is not affected (160). Therefore, the encoded 17.0 kDa (139 aa) protein is most likely involved in nucleocapsid assembly and may have a role in condensation or packaging of viral DNA. Its crucial role is reflected by the presence of ac53 orthologs in all sequenced lepidopteran and hymenopteran baculovirus genomes (Table 2 and Table 3). The homologous protein encoded by BmNPV (Bm42) is present in BVs, but is absent in ODVs (2).
Ac53a: lef-10, late expression factor 10
The gene lef-10 belongs to the group of 18 genes that support late gene expression (262). The exact function is unclear, but the LEF-10 protein (8.6 kDa, 78 aa) might be involved in promoter recognition, stabilization of late transcripts or could be associated with the virus-induced RNA polymerase complex (169). The gene has been classified as an auxiliary factor in the transcription process (97).
Ac54: vp1054, VP1054 viral capsid-associated protein
The vp1054 region produces multicistronic mRNAs from early to very late times after infection (209). VP1054 is a 42.1 kDa (365 aa) structural protein present in both BVs and ODVs required for nucleocapsid formation (209). VP1054 interacts with the 38K protein (ac98) in infected cells (281) and is encoded by a baculovirus core gene
Ac55: ac55, unknown function
Ac55 is an early gene for a hitherto unidentified 8.2 kDa (73 aa) protein without known domains. A mRNA from this ORF has been demonstrated (287).
Ac56: ac56, unknown function
Ac56 encodes a small putative protein of 9.9 kDa (84 aa) protein for which no conserved domains have been found. A transcript from this ORF was found by microarray analysis (287).
Ac57: ac57, unknown function
Ac57 is a putative early gene encoding a 19.0 kDa.(161 aa) protein. Microarray studies revealed a transcript from this ORF (287). The Ac57 protein and its orthologs in other Alphabaculoviruses form the DUF918 superfamily (176), but this gives no further clues concerning its function.
Ac58/59: ac58/59, ODV protein of unknown function:
In the closely related Rachoplusia ou (Ro)MNPV the ac58 and ac59 orthologs are fused into one ORF of 172 codons (92). This is also the case in e.g. BmNPV. Partial resequencing of the AcMNPV C6 strain confirmed that ac58 and ac59 are in fact one ORF (92).Ac58/59 encodes a 20.3 kDa (172 aa) protein. Ac58/59 specific peptides were found within or associated with the ODVs by proteomic analysis (20). Ac58/59 belongs to the ChaB superfamily, originally known in E. coli in combination with ChaA, a cation transporter. The role of ChaB proteins in baculoviruses is unclear.
Ac60: ac60, unknown function
Ac60 was predicted by sequence analysis to potentially encode a 10.1 kDa (87 aa) protein (9). Transcript levels from this ORF were reduced by 72% in a pp31 (ac36) deletion mutant (287). As in the Ac58/59 protein, Ac60 contains a ChaB superfamily domain.
Ac61: fp/25k, FP protein
FP25K (25.2 kDa, 214 aa) is a structural protein of BVs and ODVs (16). Ac61 mutants show a reduction in polyhedrin transcripts, but have wild type p10 expression levels (93). Mutations in FP25K also reduce ODV-e66 expression and transport of ODV-E66 protein to the nucleus is inhibited (16). A more general role has been proposed for FP25K in targeting and intracellular transport of viral proteins during infection (16). In BmNPV it has been shown that FP25K is required for maintaining transcriptional regulation and efficient secretion of V-CATH and maintaining a steady-state level expression during secretion (125).
During serial passage of AcMNPV in cultured insect cells spontaneous mutants occur having the "few polyhedral" (FP) phenotype. This is the result of frequent transposon insertions in this area (10). These mutants have typical characteristics (94), including a reduced number of occlusion bodies, no envelopment of nucleocapsids within the nucleus and enhanced BV production.
Ac62: lef-9, late expression factor 9
The protein LEF-9 has a molecular mass of 59.3 kDa (516 aa) and is required for late and very late gene expression (168). LEF-9 contains RNA polymerase motifs and is an essen