Time Course Analysis and Mapping of Autographa californica Nuclear Polyhedrosis Virus Transcripts

On-line monitoring of respiration in recombinant-baculovirus infected and uninfected insect cell bioreactor cultures

Respiration rates in Spodoptera frugiperda (Sf-9) cell bioreactor cultures were successfully measured on-line using two methods: The O(2) uptake rate (OUR) was determined using gas phase pO(2) values imposed by a dissolved oxygen controller and the CO(2) evolution rate (CER) was measured using an infrared detector. The measurement methods were accurate, reliable, and relatively inexpensive. The CER was routinely determined in bioreactor cultures used for the production of several recombinant proteins. Simple linear relationships between viable cell densities and both OUR and CER in exponentially growing cultures were used to predict viable cell density. Respiration measurements were also used to follow the progress of baculoviral infections in Sf-9 cultures. Infection led to increases in volumetric and per-cell respiration rates. The relationships between respiration and several other culture parameters, including viable cell density, cell protein, cell volume, glucose consumption, lactate production, viral titer, and recombinant beta-galactosidase accumulation, were examined. The extent of the increase in CER following infection and the time postinfection at which maximum CER was attained were negatively correlated with the multiplicity of infection (MOI) at multiplicities below the level required to infect all the cells in a culture. Delays in the respiration peak related to the MOI employed were correlated with delays in the peak in recombinant protein accumulation. DO levels in the range 5-100% did not exert any major effects on viable cell densities, CER, or product titer in cultures infected with a baculovirus expressing recombinant beta-galactosidase.

Bombyx mori nucleopolyhedrovirus ORF51 encodes a budded virus envelope associated protein

Bombyx mori nucleopolyhedrovirus (BmNPV) ORF 51 (Bm51) is a gene present in many lepidopteran NPVs, but its function is unknown. In this study, Bm51 was characterized. Transcripts of Bm51 were detected from 4.5 through 72 hour post infection (h p.i.) by RT-PCR. The corresponding protein was detected from 6 to 72 h p.i. in BmNPV-infected BmN cells by western blot analysis using a polyclonal antibody against Bm51. Western blot assay of occlusion-derived virus and budded virus (BV) preparations revealed that Bm51 encodes a 23-kDa structural protein that is associated with BV and is located in the envelope fraction of budded virions. The protein was temporarily called BV-E23. In addition immunofluorescence microscopy demonstrated that the protein was present within the cytoplasm and nuclei in virus-infected cells. In conclusion, the available data suggest that Bm51 is a functional ORF of BmNPV and encodes a protein expressed in the early stage of the infection cycle that is associated with the BV envelope.

Sequence analysis and organization of the Neodiprion abietis nucleopolyhedrovirus genome

Of 30 baculovirus genomes that have been sequenced to date, the only nonlepidopteran baculoviruses include the dipteran Culex nigripalpus nucleopolyhedrovirus and two hymenopteran nucleopolyhedroviruses that infect the sawflies Neodiprion lecontei (NeleNPV) and Neodiprion sertifer (NeseNPV). This study provides a complete sequence and genome analysis of the nucleopolyhedrovirus that infects the balsam fir sawfly Neodiprion abietis (Hymenoptera, Symphyta, Diprionidae). The N. abietis nucleopolyhedrovirus (NeabNPV) is 84,264 bp in size, with a G+C content of 33.5%, and contains 93 predicted open reading frames (ORFs). Eleven predicted ORFs are unique to this baculovirus, 10 ORFs have a putative sequence homologue in the NeleNPV genome but not the NeseNPV genome, and 1 ORF (neab53) has a putative sequence homologue in the NeseNPV genome but not the NeleNPV genome. Specific repeat sequences are coincident with major genome rearrangements that distinguish NeabNPV and NeleNPV. Genes associated with these repeat regions encode a common amino acid motif, suggesting that they are a family of repeated contiguous gene clusters. Lepidopteran baculoviruses, similarly, have a family of repeated genes called the bro gene family. However, there is no significant sequence similarity between the NeabNPV and bro genes. Homologues of early-expressed genes such as ie-1 and lef-3 were absent in NeabNPV, as they are in the previously sequenced hymenopteran baculoviruses. Analyses of ORF upstream sequences identified potential temporally distinct genes on the basis of putative promoter elements.

