gher yield of progeny virus (41). Deletion of the egt gene makes baculovirus-based insecticides more effective by an early reduction of the feeding damage (200).
Ac16: bv/odv-e26; structural protein
The ORF ac16 encodes a structural protein (25.9 kDa, 225 aa) present in the envelopes of BVs and ODVs, named BV/ODV-E26 or briefly E26 (14). Ac16 is an early gene and transcripts accumulate rapidly after infection (213). Ac16 transcription initiates from a cryptic promoter sequence (87). When the ac16 locus (previously called DA26) was disrupted-maintaining the N-terminus -a virus with a few polyhedra (FP) phenotype was produced, which was still infectious and showed no difference in protein synthesis when mutant and wild type virus were compared (201). However, deletion of the ac16 homolog in BmNPV (Bm8) was not successful, indicating that this ORF may be essential (24). Multiple isoforms of E26 are present in infected cells, one isoform associates with viral DNA or DNA-binding proteins, a second one associates with intracellular membranes, likely due to palmityolation (24).
More recently, it has been shown that Ac16 contains a subdomain within the acidic transcriptional activation domain for binding with IE0 and IE1. Deletion of the ac16 gene results in an increased ratio of IE0 to IE1, but there was no effect on temporal production of these proteins nor on BV production nor on DNA replication (195).
Ac17: ac17, unknown function
Ac17 gene transcripts are present from the early to the very late phases and the encoded protein (18.5 kDa, 164 aa) localized in the cytoplasm of infected cells from 6 h p.i. (7). The gene ac17, together with pe38 (ac153), he65 (ac105), gp64 (ac128), ie2 (ac151), ac16, ac25 and pcna (ac49), is activated by the transactivator IE1 in the mammalian cell line Vero E6 (161). The function of the gene remains unknown, although transcriptional control is most likely. Deletion mutants of ac17 are infectious (139).
Ac18: da41, unknown function
The ac18 gene (da41) is expressed as a 40.9 kDa (353 aa) protein. The gene is not essential for virus infection and replication at least in vitro as viable ac18 mutants are formed in bioreactors (139). The lethal dose was not affected by deleting ac18, but time to death was increased (275). Which role ac18 plays, is still unclear.
Ac19: ac19, unknown function
This gene encodes a hypothetical protein with a calculated mass of 12.2 kDa (108 aa). Although homologs are found in other baculoviruses (Table 2), the function of the gene product is unknown. This gene is represented in the transcriptome (287).
Ac20/21: actin rearrangement inducing factor-1
The gene ac20 was identified in 1994 (9) and had the potential to encode a partial homolog of ac21. Resequencing of these ORFs showed that ac20 and ac21 in fact form one ORF (92). The ac20/ac21 fusion gene codes for the 47.7 kDa (417 aa) actin rearrangement inducing factor 1 (ARIF-1). The gene is expressed after transactivation by IE-1, weakly from 2 h p.i., more abundantly after 4-6 h p.i., and not detectably at 12 h p.i. (237). ARIF-1 is a tyrosine phosphorylated protein and induces rearrangement of the actin skeleton (237) by interacting with filamentous actin (F-actin) at the plasma membrane (53). Deletion of ARIF-1 interfered with F-actin accumulation at the plasma membrane, but not with the formation of early actin cables and nuclear F-actin accumulation (53, 237). Homologs are only present in Alphabaculoviruses (Table 2).
Ac22: pif-2, per os infectivity factor 2
Ac22 encodes the 43.8 kDa (382 aa) PIF-2 protein conserved in all baculoviruses and essential for oral infectivity of midgut cells (223). Hence, PIF-2 is a per os infectivity factor. PIFs are not needed when the virus is injected into the hemolymph (as for all four known baculovirus PIF proteins). Proteomic analysis showed the presence of PIF-2 in ODVs (20) and it has a predicted N-terminal membrane anchor (249). PIF-2 is thought to be involved in binding of ODVs to midgut epithelial cells, and possibly associates with PIF-1 (203). PIF-2 is a highly conserved pro