Phenotypic and genetic analysis of Lymantria dispar nucleopolyhedrovirus few polyhedra mutants: mutations in the 25K FP gene may be caused by DNA replication errors

A seamless connection from the burst sequence to the start codon is essential for polyhedrin hyperexpression in alphabaculoviruses

Alphabaculoviruses produce a large number of occlusion bodies (OBs) in host cells during the late stage of infection. OBs are mainly composed of polyhedrin (POLH), and high-level transcription of the polh gene has been exploited to express foreign proteins in insect cells. While making Bombyx mori nucleopolyhedrovirus (BmNPV) polh mutants using a conventional transfer vector-based method, we noticed that a virus with a short sequence insertion just before the polh start codon produces fewer very small OBs. Detailed analysis of several BmNPV mutants revealed that insertions between the burst sequence and start codon markedly decrease POLH accumulation and polh transcription. We further confirmed this decrease using recombinant viruses expressing a reporter gene driven by the polh promoter. These findings underscore the critical importance of a seamless connection from the burst sequence to the start codon for baculovirus polh hyperexpression.

Interaction of Viruses with the Insect Intestine

In nature, insects face a constant threat of infection by numerous exogeneous viruses, and their intestinal tracts are the predominant ports of entry. Insects can acquire these viruses orally during either blood feeding by hematophagous insects or sap sucking and foliage feeding by insect herbivores. However, the insect intestinal tract forms several physical and immunological barriers to defend against viral invasion, including cell intrinsic antiviral immunity, the peritrophic matrix and the mucin layer, and local symbiotic microorganisms. Whether an infection can be successfully established in the intestinal tract depends on the complex interactions between viruses and those barriers. In this review, we summarize recent progress on virus-intestinal tract interplay in insects, in which various underlying mechanisms derived from nutritional status, dynamics of symbiotic microorganisms, and virus-encoded components play intricate roles in the regulation of virus invasion in the intestinal tract, either directly or indirectly. Expected final online publication date for the Annual Review of Virology, Volume 8 is September 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Loss of p24 from the Bombyx mori nucleopolyhedrovirus genome results in the formation of cuboidal occlusion bodies

Some insect viruses produce the occlusion body (OB), a large crystalline particle comprising a viral protein that occludes virions to protect them from harsh environments. The shapes and sizes of OBs are diverse depending on baculovirus species, but the detailed molecular mechanism determining them has yet to be totally clarified yet. Here we generated Bombyx mori nucleopolyhedrovirus (BmNPV) mutants of the p24 gene that encodes a viral capsid protein and found that p24-mutated BmNPVs produced cuboidal OBs with a slightly larger size than typical truncated octahedral OBs produced by wild-type BmNPVs. Meanwhile, p24 disruption has no significant impact on progeny virus production and viral pathogenicity. In addition, we experimentally demonstrated that a single amino acid substitution found in the P24 protein of the BmNPV Cubic isolate caused cuboidal OB production. These results suggest that p24 has a crucial role in generating the typical shape of OBs.

Appearances are deceptive: Three RNA viruses co-infected with the nucleopolyhedrovirus in host Lymantria dispar

The virus infection, which visually looks like typical monoinfection, in fact may hide a great complex of different species. Without detailed analysis, we may miss the important interaction between pathogens, including new species. In the current study, we found the new species inside the mix of cubic and polyhedral occlusion bodies (OBs) isolated from the gypsy moth, Lymantria dispar L. (Ld). Transmission electron microscopy (TEM) revealed that into the one cadaver were OBs which belonged to baculovirus and cypoviruses. The baculovirus produced polyhedral OBs, while cypoviruses produced polyhedral and cubic OBs. Genomic analysis detected the multiple Ld nucleopolyhedroviruses, and cypoviruses were Hubei lepidoptera virus 3 and Dendrolimus punctatus cypovirus 1. This represents the first isolation of the Hubei lepidoptera virus 3 from the gypsy moth, proposed as "Lymantria dispar cypovirus 3". The RNAseq analysis also revealed the presence of Lymantria dispar iflavirus 1. The insecticidal activity of the mixed infection was comparable to that of typical baculovirus monoinfection. Thus, we demonstrate that i) the shape of OBs identified by light microscopy cannot be a robust indicator of viral species infecting the host; ii) only specific analysis may reveal the true composition of viral infection.

