Characterization of an Autographa californica multiple nucleopolyhedrovirus mutant lacking the ac39(p43) gene

Open reading frame 39 [orf39(p43)] of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) is present in 10 isolates of the Alphabaculovirus genus. It is highly conserved in sequence and genomic location in the Group I but much less conserved in the Group II viruses. To investigate the potential role of p43 in AcMNPV infection, we constructed and characterized a p43 knockout mutant. The results showed that the p43 region was expressed as RNA from 3 h post infection to at least 24 h post infection, and its expression pattern was identical to the expression profile of its neighbouring gene, p47. P47 is an essential core gene component of the baculovirus RNA polymerase. The deletion of the p43 region was confirmed by PCR analysis of bacmid DNA and by RT-PCR analysis of RNA purified from p43 knockout infected cells. The results supported the hypothesis that a large transcript, initiating upstream of p47, includes the p43 ORF. Analyses of protein synthesis in p43 knockout infected cells clearly demonstrated that there were no obvious differences in the timing or amount of expression of P47, LEF-3, or VP39. Growth curves showed that infectious budded virus production and occlusion body formation were also not affected by the p43 knockout. We conclude that orf39(p43) is not essential for virus replication in cell culture.