Baculovirus immediately early 1, a mediator for homologous regions enhancer function in trans

Complete genome sequence analysis and genome organization of Dasychira pudibunda nucleopolyhedrovirus (DapuNPV-T1) from Turkey

The economically important pale tussock moth Dasychira pudibunda L. (Lepidoptera: Lymantriidae), known as a beech pest in Europe, primarily inflicts damage on beech leaves. In the present study, we aim to reveal the genome characteristics of Dasychira pudibunda nucleopolyhedrovirus (DapuNPV-T1), which was detected for the first time in Turkey and compare it with the reference genome and other baculovirus genomes. The DapuNPV-T1 genome was determined to be a circular, double-stranded DNA molecule with 136,920 bp and a nucleotide distribution of 54.4% G + C. Bioinformatics analysis showed that the DapuNPV-T1 genome contains 163 open reading frames with more than 150 nucleotides. Fifty-four ORFs of unknown function, 6 homologous regions (hrs), 1 AC-rich region, and 3 bro genes (bro-a, bro-b, and bro-c) were determined in the genome sequence. Comparative analysis with other baculovirus strains revealed distinctions in the DapuNPV-T1 genome based on ORF. The gene parity plot and phylogenetic analysis confirmed that DapuNPV-T1 belongs to the alphabaculovirus group Ib. In addition, the DapuNPV-T1 isolate was found to be close to the nucleopolyhedrovirus Poland isolate in Dasychira pudibunda and Orgyia pseudotsugata multiple nucleopolyhedrovirus. With this study, the first genome analysis of DapuNPV from Turkey became the second in the world to enter the literature. Comprehensive information on a wide range of isolates will provide a more detailed overview of baculoviruses and help overcome their shortcomings as biocontrol agents.

Homologous recombination risk in baculovirus expression vector system

The baculovirus expression vector system (BEVS) is widely used for producing recombinant proteins. To achieve high expression level of recombinant proteins, baculoviral elements, such as enhancers, promoters, signal peptide coding sequences and 3'-UTR, have been extensively employed. There is a recombination risk derived from homologous sequences between viral genome and functional baculovirus-derived elements associated with foreign genes. Although homologous recombination have distinct biological functions, these potential adverse recombination may trigger a DNA fragment being inverted or looped out, resulting in the production of defective viruses and eventual yields declines of recombinant proteins. However, the risk of such homologous recombination has not been systematically assessed. Here, we measured the recombination rate using a promoter-less fluorescent reporter integrated with various lengths homologous of p10 coding region. Homologous fragments longer than 60 bp possess sufficient recombination probability and exerts effect on purity and integrity of virus. Shortening the length of homologous fragments and separating homologous fragments by point mutations can effectively reduce unfavorable recombination. These findings reveal a homologous recombination risk resulted from genome-homologous baculoviral elements and propose reliable strategies reducing recombination rate to facilitate viral stability and integrity in baculovirus expression vector system.

Multiloci Manipulation of Baculovirus Genome Reveals the Pivotal Role of Homologous Regions in Viral DNA Replication, Progeny Production, and Enhancing Transcription

The engineering of viral genomes facilitates both fundamental and applied research on viruses. However, the multiloci manipulation of DNAs of viruses with large DNA genomes, such as baculoviruses, herpesviruses, and poxviruses, is technically challenging, particularly for highly homologous or repetitive sequences. Homologous regions (hrs) have multiple copies in many large DNA viruses and play pivotal roles in the viral life cycle. Here, we used synthetic biology to investigate the fundamental function of baculoviral hrs by conducting multiloci manipulation of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) DNA that contains eight hrs scattered in the genome. Using transformation-associated recombination in yeast, we generated recombinant AcMNPV genomes in which we deleted all hrs or retained a single hr (hr1, hr2, or hr3). Infectious viruses were rescued after transfecting the synthetic viral genomes into host cells, and their replication features were characterized. The results demonstrated that deletion of all hrs severely compromised viral DNA replication and progeny production, whereas retaining only a single hr was essential for efficient viral DNA replication and progeny production. The synthetic virus with hr2 or hr3 showed a growth curve similar to that of the parental virus. Transcriptomic analysis revealed that hr1, hr2, and hr3 could enhance gene transcription within a surrounding region of 14.6 kb, 13.8 kb, and 29.8 kb, respectively. Overall, this study revealed the advantages of synthetic biology in multiloci engineering and functional studies of large DNA viruses. In addition, our findings on hrs will be helpful for the design and improvement of baculovirus-based expression vectors.

