ted ORF (bro) family, which has members in many other baculoviruses either as a single gene or in multiple copies. A similar gene is also present in entomopoxvirus (3, 11). The AcMNPV bro gene is present as a single copy and encodes a protein with a predicted mass of 37.8 kDa (328 aa) with unclear function. Disruption of the bro gene has no effect on virus replication in cultured cells or on the lethal dose in insect larvae when injected as BV or per os with ODV. However, disruption of the N-terminal part of the BRO protein reduced the number of OBs (15). In contrast to Ac-MNPV, BmNPV contains five bro genes, bro-a till bro-e. The BmNPV bro-d gene is essential for virus replication in cell culture and bro-a and bro-c genes can complement each other (124). However, the absence of bro genes in several baculoviruses suggests that the requirement for bro genes may depend on the host species (15). In BmNPV, the BRO proteins reside in the nucleus until 4 h post infection (p.i.). After that time point, the proteins are found in both cytoplasm and nucleus (123). Furthermore, mutation in the leucine-rich N-terminal part of the protein results in accumulation of proteins, which suggest that this region serves as a CRM1-dependent nuclear export signal (123).
Ac3: ctl, conotoxin-like peptide
The ctl gene encodes a conotoxin-like peptide, which has a molecular mass of 5.6 kDa (53 aa). Conotoxins are neurotoxins that are present in the venom of marine snails, belonging to the genus Conus (257). Ω-conotoxins block specific types of Ca2+-channels in neurons (180), while another sub-class of conotoxins have a behavioural and anticonvulsant effect in DBA/2 mice (114). Infection with a mutant AcMNPV virus – with either a disruption or a null mutant of the gene – was not significantly different in infectivity in Sf21 cells or in virulence S. frugiperda larvae (57).
Ac4: ac4, putative enhancer activity
ORF 4 of AcMNPV is an early gene and together with five other early genes of AcMNPV, ac102, he65 (ac105), ie-1 (ac147), ac152, and pe38 (ac153) respectively, is needed to accumulate G-actin into the nucleus of Tn-368 cells (202). The expression of ac4, ie-1, and pe38 starts before the expression of ac102 or he65. The gene ac4 codes for a protein of 17.6 kDa (83 aa.) which has not been characterized thoroughly, but it has enhancer activities for cellular and viral promoters (162).
Ac5: ac5, enhancer
In BmNPV, a region upstream of the polyhedrin promoter corresponding to the 5'-ends of ac4 and ac5 of AcMNPV was shown to have enhancer capabilities (1). This enhancer activity was confirmed as the homologous region in AcMNPV resulted in increased promoter activity in luciferase-assays in combination with several full or minimal promoters: hsp70, CMVm and p35 minimal promoter in insect cells (162). In AcMNPV ac5 encodes a hypothetical protein (12.4 kDa, 109 aa), however no transcripts have been detected (287).
Ac6: lef-2, late expression factor 2
The lef-2 gene codes for late expression factor-2 (LEF-2), (23.9 kDa, 210 aa). This protein is essential for the expression from vp39 and polh promoters (216). In addition, LEF-2 as well as five other gene products (IE-1, LEF-1 LEF-3, DNA polymerase, and helicase) are required for replication of plasmid DNA containing an AcMNPV origin of replication (133). Protein-protein interaction between LEF-1 and LEF-2 is essential for this DNA replication (61) and LEF-2 binds to DNA (187). A point mutation changing an aspartic acid into an asparagine residue at amino acid 178, showed no difference in plasmid replication between mutant and wild type virus infections but showed deficiency in very late gene expression (184). Lef-2 is a baculovirus core gene.
Ac7: orf603, ORF603 peptide
The orf603 gene encodes a hitherto uncharacterized protein of 23.6 kDa (201 aa). Partial deletion of the orf603 gene did not affect BV yield in cell culture nor the dose to kill insects (72). However, a truncation of ORF603 decreased the time to death in S. frugiperda larvae (224).
Ac8: polh, major occlusion body protein
The polh gene enco