Structural and ultrastructural changes during the infection of UFL-AG-286 cells with the baculovirus AgMNPV

During infection of the permissive insect cell line UFL-AG-286 by the baculovirus Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV-2D) several morphological changes occur. By 12 h postinfection (h p.i.), the infected cells became round and exhibited a decrease in the number of cytoplasmic projections. By 24 h p.i., it was possible to detect a virogenic stroma inside the cell nucleus, and after 48 h p.i., polyhedral inclusion bodies were observed. Some of these morphological modifications are probably due to changes in the cytoskeleton of the cell and this possibility was substantiated by the observation that the distribution of actin and microtubules was dramatically modified upon infection. Several viral-induced proteins were also produced during infection and a sharp decrease in overall protein synthesis was observed. These results are very similar to those obtained with other cell lines infected with different baculoviruses, indicating a similar mechanism of infection. Copyright 1998 Academic Press.

A baculovirus gp64 early promoter is activated by host transcription factor binding to CACGTG and GATA elements

The early promoter of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus gp64 gene is active when transfected into several insect cell lines and does not require viral gene products for transcription in uninfected cells. Because previous studies have shown that the gp64 early promoter is activated above basal levels in uninfected cells, host transcription factors are likely to play a role in gp64 activation at early times postinfection. By using nuclear extracts from uninfected Sf9 cells for electrophoretic mobility shift analysis of gp64 regulatory regions, host nuclear proteins were shown to bind specifically to the upstream regulatory region of the gp64 early promoter. Host factor binding was mapped to a 24-bp sequence centered approximately 35 bp upstream of the TATA box. Two consensus eukaryotic transcription factor-binding site motifs, GATA and CACGTG, were identified within the 24-bp sequence. Competition assays using oligonucleotides containing either a GATA or a CACGTG motif and similar oligonucleotides with point mutations in these sites showed that each site is required for binding host transcription factors. To investigate the functional significance of host factor binding to GATA and CACGTG motifs, constructs containing point mutations in these motifs were examined in transient expression assays. Mutations in either or both GATA and CACGTG sites decreased reporter activity in transient expression assays, suggesting that binding of host transcription factors to these motifs is important in transcriptional regulation of the gp64 early promoter.

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.

Nucleotide sequence and characterization of the 39K gene region of Autographa californica nuclear polyhedrosis virus

The complete nucleotide sequence of the Pstl-K fragment of the Autographa californica nuclear polyhedrosis virus (AcMNPV) genome was determined. This region of the genome contains the delayed early 39K gene and V-ubi, a viral protein with homology to ubiquitine. In addition, Pstl-K potentially encodes five other proteins. Hybrid-select translation mapped nine viral-specific proteins to this fragment. Three proteins were observed with both early and late RNA; two of them comigrated with proteins directed by transcripts made from the 39K open reading frame (ORF). One protein which was only seen with hybrid-selected late RNA comigrated with ubiquitin. A correction of previously published sequence data indicates that there is an additional AUG codon upstream of the 39K ORF. This AUG codon is located 8 nucleotides downstream of the early transcription initiation site. The methionine codon is followed by codons for three amino acids and a termination codon. The 39K ORF is located 102 nucleotides downstream of the minicistron. In vitro transcription-translation experiments confirmed that the downstream AUG serves as the initiation codon for the 39K ORF. Radioimmunoprecipitation experiments indicated that the Pstl-K-encoded 39K protein reacted with antiserum raised against a previously described nuclear matrix-associated 39K protein.

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.