Statistical modeling of cell-to-cell variability in viral infection during passaging in suspension cell culture: Application in Monte-Carlo simulation

Packaging during the passaging of viruses in cell cultures yields various phenotypes and is regulated by viral protein expression in infected cells. Although such a packaging mechanism has a profound effect in controlling the virus yield, little is known about the underlying statistical models followed by virus packaging and protein expression among cells infected with the virus. A predictive framework combining identification of the probability density function (PDF) based on log-likelihood and using the PDF for Monte-Carlo simulations is developed. The Birnbaum-Saunders distribution was found to be consistent with all three-virus packaging levels, including nucleocapsids/occlusion-derived virus (ODV), ODVs/polyhedra, and polyhedra/cell for both wild-type and genetically modified AcMNPV. Next, it was demonstrated that PDF fitting could be used to compare two viruses having distinctly different genetic configurations. Finally, the identified PDF can be incorporated in RNA synthesis parameters for baculovirus infection to predict the cell-to-cell variability in protein expression using Monte-Carlo simulations. The proposed tool can be used for the estimation of uncertainty in the kinetic parameter and prediction of cell-to-cell variability for other biological systems.

Comparative analysis of error-prone replication mononucleotide repeats across baculovirus genomes

Genome replication by the baculovirus DNA polymerase often generates errors in mononucleotide repeat (MNR) sequences due to replication slippage. This results in the inactivation of genes that affects different stages of the cell infection cycle. Here we mapped these MNRs in the 59 baculovirus genomes. We found that the MNR frequencies of baculovirus genomes are different and not correlated with the genome sizes. Although the average A/T content of baculoviruses is 58.67%, the A/T MNR frequency is significantly higher than that of the G/C MNRs. Furthermore, the A7/T7 MNRs are the most frequent of those we studied. Finally, MNR frequencies in different classes of baculovirus genes, such as immediate early genes, show differences between baculovirus genomes, suggesting that the distribution and frequency of different MNRs are unique to each baculovirus species or strain. Therefore, the results of this study can help select appropriate baculoviruses for the development of biological insecticides.

Reduction of polyhedrin mRNA and protein expression levels in Sf9 and Hi5 cell lines, but not in Sf21 cells, infected with Autographa californica multiple nucleopolyhedrovirus fp25k mutants

During cell infection, the fp25k gene of baculoviruses frequently mutates, producing the few polyhedra (FP) per cell phenotype with reduced polyhedrin (polh) expression levels compared with wild-type baculoviruses. Here we report that the fp25k gene of the model baculovirus, Autographa californica multiple nucleopolyhedrovirus (AcMNPV), contains two hypermutable seven-adenine (A7) mononucleotide repeats (MNRs) that were mutated to A8 MNRs and a TTAA site that had host DNA insertions, producing fp25k mutants during Sf21 cell infection. The FP phenotype in Sf9 and Hi5 cells was more pronounced than in Sf21 cells. AcMNPV fp25k mutants produced similar levels of polyhedra or enhanced GFP, which were both under the control of the AcMNPV polh promoter for expression, in Sf21 cells but lower levels in Sf9 and Hi5 cells compared with AcMNPV with an intact fp25k gene. This correlated with the polh mRNA levels detected in each cell line. The majority of Sf21 cells infected with fp25 mutants showed high polh promoter-mediated GFP expression levels. Two cell lines subcloned from Sf21 cells that were infected with fp25k mutants showed different GFP expression levels. Furthermore, a small proportion of Hi5 cells infected with fp25k mutants showed higher production of polyhedra and GFP expression than the rest, and the latter was not correlated with increased m.o.i. Therefore, these data suggest that AcMNPV polh promoter-mediated gene expression activities differ in the three cell lines and are influenced by different cells within the cell line.

