s 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 from the hrs is found and called the non-hr, which can also serve as an origin of DNA replication (134, 138). The non-hr region is also associated with the formation of "defective interfering particles" (DIs), which are generated as an artifact in cell culture and which interfere with the replication and production of infectious BVs (145, 222). These DI particles are baculoviruses with reduced size and genome content and a higher frequency of non-hr sequences. DIs are unable to propagate autonomously.
The purpose of this review is to provide an overview of the current knowledge on the function of AcMNPV ORFs and to serve as a starting point for researchers and students to gather further detailed information on particular ORFs. The overview will also assist researchers working with other baculoviruses, which carry homologous genes. In this review, all ORFs are listed in their order of appearance in the AcMNPV genome beginning wit the ptp gene (5'-3') and named according to the GenBank file (NC_ 001623). We choose for an encyclopaedic layout in which a short description of each ORF is presented, together with a small number of selected literature references referring to key publications, and/or review papers through which further relevant literature may be found. In the current paper the description of each ORF starts with the ORF number, e.g. ac1, where ac stands for AcMNPV and "1" for the number of the ORF as indicated in the database. This ORF number is followed by the name of the gene and by the name of the gene product. A genomic map of AcMNPV is given in Fig. 1, which serves to visualize the direction of transcription for individual ORFs and their genetic environment. The indicated sizes are the predicted molecular masses and the length in amino acids (aa) for the primary translation products. Post translational modifications can of course affect the actual size of the protein. A summary of the data is presented in Table 1. In this table the ORFs are also functionally categorized into four groups: genes for vir