 as immediate-early, delayed-early, late and very late genes. Baculovirus transcripts may have 5'and 3' co-terminal ends and hence may overlap in sequence and time of expression. A canonical baculovirus transcription initiation motif (TAAG) is present in the promoter region of L and VL genes, whereas a more common motif (CAGT) is often associated with early baculovirus gene expression (recent review (213)).
The fact that for replication in cell culture only BVs are required and the ODV phenotype is dispensable allowed the development of the baculovirus expression system for production of recombinant protein in insect cells (254). Baculovirus VL genes are highly expressed but not required for virus propagation in cell culture, enabling their replacement with foreign genes. The promoters of the polyhedrin and p10 genes are extensively used as cassettes to drive the expression of single or multiple foreign genes in a baculovirus background. The development of an AcMNPV bacmid greatly facilitated the functional analysis of baculovirus genes, since it not only simplified the engineering of baculovirus expression vectors (172), but also made the construction of (knock-out) mutants much easier. The deletion of the cathepsin and the chitinase genes for instance, has improved the integrity of secreted recombinant proteins (120). The insertion of genes for the modification of glycoproteins in the Golgi system, has allowed the production of complex, mammalian-like glycoproteins in insect cells (115). AcMNPV can also be used as a delivery vector for mammalian cells and as gene therapy vector, either as a gene carrier or as a production system for other gene therapy vectors such as adeno-associated viruses (108, 174, 244). The baculovirus insect cell expression system is still being tailored and optimised to meet the demands of both the scientist as well as the commerce.
AcMNPV was isolated in 1969 by the late Dr. Patrick V. Vail and colleagues from a single virosed insect larva near Riverside (264). The insect was assigned as alfalfa looper or A. californica, but could also have been Trichoplusia ni as liquefied larvae are difficult to determine taxonomically. Later the late Dr. Lois K. Miller isolated an AcMNPV variant from A. californica on sunflower. The virus has an extremely wide host range, infecting insect species across several lepidopteran subfamilies (225). AcMNPV also replicates efficiently in cultured insect cells, such as Sf9, Sf21, Tn368, Tni High Five, and Se-UCR. Various clonal isolates of AcMNPV have been described (E2, L1, C6, HR). The genome of the AcMNPV C6 isolate was the first baculovirus genome that was sequenced completely in 1994 (GenBank: NC_001623) (9). The circular double stranded DNA genome is 133, 894 bp in size with a GC-content of 40.7 %. Partial resequencing of the AcMNPV genome ((92) led to a few modifications to the original sequence, which are not yet incorporated in the GenBank entry, resulting now in a total of 151 assigned open reading frames (ORFs). The encoded proteins range in size between 50 aa (normally set as the under limit for a baculovirus ORF) to 1221 aa (DNA helicase). All genes, except one (IE-0/IE-1), produce non-spliced transcripts. Over the years through collective effort of many laboratories around the world, functions have been assigned to many of the encoded gene products. However, the function of many ORFs -even some with orthologs in (many) other baculovirus species -remains enigmatic. At this moment in time nearly 50 baculovirus genomes have been completely sequenced and the genetic relatedness among the baculoviruses became apparent when further baculoviruses were being sequenced (266).
Eight regions with homologous repeats (hrs), each with a set of 28-mer imperfect palindromes, are present dispersed throughout the AcMNPV genome (9, 141). These hrs can act as origins of DNA replication in cell culture (135) and as enhancers of gene expression (33, 234, 271). Within the gene ac134 (p94), a sequence of direct and inverted repeats, palindromes and AT-rich regions different fr