Cell-dependent production of polyhedra and virion occlusion of Autographa californica multiple nucleopolyhedrovirus fp25k mutants in vitro and in vivo

Members of the family Baculoviridae are insect-specific dsDNA viruses that have been used for biological control of insect pests in agriculture and forestry, as well as in research and pharmaceutical protein expression in insect cells and larvae. Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the type species of the family Baculoviridae. During infection of AcMNPV in permissive cells, fp25k mutants are positively selected, leading to the formation of the few polyhedra (FP) phenotype with reduced yield of polyhedra and reduced virion occlusion efficiency, which leads to decreased oral infectivity for insects. Here we report that polyhedra of AcMNPV fp25k mutants produced from different insect cell lines and insects have differences in larval per os infectivity, and that these variations are due to different virion occlusion efficiencies in these cell lines and insects. Polyhedra of AcMNPV fp25k mutants produced from Sf cells (Sf21 and Sf9, derived from Spodoptera frugiperda) and S. frugiperda larvae had poorer virion occlusion efficiency than those from Hi5 cells (derived from Trichoplusia ni) and T. ni larvae, based on immunoblots, DNA isolation and larval oral infection analysis. AcMNPV fp25k mutants formed clusters of FP and many polyhedra (MP) in the fat body cells of both T. ni and S. frugiperda larvae. Transmission electron microscopy revealed that the nature of virion occlusion of AcMNPV fp25k mutants was dependent on the different cells of the T. ni fat body tissue. Taken together, these results indicate that the FP phenotype and virion occlusion efficiency of fp25k mutants are influenced by the host insect cells.

Removal of transposon target sites from the Autographa californica multiple nucleopolyhedrovirus fp25k gene delays, but does not prevent, accumulation of the few polyhedra phenotype

Low-cost, large-scale production of the baculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV) using continuous insect cell culture is seriously hindered by the accumulation of AcMNPV mutants. Specifically, few-polyhedra (FP) mutants, with a reduced yield of occluded virus (polyhedra) and decreased infectivity, usually accumulate upon passaging in cell culture. FP mutations result from transposon insertions in the baculovirus fp25k gene, leading to significantly reduced levels of FP25K protein synthesis. This study evaluated the effects of removing the transposon insertion sites from the wild-type baculovirus fp25k gene; the mutated virus was denoted Ac-FPm. Specifically, this study involved a detailed comparison of wild-type (WT) AcMNPV and Ac-FPm with regard to the proportion of cells having polyhedra, number of polyhedra per cell, the fraction of empty polyhedra, number of occlusion-derived viruses per polyhedron, number of nucleocapsids in the nuclei, FP25K protein synthesis and genetic analysis of the fp25k gene. Removal of TTAA transposon insertion sites from the fp25k gene stabilized FP25K protein synthesis and delayed the appearance of the FP phenotype from passage 5 to passage 10. Electron micrographs revealed that more virus particles were found inside the nuclei of cells infected with Ac-FPm than in the nuclei of cells infected with WT AcMNPV (at passage 10). Abnormalities, however, were observed in envelopment of nucleocapsids and virus particle occlusion within Ac-FPm polyhedra. Thus, the FP phenotype appeared in spite of continued FP25K protein synthesis, suggesting that mechanisms other than fp25k gene disruption can lead to the FP phenotype.

A new nucleopolyhedrovirus strain (LdMNPV-like virus) with a defective fp25 gene from Lymantria xylina (Lepidoptera: Lymantriidae) in Taiwan

A new multiple nucleopolyhedrovirus strain was isolated from casuarina moth, Lymantria xylina Swinhoe, (Lepidoptera: Lymantriidae) in Taiwan. This Lymantria-derived virus can be propagated in IPLB-LD-652Y and NTU-LY cell lines and showed a few polyhedra (occlusion bodies) CPE in the infected cells. The restriction fragment length polymorphism (RFLP) profiles of whole genome indicated that this virus is distinct from LyxyMNPV and the virus genome size was approximately 139 kbps, which was smaller than that of LyxyMNPV. The molecular phylogenetic analyses of three important genes (polyhedrin, lef-8 and lef-9) were performed. Polyhedrin, LEF-8 and LEF-9 putative amino acid analyses of this virus revealed that this virus belongs to Group II NPV and closely related to LdMNPV than to LyxyMNPV. The phylogenetic distance analysis was further clarified the relationship to LdMNPV and this virus provisionally named LdMNPV-like virus. A significant deletion of a 44bp sequence found in LdMNPV-like virus was noted in the fp25k sequences of LdMNPV and LyxyMNPV and may play an important role in the few polyhedra CPE. In ultrastructural observations, the nuclei of the infected LD host cells contained large occlusion bodies (OBs), and few OBs, which presented as one or two OBs in a nucleus that was otherwise filled with free nuclocapsids and virions. We concluded that this LdMNPV-like virus is a new LdMNPV strain from L. xylina.

