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

O Argaud, L Croizier, M Lopez-Ferber and G Croizier Unite de Genetique des Virus, Station de Recherches de Pathologie Comparee, INRA-URA CNRS 2209, Saint Christol-Les Ales, France. Autographa californica nucleopolyhedrovirus (AcMNPV) does not replicate in Bombyx mori cells (Bm5, BmN). We have shown previously that when a short DNA sequence within AcMNPV ORF95, which encodes the viral helicase P143, is replaced with the colinear region of B. mori nucleopolyhedrovirus (BmNPV), AcMNPV gains the ability to replicate in Bm5 cells. To determine the mutational events in the p143 gene required to allow AcMNPV replication in B. mori cells, AcMNPV recombinants produced in Sf9 cells were screened in vivo in B. mori larvae, which are more permissive to baculovirus infection than B. mori cell lines. Eight combinations of mutations were tested and characterization of viral DNA extracted from dead larvae showed that amino acid changes at position 564 and 577 are required to kill B. mori larvae.