Regulation of host RNA levels during baculovirus infection

During infection of the permissive host insect cell line Spodoptera frugiperda IPLB-SF-21 by the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV), the levels of host actin, histone, and heat shock 70 (hsp70) RNAs are reduced substantially. Reduction of the host RNA levels occurs primarily during a narrow window of the replication process, from approximately 12 to 18 hr postinfection (p.i.), corresponding to the phase in which the extracellular form of the virus buds into the media. A late viral protein appears to be required for this reduction since cycloheximide, an inhibitor of cytosolic protein synthesis, and aphidicolin, an inhibitor of host and viral DNA polymerases, inhibit the reduction of actin and histone RNA levels. A cDNA corresponding to the carboxyl half of the S. frugiperda mitochondrial cytochrome c oxidase subunit III (COIII) gene was isolated, sequenced, and characterized. Two differentially regulated mitochondrial transcripts of this gene are observed. The level of the larger of these transcripts, which is dependent on active cytosolic protein synthesis, is reduced during virus infection in a fashion similar to that of the nuclear host genes. The smaller COIII transcript is stable until at least 24 hr p.i. but the level of this RNA eventually declines by 48 hr p.i.

Multiple early transcripts and splicing of the Autographa californica nuclear polyhedrosis virus IE-1 gene

The immediate-early IE-1 gene of Autographa californica nuclear polyhedrosis virus was cloned, and its nucleotide sequence was determined. Sequence analysis indicated that this gene would encode a protein of 582 amino acids with a predicted molecular weight of 66,822. Analysis of IE-1 gene expression during baculovirus infection identified two transcripts. One, 1.9 kilobases (kb), was expressed at constant steady-state levels throughout infection, whereas the other, 2.1 kb, was expressed only early in infection. Analysis of IE-1 cDNA clones demonstrated that the 2.1-kb transcript contained the entire 1.9-kb transcript (exon 1) plus an additional 5' end (exon 0). Genomic Southern analysis placed the exon 0 sequences on the EcoRI B fragment, 4 kilobase pairs upstream of exon 1. Sequencing of the upstream region identified an open reading frame whose 5' end was identical to the exon 0 sequences in the cDNAs. Examination of the genomic DNA sequences around the exon-exon junction revealed sequences similar to published consensus splice acceptor and donor sequences. This is the first example of splicing of any viral transcript during baculovirus infection.

Characterization of an early gene accelerating expression of late genes of the baculovirus Autographa californica nuclear polyhedrosis virus

The region of the Autographa californica nuclear polyhedrosis virus (AcMNPV) encompassing the EcoRI T fragment (29.0 to 30.1 map units) was characterized by DNA sequencing, transcriptional mapping, and site-directed mutagenesis. The largest transcript from this region, an early 1.7-kilobase (kb) poly(A)+ RNA, encompassed three tandem, nonoverlapping open reading frames (ORFs). The largest of these ORFs, ETL, was proximal to the 5' end of the transcript and had the capacity to encode a 28-kilodalton (kDa) polypeptide. A recombinant virus, vETL beta gal, containing the Escherichia coli beta-galactosidase (beta gal) gene fused to the N-terminal two-thirds of the ETL ORF, produced blue plaques in the presence of a chromogenic indicator of beta gal and wild-type levels of polyhedra in cell culture. This recombinant was also infectious in insect larvae by oral administration of occluded virus. Comparison of vETL beta gal and wild-type viral proteins pulse-labeled at various times postinfection (p.i.) revealed (i) absence of a virus-induced 28-kDa polypeptide, (ii) early expression of a large (approximately 130-kDa) polypeptide which may be the ETL-beta gal fusion protein, (iii) a delay in expression of early 35 and 40-kDa polypeptides, and (iv) a 4- to 6-h delay in the expression of late proteins in vETL beta gal-infected cells. Cycloheximide did not inhibit synthesis of the 1.7-kb RNA but did inhibit its shutoff, which occurs at 12 h p.i. in the absence of inhibitors. Thus, the ETL gene product is apparently an early 28-kDa protein which is necessary, directly or indirectly, for timely expression of many other AcMNPV genes. The promoter-leader regions of the 1.7-kDa transcript showed significant sequence similarities to the leader of the AcMNPV IE-1 gene. The middle ORF within the 1.7-kb transcript, ETM, would encode a hydrophobic polypeptide of 113 amino acid residues. ETS, a small ORF within and proximal to the 3' end of the 1.7-kb transcript, was also transcribed as a set of smaller (approximately 0.5-kb) RNAs initiated heterogeneously in the region between ETL and ETS and persisting throughout infection.