Accumulation of few-polyhedra mutants upon serial passage of Anticarsia gemmatalis multiple nucleopolyhedrovirus in cell culture

Anticarsia gemmatalis nucleopolyhedrovirus (AgMNPV) has been widely used to control the velvetbean caterpillar, Anticarsia gemmatalis, in Brazil. To date, AgMNPV has been produced by larval infection and, due to in vivo production limitations and the continuing high demand for the biopesticide, attempts should be made to develop in vitro production of this virus. In order to investigate the effects caused by serial passage of AgMNPV in cell culture, we carried out a total of ten passages and analyzed the morphological and the genomic changes of the virus. After six passages, the many-polyhedra (MP) phenotype started to switch to the few-polyhedra (FP) phenotype which rapidly accumulated in the virus population. Ultrastructural analysis showed typical signs of FP mutant formation such as decrease in the number of polyhedra per cell, polyhedra aberrant morphology and low numbers of virions occluded in the protein matrix. Also enhanced BV production was observed from the fifth passage indicating that FP mutants were becoming predominant in comparison to the wild type virus. Restriction endonuclease analysis of the viral DNA revealed that lower and higher passages had similar profiles indicating that there were no large insertions or deletions or rearrangements in their genomes and indicating the generation of FP mutants instead of defective interfering viruses.

Genomic sequence analysis of a nucleopolyhedrovirus isolated from the diamondback moth, Plutella xylostella

The CL3 plaque isolate of Plutella xylostella multiple nucleopolyhedrovirus (PlxyMNPV-CL3) exhibits a high degree of genetic similarity with the Autographa californica MNPV (AcMNPV) but is significantly more virulent against the diamondback moth, P. xylostella, than AcMNPV. To identify genetic differences between PlxyMNPV-CL3 and AcMNPV that may account for the difference in virulence against P. xylostella, the genome sequence of the CL3 plaque isolate of PlxyMNPV was determined and compared to the genome sequence of AcMNPV isolate C6. The PlxyMNPV genome is 134,417 bp, 523 bp larger than the AcMNPV-C6 genome, and the nucleotide sequence is almost completely co-linear with that of AcMNPV-C6. Of the 153 open reading frames (ORFs) identified in PlxyMNPV, 151 had homologues in AcMNPV-C6, with a mean amino acid sequence identity of 98.5%. The PlxyMNPV genome possessed two features previously reported for other variants of AcMNPV: (1) an extra baculovirus repeated orf (bro) sequence located between the plxy29/ac30 and sod ORFs, and (2) the deletion of the AcMNPV pnk/pnl polynucleotide kinase/ligase gene. In addition, an 817 bp insert of unknown origin located between the fp25K and lef-9 genes was discovered. This insert contained two small ORFs and was detected in both tissue culture- and larvae-derived PlxyMNPV DNA by PCR. Finally, the PlxyMNPV-CL3 ie-2 gene encodes a product with a low level (37.3%) of amino acid sequence identity with the AcMNPV-C6 ie-2 product. PlxyMNPV-CL3 apparently acquired this variant ie2 gene by recombination with an undescribed nucleopolyhedrovirus.

Dual mutations in the Autographa californica nucleopolyhedrovirus FP-25 and p35 genes result in plasma-membrane blebbing in Trichoplusia ni cells

Spodoptera frugiperda cells infected with Autographa californica nucleopolyhedrovirus (AcMNPV) lacking a functional anti-apoptotic p35 protein undergo apoptosis. However, such mutants replicate normally in Trichoplusia ni (TN-368) cells. An AcMNPV plaque isolate (AcdefrT) was identified during propagation of a virus deficient in p35 in TN-368 cells. This virus exhibited enhanced budded-particle formation in TN-368 cells, but was partially defective for polyhedra production in the same cells. Virus replication in AcdefrT-infected TN-368 cells was accompanied by extensive plasma-membrane blebbing and caspase activation late in infection, both features of apoptosis. Rescue of the p35 locus of AcdefrT continued to result in a reduction in polyhedra and increase in budded virus production in TN-368 cells, but no plasma-membrane blebbing was observed. The mutation was mapped to the FP-25 gene locus. This gene mutation combined with the non-functional p35 was found to be responsible for the cell-blebbing effect observed in AcdefrT-infected TN-368 cells.

