**1. Introduction**

Baculoviridae is a diverse family of insect viruses with large, double-stranded, circular DNA genomes packaged in rod-shaped, enveloped nucleocapsids. A characteristic feature of baculoviruses is the production of paracrystalline occlusion bodies (OBs) which surround the assembled virions at late times of infection. Baculoviruses produce lethal infections in their hosts and OBs protect the virions in the environment after death of the insect until uptake by another susceptible host. Other insect viruses also produce OBs such as entomopoxviruses (EPV) and cytoplasmic polyhedrosis viruses (CPV). EPV and CPV replicate in the cytoplasm of infected cells; in contrast, baculoviruses replicate within the nucleus. According to the size and shape of OBs, baculoviruses were traditionally classified into two genera: nucleopolyhedrovirus (NPV), which produce large OBs known as polyhedra, and granulovirus (GV), which produce small ovoid OBs or granules. The major proteins that form each class of OB are known as polyhedrin and granulin, respectively. Recently, after several baculoviral genomes have been sequenced, a new classification based on the phylogenetic relationships between species within the family was accepted [1-2]. Four genera were defined: *Alphabaculovirus* (lepidopteran NPV), *Betabaculovirus* (lepidopter‐ an GV), *Gammabaculovirus* (hymenopteran NPV) and *Deltabaculovirus* (dipteran NPV). Baculoviruses are not infectious to vertebrates and they were known from far before they were recognized as viral entities as they produced disease outbreaks in the silkworm rearing. There are more than 600 species described in the literature [3] and some of them are widely used as bioinsecticides to control insect pests in agriculture and forestry [4]. A

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feature that differentiates baculovirus from other DNA viruses that replicate in the nuclei of infected cells is that they encode a novel DNA-dependent RNA polymerase. This enzyme, of uncertain evolutionary origin, is responsible for transcription of baculovirus late and very late genes. At this time of the infection the transcription of most cellular genes is shutoff and the synthesis of the polyhedrin/granulin becomes prominent to finally account for up to 95% of total cellular protein production. This high capacity of protein synthesis has been exploited for the development of baculoviruses as vectors for expres‐ sion of foreign proteins.

in changes operated in the expression of host genes and the progression of the cell cycle.

Baculovirus Gene Expression http://dx.doi.org/10.5772/56955 59

Apparently, the nucleocapsids of NPVs enter the nucleus through the nuclear pores, whereas the genome of GVs is probably injected [6]. Virions of baculoviruses are devoid of histones, in turn the DNA in the nucleocapsid is packed in association with viral protein P6.9, a basic DNA-binding protein. This small polypeptide is rich in arginine, serine and threonine residues, a feature similar to proteins called protamines present in the nuclei of spermatids in many animals and plants. The positive arginine residues in protamines neutralize the negative charges in the DNA backbone while serine and threonine medi‐ ate interaction between protamine molecules, resulting in a high condensation of genom‐ ic DNA. Once it is uncoated into the nucleus, the DNA dissociates from P6.9 through the phosphorylation of the protein. During infection the viral DNA appears to be organized in the form of nucleosomal-like structures in association with P6.9, as suggested by

In general, genes of DNA viruses are transcribed in a temporal sequence and the process is highly regulated by infection-derived mechanisms and proteins from both host and viral origin. This stepwise mode of gene expression ensures the availability of gene products required for the progression into the next phase of the infection. Baculoviruses express their genes in three successive phases designated as early, late and very late (figure 1). Early genes are transcribed by the host RNA polymerase II before virus DNA replica‐ tion, while late and very late genes are transcribed by a virus-encoded RNA polymerase, after starting of viral DNA replication. Products of a number of early genes are required for virus DNA synthesis and for expression of late/very late genes, and at least one late gene product is also needed for expression of very late genes. The progression of the infection into the late phase correlates with the transcriptional shutoff of cellular and early viral genes. Genes belonging to the different temporal classes are encoded in both DNA strands without any associative distribution in the genome. A number of baculovirus genes contains promoters with sequence elements characteristic of both early and late classes. Their transcription is regulated independently at each temporal phase which ensures their expression throughout the infection [8]. Most of our knowledge of baculovirus gene regulation comes from studies in *Autographa californica* nucleopolyhedrovirus (AcMNPV), which is the type species of the family. This alphabaculovirus has a wide host range and causes productive infections in permissive insect cell-lines Sf21 (and its clonal isolate Sf9), derived from *Spodoptera frugiperda* [9] and TN368, from *Trichoplusia ni* [10]. The genome of AcMNPV encodes 154 predicted open reading frames (ORFs) [11]. Roughly, one half corresponds to late genes, according to the sequence elements present in their promoters. About 25 gene products, mostly of early genes, have functions directly or indirectly related

At late times, ODVs become occluded into OBs in the periphery of the VS.

experiments of micrococcal nuclease digestion of isolated nuclei [7].

**4. Transcription program of baculovirus genes**

to gene expression.