A synthetic peptide to the predicted 6.9K translation product of the HindIII-H/EcoRI-D region of the AcNPV genome induces antibodies to the basic DNA-binding protein

The sequence of the HindIII-H/EcoRI-D region of the AcNPV genome revealed a 55 codon methionine-initiated open reading frame (Wilson, M.E., Mainprize, T.H., Friesen, P.D. and Miller, L.K., 1987, J. Virol. 61, 661-666). This open reading frame predicts a 6.9K translation product with an amino acid composition nearly identical to the basic virion DNA-binding protein. A synthetic peptide was prepared to a region of the predicted 6.9K protein and used to induce antibodies in rabbits. Western blots of viral proteins, infected cell-specific proteins (24 h p.i.), and basic chromatin-associated proteins (24 h p.i.) were probed with anti-6.9K antisera. In all immunoblots, the anti-6.9K antisera were specific for the basic DNA-binding protein. This investigation provides further evidence that the basic DNA-binding protein of the virion is a viral-encoded gene transcribed from the HindIII-H/EcoRI-D region of the genome.

Overlapping sets of viral RNAs reflect the array of polypeptides in the EcoRI J and N fragments (map positions 81.2 to 85.0) of the Autographa californica nuclear polyhedrosis virus genome

In several parts of the Autographa californica nuclear polyhedrosis virus (AcNPV) genome, nested sets of overlapping RNAs with common 3' or 5' termini have been recognized. In the present report, the pattern of viral transcription and the arrangement of viral gene products in the region of 81.2 to 85.0 map units were investigated. In this segment of the AcNPV genome, at least nine size classes of viral RNA were identified which ranged in size from 1.3 kilobases (kb) to 4.6 kb and exhibited common 3' termini. The detailed restriction map and the nucleotide sequence of this part of the AcNPV genome were determined. Computer analyses revealed several open reading frames (ORFs) on the rightward-transcribed strand with potential TATA and CAAT signals preceding many of the potential ORFs and the 5' termini of some of the mapped RNAs. The leftward-transcribed strand was devoid of major ORFs. The presumptive polypeptides encoded by the larger ORFs ranged in size from 11.3 to 55.6 kilodaltons (kDa). The amino acid sequence of the presumptive polypeptide encoded by ORF3, a 33.6-kDa molecule, exhibited an unusual, clustered 16-fold repeat of the dipeptide arginine-serine in a protein that showed an overall preponderance of basic amino acids. The results of in vitro translation experiments with hybrid-selected RNAs homologous to internal subfragments of the 81.2- to 85.0-map-unit region yielded polypeptides of approximately 28, 34 to 36, and 48 to 50 kDa, which were close in size to the lengths of the major ORFs derived from the nucleotide sequence. The localizations of individual size classes of RNAs in the 81.2- to 85.0-map-unit region of the viral genome were determined precisely at the 3' and 5' termini by S1 protection analyses. Within a sequence of eight nucleotides, all RNAs had the same 3' terminus, which lay close to multiple polyadenylation signals. The initiation sites of the nine different RNA size classes were precisely mapped. As the cap sites of the smaller RNAs (less than 1.8 kb) were determined by S1 protection analyses, a multitude of RNA initiation sites became apparent. It was also shown that the different RNA size classes in the 81.2- to 85.0-map-unit region were detectable as early as 2 h and at least until 36 to 48 h after infection. In unselected cytoplasmic RNA, the size classes of viral RNAs specific for the EcoRI J fragment were detectable early as well as late after infection, although at early times the larger RNAs were detectable in smaller amounts.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of baculovirus p10 synthesis using monoclonal antibodies

A series of monoclonal antibodies were produced against virion proteins of the multicapsid nuclear polyhedrosis virus of Orgyia pseudotsugata (OpMNPV). Four of these antibodies reacted with a protein of 14 kd on Western blots of electrophoretically separated OpMNPV virion proteins. These antibodies were used to identify immunoreactive clones from a lambda gt11 expression library of OpMNPV DNA. By hybridization of insert DNA from the lambda gt11 clones to blots of digests of OpMNPV genomic DNA, and by sequencing the ends of the lambda gt11 inserts, these clones were shown to contain a portion of the p10 gene. The regions containing epitopes recognized by the four monoclonal antibodies were located using fusion proteins made from selected portions of the p10 reading frame in a trpE vector. One of the p10 antibodies was used to characterize p10 synthesis in infected Lymantria dispar cells by using Western blots and immunofluorescent staining. The p10 protein was detected with immunofluorescent microscopy at 14 hr postinfection and by 20 hr it formed intensely staining cytoplasmic structures. On Western blots of infected cells, two forms of p10 (of about 14 and 15 kd) were observed. One of the p10 monoclonal antibodies showed a strong cross-reaction with cytoskeletal structures in uninfected insect cells and rat fibroblasts.