Functional analysis of FP25K of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus

The fp25k gene of Helicoverpa armigera single nucleocapsid nucleopolyhedrovirus (HearNPV) was studied. HearNPV fp25k gene transcription was found starting from about 18 h post-infection, and protein could be detected from the same time with antiserum against FP25K. To study the function of HearNPV fp25k, a recombinant HearNPV (HaBacWD11) with an enhanced green fluorescent protein (GFP) gene replacing the fp25k was constructed using HaBacHZ8, a bacmid of HearNPV that lacks the polyhedrin gene. Growth curve analysis showed that HaBacWD11 produced higher titres of budded viruses (BVs) than its wild-type counterpart HaBacHZ8-GFP. Electron microscopic analysis indicated that at the late stage of infection, the number of intranuclear enveloped nucleocapsids in HaBacWD11-infected cells was much less than that of HaBacHZ8-GFP. A rescue recombinant virus HaBacWD14 was constructed by reintroducing fp25k gene into HaBacWD11. The growth curve and electron microscopic analysis of the rescued recombinant confirmed that the increase of BV yield and the decrease of the virion production in infected cells were the result of fp25k deletion. The expression of membrane fusion protein (Ha133) and ODV-E66 were studied using the FP25K mutants HaBacWD11 and HaBacHZ8-GFP. Unlike FP25K mutants in Autographa californica multicapsid NPV (AcMNPV), which caused an increase in the expression of membrane fusion protein GP64 and a decrease of ODV-E66, no obvious changes at the expression level of Ha133 and ODV-E66 were observed in HearNPV FP25K mutant.

The Lymantria dispar nucleopolyhedrovirus contains the capsid-associated p24 protein gene

During the course of investigations on a wild-type strain of Lymantria dispar multinucleocapsid nucleopolyhedrovirus (LdMNPV), a region of the viral genome was analyzed and found to contain 697 bp that is lacking in the sequenced strain (5-6) of LdMNPV (Kuzio et al., Virology 253, 17-34, 1999). The sequenced strain of LdMNPV contains a mutation in the 25 K few polyhedra (FP) gene, and exhibits the phenotype of a FP mutant. The additional sequence was located at approximately 81.4 map units within the viral genome, and was found in 10 different wild-type LdMNPV genotypic variants analyzed. Since the additional sequence wasfound in all wild-type virus strains analyzed, this sequence should be included in the representative LdMNPV genome. Sequence analysis of the genomic region containing the additional sequences revealed the presence of a homologue of the Autographa californica MNPV capsid-associated p24 gene (ORF 129). This gene, absent in LdMNPV isolate 5-6, is also present in the Orgyia pseudotsugata MNPV, Bombyx mori NPV, Spodoptera exigua MNPV, S. litura MNPV, Mamestra configurata MNPV, Helicoverpa armigera SNPV, H. zea SNPV, Buzura suppressaria SNPV, Xestia c-nigrum granulovirus, Plutella xylostella GV, and Cydia pomonella GV.

Phenotypic and genotypic analysis of Helicoverpa armigera nucleopolyhedrovirus serially passaged in cell culture

Rapid accumulation of few polyhedra (FP) mutants was detected during serial passaging of Helicoverpa armigera nucleopolyhedrovirus (HaSNPV) in cell culture. 100% FP infected cells were observed by passage 6. The specific yield decreased from 178 polyhedra per cell at passage 2 to two polyhedra per cell at passage 6. The polyhedra at passage 6 were not biologically active, with a 28-fold reduction in potency compared to passage 3. Electron microscopy studies revealed that very few polyhedra were produced in an FP infected cell (<10 polyhedra per section) and in most cases these polyhedra contained no virions. A specific failure in the intranuclear nucleocapsid envelopment process in the FP infected cells, leading to the accumulation of naked nucleocapsids, was observed. Genomic restriction endonuclease digestion profiles of budded virus DNA from all passages did not indicate any large DNA insertions or deletions that are often associated with such FP phenotypes for the extensively studied Autographa californica nucleopolyhedrovirus and Galleria mellonella nucleopolyhedrovirus. Within an HaSNPV 25K FP gene homologue, a single base-pair insertion (an adenine residue) within a region of repetitive sequences (seven adenine residues) was identified in one plaque-purified HaSNPV FP mutant. Furthermore, the sequences obtained from individual clones of the 25K FP gene PCR products of a late passage revealed point mutations or single base-pair insertions occurring throughout the gene. The mechanism of FP mutation in HaSNPV is likely similar to that seen for Lymantria dispar nucleopolyhedrovirus, involving point mutations or small insertions/deletions of the 25K FP gene.

