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

Real-Time Expression Analysis of Selected Anticarsia gemmatalis multiple nucleopolyhedrovirus Gene Promoters during Infection of Permissive, Semipermissive and Nonpermissive Cell Lines

Baculovirus infection follows a transcriptionally controlled sequence of gene expression that occurs by activation of different viral gene promoter sequences during infection. This sequence of promoter activation may be disrupted by cellular defenses against viral infection, which might interfere with viral progeny formation. In this work, the activity of the ie1, gp64, lef-1, vp39, p6.9 and polh promoters of the Anticarsia gemmatalis multiple nucleopolyhedrovirus was assessed during infection of permissive, semipermissive and nonpermissive cell lines by a novel methodology that detects reporter protein luminescence in real-time. This technique allowed us to characterize in rich detail the AgMNPV promoters in permissive cell lines and revealed differential profiles of expression in cells with limited permissivity that correlate well with limitations in viral DNA replication. Semipermissive and nonpermissive cell lines presented delays and restrictions in late and very late promoter expression. Cells undergoing apoptosis did not inhibit late gene expression; however, viral progeny formation is severely affected. This work demonstrates the application of the real-time luminescence detection methodology and how the promoter expression profile may be used to diagnose cellular permissivity to baculovirus infection.

An additional copy of the homologous region (hr1) sequence in the Autographa californica multinucleocapsid polyhedrosis virus genome promotes hyperexpression of foreign genes

The Autographa californica multinucleocapsid nuclear polyhedrosis virus genome contains nine homologous region (hr1, hr1a, hr2, hr2a, hr3, hr4a, hr4b, hr4c, and hr5) sequences that are thought to be involved in viral replication and activation of transcription. Our results show that the 750 bp hr1 sequence is capable of functioning as an enhancer of transcription of foreign genes from the homologous late polyhderin gene promoter and the heterologous Drosophila heat shock protein (hsp70) promoter in insect cells. Introduction of an additional copy of the complete hr1 element downstream to the polyhedrin locus in the viral genome, while not affecting the stability of the recombinant virus for at least 30 serial passages, led to hyperexpression of reporter genes. The enhancement in the expression levels of foreign genes varied from 40 to 90-fold depending on the promoter used.

Novel Sp family-like transcription factors are present in adult insect cells and are involved in transcription from the polyhedrin gene initiator promoter

We earlier documented the involvement of a cellular factor, polyhedrin (polh) promoter-binding protein, in transcription from the Autographa californica nuclear polyhedrosis virus polh gene promoter. Sequences upstream of the polh promoter were found to influence polh promoter-driven transcription. Analysis of one such region, which could partially compensate for the mutated polh promoter and also activate transcription from the wild-type promoter, revealed a sequence (AcSp) containing a CACCC motif and a loose GC box resembling the binding motifs of the transcription factor Sp1. AcSp and the consensus Sp1 sequence (cSp) specifically bound factor(s) in HeLa and Spodoptera frugiperda (Sf9) insect cell nuclear extracts to generate identical binding patterns, indicating the similar nature of the factor(s) interacting with these sequences. The AcSp and cSp oligonucleotides enhanced in vivo expression of a polh promoter-driven luciferase gene. In vivo mopping of these factor(s) significantly reduced transcription from the polh promoter. Recombinant viruses carrying deletions in the upstream AcSp sequence confirmed the requirement of these factor(s) in polh promoter-driven transcription in the viral context. We demonstrate for the first time DNA-protein interactions involving novel members of the Sp family of proteins in adult insect cells and their involvement in transcription from the polh promoter.

The host factor polyhedrin promoter binding protein (PPBP) is involved in transcription from the baculovirus polyhedrin gene promoter

Hypertranscription and temporal expression from the Autographa californica nuclear polyhedrosis (AcNPV) baculovirus polyhedrin promoter involves an alpha-amanitin-resistant RNA polymerase and requires a trans-acting viral factor(s). We previously reported that a 30-kDa host factor, polyhedrin promoter binding protein (PPBP), binds with unusual affinity, specificity, and stability to the transcriptionally important motif AATAAATAAGTATT within the polyhedrin (polh) initiator promoter and also displays coding strand-specific single-stranded DNA (ssDNA)-binding activity (S. Burma, B. Mukherjee, A. Jain, S. Habib, and S. E. Hasnain, J. Biol. Chem. 269:2750-2757, 1994; B. Mukherjee, S. Burma, and S. E. Hasnain, J. Biol. Chem. 270:4405-4411, 1995). We now present evidence which indicates that an additional factor(s) is involved in stabilizing PPBP-duplex promoter and PPBP-ssDNA interactions. TBP (TATA box binding protein) present in Spodoptera frugiperda (Sf9) cells is characteristically distinct from PPBP and does not interact directly with the polh promoter. Replacement of PPBP cognate sequences within the polh promoter with random nucleotides abolished PPBP binding in vitro and also failed to express the luciferase reporter gene in vivo. Phosphocellulose fractions of total nuclear extract from virus-infected cells which support in vitro transcription from the polh promoter contain PPBP activity. When PPBP was sequestered by the presence of oligonucleotides containing PPBP cognate sequence motifs, in vitro transcription of a C-free reporter cassette was affected but was restored by the exogenous addition of nuclear extract containing PPBP. When PPBP was mopped out in vivo by a plasmid carrying PPBP cognate sequence present in trans, polh promoter-driven expression of the luciferase reporter was abolished, demonstrating that binding of PPBP to the polh promoter is essential for transcription.