Divergent transcription of early 35- and 94-kilodalton protein genes encoded by the HindIII K genome fragment of the baculovirus Autographa californica nuclear polyhedrosis virus

The organization of viral genes within the 3.7-kilobase-pair HindIII-K/EcoRI-S region of the Autographa californica nuclear polyhedrosis virus genome (85 to 88 map units) was determined by using a combination of nucleotide sequencing, transcriptional mapping, and in vitro translation of hybrid selected RNA. Two nonoverlapping genes, extending in opposite directions and encoding polypeptides with molecular weights of 35,000 and 94,000 (35K and 94K polypeptides), were identified. Unspliced, messenger-active RNAs were transcribed from both genes early (2 h) after infection. Indicative of immediate-early genes, transcription of the divergent RNAs was unaffected by the protein synthesis inhibitor, cycloheximide. Late in infection, abundant RNAs were transcribed from promoters located at least 2.5 kilobase pairs upstream from the gene encoding the 35K polypeptide. These transcripts completely overlapped both the 35K and 94K polypeptide genes but apparently lacked protein-coding potential, suggesting that the transcripts may play a role in the suppression of early viral gene expression.

Location, transcription, and sequence of a baculovirus gene encoding a small arginine-rich polypeptide

The nucleotide sequence of the cDNA of an abundant late 0.5-kilobase transcript of Autographa californica nuclear polyhedrosis virus revealed a small open reading frame encoding an arginine-rich 6.9-kilodalton protein. The predicted amino acid composition of the 6.9-kilodalton protein was essentially identical to that of the core protein of viral nucleocapsids. The precise location of the 5' and 3' ends of the transcript were confirmed by S1 nuclease and primer extension analyses. Multiple overlapping transcripts through this region include three early and three abundant late RNAs which are transcribed counterclockwise and one transient RNA which is transcribed clockwise with respect to the physical map of the virus.

Variation in the temporal expression of overlapping baculovirus transcripts

To investigate gene expression from the Autographa californica nuclear polyhydrolysis virus genome (AcNPV), complementary DNA (cDNA) was synthesized from polyadenylated RNA transcribed at 2 h and 10 h postinfection (p.i.) and then cloned into Escherichia coli using plasmid pUC-9. Eighteen 2 h cDNA plasmids were homologous to five distinct regions of the viral genome, while forty-nine 10 h cDNA plasmids were homologous to 15 regions including the five regions transcribed at 2 h. Temporal expression of polyadenylated RNA transcribed from diverse regions of the genome was examined using Northern blot hybridization with the above 2 and 10 h cDNA probes. All regions displayed overlapping sets of RNAs. In addition to HindIII-I/EcoRI-F (IF) and HindIII-B2/EcoRI-H (B2H), several, but not all, regions showed a sequential appearance of higher molecular weight RNAs as the infection progressed. Each overlapping set of RNAs exhibited unique characteristics including variations in the number of overlapping transcripts, their temporal regulation, and their relative abundance during the course of infection.

