Deletion of pe38 attenuates AcMNPV genome replication, budded virus production, and virulence in heliothis virescens

The pe38 gene product of Autographa californica M nucleopolyhedrovirus (AcMNPV) has been shown to be involved in transcriptionally transactivating viral genes and augmenting viral DNA replication in transient assays. To assess the role of pe38 during infection, we generated a knockout virus, Δ pe38-E9/E9, in which the pe38 open reading frame was replaced with that of the green fluorescent protein. We compared mutant and wild-type (WT) viral replication in insect cell culture and virulence in Heliothis virescens larvae. Compared to WT, Δ pe38-E9/E9 budded virus (BV) production was delayed by at least 3 h, and BV yields were reduced over 99%. Similarly, Δ pe38-E9/E9 DNA synthesis levels were greatly reduced relative to those of WT, but onset of DNA replication was the same for both viruses. In bioassays, nearly sevenfold more Δ pe38-E9/E9 virus than WT virus was required to achieve an LD 50 when administered orally, but not hemocoelically. These results support the hypothesis that the kinetics of AcMNPV BV production greatly impact virulence in larvae infected orally (the natural route of infection) and that PE38 is an important, but not essential, factor in viral DNA synthesis and BV production.