Two key mutations in the host-range specificity domain of the p143 gene of Autographa californica nucleopolyhedrovirus are required to kill Bombyx mori larvae

This study identified the specific mutations in the Autographa californica nucleopolyhedrovirus (AcMNPV) p143 helicase gene required to expand its host range to Bombyx mori (silkworm) larvae. The researchers performed site-directed mutagenesis on a 399 base pair fragment of the p143 gene, creating eight mutation combinations based on differences between AcMNPV and the naturally host-adapted Bombyx mori nucleopolyhedrovirus (BmNPV). Recombinant viruses were screened by transfecting Spodoptera frugiperda (Sf9) cells, then testing the resulting supernatants in B. mori larvae, which proved more permissive than B. mori cell lines. Analysis of viral DNA extracted from infected larvae revealed that exactly two amino acid substitutions—serine to asparagine at position 564 and phenylalanine to leucine at position 577—were both necessary and sufficient to enable AcMNPV to replicate in and kill B. mori larvae. This work demonstrates that minimal genetic changes in the host-range specificity domain of the p143 helicase can dramatically alter baculovirus host tropism, with implications for developing baculoviruses as selective bioinsecticides.

Key findings

• Two specific amino acid substitutions in the p143 helicase gene (S564N and F577L) are required to extend AcMNPV infectivity to Bombyx mori larvae
• Each mutation alone was insufficient; both amino acid changes were necessary for viral replication and larval mortality
• B. mori larvae were significantly more permissive to mutant baculovirus infection than B. mori cell lines, allowing identification of host-range determinants
• Recombinant viruses containing the required mutations produced characteristically small (1 μm) cuboidal polyhedra, distinct from BmNPV inclusions
• The p143 helicase gene serves as a critical host-range specificity determinant controlling baculovirus tropism