Nucleotide sequencing and transcriptional mapping of the Orgyia pseudotsugata multicapsid nuclear polyhedrosis virus p10 gene

A 32P-labeled cloned DNA fragment (AcMNPV HindIII-Q) containing one of the repeated sequences and a portion of the p10 gene from Autographa californica multicapsid nuclear polyhedrosis virus (AcMNPV) was used to probe Southern blots containing restriction endonuclease digests of Orgyia pseudotsugata muticapsid nuclear polyhedrosis virus (OpMNPV) DNA. A single 3.6-kb fragment, OpMNPV HindIII-Q, was hybridized. The OpMNPV HindIII-Q fragment was cloned into pUC-18, mapped with restriction endonucleases, and reprobed with the AcMNPV HindIII-Q fragment. A small region of ca. 700 bp, near the left end of the cloned fragment, was cross-hybridized. DNA sequencing in this region revealed an open reading frame of 279 bp which shares detectable homology with the p10 gene of AcMNPV. The sequences downstream from the p10 gene in both viruses also contain long open reading frames which share homology. Northern blot analysis of RNA from OpMNPV infected O. leucostigma cells was used to define the temporal and spatial organization of transcripts from this region. S1 analysis of both termini of the major p10 mRNA indicates nontranslated regions of 52-53 bases at the 5' end and 175 bases at the 3' end. The 5'-mRNA start site was located within a 12-nucleotide sequence which is conserved in all late hyperexpressed baculovirus genes.

Functional mapping of a trans-activating gene required for expression of a baculovirus delayed-early gene

The temporal regulation of an early gene of the baculovirus Autographa californica nuclear polyhedrosis virus was examined. We constructed a plasmid (plasmid 39CAT) in which the bacterial gene for chloramphenicol acetyltransferase was placed under the control of the promoter for the gene for a A. californica nuclear polyhedrosis virus 39,000-dalton protein (39K). A transient expression assay of plasmid 39CAT revealed that the 39K gene was expressed in infected cells but not in uninfected cells, indicating that the 39K gene should be classified as a delayed-early gene. The 39K promoter also efficiently directed the synthesis of chloramphenicol acetyltransferase when the plasmid was cotransfected with viral DNA which had been restricted with several restriction enzymes. To map the location of the gene(s) required for the synthesis of 39K, plasmid 39CAT was cotransfected with purified restriction fragments of A. californica nuclear polyhedrosis virus DNA. Fragments which mapped between 90.7 and 100.8 map units induced plasmid 39CAT. Plasmid pEcoRI-B, containing EcoRI fragment B (90 to 100 map units), activated plasmid 39CAT. Functional mapping of plasmid pEcoRI-B indicated that the essential region was located between 95.0 and 97.5 map units. The 5' end of this gene was mapped, and the chloramphenicol acetyltransferase gene was inserted under the control of its promoter. Transient assay experiments indicated that the trans-acting regulatory gene was expressed in uninfected cells and is therefore an immediate-early gene. This gene was named IE-1.

Physical mapping of temporally regulated, overlapping transcripts in the region of the 10K protein gene in Autographa californica nuclear polyhedrosis virus

We investigated transcriptional activity in the region of a gene for a major late protein (10 kilodaltons) of Autographa californica nuclear polyhedrosis virus. This 10K protein gene spans an HindIII cleavage site, with the 5' end of the gene located in the HindIII-Q fragment and the 3' end in the HindIII-P fragment. Northern blot analysis showed that there were at least four transcripts mapping in this region, two that are present maximally at 12 h (1,500 and 1,100 bases in length) and two that are present at 24 h (750 and 2,500 bases in length). Northern blot analysis also suggested that these transcripts overlap. S1 mapping identified the precise positions of the transcripts and confirmed the overlap. The 1,500- and 1,100-base transcripts share a common 5' end, about 1,056 bases upstream from the HindIII-P-Q boundary; the 750- and 2,500-base transcripts share a common 5' end, about 303 bases upstream from the HindIII-P-Q boundary. The 1,100-base transcript terminates in the Q fragment, about 234 bases from the HindIII-P-Q boundary. The other three transcripts all end in the P fragment. Both the 1,500- and the 750-base transcripts terminate approximately 240 bases from the HindIII-P-Q boundary. The 2,500-base transcript exhibits 3'-end heterogeneity, ending around 2,046 bases from the HindIII-P-Q boundary. Therefore, three different transcripts (1,500, 750, and 2,500 bases in length) contain the complete 10K protein coding region. A fourth transcript (1,100 bases in length) overlaps the 750-base 10K protein gene transcript by 70 bases.