Abnormal formation of polyhedra resulting from a single mutation in the polyhedrin gene of Autographa californica multicapsid nucleopolyhedrovirus

A spontaneous mutant that produces a single abnormally large cubic polyhedron per infected cell was isolated from a polyhedra-positive recombinant Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV). Both wild-type and mutant virus produce two forms of virus particles, budded virions and occluded virions. However, occluded virions are not found within the polyhedra of cells infected with mutant virus, as with the wild-type virus. These large cubic polyhedra do not have the typical lattice-like structure normally seen in wild-type polyhedra and are noninfectious. Spodoptera frugiperda 9 (SF9) cells which were infected with this virus had low infectivity to larvae. No significant alterations were found in the viral genome by restriction enzyme analysis, and no mutations were found in the 25K gene. A single point mutation resulting in an amino acid change of Gly25 to Asp was identified in the polyhedrin gene. A transfer vector containing the entire polyhedrin gene including the point mutation was constructed and used to cotransfect Sf9 cells with a polyhedron-negative recombinant virus. Large cubic polyhedra were once again observed, confirming that the Gly25 to Asp mutation is responsible for the formation of abnormal polyhedra.

Identification of a novel Lymantria dispar nucleopolyhedrovirus mutant that exhibits abnormal polyhedron formation and virion occlusion

In previous studies on the formation of Lymantria dispar nuclear polyhedrosis virus (LdMNPV) few polyhedra (FP) mutants, several polyhedron formation mutants (PFM) were identified that appeared to be unique. These viral mutants are being characterized to investigate the processes of polyhedron formation and virion occlusion. LdMNPV isolate PFM-1 is one of these mutants, and is described in this report. Genetic techniques were used to determine if isolate PFM-1 contained a mutation in the polyhedrin or 25K FP gene. Wild-type viruses were recovered after coinfection of Ld652Y cells with isolate PFM-1 and a FP mutant, and with isolates PFM-1 and PFM-C (isolate PFM-C contains a mutation in the polyhedrin gene). These viruses were analyzed by genomic restriction endonuclease digestion and found to be chimeras of the original PFMs used in the coinfections. Marker rescue studies mapped the mutation in isolate PFM-1 to a genomic region that does not include the polyhedrin or 25K FP genes. Isolate PFM-1 produced approximately 14-fold fewer polyhedra than LdMNPV isolate A21-MPV, an isolate that produces wild-type levels of polyhedra, and approximately 2-fold more polyhedra compared to the FP isolate 122-2. Polyhedra generated by isolate PFM-1 were normal in size and shape but contained very few viral nucleocapsids. The same amount of budded virus (BV) was released from cells infected with isolates PFM-1 and A21-MPV. In contrast, isolate 122-2 yielded significantly more BV than isolates PFM-1 and A21-MPV.

Molecular analysis of an enhancin gene in the Lymantria dispar nuclear polyhedrosis virus

A Lymantria dispar nuclear polyhedrosis virus (LdMNPV) gene has been identified that encodes a homolog to the granulovirus (GV) enhancin proteins that are capable of enhancing the infection of other baculoviruses. Enhancin genes have been identified and sequenced for three species of GVs but have not been found in any other nuclear polyhedrosis virus to date. The LdMNPV enhancin gene is located between 67.6 and 70.1 kbp on the viral genome. Northern and primer extension analyses of viral RNAs indicate that the enhancin gene transcripts are expressed at late times postinfection from a consensus baculovirus late promoter. The LdMNPV enhancin exhibits 29% amino acid identity to the enhancin proteins of the Trichoplusia ni, Pseudaletia unipuncta, and Helicoverpa armigera GVs. All four proteins contain a conserved zinc-binding domain characteristic of metalloproteases. A recombinant virus (enhancin::cat) was constructed in which the LdMNPV enhancin gene was inactivated by insertion mutagenesis in order to ascertain the effect of the enhancin protein on viral potency. The bioassay results indicate that disruption of the enhancin gene in the LdMNPV results in a reduction in viral potency.