Autographa californica Multiple Nucleopolyhedrovirus me53 ( ac140 ) Is a Nonessential Gene Required for Efficient Budded-Virus Production

me53 is a highly conserved baculovirus gene found in all lepidopteran baculoviruses that have been fully sequenced to date. The putative ME53 protein contains a zinc finger domain and has been previously described as a major early transcript. We generated anme53 -null bacmid (AcΔme53GFP), as well as a repair virus (AcRepME53:HA-GFP) carryingme53 with a C-terminal hemagglutinin (HA) tag, under the control of its native early and late promoter elements. Sf9 and BTI-Tn-5b1 cells transfected with AcΔme53GFP resulted in a 3-log reduction in budded-virus (BV) production compared to both the parentalAutographa californica multiple nucleopolyhedrosis virus and the repair bacmids, demonstrating that althoughme53 is not essential for replication, replication is compromised in its absence. Our data also suggest thatme53 does not affect DNA replication. Cell fractionation showed that ME53 is found in both the nucleus and the cytoplasm as early as 6 h postinfection. Deletion of the early transcriptional start site resulted in a 10- to 360-fold reduction of BV yield; however, deletion of the late promoter (ATAAG) resulted in a 160- to 1,000-fold reduction, suggesting that, in the context of BV production, ME53 is required both early and late in the infection cycle. Additional Western blot analysis of purified virions from the repair virus revealed that ME53:HA is associated with both BV and occlusion-derived virions. Together, these results indicate thatme53 , although not essential for viral replication, is required for efficient BV production.