Temporal regulation of baculovirus RNA: overlapping early and late transcripts

Analysis of the temporal sequence of RNA transcription within the Autographa californica nuclear polyhedrosis virus genome revealed individual transcription units composed of overlapping early or late RNAs, or both. High-resolution S1 nuclease mapping within the 3.0-kilobase HindIII-K fragment located five overlapping RNAs (1.07, 1.38, 2.63, 3.16, and 3.50 kilobases) transcribed in the same direction and terminated at a common 3' site. The smallest RNAs appeared early but were replaced in time by successively larger RNAs initiated further upstream. Primer extension analysis supported the contention that this temporal regulation involved the sequential activation of upstream promoters and the coordinate deactivation of downstream promoters. As such, transcription from upstream genes may suppress that of downstream genes via transcriptional interference (promoter occlusion) and thereby facilitate sequential expression of different viral functions. In contrast, overlapping RNAs with extended 3' ends were transcribed from the abundantly expressed p10 and polyhedrin genes mapping to the HindIII-Q,P/EcoRI-P and HindIII-V/EcoRI-I fragments, respectively. These RNAs were synthesized maximally during the very late occlusion phase and consisted of a major small transcript and several larger but less abundant transcripts.

Size and map locations of early transcription products on the Autographa californica nuclear polyhedrosis virus genome

The size classes and map locations of early transcripts of Autographa californica nuclear polyhedrosis virus were identified by probing Northern blots with cloned viral DNA fragments. Twelve abundant and 30 minor transcripts were detected by 4 hr postinfection. There was an increase in the abundancy and number of virus-specific transcripts between 6 and 8 hr postinfection, coincident with the onset of DNA replication. The change in transcription pattern did not occur in cells infected at the nonpermissive temperature with a DNA-negative mutant indicating that these early transcripts were synthesized prior to the initiation of viral DNA replication. Two early transcripts, 4.6 and 1.1 kb, mapped to the genomic region of the temperature-sensitive lesion, between 60.1 and 62.0%.

Transcription of overlapping sets of RNAs from the genome of Autographa californica nuclear polyhedrosis virus: a novel method for mapping RNAs

The insect baculovirus Autographa californica nuclear polyhedrosis virus (AcNPV) contains a double-stranded, supercoiled circular genome of 126 to 129 kilobase pairs in length. In cultured Spodoptera frugiperda insect cells the virus replications, and early and late phases of viral genome expression are discernible. We previously mapped 5 early and at least 32 late different viral polypeptides on the viral genome (H. Esche, H. Lübbert, B. Siegmann, and W. Doerfler, EMBO J. 1:1629-1633, 1982). However, at the same time we located 11 early and more than 90 late different size-classes of viral RNA on the AcNPV genome. Evidence for extensive RNA splicing in this virus system has not been adduced, although tiny splices cannot yet be ruled out (H. Lübbert and W. Doerfler, J. Virol. 50:497-506, 1984). The large number of AcNPV transcripts and the apparent lack of splicing have raised tantalizing questions about the mechanisms involved in the expression of AcNPV DNA and its regulation. It is also unknown how the widely differing numbers of RNAs and polypeptides can be correlated. For this reason, we have started to analyze in detail the map locations of some of the RNA size-classes in three different segments of the viral genome. For this purpose a novel method has been devised which will prove useful for the analyses of transcriptional patterns in complex viral genomes. The EcoRI fragments J, O through F, and Q, comprising viral DNA segments between 81.8 and 86.4, 32.6 and 41.0, and 88.2 and 89.7 map units, respectively, were investigated. Surprisingly, overlapping sets of viral RNAs of various lengths and with apparently common 3' termini in EcoRI fragments J (seven size-classes) and O through F (four size-classes) or with common 5' termini in EcoRI fragment Q (two size-classes) have been detected. At present, the functional significance of this mode of transcription is unknown. EcoRI fragment Q of AcNPV DNA encodes a 10,000-molecular-weight polypeptide which is expressed abundantly late after infection. The function of this protein has not yet been elucidated. The promoter and 5' part of the gene for the 10,000-molecular-weight polypeptide have been sequenced, and we have shown that at least two RNAs of different lengths are transcribed in this region and initiated at one site of three nucleotides. Studies on the expression of the AcNPV genome have revealed interesting properties not commonly found in other eucaryotic systems.