Exploration of an Actin Promoter-Based Transient Expression Vector to Trace the Cellular Localization of Nucleorhabdovirus Proteins in Leafhopper Cultured Cells

Continuously cultured cell lines derived from planthopper and leafhopper have greatly facilitated the investigation of rice viruses transmitted by these insects. However, the lack of a suitable transient expression vector has limited their utility. Here, by cloning and analyzing the promoter sequence of the gene encoding cytoplasmic actin from the leafhopper Nephotettix cincticeps, we successfully developed the first efficient transient expression vector for cultured leafhopper cells, which can also be used to express exogenous proteins in other insect culture cell lines, including those derived from Recilia dorsalis leafhopper and Spodoptera frugiperda (Sf9). Furthermore, insertion of the Hr5 viral enhancer element and knockdown of the endogenous Dicer2 gene notably improved the vector's expression efficiency in leafhopper cells. Using the optimized vector, we have for the first time traced the cellular localization of the proteins encoded by rice yellow stunt virus (RYSV) in cells of its insect vector and demonstrated that P6 protein is a component of the viroplasm.

Genomic diversity in European Spodoptera exigua multiple nucleopolyhedrovirus isolates

Key virus traits such as virulence and transmission strategies rely on genetic variation that results in functional changes in the interactions between hosts and viruses. Here, comparative genomic analyses of seven isolates of Spodoptera exigua multiple nucleopolyhedrovirus (SeMNPV) with differing phenotypes were employed to pinpoint candidate genes that may be involved in host-virus interactions. These isolates obtained after vertical or horizontal transmission of infection in insects differed in virulence. Apart from one genome containing a piggyBac transposon, all European SeMNPV isolates had a similar genome size and content. Complete genome analyses of single nucleotide polymorphisms and insertions/deletions identified mutations in 48 ORFs that could result in functional changes. Among these, 13 ORFs could be correlated with particular phenotypic characteristics of SeMNPV isolates. Mutations were found in all gene functional classes and most of the changes we highlighted could potentially be associated with differences in transmission. The regulation of DNA replication (helicase, lef-7) and transcription (lef-9, p47) might be important for the establishment of sublethal infection prior to and following vertical transmission. Virus-host cell interactions also appear instrumental in the modulation of viral transmission as significant mutations were detected in virion proteins involved in primary (AC150) or secondary infections (ME35) and in apoptosis inhibition (IAP2, AC134). Baculovirus populations naturally harbour high genomic variation located in genes involved at different levels of the complex interactions between virus and host during the course of an infection. The comparative analyses performed here suggest that the differences in baculovirus virulence and transmission phenotypes involve multiple molecular pathways.

The progress and future of enhancing antiviral capacity by transgenic technology in the silkworm Bombyx mori

Bombyx mori is a common lepidopteran model and an important economic insect for silk production. B. mori nucleopolyhedrovirus (BmNPV) is a typical pathogenic baculovirus that causes serious economic losses in sericulture. B. mori and BmNPV are a model of insect host and pathogen interaction including invasion of the host by the pathogen, host response, and enhancement of host resistance. The antiviral capacity of silkworms can be improved by transgenic technology such as overexpression of an endogenous or exogenous antiviral gene, RNA interference of the BmNPV gene, or regulation of the immune pathway to inhibit BmNPV at different stages of infection. Antiviral capacity could be further increased by combining different methods. We discuss the future of an antiviral strategy in silkworm, including possible improvement of anti-BmNPV, the feasibility of constructing transgenic silkworms with resistance to multiple viruses, and the safety of transgenic silkworms. The silkworm model could provide a reference for disease control in other organisms.

Resistance to BmNPV via overexpression of an exogenous gene controlled by an inducible promoter and enhancer in transgenic silkworm, Bombyx mori

The hycu-ep32 gene of Hyphantria cunea NPV can inhibit Bombyx mori nucleopolyhedrovirus (BmNPV) multiplication in co-infected cells, but it is not known whether the overexpression of the hycu-ep32 gene has an antiviral effect in the silkworm, Bombyx mori. Thus, we constructed four transgenic vectors, which were under the control of the 39 K promoter of BmNPV (39 KP), Bombyx mori A4 promoter (A4P), hr3 enhancer of BmNPV combined with 39 KP, and hr3 combined with A4P. Transgenic lines were created via embryo microinjection using practical diapause silkworm. qPCR revealed that the expression level of hycu-ep32 could be induced effectively after BmNPV infection in transgenic lines where hycu-ep32 was controlled by hr3 combined with 39 KP (i.e., HEKG). After oral inoculation of BmNPV with 3 × 10⁵ occlusion bodies per third instar, the mortality with HEKG-B was approximately 30% lower compared with the non-transgenic line. The economic characteristics of the transgenic lines remained unchanged. These results suggest that overexpression of an exogenous antiviral gene controlled by an inducible promoter and enhancer is a feasible method for breeding silkworms with a high antiviral capacity.