Involvement of host factors in transcription from baculovirus very late promoters -- a review

The baculovirus expression vector system has emerged as the system of choice for the expression of a number of heterologous genes of both prokaryotic and eukaryotic origin. This system utilizes the baculovirus very late, hyperactive polyhedrin and p10 promoters to drive the transcription of foreign genes. Regulation of transcription from these promoters is presently not well understood even though a number of viral gene products that may be important for transcription have been identified. Fresh insight into host-virus interactions during baculovirus pathogenesis is now offered by the identification of insect host factors that interact with transcriptionally essential motifs of these promoters as well as cis-acting enhancer-like elements upstream from the promoter.

Heterologous promoter recognition leading to high-level expression of cloned foreign genes in Bombyx mori cell lines and larvae

The baculovirus expression system using the Autographa californica nuclear polyhedrosis virus (AcNPV) has been extensively utilized for high-level expression of cloned foreign genes, driven by the strong viral promoters of polyhedrin (polh) and p10 encoding genes. A parallel system using Bombyx mori nuclear polyhedrosis virus (BmNPV) is much less exploited because the choice and variety of BmNPV-based transfer vectors are limited. Using a transient expression assay, we have demonstrated here that the heterologous promoters of the very late genes polh and p10 from AcNPV function as efficiently in BmN cells as the BmNPV promoters. The location of the cloned foreign gene with respect to the promoter sequences was critical for achieving the highest levels of expression, following the order + 35 > + 1 > -3 > -8 nucleotides (nt) with respect to the polh or p10 start codons. We have successfully generated recombinant BmNPV harboring AcNPV promoters by homeologous recombination between AcNPV-based transfer vectors and BmNPV genomic DNA. Infection of BmN cell lines with recombinant BmNPV showed a temporal expression pattern, reaching very high levels in 60-72 h post infection. The recombinant BmNPV harboring the firefly luciferase-encoding gene under the control of AcNPV polh or p10 promoters, on infection of the silkworm larvae led to the synthesis of large quantities of luciferase. Such larvae emanated significant luminiscence instantaneously on administration of the substrate luciferin resulting in 'glowing silkworms'. The virus-infected larvae continued to glow for several hours and revealed the most abundant distribution of virus in the fat bodies. In larval expression also, the highest levels were achieved when the reporter gene was located at + 35 nt of the polh.

Overexpression of a functional NMDA receptor subunit (NMDAR1) in baculovirus-infected Trichoplusia ni insect cells

For overexpression of the N-methyl-D-aspartate (NMDA) receptor subunit 1b (NMDAR1b), its corresponding cDNA was extended by codons for six histidine residues at the 3'-end, cloned into a baculovirus transfer vector and integrated into the viral genome. Infection of Trichoplusia ni insect cells (High FiveTM cells) with recombinant baculovirus resulted in the production of 126- and 105-kDa NR 1b proteins in the cell membrane fraction. Enzymatic deglycosylation with PNGase F as well as infection of the insect cells in the presence of tunicamycin revealed that the two proteins represented the N-glycosylated and non-glycosylated forms of NMDAR1b, respectively. The recombinant NR1b protein was also identified with immunocytochemical methods employing a monoclonal antibody which recognized the six histidine residues. The affinity of this histidine tag to nickel ions was used for the purification of the NR1b protein. The glycine binding site of the subunit was successfully identified and analyzed with the specific antagonist 5,7-[3-3H]dichlorokynurenate (DCKA). The observed binding characteristics were similar to those obtained for native NMDA receptors. Whereas in electrophysiological measurements a functional NMDA receptor channel could not be found in infected insect cells, its expression was demonstrated in the Xenopus oocyte system after injection of the NMDAR1b cDNA construct.

A 30-kDa host protein binds to two very-late baculovirus promoters

A 30-kDa host factor (polyhedrin-promoter-binding protein; PPBP) specifically binds to sequences critical for transcription from the baculovirus polyhedrin (p29) gene initiator promoter [Burma, S., Mukherjee, B., Jain, A., Habib, S. & Hasnain, S. E. (1994) J. Biol. Chem. 269, 2750-2757; Mukherjee, B., Burma, S. & Hasnain, S. E. (1995) J. Biol. Chem. 270, 4405-4411]. A host factor also binds, in gel shift assays, to the very-late p10 gene promoter through DNA sequence motifs similar to the PPBP.p29 interaction. The p10 host factor complex was specifically competed out with oligonucleotides containing p29 cognate sequence motifs AATAAA and TAAGTATT, but this did not occur when these motifs were replaced with random sequences. From ultraviolet cross-linking analysis, the molecular mass of this host factor was estimated to be approximately 30 kDa. Experiments were performed to investigate if this host factor displayed any differences in affinity and turnover with respect to the p29 and p10 untranslated leader sequences known to be important for temporal fine tuning and the late burst of transcription. Half-life determination of the p10-binding protein revealed similar binding affinities for the initiator elements of both the promoters, but higher affinity for the p10 5'-untranslated region (approximately 30 min versus approximately 10 min). The involvement of a similar host factor binding to both the p10 and p29 promoters indicates the possibility of a similar mode of transcription initiation from these two very-late promoters.