P13 of Leucania separata multiple nuclear polyhedrosis virus affected the polyhedra and budded virions yields of AcMNPV

Evolutionary analysis and biological characterization of a novel alphabaculovirus isolated from Mythimna separata

Baculoviruses are insect-specific pathogens. Novel baculovirus isolates provide new options for the biological control of pests. Therefore, research into the biological characteristics of newly isolated baculoviruses, including accurate classification and nomenclature, is important. In this study, a baculovirus was isolated from Mythimna separata and its complete genome sequence was determined by next-generation sequencing. The double-stranded DNA genome was 153 882 bp in length, encoding 163 open reading frames. The virus was identified as a variant of Mamestra brassicae multiple nucleopolyhedrovirus (MbMNPV) and designated Mamestra brassicae multiple nucleopolyhedrovirus CHN1 (MbMNPV-CHN1) according to ultrastructural analysis, genome comparison and phylogenetic analysis. Phylogenetic inference placed MbMNPV-CHN1 in a clade containing isolates of MacoNPV-A, MacoNPV-B and MbMNPV, which we have designated the Mb-McNPV group. The genomes of isolates in the Mb-McNPV group exhibited a high degree of collinearity with relatively minor differences in the content of annotated open reading frames. The development of codon usage bias in the Mb-McNPV group was affected mainly by natural selection. MbMNPV-CHN1 shows high infectivity against seven species of Lepidoptera. The yield of MbMNPV-CHN1 in the fourth- and fifth-instar M. separata larvae was 6.25×109-1.23×1010 OBs/cadaver. Our data provide insights into the classification, host range and virulence differences among baculoviruses of the Mb-McNPV group, as well as a promising potential new baculoviral insecticide.

Rare codons in uORFs of baculovirus p13 gene modulates downstream gene expression

The p13 gene is a group II nucleopolyhedroviruses (NPVs) specific gene and featured by containing upstream mini ORFs (uORF) in its 5' UTR region. However, there are almost no reports published on the functions of the uORFs of p13 gene. In this study, the Luciferase Reporter Assay System was employed to investigate how the mini ORFs of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) p13 gene (Ha-p13) and its rare codons regulated the downstream gene expression. After the coding sequence of uORFs in the Ha-p13 gene was fused to the luciferase reporter gene in the expression vector pGL3 and the plasmid DNA was then transfected into the Hz-AM1 cells, the translation of the fusion protein could be initiated from the start codon of the uORFs. The uAUG and its context in uORF2 seemed to be more efficient for translation initiation than that in uORF1. Mutation of the start codons in one or both of uORFs (uORF1 or uORF2) could significantly increase the expression of the downstream reporter gene. The start codon mutation in uORF1 produced a higher reporter gene expression than that in uORF2, indicating that the uORF1 could be a stronger inhibitor than the uORF2, and the length of uORFs seemed not to be crucial for down-regulating translation. The expression of both uORFs could co-regulate the associated gene expression. Substituting the rare codons in uORF1, uORF2 or both with less rare codons dramatically increased the expression of the downstream reporter gene. Rare codon mutations in both uORFs were much more efficient in up-regulating the associate gene expression than mutations in either of the two uORFs alone.