMNPV and OpMNPV. The p26 mRNA is transcribed through the p26 gene and coterminates with the p10 gene resulting in a mRNA containing copies of both genes. The reading frames of the OpMNPV and AcMNPV p26 genes showed 47% amino acid sequence homology which is clustered in six regions with over 65% amino acid homology. There was a distinct bias toward incorporation of G/C-rich codons in the OpMNPV p26 gene. No DNA homology was observed between the region upstream of the p26 gene in AcMNPV and OpMNPV. In AcMNPV, this region contains the homologous repeated (hr) sequence hr5. Hybridization of a plasmid containing an AcMNPV-repeated sequence (hr5) to Southern blots of the OpMNPV genome indicated that this repeated sequence is lacking in OpMNPV.