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The betabaculovirus originally called Pseudaletia (Mythimna) sp. granulovirus #8 (MyspGV#8) was examined by electron microscopy, host barcoding PCR, and determination of the nucleotide sequence of its genome. Scanning and transmission electron microscopy revealed that the occlusion bodies of MyspGV#8 possessed the characteristic size range and morphology of betabaculovirus granules. Barcoding PCR using cytochrome oxidase I primers with DNA from the MyspGV#8 collection sample confirmed that it had been isolated from the true armyworm, Mythimna unipuncta (Lepidoptera: Noctuidae) and therefore was renamed MyunGV#8. The MyunGV#8 genome was found to be 144,673 bp in size with a nucleotide distribution of 49.9% G+C, which was significantly smaller and more GC-rich than the genome of Pseudaletia unipuncta granulovirus H (PsunGV-H), another M. unipuncta betabaculovirus. A phylogeny based on concatenated baculovirus core gene amino acid sequence alignments placed MyunGV#8 in clade a of genus Betabaculovirus. Kimura-2-parameter nucleotide distances suggested that MyunGV#8 represents a virus species different and distinct from other species of Betabaculovirus. Among the 153 ORFs annotated in the MyunGV#8 genome, four ORFs appeared to have been obtained from or donated to the alphabaculovirus lineage represented by Leucania separata nucleopolyhedrovirus AH1 (LeseNPV-AH1) during co-infection of Mythimna sp. larvae. A set of 33 ORFs was identified that appears only in other clade a betabaculovirus isolates. This clade a-specific set includes an ORF that encodes a polypeptide sequence containing a CIDE_N domain, which is found in caspase-activated DNAse/DNA fragmentation factor (CAD/DFF) proteins. CAD/DFF proteins are involved in digesting DNA during apoptosis.

Lepidopteran nucleopolyhedroviruses are members of the Baculoviridae and have been categorized as having two morphotypes of occluded virions: multiple nucleocapsids or single nucleocapsids within the virion envelope. Although it is a definitive characteristic of specific viruses, it appears to lack a defined genetic basis and is independent of virus phylogeny. This review summarizes the factors that appear to influence this trait and the role that it may play in virus biology.

A baculovirus isolate from a USDA Forest Service collection was characterized by electron microscopy and analysis of its genome sequence. The isolate, formerly referred to as Pseudoletia (Mythimna) sp. nucleopolyhedrovirus #7 (MyspNPV#7), was determined by barcoding PCR to derive from the host species Mythimna unipuncta (true armyworm) and was renamed Mythimna unipuncta nucleopolyhedrovirus #7 (MyunNPV#7). The occlusion bodies (OBs) and virions exhibited a size and morphology typical for OBs produced by the species of genus Alphabaculovirus, with occlusion-derived virions consisting of 2–5 nucleocapsids within a single envelope. The MyunNPV#7 genome was determined to be 148,482 bp with a 48.58% G+C nucleotide distribution. A total of 159 ORFs of 150 bp or larger were annotated in the genome sequence, including the 38 core genes of family Baculoviridae. The genome contained six homologous repeat regions (hrs) consisting of multiple copies of a 34-bp imperfect palindrome. Phylogenetic inference from concatenated baculovirus core gene amino acid sequence alignments placed MyunNPV#7 with group II alphabaculoviruses isolated from other armyworm and cutworm host species of lepidopteran family Noctuidae. MyunNPV#7 could be distinguished from other viruses in this group on the basis of differences in gene content and order. Pairwise nucleotide distances suggested that MyunNPV#7 represents a distinct species in Alphabaculovirus. The MyunNPV#7 genome was found to contain two copies of the late expression factor-7 (lef-7) gene, a feature not reported for any other baculovirus genome to date. Both copies of lef-7 encoded an F-box domain, which is required for the function of LEF-7 in baculovirus DNA replication.

The genome sequence of a baculovirus from Hemileuca sp. was determined. The genome is 140,633 kb, has a G+C content of 38.1 %, and encodes 137 putative open-reading frames over 50 amino acids. 126 of these ORFs showed similarity to other baculovirus genes in the database including all 37 core genes. Of the remaining 11 predicted genes, one is related to a lepidopteran serpin gene. This is the first report of a baculovirus encoding a member of this family of serine protease inhibitors, and to our knowledge the first report of a viral serpin outside the Poxviridae. The genome also contained three homologous repeat sequences. Phylogenetic analysis indicated that the virus is a group II Alphabaculovirus and belongs to a lineage that includes Orgyia leucostigma, Ectropis obliqua, Apocheima cinerarium, and Euproctis pseudoconspersa nucleopolyhedroviruses.

To determine the diversity of retroviruses (errantiviruses) in cell lines used for baculovirus expression, degenerate primers were designed complementary to conserved regions of lepidopteran errantivirus reverse transcriptase genes. These primers were used to PCR amplify sequences from DNA isolated from Spodoptera frugiperda (Sf-9) and Trichoplusia ni (Hi-5) cell lines. Cloning, sequencing, and phylogenetic analysis of over 20 PCR products from each cell line demonstrated the presence of diverse populations of retrovirus sequences comprising at least six major lineages.

By use of a transient replication assay, nine genes involved in DNA replication were identified in the genome of the Autographa californica baculovirus. Six genes encoding helicase, DNA polymerase, IE-1, LEF-1, LEF-2, and LEF-3 are essential for DNA replication while three genes encoding P35, IE-2, and PE38 stimulate DNA replication. No stimulation by the AcMNPV pcna gene, encoding a protein with sequence homology to proliferating-cell nuclear antigen, was observed. A pattern of amino acids found in a number of single-stranded-DNA-binding proteins was identified in the carboxyl-terminal region of IE-1.

The production and characterization of nuclear extracts from uninfected Spodoptera frugiperda cells, capable of accurately initiating transcription of baculovirus immediate early genes in vitro, are described. Optimal in vitro transcription was dependent on the presence of a TATA box promoter element and was abolished by α-amanitin. Nuclear extracts from the S. frugiperda cells primed with plasmid DNA containing the adenovirus major late promoter produced run-off transcripts of the size predicted for initiation from the adenovirus promoter. In addition, nuclear extracts prepared from a human cell line accurately initiated transcription from the promoter of the baculovirus immediate early gene encoding gp64. Primer extension analysis showed that transcripts derived from the gp64 gene promoter using both the S. frugiperda and human cell nuclear extracts initiated at the same nucleotide as transcripts produced in vivo.